BCH 306
ANALYTICAL METHODS IN BIOCHEMISTRY
At the end of the course, students should be able to explain principles of instrumentation in biochemistry; relate attainable precision to the analytical method employed; justify selection of methods for particular biochemical investigations; explain the theoretical basis of major biochemical instruments; and perform specified analytical investigations.
Accuracy; precision; calibration; sensitivity; selectivity; cell culture; immunoassay; blotting; isotope; electrophoresis; manometry; centrifugation; chromatography; retention factor; HPLC; spectroscopy; absorbance; Beer-Lambert law; fluorescence; NMR; mass spectrometry; GC-MS; polarography; potentiometry; thermocycler; PCR; validation.
Principles of Biochemical Analysis and Instrumentation
1.1 Analytical biochemistry
Analytical biochemistry applies physical, chemical and biological principles to detect, identify, separate and quantify molecules or cellular components. A useful method begins with a clearly defined analytical question.
1.2 Components of an instrument
Many instruments contain a source or stimulus, a sample-handling region, a selector or separation component, a detector or transducer, signal-processing electronics and a readout. The exact arrangement depends on what physical property is measured.
1.3 Calibration
Calibration establishes the relationship between instrument response and known standards. Unknown samples should normally be measured within the validated working range rather than by uncontrolled extrapolation.
1.4 Accuracy and precision
Accuracy describes closeness to an accepted or true value, whereas precision describes agreement among repeated measurements. A method can be precise but systematically inaccurate.
| Term | Meaning | Typical concern |
|---|---|---|
| Accuracy | Closeness to accepted value | Bias/systematic error |
| Precision | Agreement among repeats | Random variation |
| Sensitivity | Change in response per change in analyte | Ability to distinguish concentration changes |
| Selectivity | Ability to measure analyte despite interferents | Cross-reaction/matrix interference |
1.5 Sensitivity and detection
Sensitivity describes how strongly response changes with analyte concentration. The detection limit concerns the smallest amount distinguishable from background under defined conditions.
1.6 Specificity and selectivity
A highly selective method distinguishes the analyte from interfering substances. Biological assays often achieve selectivity through antibodies, enzymes, receptors or sequence complementarity.
1.7 Random and systematic error
Random error causes unpredictable scatter and primarily affects precision. Systematic error shifts results consistently and primarily affects accuracy.
1.8 Method selection
The preferred analytical method depends on analyte properties, concentration, sample complexity, required precision, available equipment, throughput, cost and whether structural or quantitative information is needed.
1.9 Quality control
Blanks, standards, controls, replicates and instrument checks reveal contamination, drift and procedural failure. Reliable results depend on quality control throughout the analytical process, not only on sophisticated equipment.
Analytical work should be designed backwards from the biological question. If the aim is to quantify a known protein, an immunoassay may be appropriate; if the aim is to separate unknown metabolites before identification, chromatography coupled to spectroscopy or mass spectrometry may be more informative. Instrument sophistication alone does not determine suitability.
Chapter Summary
- Analytical biochemistry: Analytical biochemistry applies physical, chemical and biological principles to detect, identify, separate and quantify molecules or cellular components.
- Components of an instrument: Many instruments contain a source or stimulus, a sample-handling region, a selector or separation component, a detector or transducer, signal-processing electronics and a readout.
- Calibration: Calibration establishes the relationship between instrument response and known standards.
- Accuracy and precision: Accuracy describes closeness to an accepted or true value, whereas precision describes agreement among repeated measurements.
- Sensitivity and detection: Sensitivity describes how strongly response changes with analyte concentration.
- Specificity and selectivity: A highly selective method distinguishes the analyte from interfering substances.
- Random and systematic error: Random error causes unpredictable scatter and primarily affects precision.
- Method selection: The preferred analytical method depends on analyte properties, concentration, sample complexity, required precision, available equipment, throughput, cost and whether structural or quantitative information is needed.
- Quality control: Blanks, standards, controls, replicates and instrument checks reveal contamination, drift and procedural failure.
Review and Examination Questions
- Explain the fundamental analytical principle underlying principles of biochemical analysis and instrumentation.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on principles of biochemical analysis and instrumentation.
Sample Collection, Preparation and Laboratory Quality
2.1 Representative sampling
An analytical result is meaningful only if the specimen represents the material or biological condition under investigation. Sampling design therefore precedes instrument choice.
2.2 Pre-analytical variables
Collection time, anticoagulant, temperature, delay before processing, light exposure, haemolysis and repeated freeze-thaw cycles can alter biochemical measurements.
2.3 Homogenisation
Tissues may be disrupted mechanically or chemically to release intracellular components. The method should preserve the analyte while achieving adequate extraction.
2.4 Buffers
Buffers stabilise pH during extraction and analysis. Buffer composition, ionic strength and temperature can influence proteins, membranes and enzyme activity.
2.5 Clarification
Centrifugation or filtration removes particulate material when a clear supernatant is required for subsequent analysis.
2.6 Controls
Positive controls demonstrate that a procedure can detect the expected signal; negative controls help reveal contamination or nonspecific response.
2.7 Replicates
Technical replicates assess measurement reproducibility, while biological replicates capture variation among independent biological samples.
2.8 Standard operating procedures
Written procedures improve consistency by defining specimen handling, reagent preparation, instrument settings, calculations, acceptance criteria and documentation.
2.9 Laboratory safety
Biological specimens, chemicals, compressed gases, high voltage, ultraviolet radiation and radioactive materials require hazard-specific controls, appropriate personal protective equipment and institutional procedures.
Sample preparation is part of the measurement system. A perfectly calibrated detector cannot recover information destroyed by poor specimen handling. Enzyme activity may fall during warm storage, metabolites may oxidise, cells may change gene expression after prolonged handling, and repeated freeze-thaw cycles can alter proteins.
Chapter Summary
- Representative sampling: An analytical result is meaningful only if the specimen represents the material or biological condition under investigation.
- Pre-analytical variables: Collection time, anticoagulant, temperature, delay before processing, light exposure, haemolysis and repeated freeze-thaw cycles can alter biochemical measurements.
- Homogenisation: Tissues may be disrupted mechanically or chemically to release intracellular components.
- Buffers: Buffers stabilise pH during extraction and analysis.
- Clarification: Centrifugation or filtration removes particulate material when a clear supernatant is required for subsequent analysis.
- Controls: Positive controls demonstrate that a procedure can detect the expected signal; negative controls help reveal contamination or nonspecific response.
- Replicates: Technical replicates assess measurement reproducibility, while biological replicates capture variation among independent biological samples.
- Standard operating procedures: Written procedures improve consistency by defining specimen handling, reagent preparation, instrument settings, calculations, acceptance criteria and documentation.
- Laboratory safety: Biological specimens, chemicals, compressed gases, high voltage, ultraviolet radiation and radioactive materials require hazard-specific controls, appropriate personal protective equipment and institutional procedures.
Review and Examination Questions
- Explain the fundamental analytical principle underlying sample collection, preparation and laboratory quality.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on sample collection, preparation and laboratory quality.
Tissue and Cell Culture Techniques
3.1 Culture concept
Cell culture maintains cells outside the organism under controlled nutritional and physicochemical conditions. Tissue culture is a broader term that can include cells, tissue fragments or organ-like preparations.
3.2 Aseptic technique
Sterility is maintained by disinfected work areas, sterile materials, careful handling and suitable containment. Microbial contamination can change cell behaviour even before it becomes visually obvious.
3.3 Culture media
Media provide salts, energy substrates, amino acids, vitamins and other components. Serum or defined supplements may supply hormones, attachment factors and growth signals.
3.4 Primary and continuous cultures
Primary cultures originate directly from tissues and often retain many differentiated features but have limited lifespan. Continuous cell lines can proliferate for extended periods and may differ substantially from their tissue of origin.
3.5 Adherent and suspension culture
Some cells require attachment to a treated surface, whereas others grow suspended in medium. Culture vessel and harvesting methods depend on growth behaviour.
3.6 Incubation conditions
Temperature, pH, gas composition, humidity and osmolality must suit the cell type. Mammalian culture commonly uses controlled carbon dioxide to support bicarbonate buffering.
3.7 Subculture
Growing cells must be transferred or diluted before excessive density, nutrient depletion or waste accumulation alters physiology.
3.8 Viability
Cell number and viability can be assessed by dye exclusion, metabolic assays or other validated methods. Viability measurements are essential when comparing treatment effects.
