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.

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

FT- IR Instrument

IR Sources:Nernst Glower orNichrome WireOr Globar

Thermal Detectors:Thermocouple orPyroelectric cell

Laser (red) to maintain alignment and keep detector calibrated

No optics other than mirrors, due to interferometer (Jacquinot Advantage)

Fourier Transform of the Interferogram

Sample preparation

No good solvents transparent in this region - best to do pure substances.

Gas - use special gas cellLiquid - neat, 0.01mm thickness

between two salt platesSolid - 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