Overview
Measuring light emission, transmittance and scattering are among the most important techniques used in the modern biochemistry laboratory. They form the basis of colorimeters, spectrophotometers, chemiluminescence assays, and ELISA. The ease and accuracy of measuring light interactions makes these methods the choice for a huge range of quantitative analyses.
This lesson covers the core optical concepts β incident, transmitted, absorbed and scattered light β and explains the instruments and principles (colorimetry, Beer-Lambert's law, spectrophotometry, chemiluminescence, fluorescence and flame photometry) built on them.
Learning Objectives
After this lesson you will be able toβ¦- Describe the functional aspects of a colorimeter.
- Describe spectrophotometry and how it differs from colorimetry.
- Explain the functional aspects of chemiluminescence.
- Explain the concepts of incident, transmitted, absorbed and scattered light.
- Describe the Beer-Lambert's law and its practical application.
Clinical Story
Why This MattersA doctor orders a routine glucose and cholesterol panel for a patient. Behind the scenes, the automated analyzer relies entirely on colorimetric and spectrophotometric principles β measuring how much light a coloured reaction product absorbs β to convert a chemical reaction into the numeric result printed on the report. Understanding these optical principles is essential for troubleshooting inaccurate results.
Core Concepts
Incident light is the beam directed at the cuvette from the light source. Transmitted light is the light that passes through the cuvette and emerges on the other side. Absorbed light (absorbance) describes light absorbed by the reaction mixture, calculated as Absorbance (A) = Incident light (I) β Transmitted light (T) when scattering is zero. Scattered light is light reflected or scattered by opaque particles rather than absorbed, and is used to measure turbidity, such as in immunoturbidimetric reactions.
Colorimetry measures the absorbance of a reaction mixture at a fixed wavelength. It requires the analyte to produce a coloured product, or a measurable change in absorbance (e.g. UV kinetic methods). Absorbance = logββ(I/T). The same principle underlies colorimeters, semi-auto analyzers and auto analyzers; the difference lies in how much of the pipetting, mixing and plotting is automated.
Beer-Lambert's law relates absorbance to the intensity of incident light, path length, and optical density of the medium: A = Ξ΅bc, where A = absorbance, Ξ΅ = molar absorptivity (L/mol/cm), b = path length of the cuvette, and c = concentration of the compound (mol/L). Since path length is fixed in a colorimeter (set to zero using a distilled water blank), absorbance becomes directly proportional to analyte concentration.
Unlike colorimetry, which reads absorbance at one fixed wavelength, spectrophotometry reads absorbance across a range of wavelengths (every 5β10 nm), producing a data stream or spectral graph of absorbance versus wavelength. This allows identification of the peak absorption wavelength of a compound and comparison of spectra between substances.
Chemiluminescence is light production during a chemical reaction (e.g. luminol + hydrogen peroxide β 3-aminophthalate + light), exploited in immunochemiluminescence assays for hormone measurement, with sensitivity approaching radioimmunoassay (RIA). Fluorescence is the ability of a substance to emit visible light after absorbing UV or visible light β an electron is excited to a higher orbital and, on returning to its ground state, releases lower-energy (longer wavelength) light. Tryptophan residues in proteins display fluorescence useful for studying protein folding.
Laboratory Principle
A light source passes through a filter or diffraction grating that isolates the required wavelength, then through the cuvette containing the coloured sample. The transmitted light strikes a photovoltaic cell, generating a current inversely proportional to absorbance. Since absorbance is directly proportional to analyte concentration (per Beer-Lambert's law), the instrument converts this photocurrent into a concentration reading via a calibration curve plotted from known standards.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Distilled water blank | Analytical grade | Zeroing the colorimeter / spectrophotometer | Room temperature |
| Luminol | Chemiluminescence reagent | Reaction with H2O2 to generate light for immunoassays | 2β8Β°C, protect from light |
| Hydrogen peroxide (H2O2) | Oxidant reagent grade | Chemiluminescent reaction partner with luminol | 2β8Β°C, protect from light |
| Serial dilution standards | Known concentration series | Constructing the calibration curve | As per analyte stability requirements |
Step-by-Step Procedure
Set the filter or diffraction grating to the wavelength at which the coloured reaction product absorbs maximally.
Fill the cuvette with distilled water (the blank) and set the instrument to zero absorbance, negating the fixed path length factor.
Read absorbance of a serial dilution of known standard concentrations and plot absorbance against concentration to obtain the calibration curve.