3.9 Applications
Cell culture supports studies of metabolism, toxicity, signalling, drug response, enzyme expression, molecular biology and production of biological materials.
3.10 Limitations
Cultured cells are simplified models. Passage number, genetic drift, contamination and loss of tissue architecture can limit extrapolation to an intact organism.
Cell culture illustrates the tension between experimental control and biological realism. Defined conditions make mechanisms easier to study, but removal from tissue architecture changes signalling and metabolism. Conclusions should therefore match the model actually used.
Chapter Summary
- Culture concept: Cell culture maintains cells outside the organism under controlled nutritional and physicochemical conditions.
- Aseptic technique: Sterility is maintained by disinfected work areas, sterile materials, careful handling and suitable containment.
- Culture media: Media provide salts, energy substrates, amino acids, vitamins and other components.
- Primary and continuous cultures: Primary cultures originate directly from tissues and often retain many differentiated features but have limited lifespan.
- Adherent and suspension culture: Some cells require attachment to a treated surface, whereas others grow suspended in medium.
- Incubation conditions: Temperature, pH, gas composition, humidity and osmolality must suit the cell type.
- Subculture: Growing cells must be transferred or diluted before excessive density, nutrient depletion or waste accumulation alters physiology.
- Viability: Cell number and viability can be assessed by dye exclusion, metabolic assays or other validated methods.
- Applications: Cell culture supports studies of metabolism, toxicity, signalling, drug response, enzyme expression, molecular biology and production of biological materials.
- Limitations: Cultured cells are simplified models.
Review and Examination Questions
- Explain the fundamental analytical principle underlying tissue and cell culture techniques.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on tissue and cell culture techniques.
Immunoassays
4.1 Antigen-antibody recognition
Immunoassays exploit selective binding between antibodies and antigens. The analytical signal reports formation of an immune complex.
4.2 Affinity and avidity
Affinity describes the strength of one binding interaction, whereas avidity reflects the combined strength of multiple interactions. Both influence assay performance.
4.3 Competitive assay
In a competitive format, sample analyte competes with labelled or immobilised analyte for limited antibody binding. Signal often varies inversely with analyte concentration.
4.4 Sandwich assay
A capture antibody binds analyte and a second detection antibody recognises another epitope. Signal commonly increases with analyte concentration.
4.5 ELISA
Enzyme-linked immunosorbent assays use an enzyme label to generate a measurable product. Formats include direct, indirect, sandwich and competitive designs.
4.6 Standards and controls
Quantitative immunoassays require standards of known concentration and appropriate controls. The calibration curve may be nonlinear and should be analysed using a suitable model.
4.7 Interference
Cross-reactivity, heterophile antibodies, matrix effects and extremely high analyte concentrations can distort results. Assay specificity must therefore be experimentally established.
4.8 Applications
Immunoassays quantify hormones, proteins, antibodies, antigens, drugs and other molecules in research and diagnostic settings.
Immunoassay selectivity is powerful but conditional. An antibody recognises molecular features rather than a printed analyte name; related molecules or altered epitopes can therefore produce unexpected responses. Calibration and specificity controls are as important as the antibody itself.
Chapter Summary
- Antigen-antibody recognition: Immunoassays exploit selective binding between antibodies and antigens.
- Affinity and avidity: Affinity describes the strength of one binding interaction, whereas avidity reflects the combined strength of multiple interactions.
- Competitive assay: In a competitive format, sample analyte competes with labelled or immobilised analyte for limited antibody binding.
- Sandwich assay: A capture antibody binds analyte and a second detection antibody recognises another epitope.
- ELISA: Enzyme-linked immunosorbent assays use an enzyme label to generate a measurable product.
- Standards and controls: Quantitative immunoassays require standards of known concentration and appropriate controls.
- Interference: Cross-reactivity, heterophile antibodies, matrix effects and extremely high analyte concentrations can distort results.
- Applications: Immunoassays quantify hormones, proteins, antibodies, antigens, drugs and other molecules in research and diagnostic settings.
Review and Examination Questions
- Explain the fundamental analytical principle underlying immunoassays.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on immunoassays.
Blotting Techniques
5.1 General principle
Blotting transfers separated biomolecules from a gel or other matrix to a durable membrane where specific targets can be detected.
5.2 Southern blot
Southern blotting detects specific DNA sequences after DNA separation and transfer to a membrane, using a complementary labelled nucleic-acid probe.
5.3 Northern blot
Northern blotting detects RNA species and can provide information about transcript size and relative abundance.
5.4 Western blot
Western blotting detects proteins, typically after electrophoretic separation, transfer and antibody-based recognition.
5.5 Transfer
Transfer can be driven electrically or by capillary action depending on the technique. Efficient transfer must preserve spatial separation.
5.6 Blocking
In immunoblotting, membrane sites not occupied by target protein are blocked to reduce nonspecific antibody binding.
5.7 Detection
Signals may be generated by colour reactions, chemiluminescence, fluorescence or other labels. Signal should be acquired within a useful quantitative range.
5.8 Controls
Loading controls, positive controls and negative controls help distinguish biological differences from unequal loading, transfer failure or nonspecific binding.
5.9 Interpretation
A band at an expected position supports target detection only when specificity and controls are adequate; band intensity is not automatically a valid concentration measurement.
Blotting combines separation with molecular recognition. The gel answers where molecules migrated, transfer makes them accessible on a membrane, and the probe or antibody asks whether the target is present. Failure at any stage can create a false conclusion.
Chapter Summary
- General principle: Blotting transfers separated biomolecules from a gel or other matrix to a durable membrane where specific targets can be detected.
- Southern blot: Southern blotting detects specific DNA sequences after DNA separation and transfer to a membrane, using a complementary labelled nucleic-acid probe.
- Northern blot: Northern blotting detects RNA species and can provide information about transcript size and relative abundance.
- Western blot: Western blotting detects proteins, typically after electrophoretic separation, transfer and antibody-based recognition.
- Transfer: Transfer can be driven electrically or by capillary action depending on the technique.
- Blocking: In immunoblotting, membrane sites not occupied by target protein are blocked to reduce nonspecific antibody binding.
- Detection: Signals may be generated by colour reactions, chemiluminescence, fluorescence or other labels.
- Controls: Loading controls, positive controls and negative controls help distinguish biological differences from unequal loading, transfer failure or nonspecific binding.
- Interpretation: A band at an expected position supports target detection only when specificity and controls are adequate; band intensity is not automatically a valid concentration measurement.
Review and Examination Questions
- Explain the fundamental analytical principle underlying blotting techniques.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on blotting techniques.
Isotopic and Radiochemical Techniques
6.1 Isotopes
Isotopes are atoms of the same element with the same proton number but different neutron numbers. Radioisotopes undergo spontaneous nuclear decay.
6.2 Activity
Radioactive activity is the rate of nuclear disintegration. The SI unit is the becquerel, corresponding to one disintegration per second.
6.3 Half-life
Half-life is the time required for the number of undecayed radioactive nuclei, and therefore activity under simple conditions, to fall to one half of its initial value.
Question: A radioactive tracer initially has activity 800 Bq and a half-life of 6 hours. What activity remains after 18 hours?
Given: 18 hours = 3 half-lives.
Formula: A = A0(1/2)n.
Solution: 800 × (1/2)3 = 100 Bq.
Answer: 100 Bq.
6.4 Tracer principle
A labelled molecule can be followed through a biochemical system when the isotope does not substantially alter its biological behaviour.
6.5 Detection
Radioactivity may be detected using devices such as Geiger-Müller counters, scintillation counters or specialised imaging systems, depending on radiation type and experimental design.
6.6 Autoradiography
Radioactive emissions can expose a detector or imaging medium, revealing the spatial distribution of labelled molecules.
6.7 Applications
Isotopic methods have been used to trace metabolic pathways, measure molecular turnover, study ligand binding and detect labelled nucleic acids or proteins.
6.8 Safety
Radiochemical work requires authorised facilities, training, contamination monitoring, shielding appropriate to radiation type, controlled waste management and minimisation of exposure.
6.9 Non-radioactive alternatives
Fluorescent, chemiluminescent and stable-isotope approaches can replace radioactivity in many applications, depending on sensitivity and experimental needs.
