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🔬Modern Optics

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15.2 Absorption spectroscopy techniques

3 min readLast Updated on July 22, 2024

Absorption spectroscopy measures how materials absorb light at different wavelengths. It's a powerful tool for identifying and quantifying substances in various fields. The Beer-Lambert law is key, linking light absorption to sample concentration and path length.

This technique has wide-ranging applications, from environmental monitoring to pharmaceutical analysis. Factors like concentration, path length, and wavelength affect absorption spectra, influencing the accuracy and sensitivity of measurements. It's a versatile, non-destructive method for chemical analysis.

Principles and Instrumentation

Principles of absorption spectroscopy

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  • Measures absorption of electromagnetic radiation by a sample as a function of wavelength or frequency
  • Incident light attenuated by the sample, transmitted light measured
  • Absorption occurs when energy of incident photons matches energy difference between ground state and excited state of sample molecules
  • Allows identification and quantification of chemical species based on their characteristic absorption spectra (UV-Vis, IR)
  • Non-destructive technique, sample can be recovered after analysis
  • Sensitivity and selectivity depend on the choice of wavelength and sample preparation

Beer-Lambert law in quantitative analysis

  • Relates attenuation of light to properties of material through which light is traveling
  • Absorbance (AA) directly proportional to concentration (cc) of absorbing species and path length (ll) of sample: A=εlcA = \varepsilon lc
    • ε\varepsilon is molar attenuation coefficient, constant characteristic of absorbing species at particular wavelength
  • Prepare series of standard solutions with known concentrations of analyte
  • Measure absorbance of standard solutions and create calibration curve (absorbance vs. concentration)
  • Measure absorbance of unknown sample and determine concentration using calibration curve
  • Allows accurate quantification of analytes in various matrices (solutions, gases)

Factors affecting absorption spectra

  • Concentration
    • Higher concentrations lead to increased absorbance, more molecules available to absorb light
    • Deviations from linearity may occur at high concentrations due to aggregation or saturation effects
  • Path length
    • Longer path lengths result in increased absorbance, light interacts with more sample molecules
    • Doubling path length doubles absorbance, as per Beer-Lambert law
    • Choose appropriate path length to ensure absorbance falls within linear range of instrument (0.1-1.0)
  • Wavelength of incident light
    • Absorption occurs when energy of incident photons matches energy difference between molecular states
    • Different compounds absorb light at different wavelengths, leading to characteristic absorption spectra
    • Wavelength of maximum absorption (λmax\lambda_{max}) often used for quantitative analysis
    • Choose wavelength with high molar attenuation coefficient for increased sensitivity

Applications of absorption spectroscopy

  • Environmental monitoring
    • Determine concentration of pollutants in water or air samples (heavy metals, organic contaminants)
    • Monitor water quality in rivers, lakes, and oceans
    • Assess air quality in urban and industrial areas
  • Pharmaceutical analysis
    • Verify identity and purity of drug compounds
    • Quantify active ingredients and impurities in formulations
    • Ensure compliance with regulatory standards (USP, EP)
  • Biochemical assays
    • Quantify concentration of biomolecules in biological samples (proteins, nucleic acids)
    • Monitor enzymatic reactions and kinetics
    • Study protein-ligand interactions and binding affinity
  • Material science
    • Characterize optical properties of materials (semiconductors, polymers)
    • Investigate electronic transitions and band structure
    • Assess purity and composition of materials
  • Food and beverage industry
    • Determine nutritional content of food products (vitamins, minerals)
    • Monitor fermentation processes in beer and wine production
    • Detect adulteration and contamination in food samples


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.