IR Spectroscopy: Qualitative and Quantitative Analysis

Key Concepts

• Qualitative Analysis

  • Requires a pure compound for accurate identification.
  • Uses group frequencies as a guide to identify functional groups.
  • Employs a correlation chart for specific functional group analysis.
  • Computerized spectral search engines can assist.
  • IR assignments should be used alongside chemical, physical, or other spectroscopic properties.
  • Fingerprint Region (650 to 1200 cm⁻¹)
    • Matching spectra in this region is strong evidence of identical compounds.

• Quantitative Analysis

  • Limited use but can yield reasonable results with care.
  • Trace detection limits possible (parts per million) especially for gases.
  • Absorption in the IR region is common across most compounds including solids, liquids, and gases.
  • Challenges include poor sensitivity and need for relatively high solution concentrations (1-2% weight/weight percent).
  • High concentrations can lead to deviations from Beer’s Law.
  • Narrow absorption bands and complex spectra with overlapping bands may cause polychromatic errors.

Practical Application

• IR Spectrum Analysis
  • Spectra Components
    • Percent Transmittance (%T) on the y-axis.
    • Wavelength or wave number on the x-axis.
  • Measurement Approach
    • Absorption calculated as log(T₀/Tₛ).
    • T₀: Full intensity, Tₛ: Intensity at the peak.
    • Intensity comparison is crucial for determining absorption details.

Important Considerations

• When using IR for both qualitative and quantitative analysis, awareness of sample purity, concentration limits, and potential for spectral overlap is crucial to ensure accurate results.
• Understanding the basics of how spectra are represented and measured is essential for correctly interpreting IR data.