Radioisotopes are exceptionally useful tracers because nuclear decay can be detected independently of ordinary chemical concentration measurements. The same property creates a radiation hazard, so experimental sensitivity must be balanced with strict exposure and contamination control.
Chapter Summary
- Isotopes: Isotopes are atoms of the same element with the same proton number but different neutron numbers.
- Activity: Radioactive activity is the rate of nuclear disintegration.
- Half-life: Half-life is the time required for the number of undecayed radioactive nuclei, and therefore activity under simple conditions, to fall to one half of its initial value.
- Tracer principle: A labelled molecule can be followed through a biochemical system when the isotope does not substantially alter its biological behaviour.
- Detection: Radioactivity may be detected using devices such as Geiger-Müller counters, scintillation counters or specialised imaging systems, depending on radiation type and experimental design.
- Autoradiography: Radioactive emissions can expose a detector or imaging medium, revealing the spatial distribution of labelled molecules.
- Applications: Isotopic methods have been used to trace metabolic pathways, measure molecular turnover, study ligand binding and detect labelled nucleic acids or proteins.
- Safety: Radiochemical work requires authorised facilities, training, contamination monitoring, shielding appropriate to radiation type, controlled waste management and minimisation of exposure.
- Non-radioactive alternatives: Fluorescent, chemiluminescent and stable-isotope approaches can replace radioactivity in many applications, depending on sensitivity and experimental needs.
Review and Examination Questions
- Explain the fundamental analytical principle underlying isotopic and radiochemical techniques.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on isotopic and radiochemical techniques.
Electrophoresis
7.1 Principle
Electrophoresis separates charged molecules by their migration in an electric field. Mobility depends on charge, size, shape, field strength and properties of the supporting medium.
7.2 Agarose gels
Agarose gels are widely used for nucleic acids. Pore size depends on gel concentration and influences the range of fragments resolved.
7.3 Polyacrylamide gels
Polyacrylamide provides smaller, controllable pores and high resolving power for proteins and smaller nucleic acids.
7.4 SDS-PAGE
SDS denatures many proteins and gives them a broadly similar negative charge-to-mass ratio, so migration primarily reflects polypeptide size.
7.5 Native PAGE
Native electrophoresis preserves more of a protein's conformation and intrinsic charge, so migration depends on size, shape and charge.
7.6 Isoelectric focusing
Proteins migrate through a pH gradient until their net charge approaches zero at the isoelectric point.
7.7 Buffers and voltage
Buffer ions carry current and maintain pH. Excessive voltage can produce heating, distorted bands and loss of resolution.
7.8 Staining
Separated molecules may be visualised using dyes, fluorescent stains or specific detection methods.
7.9 Molecular-size estimation
Migration of standards can provide an empirical calibration for estimating molecular size, but estimates depend on method assumptions.
7.10 Applications
Electrophoresis is used for purity assessment, molecular-size estimation, isoform analysis, nucleic-acid fragment analysis and preparation for blotting.
Electrophoretic resolution depends on both molecule and matrix. Increasing gel concentration reduces effective pore size; denaturing agents change molecular conformation; buffer pH changes charge; and excessive heating broadens bands. A gel is therefore an engineered separation environment.
Chapter Summary
- Principle: Electrophoresis separates charged molecules by their migration in an electric field.
- Agarose gels: Agarose gels are widely used for nucleic acids.
- Polyacrylamide gels: Polyacrylamide provides smaller, controllable pores and high resolving power for proteins and smaller nucleic acids.
- SDS-PAGE: SDS denatures many proteins and gives them a broadly similar negative charge-to-mass ratio, so migration primarily reflects polypeptide size.
- Native PAGE: Native electrophoresis preserves more of a protein's conformation and intrinsic charge, so migration depends on size, shape and charge.
- Isoelectric focusing: Proteins migrate through a pH gradient until their net charge approaches zero at the isoelectric point.
- Buffers and voltage: Buffer ions carry current and maintain pH.
- Staining: Separated molecules may be visualised using dyes, fluorescent stains or specific detection methods.
- Molecular-size estimation: Migration of standards can provide an empirical calibration for estimating molecular size, but estimates depend on method assumptions.
- Applications: Electrophoresis is used for purity assessment, molecular-size estimation, isoform analysis, nucleic-acid fragment analysis and preparation for blotting.
Review and Examination Questions
- Explain the fundamental analytical principle underlying electrophoresis.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on electrophoresis.
Centrifugation and Ultracentrifugation
8.1 Sedimentation
Centrifugation accelerates sedimentation by applying centrifugal force. Sedimentation depends on particle size, shape, density, medium density and viscosity.
8.2 RCF and RPM
Revolutions per minute describe rotor speed, whereas relative centrifugal force expresses acceleration relative to gravity and also depends on rotor radius.
Question: A rotor has radius 10 cm and runs at 10,000 rpm. Estimate RCF using RCF = 1.118 × 10−5 × r(cm) × rpm².
Solution: RCF = 1.118 × 10−5 × 10 × (10,000)² = 11,180.
Answer: Approximately 11,180 × g.
8.3 Differential centrifugation
Sequential centrifugation at increasing forces separates components according to differences in sedimentation rate, commonly producing enriched cellular fractions.
8.4 Density-gradient centrifugation
A density gradient improves separation. In rate-zonal methods particles separate mainly by sedimentation rate before pelleting; in isopycnic methods they migrate toward positions matching their buoyant density.
8.5 Rotors
Fixed-angle and swinging-bucket rotors produce different sedimentation paths and are selected according to the separation required.
8.6 Ultracentrifugation
Very high centrifugal fields permit separation or analysis of small particles, macromolecules and organelles.
8.7 Sedimentation coefficient
The sedimentation coefficient relates sedimentation velocity to applied acceleration and is commonly expressed in Svedberg units.
8.8 Applications
Centrifugation supports cell fractionation, clarification, organelle isolation, macromolecular purification and analytical studies.
8.9 Practical safety
Tubes must be compatible, balanced and inspected; rotors require correct loading and maintenance because high-speed rotor failure is hazardous.
RCF should be reported rather than RPM when reproducibility across centrifuges matters. Two rotors operating at the same RPM can generate different centrifugal accelerations because radius differs. This is a common practical source of confusion.
Chapter Summary
- Sedimentation: Centrifugation accelerates sedimentation by applying centrifugal force.
- RCF and RPM: Revolutions per minute describe rotor speed, whereas relative centrifugal force expresses acceleration relative to gravity and also depends on rotor radius.
- Differential centrifugation: Sequential centrifugation at increasing forces separates components according to differences in sedimentation rate, commonly producing enriched cellular fractions.
- Density-gradient centrifugation: A density gradient improves separation.
- Rotors: Fixed-angle and swinging-bucket rotors produce different sedimentation paths and are selected according to the separation required.
- Ultracentrifugation: Very high centrifugal fields permit separation or analysis of small particles, macromolecules and organelles.
- Sedimentation coefficient: The sedimentation coefficient relates sedimentation velocity to applied acceleration and is commonly expressed in Svedberg units.
- Applications: Centrifugation supports cell fractionation, clarification, organelle isolation, macromolecular purification and analytical studies.
- Practical safety: Tubes must be compatible, balanced and inspected; rotors require correct loading and maintenance because high-speed rotor failure is hazardous.
Review and Examination Questions
- Explain the fundamental analytical principle underlying centrifugation and ultracentrifugation.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on centrifugation and ultracentrifugation.
Manometry and Gas-Exchange Measurements
9.1 Manometry
Manometry measures pressure changes in a closed or controlled system. In classical biochemistry it was used to follow gas consumption or production during biological reactions.
9.2 Pressure-volume relationship
Gas pressure changes reflect changes in gas amount when temperature and volume are appropriately controlled. Interpretation follows gas-law principles.
9.3 Warburg apparatus
The classical Warburg manometer allowed measurement of oxygen uptake or carbon dioxide production by tissues, cells or enzyme systems.
9.4 Carbon dioxide absorption
Alkali can absorb carbon dioxide, allowing pressure change to be interpreted primarily as oxygen consumption in suitable experimental arrangements.
9.5 Controls
Thermal equilibration, blanks and matched vessels are essential because pressure is highly sensitive to temperature and non-biological gas changes.
9.6 Applications
Manometry historically contributed to studies of respiration, fermentation and enzyme reactions involving gases.
9.7 Modern context
Oxygen electrodes and modern respirometry often provide more convenient measurements, but manometry remains important for understanding the physical basis of gas-exchange analysis.
