Infrared Radiation 780 nm - 1000 m Near, Mid and Far Most used - 2.5-15 m IR spectra contain qualitative information, can also be used for quantitative analysis.

Energy Level Diagram

Types of Molecular Vibrations Vibration must result in net change of dipole moment for it to be “IR active”

IR spectrum of CO2

Laser (red) to maintain alignment and keep detector calibrated FT- IR Instrument IR Sources: Nernst Glower or Nichrome Wire Or Globar No optics other than mirrors, due to interferometer (Jacquinot Advantage) Thermal Detectors: Thermocouple or Pyroelectric cell

Fourier Transform of the Interferogram

Sample preparation No good solvents transparent in this region - best to do pure substances. Gas - use special gas cell Liquid - neat, 0.01mm thickness between two salt plates Solid - particles must be <  to avoid scattering radiation.

More Sample Preparation Pellet - grind sample with KBr powder, then press into a transparent disc. Mull - analyte is dispersed into a mineral oil (Nujol) or a fluorinated polymer

Reflectance measurements DRIFTS - Diffuse Reflectance Infrared FT Spectroscopy ATR -Atenuated Total Reflectance

ATR - Golden Gate IR beam focused on sample through germanium crystal, onto sample which is held down with diamond anvil. IR beam reflects off sample, some of the wave penetrates into surface so reflected light is comparable to what would be transmitted through a sample cuvette.

ATR Evanescent Wave  for Ge is ~ 4  for your sample will be different, so reflection will take place at the interface

Qualitative Analysis - Spectral Interpretation Group Frequency range - 3600-1200 cm-1 Fingerprint region - 1200-700 cm-1 Spectral Libraries are available for matching your results to known spectra. Forensics – ex, single strands of fiber Surface Studies

FT-IR Quantitative Analysis Not common, because of overlapping signals from mixed compounds. Also, deviations from Beer’s Law happen at these wavelengths.

Near Infrared Spectroscopy 770 nm-2500 nm Quantitative, not qualitative Liquids or Solids (DRIFTS)

Applications of NIR Pharmaceutical Powders Process Analytical Technology (PAT) - online in situ process monitoring