Place the reacted sample in the cuvette and measure absorbance under identical conditions used for the standards.
Plot the sample's absorbance on the calibration curve (manually in a colorimeter, automatically in an auto-analyzer) to determine analyte concentration.
Flow Diagram
Quality Control
Verify wavelength accuracy and photometric linearity of the colorimeter/spectrophotometer periodically using certified reference filters. Re-zero with a distilled water blank before each batch, and re-run the calibration curve whenever reagent lots change.
Participate in external instrument performance verification schemes to confirm accuracy of absorbance readings across the laboratory's full range of colorimetric assays.
Reference Values
Key Optical Parametersβ οΈ These are technical optical parameters, not patient reference ranges. Instrument-specific calibration values may vary by manufacturer.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Absorbance reading outside calibration curve range | Result may be inaccurate (non-linear region) | Dilute sample and re-test within the linear range |
| Turbid or lipemic sample | Scattered light interferes with true absorbance reading | Ultracentrifuge or use a blank correction for turbidity |
| Drift in blank absorbance | Instrument calibration or cuvette contamination issue | Re-zero with fresh blank; clean or replace cuvette |
Common Errors & How to Avoid Them
Cause: Beer-Lambert's law only holds true within a limited concentration range; very high concentrations cause non-linearity.
Prevention: Dilute high-concentration samples and re-test within the validated linear range.
Cause: Contaminated or damaged cuvette surfaces scatter light, causing falsely elevated absorbance.
Prevention: Handle cuvettes by the ridged (frosted) sides only, and clean with lint-free tissue before use.
Cause: Bubbles scatter incident light, giving spuriously high absorbance readings.
Prevention: Gently tap or inspect the cuvette to ensure no bubbles are present before reading.
Laboratory Tips from the Bench
Always re-zero the instrument with a fresh distilled water blank before each new batch of samples β drift over time is a common source of systematic error.
Use quartz cuvettes for UV and fluorescence work, since standard glass cuvettes absorb UV light and will distort results.
Remember "A = Ξ΅bc" as "A Big Cat" β Absorbance = (molar absorptivity) Γ (path length b) Γ (concentration c).
Important Notes
Turbid or lipemic samples scatter light in addition to absorbing it, which can falsely elevate colorimetric readings unless corrected or the sample is pre-treated.
Immunochemiluminescence assays can achieve sensitivity comparable to radioimmunoassay without the hazards and disposal issues of radioactive material, making them the preferred method for many hormone assays today.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeA technologist notices that glucose results across an entire morning batch are trending 15% higher than expected compared to the internal QC target, despite fresh reagents.
A non-zero blank absorbance indicates the instrument was not properly zeroed with distilled water before the batch, introducing a constant positive offset that inflated every subsequent reading.
- βAlways re-zero the instrument with a fresh blank before starting a batch.
- βQC values outside range should trigger a check of blank absorbance before troubleshooting reagents.
- βA constant offset across all QC levels suggests a systematic (not random) error source.
Frequently Asked Questions
The blank removes the constant contribution of the cuvette's path length and any background absorbance from the solvent, so that the only remaining factor affecting the reading is the optical density of the analyte itself.
Because it reads absorbance across a range of wavelengths rather than one fixed wavelength, it produces a full spectral profile useful for identifying compounds and confirming peak absorption wavelength.
Each enzyme-catalysed chemiluminescent reaction produces a detectable photon, and sensitive photon-counting sensors can measure extremely small numbers of reactions, giving quantitation comparable to radioactive methods without the safety and disposal burden.
Quick Revision
10-Minute ReviewKey Takeaways
- Incident, transmitted, absorbed, and scattered light are the four key optical concepts underlying laboratory photometry.
- Colorimetry measures absorbance at a fixed wavelength; spectrophotometry scans across a range.
- Beer-Lambert's law (A = Ξ΅bc) is the mathematical basis converting absorbance into concentration.
- Chemiluminescence and fluorescence are light-emission-based techniques used in immunoassays and protein studies.
- Scattered light is exploited to measure turbidity, distinct from true absorbance.
- Proper blanking and cuvette care are essential to avoid systematic photometric errors.
Competency Checklist
Track Your MasteryReferences
- NIOS Biochemistry Course Material, Lesson 18: Spectrophotometry, Light Emission and Scattering Analytical Technique.
- Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, latest edition.
- Varley's Practical Clinical Biochemistry, latest edition.