Manometry teaches a general analytical principle: an indirect signal must be connected to the biochemical event through a physical model. Pressure change is not itself respiration; experimental design makes it interpretable as oxygen consumption or gas production.
Chapter Summary
- Manometry: Manometry measures pressure changes in a closed or controlled system.
- Pressure-volume relationship: Gas pressure changes reflect changes in gas amount when temperature and volume are appropriately controlled.
- Warburg apparatus: The classical Warburg manometer allowed measurement of oxygen uptake or carbon dioxide production by tissues, cells or enzyme systems.
- Carbon dioxide absorption: Alkali can absorb carbon dioxide, allowing pressure change to be interpreted primarily as oxygen consumption in suitable experimental arrangements.
- Controls: Thermal equilibration, blanks and matched vessels are essential because pressure is highly sensitive to temperature and non-biological gas changes.
- Applications: Manometry historically contributed to studies of respiration, fermentation and enzyme reactions involving gases.
- Modern context: Oxygen electrodes and modern respirometry often provide more convenient measurements, but manometry remains important for understanding the physical basis of gas-exchange analysis.
Review and Examination Questions
- Explain the fundamental analytical principle underlying manometry and gas-exchange measurements.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on manometry and gas-exchange measurements.
Paper and Thin-Layer Chromatography
10.1 Chromatographic principle
Chromatography separates mixture components through different distributions between a stationary phase and a mobile phase.
10.2 Paper chromatography
Cellulose paper supports separation as solvent moves by capillary action. Solutes partition differently according to their interactions with the aqueous/stationary environment and mobile solvent.
10.3 Thin-layer chromatography
TLC uses a thin layer of adsorbent such as silica on a support. Components migrate at different rates according to adsorption and solubility.
10.4 Rf value
The retention factor is the distance travelled by a solute divided by the distance travelled by the solvent front under the same run conditions.
Question: A solute spot travels 5.4 cm while the solvent front travels 9.0 cm. Calculate Rf.
Formula: Rf = distance travelled by solute ÷ distance travelled by solvent front.
Solution: 5.4/9.0 = 0.60.
Answer: Rf = 0.60.
10.5 Development
The origin is placed above the solvent level, the chamber is closed, and solvent ascends until a suitable separation distance is reached.
10.6 Visualisation
Colourless compounds can be detected using specific chemical reagents, fluorescence or ultraviolet illumination where appropriate.
10.7 Identification
Rf values can support identification only when standards are run under the same conditions; they are not universal constants.
10.8 Applications
Paper and TLC methods are useful for teaching and screening amino acids, sugars, lipids, pigments and other small molecules.
Rf values are operational measurements. They depend on stationary phase, solvent, temperature, chamber saturation and sample loading. A spot matching a standard under identical conditions is useful evidence, but an Rf copied from another solvent system is not directly comparable.
Chapter Summary
- Chromatographic principle: Chromatography separates mixture components through different distributions between a stationary phase and a mobile phase.
- Paper chromatography: Cellulose paper supports separation as solvent moves by capillary action.
- Thin-layer chromatography: TLC uses a thin layer of adsorbent such as silica on a support.
- Rf value: The retention factor is the distance travelled by a solute divided by the distance travelled by the solvent front under the same run conditions.
- Development: The origin is placed above the solvent level, the chamber is closed, and solvent ascends until a suitable separation distance is reached.
- Visualisation: Colourless compounds can be detected using specific chemical reagents, fluorescence or ultraviolet illumination where appropriate.
- Identification: Rf values can support identification only when standards are run under the same conditions; they are not universal constants.
- Applications: Paper and TLC methods are useful for teaching and screening amino acids, sugars, lipids, pigments and other small molecules.
Review and Examination Questions
- Explain the fundamental analytical principle underlying paper and thin-layer chromatography.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on paper and thin-layer chromatography.
Column Chromatography
11.1 Column format
A stationary phase is packed into a column and mobile phase passes through it. Sample components move at rates determined by their interactions with the stationary phase.
11.2 Adsorption chromatography
Compounds bind to a solid stationary phase with different strengths and are eluted by changing solvent conditions.
11.3 Ion exchange
Charged groups on the stationary phase bind oppositely charged analytes. Salt concentration or pH can be changed to elute bound molecules.
11.4 Size exclusion
Porous beads separate molecules by hydrodynamic size. Larger molecules access less pore volume and generally elute earlier.
11.5 Affinity chromatography
An immobilised ligand selectively captures a target molecule based on biological or chemical recognition.
11.6 Fractions
Column eluate is collected in fractions. Each fraction can be analysed for absorbance, activity or another target-specific property.
11.7 Resolution
Resolution depends on particle size, column dimensions, flow rate, sample volume and the selectivity of the separation mechanism.
11.8 Applications
Column methods are central to purification of proteins, nucleic acids, metabolites and other biochemical compounds.
Column chromatography demonstrates the trade-off among resolution, speed and recovery. Narrow peaks and good separation are desirable, but very slow or harsh methods may reduce biological activity. Purification is therefore an optimisation problem, not simply a quest for the longest column.
Chapter Summary
- Column format: A stationary phase is packed into a column and mobile phase passes through it.
- Adsorption chromatography: Compounds bind to a solid stationary phase with different strengths and are eluted by changing solvent conditions.
- Ion exchange: Charged groups on the stationary phase bind oppositely charged analytes.
- Size exclusion: Porous beads separate molecules by hydrodynamic size.
- Affinity chromatography: An immobilised ligand selectively captures a target molecule based on biological or chemical recognition.
- Fractions: Column eluate is collected in fractions.
- Resolution: Resolution depends on particle size, column dimensions, flow rate, sample volume and the selectivity of the separation mechanism.
- Applications: Column methods are central to purification of proteins, nucleic acids, metabolites and other biochemical compounds.
Review and Examination Questions
- Explain the fundamental analytical principle underlying column chromatography.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on column chromatography.
Gas Chromatography
12.1 Principle
Gas chromatography separates volatile compounds between a gaseous mobile phase and a stationary phase within a column.
12.2 Carrier gas
An inert carrier gas transports vaporised analytes through the column at controlled flow.
12.3 Injector
The sample is introduced into a heated inlet where suitable analytes are vaporised rapidly.
12.4 Column and oven
Capillary or packed columns reside in a temperature-controlled oven. Temperature programming can improve separation of compounds with different volatilities.
12.5 Retention time
A compound's retention time reflects its volatility and interaction with the stationary phase under defined conditions.
12.6 Detectors
Different detectors respond to different chemical properties. Flame-ionisation detection is common for many organic compounds, while mass spectrometry provides structural information.
12.7 Derivatisation
Nonvolatile or thermally unstable biochemical compounds may require chemical derivatisation to make them suitable for GC.
12.8 Applications
GC can analyse fatty-acid derivatives, solvents, volatile metabolites and other compounds compatible with gas-phase separation.
GC is strongest when analytes can enter the gas phase without decomposition. Biochemical compounds with polar functional groups may require derivatisation, which changes their chromatographic behaviour but can make otherwise unsuitable molecules volatile enough for analysis.
Chapter Summary
- Principle: Gas chromatography separates volatile compounds between a gaseous mobile phase and a stationary phase within a column.
- Carrier gas: An inert carrier gas transports vaporised analytes through the column at controlled flow.
- Injector: The sample is introduced into a heated inlet where suitable analytes are vaporised rapidly.
- Column and oven: Capillary or packed columns reside in a temperature-controlled oven.
- Retention time: A compound's retention time reflects its volatility and interaction with the stationary phase under defined conditions.
- Detectors: Different detectors respond to different chemical properties.
- Derivatisation: Nonvolatile or thermally unstable biochemical compounds may require chemical derivatisation to make them suitable for GC.
- Applications: GC can analyse fatty-acid derivatives, solvents, volatile metabolites and other compounds compatible with gas-phase separation.
Review and Examination Questions
- Explain the fundamental analytical principle underlying gas chromatography.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on gas chromatography.
High-Performance Liquid Chromatography
13.1 HPLC principle
HPLC drives liquid mobile phase through a tightly packed stationary phase under high pressure, enabling efficient and reproducible separations.
13.2 Pump
The pump delivers mobile phase at controlled flow and pressure. Stable flow is essential for reproducible retention.
