15.2 Absorption spectroscopy techniques

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 ($A$) directly proportional to concentration ($c$) of absorbing species and path length ($l$) of sample: $A = \varepsilon lc$
  • $\varepsilon$ 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 ($\lambda_{max}$) 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