13.3 Injector and autosampler
A defined sample volume is introduced into the flowing mobile phase manually or automatically.
13.4 Columns
Stationary phases are selected according to the separation mechanism. Reversed-phase HPLC is widely used for molecules differing in hydrophobicity.
13.5 Isocratic and gradient elution
Isocratic runs maintain constant mobile-phase composition, while gradients change composition during the run to elute compounds spanning a wider range of retention.
13.6 Detectors
UV-visible absorbance, fluorescence, refractive-index and mass-spectrometric detectors can be coupled to HPLC depending on analyte properties.
13.7 Chromatogram
Peak retention and area provide qualitative and quantitative information when calibration, resolution and detector response are appropriate.
| Chromatographic feature | Typical analytical meaning |
|---|---|
| Retention time | Supports identification under fixed conditions |
| Peak area | Often proportional to amount within calibrated range |
| Peak width | Reflects dispersion/efficiency |
| Resolution | Degree of separation between neighbouring peaks |
13.8 Applications
HPLC is used for amino acids, peptides, nucleotides, drugs, metabolites and many other biochemical analytes.
13.9 Method development
Mobile-phase composition, pH, column chemistry, flow rate and temperature are optimised to obtain adequate selectivity, peak shape and analysis time.
HPLC performance emerges from interactions among stationary phase, mobile phase and analyte. A change in pH can alter ionisation; organic-solvent proportion can alter hydrophobic retention; flow rate affects analysis time and efficiency. Method development systematically adjusts these variables.
Chapter Summary
- HPLC principle: HPLC drives liquid mobile phase through a tightly packed stationary phase under high pressure, enabling efficient and reproducible separations.
- Pump: The pump delivers mobile phase at controlled flow and pressure.
- Injector and autosampler: A defined sample volume is introduced into the flowing mobile phase manually or automatically.
- Columns: Stationary phases are selected according to the separation mechanism.
- Isocratic and gradient elution: Isocratic runs maintain constant mobile-phase composition, while gradients change composition during the run to elute compounds spanning a wider range of retention.
- Detectors: UV-visible absorbance, fluorescence, refractive-index and mass-spectrometric detectors can be coupled to HPLC depending on analyte properties.
- Chromatogram: Peak retention and area provide qualitative and quantitative information when calibration, resolution and detector response are appropriate.
- Applications: HPLC is used for amino acids, peptides, nucleotides, drugs, metabolites and many other biochemical analytes.
- Method development: Mobile-phase composition, pH, column chemistry, flow rate and temperature are optimised to obtain adequate selectivity, peak shape and analysis time.
Review and Examination Questions
- Explain the fundamental analytical principle underlying high-performance liquid chromatography.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on high-performance liquid chromatography.
UV-Visible Spectrophotometry
14.1 Absorption
Molecules absorb ultraviolet or visible radiation when photon energy promotes electronic transitions. The wavelength pattern depends on molecular structure.
14.2 Beer-Lambert law
Within an appropriate range, absorbance is proportional to molar absorptivity, optical path length and concentration.
Question: A compound has molar absorptivity 12,000 L mol−1 cm−1. Its absorbance in a 1.00 cm cuvette is 0.600. Find concentration.
Formula: A = εbc; therefore c = A/(εb).
Solution: c = 0.600/(12,000 × 1.00) = 5.0 × 10−5 mol L−1.
Answer: 5.0 × 10−5 M.
14.3 Instrument components
A spectrophotometer includes a radiation source, wavelength selector, sample holder, detector and signal-processing system.
14.4 Blank
A reagent blank accounts for absorbance contributed by solvent, cuvette and reagents other than the analyte.
14.5 Calibration curve
When molar absorptivity is unknown or matrix effects are important, standards of known concentration can establish the response-concentration relationship.
Standards should bracket the expected unknown concentration. A blank establishes baseline response. Replicate standards reveal imprecision, while quality-control samples that are not used to fit the calibration curve provide an independent check on analytical performance.
14.6 Protein and nucleic acids
Proteins can absorb near 280 nm because of aromatic residues, while nucleic acids absorb strongly near 260 nm; quantitative interpretation requires appropriate assumptions and purity considerations.
14.7 Limitations
Stray light, scattering, dirty cuvettes, chemical equilibria and concentrations outside the linear range can violate simple Beer-Lambert behaviour.
14.8 Applications
UV-visible methods support enzyme kinetics, concentration measurement, reaction monitoring and analysis of coloured biochemical products.
Absorbance is logarithmically related to transmitted light but linearly related to concentration only under Beer-Lambert conditions. Dilution of an over-range sample is therefore often more defensible than extrapolating far beyond calibrated absorbance.
Chapter Summary
- Absorption: Molecules absorb ultraviolet or visible radiation when photon energy promotes electronic transitions.
- Beer-Lambert law: Within an appropriate range, absorbance is proportional to molar absorptivity, optical path length and concentration.
- Instrument components: A spectrophotometer includes a radiation source, wavelength selector, sample holder, detector and signal-processing system.
- Blank: A reagent blank accounts for absorbance contributed by solvent, cuvette and reagents other than the analyte.
- Calibration curve: When molar absorptivity is unknown or matrix effects are important, standards of known concentration can establish the response-concentration relationship.
- Protein and nucleic acids: Proteins can absorb near 280 nm because of aromatic residues, while nucleic acids absorb strongly near 260 nm; quantitative interpretation requires appropriate assumptions and purity considerations.
- Limitations: Stray light, scattering, dirty cuvettes, chemical equilibria and concentrations outside the linear range can violate simple Beer-Lambert behaviour.
- Applications: UV-visible methods support enzyme kinetics, concentration measurement, reaction monitoring and analysis of coloured biochemical products.
Review and Examination Questions
- Explain the fundamental analytical principle underlying uv-visible spectrophotometry.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on uv-visible spectrophotometry.
Infrared and Fourier-Transform Infrared Spectroscopy
15.1 Infrared absorption
Infrared radiation excites molecular vibrations. Absorption frequencies depend on bond strength, atomic masses and molecular environment.
15.2 Functional groups
Characteristic absorption regions can reveal functional groups such as O–H, N–H, C=O and other bonds.
15.3 Fingerprint region
The lower-frequency fingerprint region contains complex patterns useful for comparing and identifying compounds.
15.4 FTIR principle
Fourier-transform infrared instruments collect an interferogram containing information from many wavelengths and mathematically transform it into an infrared spectrum.
15.5 Interferometer
A Michelson-type interferometer modulates infrared radiation before it reaches the sample, enabling rapid collection and good signal-to-noise performance.
15.6 Sample handling
Liquids, solids, films and biological materials can be analysed using suitable transmission or attenuated-total-reflectance arrangements.
15.7 Applications
FTIR can characterise functional groups, monitor chemical changes and investigate proteins, lipids and other biomaterials.
15.8 Limitations
Water absorbs strongly in parts of the infrared region, overlapping bands complicate interpretation, and definitive structural identification often requires complementary methods.
FTIR converts an interferogram into a spectrum mathematically. The instrument does not simply scan one wavelength after another in the same manner as a basic dispersive spectrometer. This distinction explains the speed and signal advantages associated with Fourier-transform acquisition.
Chapter Summary
- Infrared absorption: Infrared radiation excites molecular vibrations.
- Functional groups: Characteristic absorption regions can reveal functional groups such as O–H, N–H, C=O and other bonds.
- Fingerprint region: The lower-frequency fingerprint region contains complex patterns useful for comparing and identifying compounds.
- FTIR principle: Fourier-transform infrared instruments collect an interferogram containing information from many wavelengths and mathematically transform it into an infrared spectrum.
- Interferometer: A Michelson-type interferometer modulates infrared radiation before it reaches the sample, enabling rapid collection and good signal-to-noise performance.
- Sample handling: Liquids, solids, films and biological materials can be analysed using suitable transmission or attenuated-total-reflectance arrangements.
- Applications: FTIR can characterise functional groups, monitor chemical changes and investigate proteins, lipids and other biomaterials.
- Limitations: Water absorbs strongly in parts of the infrared region, overlapping bands complicate interpretation, and definitive structural identification often requires complementary methods.
Review and Examination Questions
- Explain the fundamental analytical principle underlying infrared and fourier-transform infrared spectroscopy.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on infrared and fourier-transform infrared spectroscopy.
Fluorimetry
16.1 Fluorescence
Some molecules absorb light at one wavelength and emit light at a longer wavelength after rapid relaxation from an excited electronic state.
16.2 Excitation and emission
A fluorimeter selects an excitation wavelength and measures emitted light, commonly at approximately right angles to reduce transmitted excitation light reaching the detector.
16.3 Stokes shift
The difference between excitation and emission maxima helps separate emitted fluorescence from excitation radiation.
16.4 Sensitivity
Fluorescence methods can be highly sensitive because emitted light is measured against a relatively low background.
16.5 Quantum yield
Fluorescence intensity depends on the probability that absorbed energy is emitted as fluorescence rather than lost through other processes.
16.6 Quenching
Molecular collisions, complex formation, oxygen and other processes can reduce fluorescence intensity and must be controlled.
16.7 Intrinsic fluorescence
Aromatic amino acids, especially tryptophan, can provide information about protein concentration, folding and local environment.
16.8 Fluorescent labels
Non-fluorescent analytes can be measured after reaction with fluorogenic reagents or attachment of fluorescent probes.
16.9 Applications
Fluorimetry is used in enzyme assays, binding studies, nucleic-acid quantification, immunoassays and cellular imaging-related measurements.
Fluorescence sensitivity does not eliminate the need for calibration. Quenching, photobleaching, inner-filter effects and environmental changes can make intensity depart from simple proportionality to concentration.
Chapter Summary
- Fluorescence: Some molecules absorb light at one wavelength and emit light at a longer wavelength after rapid relaxation from an excited electronic state.
- Excitation and emission: A fluorimeter selects an excitation wavelength and measures emitted light, commonly at approximately right angles to reduce transmitted excitation light reaching the detector.
- Stokes shift: The difference between excitation and emission maxima helps separate emitted fluorescence from excitation radiation.
- Sensitivity: Fluorescence methods can be highly sensitive because emitted light is measured against a relatively low background.
- Quantum yield: Fluorescence intensity depends on the probability that absorbed energy is emitted as fluorescence rather than lost through other processes.
- Quenching: Molecular collisions, complex formation, oxygen and other processes can reduce fluorescence intensity and must be controlled.
- Intrinsic fluorescence: Aromatic amino acids, especially tryptophan, can provide information about protein concentration, folding and local environment.
- Fluorescent labels: Non-fluorescent analytes can be measured after reaction with fluorogenic reagents or attachment of fluorescent probes.
- Applications: Fluorimetry is used in enzyme assays, binding studies, nucleic-acid quantification, immunoassays and cellular imaging-related measurements.
Review and Examination Questions
- Explain the fundamental analytical principle underlying fluorimetry.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on fluorimetry.
Nuclear Magnetic Resonance Spectroscopy
17.1 Nuclear spin
Certain nuclei possess spin and magnetic moments. In an external magnetic field, their energy states become distinguishable.
17.2 Resonance
Radiofrequency energy can induce transitions between nuclear spin states when the applied frequency matches the energy difference.
17.3 Chemical shift
Electrons shield nuclei from the external field to different extents, causing resonance positions that reflect chemical environment.
17.4 Spin-spin coupling
Magnetic interactions between neighbouring nuclei can split signals and provide information about molecular connectivity.
17.5 Integration
For suitable quantitative proton spectra, integrated signal area can reflect the relative number of contributing nuclei.
17.6 Sample and solvent
NMR commonly uses deuterated solvents to reduce solvent proton signals and provide field-locking capability.
17.7 Biochemical applications
NMR can determine structures of small molecules, investigate protein and nucleic-acid structure, study molecular interactions and profile metabolites.
17.8 Strengths
NMR is non-destructive and information-rich, but it generally requires more sample than highly sensitive mass-spectrometric methods.
17.9 Instrumentation
Major components include a strong magnet, radiofrequency transmitter and receiver, probe, field-control electronics and computational Fourier transformation.
NMR and mass spectrometry provide different kinds of structural evidence. NMR reports nuclei in their chemical environments and can describe connectivity and dynamics; MS reports ion mass-to-charge values and fragmentation. Combining them can resolve structures that either method alone leaves uncertain.
Chapter Summary
- Nuclear spin: Certain nuclei possess spin and magnetic moments.
- Resonance: Radiofrequency energy can induce transitions between nuclear spin states when the applied frequency matches the energy difference.
- Chemical shift: Electrons shield nuclei from the external field to different extents, causing resonance positions that reflect chemical environment.
- Spin-spin coupling: Magnetic interactions between neighbouring nuclei can split signals and provide information about molecular connectivity.
- Integration: For suitable quantitative proton spectra, integrated signal area can reflect the relative number of contributing nuclei.
- Sample and solvent: NMR commonly uses deuterated solvents to reduce solvent proton signals and provide field-locking capability.
- Biochemical applications: NMR can determine structures of small molecules, investigate protein and nucleic-acid structure, study molecular interactions and profile metabolites.
- Strengths: NMR is non-destructive and information-rich, but it generally requires more sample than highly sensitive mass-spectrometric methods.
- Instrumentation: Major components include a strong magnet, radiofrequency transmitter and receiver, probe, field-control electronics and computational Fourier transformation.
Review and Examination Questions
- Explain the fundamental analytical principle underlying nuclear magnetic resonance spectroscopy.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on nuclear magnetic resonance spectroscopy.
Mass Spectrometry
18.1 Principle
Mass spectrometry converts analytes into gas-phase ions, separates ions according to mass-to-charge ratio and records their abundance.
18.2 Ionisation
Ionisation methods are chosen to suit analyte properties. Electron ionisation is common in GC-MS, while electrospray and MALDI are widely used for biomolecules.
18.3 Mass analyser
Analysers such as quadrupoles, time-of-flight devices and ion traps separate ions according to mass-to-charge behaviour using different physical principles.
18.4 Detector
The detector converts arriving ions into electrical signals from which a mass spectrum is generated.
18.5 Molecular mass
A molecular ion or related ion series can provide molecular-mass information, subject to charge state, adducts and fragmentation.
18.6 Fragmentation
Controlled fragmentation provides structural information because bond cleavage generates diagnostic product ions.
18.7 Proteomics
Peptides produced by protein digestion can be analysed by tandem mass spectrometry to identify proteins and investigate modifications.
18.8 Metabolomics
Mass spectrometry coupled to chromatography can measure complex mixtures of metabolites with high sensitivity.
18.9 Limitations
Ion suppression, matrix effects, isobaric compounds and identification uncertainty require appropriate standards, separation and data analysis.
Mass spectra must be interpreted in terms of ions rather than neutral molecules alone. Charge states, isotope patterns, adducts and fragments can create several peaks from one analyte. Modern biomolecular MS therefore relies on both instrumentation and computational interpretation.
Chapter Summary
- Principle: Mass spectrometry converts analytes into gas-phase ions, separates ions according to mass-to-charge ratio and records their abundance.
- Ionisation: Ionisation methods are chosen to suit analyte properties.
- Mass analyser: Analysers such as quadrupoles, time-of-flight devices and ion traps separate ions according to mass-to-charge behaviour using different physical principles.
- Detector: The detector converts arriving ions into electrical signals from which a mass spectrum is generated.
- Molecular mass: A molecular ion or related ion series can provide molecular-mass information, subject to charge state, adducts and fragmentation.
- Fragmentation: Controlled fragmentation provides structural information because bond cleavage generates diagnostic product ions.
- Proteomics: Peptides produced by protein digestion can be analysed by tandem mass spectrometry to identify proteins and investigate modifications.
- Metabolomics: Mass spectrometry coupled to chromatography can measure complex mixtures of metabolites with high sensitivity.
- Limitations: Ion suppression, matrix effects, isobaric compounds and identification uncertainty require appropriate standards, separation and data analysis.
Review and Examination Questions
- Explain the fundamental analytical principle underlying mass spectrometry.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on mass spectrometry.
Gas Chromatography-Mass Spectrometry
19.1 Hyphenated technique
GC-MS combines chromatographic separation with mass-spectrometric detection, providing both retention behaviour and mass spectral information.
19.2 Interface
Compounds eluting from the GC column enter the mass spectrometer through an interface designed to preserve separation and maintain vacuum requirements.
19.3 Electron ionisation
Electron ionisation produces reproducible fragmentation patterns useful for library matching of many volatile organic compounds.
19.4 Identification
Confidence increases when retention information and mass spectra agree with authentic standards or validated spectral libraries.
19.5 Quantification
An internal standard can compensate for variation in extraction, injection and instrumental response when properly selected.
19.6 Derivatisation
Many polar metabolites require derivatisation before GC-MS to improve volatility and thermal stability.
19.7 Applications
GC-MS is used for organic acids, fatty-acid derivatives, steroids, environmental compounds and metabolomic analyses compatible with gas chromatography.
19.8 Quality assurance
Blanks, calibration standards, quality-control samples and checks for carryover are essential in long analytical sequences.
Hyphenated instrumentation increases confidence by combining separation and detection. A GC peak alone may be nonspecific and a mass spectrum from a mixture may be difficult to interpret; GC-MS first reduces mixture complexity and then obtains spectral information for each eluting component.
Chapter Summary
- Hyphenated technique: GC-MS combines chromatographic separation with mass-spectrometric detection, providing both retention behaviour and mass spectral information.
- Interface: Compounds eluting from the GC column enter the mass spectrometer through an interface designed to preserve separation and maintain vacuum requirements.
- Electron ionisation: Electron ionisation produces reproducible fragmentation patterns useful for library matching of many volatile organic compounds.
- Identification: Confidence increases when retention information and mass spectra agree with authentic standards or validated spectral libraries.
- Quantification: An internal standard can compensate for variation in extraction, injection and instrumental response when properly selected.
- Derivatisation: Many polar metabolites require derivatisation before GC-MS to improve volatility and thermal stability.
- Applications: GC-MS is used for organic acids, fatty-acid derivatives, steroids, environmental compounds and metabolomic analyses compatible with gas chromatography.
- Quality assurance: Blanks, calibration standards, quality-control samples and checks for carryover are essential in long analytical sequences.
Review and Examination Questions
- Explain the fundamental analytical principle underlying gas chromatography-mass spectrometry.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on gas chromatography-mass spectrometry.
Polarography, Potentiometry and Electrometric Measurements
20.1 Electroanalytical principle
Electrochemical methods relate electrical potential, current, charge or conductivity to chemical composition or reaction processes.
20.2 Polarography
Classical polarography records current as applied potential changes at a suitable electrode system, historically including the dropping-mercury electrode.
20.3 Current-potential curve
Electroactive species undergo oxidation or reduction at characteristic potential ranges, while current can relate to analyte concentration under controlled mass transport.
20.4 Potentiometry
Potentiometric methods measure the potential difference between an indicator electrode and a reference electrode with negligible current flow.
20.5 pH electrode
A glass electrode responds to hydrogen-ion activity relative to a reference system, allowing pH measurement after calibration with appropriate buffers.
20.6 Ion-selective electrodes
Selective membranes can provide potential responses related to activities of particular ions.
20.7 Nernst relationship
Electrode potential depends logarithmically on ionic activity under ideal conditions, forming the theoretical basis of many potentiometric measurements.
20.8 Conductometric context
Conductivity measurements assess the ability of a solution to carry electric current and therefore depend on ionic concentration and mobility.
20.9 Applications
Electrometric methods support pH measurement, ion analysis, redox studies, oxygen measurements and monitoring of biochemical reactions.
Electrochemical measurements are governed by activities and interfacial processes, not merely by bulk concentration. Calibration with standards resembling the sample matrix helps translate electrode response into useful analytical values.
Chapter Summary
- Electroanalytical principle: Electrochemical methods relate electrical potential, current, charge or conductivity to chemical composition or reaction processes.
- Polarography: Classical polarography records current as applied potential changes at a suitable electrode system, historically including the dropping-mercury electrode.
- Current-potential curve: Electroactive species undergo oxidation or reduction at characteristic potential ranges, while current can relate to analyte concentration under controlled mass transport.
- Potentiometry: Potentiometric methods measure the potential difference between an indicator electrode and a reference electrode with negligible current flow.
- pH electrode: A glass electrode responds to hydrogen-ion activity relative to a reference system, allowing pH measurement after calibration with appropriate buffers.
- Ion-selective electrodes: Selective membranes can provide potential responses related to activities of particular ions.
- Nernst relationship: Electrode potential depends logarithmically on ionic activity under ideal conditions, forming the theoretical basis of many potentiometric measurements.
- Conductometric context: Conductivity measurements assess the ability of a solution to carry electric current and therefore depend on ionic concentration and mobility.
- Applications: Electrometric methods support pH measurement, ion analysis, redox studies, oxygen measurements and monitoring of biochemical reactions.
Review and Examination Questions
- Explain the fundamental analytical principle underlying polarography, potentiometry and electrometric measurements.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on polarography, potentiometry and electrometric measurements.
Thermocycler and PCR Instrumentation
21.1 Thermocycler
A thermocycler rapidly and reproducibly changes sample temperature through programmed cycles, providing the physical platform for polymerase chain reaction.
21.2 PCR components
A typical PCR contains template DNA, two primers, thermostable DNA polymerase, deoxyribonucleoside triphosphates, magnesium ions and an appropriate buffer.
21.3 Denaturation
Heating separates double-stranded DNA into template strands.
21.4 Annealing
Cooling permits primers to hybridise to complementary target sequences. Temperature must balance specificity and efficient primer binding.
21.5 Extension
A thermostable DNA polymerase extends primers in the 5' to 3' direction at a temperature suitable for the enzyme.
21.6 Amplification
Repeated cycles can produce exponential target amplification during the efficient phase of the reaction, although amplification eventually departs from ideal doubling.
21.7 Controls
A no-template control helps detect contamination, while a positive control helps distinguish reaction failure from a truly negative sample.
21.8 Applications
PCR supports detection, cloning, genotyping, sequencing workflows and many molecular-biochemistry investigations.
21.9 Real-time extension
Quantitative real-time PCR instruments combine thermal cycling with fluorescence monitoring, allowing amplification to be followed during the reaction.
A thermocycler is an instrument for controlled temperature cycling; PCR is the biochemical reaction performed within it. Good PCR design depends on primer specificity, template quality, magnesium concentration and controls as well as accurate temperature control.
Chapter Summary
- Thermocycler: A thermocycler rapidly and reproducibly changes sample temperature through programmed cycles, providing the physical platform for polymerase chain reaction.
- PCR components: A typical PCR contains template DNA, two primers, thermostable DNA polymerase, deoxyribonucleoside triphosphates, magnesium ions and an appropriate buffer.
- Denaturation: Heating separates double-stranded DNA into template strands.
- Annealing: Cooling permits primers to hybridise to complementary target sequences.
- Extension: A thermostable DNA polymerase extends primers in the 5' to 3' direction at a temperature suitable for the enzyme.
- Amplification: Repeated cycles can produce exponential target amplification during the efficient phase of the reaction, although amplification eventually departs from ideal doubling.
- Controls: A no-template control helps detect contamination, while a positive control helps distinguish reaction failure from a truly negative sample.
- Applications: PCR supports detection, cloning, genotyping, sequencing workflows and many molecular-biochemistry investigations.
- Real-time extension: Quantitative real-time PCR instruments combine thermal cycling with fluorescence monitoring, allowing amplification to be followed during the reaction.
Review and Examination Questions
- Explain the fundamental analytical principle underlying thermocycler and pcr instrumentation.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on thermocycler and pcr instrumentation.
Method Selection, Validation and Integrated Instrumentation
22.1 Analytical question
Method choice begins by asking whether the goal is identification, quantification, purification, structural characterisation, localisation or measurement of biological activity.
22.2 Matrix
A pure standard, cell extract, plasma sample and tissue homogenate present different interference problems. Sample preparation must match the matrix.
22.3 Precision requirement
A screening experiment may tolerate more uncertainty than a quantitative clinical or kinetic study. Required precision should therefore be specified before data collection.
22.4 Orthogonal methods
Independent methods based on different physical principles provide stronger evidence than repeated measurements using one principle.
22.5 Validation
Useful validation characteristics include accuracy, precision, linearity or working range, selectivity, sensitivity, recovery, robustness and detection/quantification limits where applicable.
22.6 Instrument maintenance
Calibration, preventive maintenance, lamp or column condition, detector performance and software settings can influence results over time.
22.7 Documentation
Raw data, sample identity, reagent lots, calibration information, calculations and deviations should be recorded so the analytical process can be reconstructed.
22.8 Integrated platforms
Modern instruments often couple separation to detection, as in GC-MS or LC-MS, or combine thermal control with optical detection, as in real-time PCR.
22.9 Responsible interpretation
Instrument output is evidence, not an automatic conclusion. Peaks, bands and numerical readouts must be interpreted using controls, calibration, method limitations and biological context.
Method validation is evidence that a procedure is fit for its intended purpose. The relevant performance characteristics depend on the question. A qualitative presence/absence test and a quantitative concentration assay need different validation emphasis.
Chapter Summary
- Analytical question: Method choice begins by asking whether the goal is identification, quantification, purification, structural characterisation, localisation or measurement of biological activity.
- Matrix: A pure standard, cell extract, plasma sample and tissue homogenate present different interference problems.
- Precision requirement: A screening experiment may tolerate more uncertainty than a quantitative clinical or kinetic study.
- Orthogonal methods: Independent methods based on different physical principles provide stronger evidence than repeated measurements using one principle.
- Validation: Useful validation characteristics include accuracy, precision, linearity or working range, selectivity, sensitivity, recovery, robustness and detection/quantification limits where applicable.
- Instrument maintenance: Calibration, preventive maintenance, lamp or column condition, detector performance and software settings can influence results over time.
- Documentation: Raw data, sample identity, reagent lots, calibration information, calculations and deviations should be recorded so the analytical process can be reconstructed.
- Integrated platforms: Modern instruments often couple separation to detection, as in GC-MS or LC-MS, or combine thermal control with optical detection, as in real-time PCR.
- Responsible interpretation: Instrument output is evidence, not an automatic conclusion.
Review and Examination Questions
- Explain the fundamental analytical principle underlying method selection, validation and integrated instrumentation.
- Describe the major components of the relevant apparatus or instrument and state their functions.
- Outline a suitable experimental procedure and the controls required.
- Identify important sources of error and explain how they can be minimised.
- State major biochemical applications of the method.
- Compare this method with another technique that can answer a related analytical question.
- Explain how calibration, standards or controls affect interpretation.
- Write a structured examination essay on method selection, validation and integrated instrumentation.
Laboratory Practical Guide
Practical 1: UV-Visible Calibration Curve
Prepare a blank and a series of standards spanning the expected concentration range. Zero or baseline the instrument appropriately, measure standards at the selected wavelength, plot response against concentration, assess the usable linear region, and determine unknowns only within the validated range. Replicate measurements should be used to estimate precision.
Using dirty cuvettes, fingerprints in the optical path, bubbles, unmatched cuvettes, wrong blank, incorrect wavelength, samples above the linear range, and failure to mix standards uniformly.
Practical 2: Paper or Thin-Layer Chromatography
Apply small concentrated spots to a marked origin, develop in a closed chamber with the origin above the solvent, mark the solvent front immediately after removal, visualise compounds with an appropriate method and calculate Rf values relative to standards run in the same system.
Practical 3: Electrophoresis
Prepare the gel at a concentration suited to the target size range, load standards and samples carefully, apply the correct polarity and voltage, terminate the run before target molecules leave the gel, stain or otherwise detect the separated bands, and interpret sample migration against controls and molecular-size markers.
Practical 4: Differential Centrifugation
Homogenise tissue under conditions that preserve organelles, remove large debris at low centrifugal force, then subject supernatants to progressively higher forces. Record RCF, duration and temperature for each step. Recognise that each pellet is enriched rather than absolutely pure.
Practical 5: ELISA Principle
Arrange standards, controls and samples according to the assay format. Follow specified incubation, washing and detection steps consistently. Construct the required calibration relationship and verify that control results meet acceptance criteria before reporting unknowns.
Practical 6: PCR and Thermocycler Familiarisation
Identify the thermocycler block, heated lid and programming interface. Relate denaturation, annealing and extension temperatures to molecular events. A teaching PCR should include a positive control and a no-template control. Post-amplification analysis should be physically organised to reduce contamination of pre-PCR areas.
Practical 7: HPLC Instrument Familiarisation
Identify solvent reservoirs, degassing system where present, pump, injector/autosampler, column, detector, data system and waste line. Trace the mobile-phase flow path. Explain why pressure, solvent compatibility, column equilibration and sample cleanliness matter before any run is started.
Practical 8: pH Measurement
Calibrate the pH meter with appropriate standard buffers that bracket the expected sample pH. Rinse the electrode between solutions without abrasive wiping, allow the reading to stabilise and record temperature where relevant. Store the electrode according to manufacturer guidance rather than allowing the sensing bulb to dry.
Integrated Quantitative Problems
Dilution
Question: Prepare 100 mL of a 0.20 M solution from a 1.0 M stock.
Formula: C1V1 = C2V2.
Solution: V1 = (0.20 × 100)/1.0 = 20 mL. Dilute 20 mL stock to a final volume of 100 mL.
Answer: 20 mL stock, made up to 100 mL total volume.
Coefficient of Variation
Question: Replicate measurements have mean 50.0 units and standard deviation 1.5 units. Calculate CV.
Formula: CV% = SD/mean × 100.
Solution: 1.5/50.0 × 100 = 3.0%.
Answer: CV = 3.0%.
Serial Dilution
Three successive 1:10 dilutions produce an overall dilution of 1:1000. If the final tube measures 4.0 µg/mL and the method is valid at that concentration, the corresponding original concentration is 4.0 × 1000 = 4000 µg/mL, provided no additional preparation factors apply.
Choosing an Analytical Method
| Analytical question | Suitable starting method | Reason |
|---|---|---|
| Separate DNA fragments by size | Agarose electrophoresis | Size-dependent migration through gel |
| Quantify a specific antigen | Validated immunoassay | Antibody-based selectivity |
| Resolve and quantify nonvolatile metabolites | HPLC with suitable detector | High-efficiency liquid separation |
| Identify volatile organic compounds | GC-MS | Gas-phase separation plus mass spectra |
| Characterise functional groups | FTIR | Vibrational absorption |
| Determine detailed chemical environment | NMR | Nuclear resonance and chemical shifts |
Comprehensive Examination Revision
- Differentiate accuracy, precision, sensitivity, selectivity and detection limit.
- Explain calibration and the roles of blanks, standards and controls.
- Discuss tissue and cell culture techniques, including asepsis, media, subculture and viability.
- Compare competitive and sandwich immunoassays and explain ELISA.
- Compare Southern, Northern and Western blotting.
- Explain isotope half-life, tracer methodology and radiochemical safety.
- Discuss the principles, instrumentation and applications of electrophoresis.
- Differentiate SDS-PAGE, native PAGE and isoelectric focusing.
- Explain differential, rate-zonal and isopycnic centrifugation.
- Distinguish RPM from RCF and solve an RCF calculation.
- Explain the principle and biochemical application of manometry.
- Compare paper chromatography and thin-layer chromatography and calculate Rf.
- Discuss column chromatography, including ion exchange, size exclusion and affinity methods.
- Explain the instrumentation and applications of gas chromatography.
- Discuss HPLC components, isocratic/gradient elution and chromatogram interpretation.
- State and apply the Beer-Lambert law.
- Explain UV-visible spectrophotometer components and sources of analytical error.
- Discuss infrared spectroscopy and the operating principle of FTIR.
- Explain fluorescence, Stokes shift, quenching and fluorimetric instrumentation.
- Discuss the principles and biochemical applications of NMR.
- Explain ionisation, mass analysis, detection and fragmentation in mass spectrometry.
- Discuss GC-MS as a hyphenated analytical technique.
- Explain polarography and potentiometry and state the significance of the Nernst relationship.
- Describe pH-electrode calibration and important sources of error.
- Explain thermocycler operation and relate each PCR temperature stage to its molecular event.
- Discuss method validation and why analytical method selection depends on the intended investigation.
- Design an analytical strategy to identify and quantify an unknown biochemical compound in a complex sample.
Selected References
- National Universities Commission. Core Curriculum and Minimum Academic Standards for the Nigerian University System: Sciences. BCH 306.
- Wilson, K. and Walker, J. Principles and Techniques of Biochemistry and Molecular Biology.
- Harris, D. C. Quantitative Chemical Analysis.
- Skoog, D. A., Holler, F. J. and Crouch, S. R. Principles of Instrumental Analysis.
- Nelson, D. L. and Cox, M. M. Lehninger Principles of Biochemistry.
- Scopes, R. K. Protein Purification: Principles and Practice.
- Freshney, R. I. Culture of Animal Cells.