Fourier Transform Infrared (ft-ir) Spectroscopy Theory and Applications

FT-IR Application Advantages

• Opaque or cloudy samples
• Energy limiting accessories such as diffuse reflectance or FT-IR microscopes
• High resolution experiments (as high as 0.001 cm-1 resolution)
• Trace analysis of raw materials or finished products
• Depth profiling and microscopic mapping of samples
• Kinetics reactions on the microsecond time-scale
• Analysis of chromatographic and thermogravimetric sample fractions

FT-IR Terms and Definitions

Resolution (common definition) –

The separation of the various spectral wavelengths, usually defined in wavenumbers (cm-1).

A setting of 4 to 8 cm-1 is sufficient for most solid and liquid samples. Gas analysis experiments may need a resolution of 2 cm-1 or higher. Higher resolution experiments will have lower signal-to-noise.

FT-IR Terms and Definitions

Resolution – FT/IR Case

A spectrum is said to be collected at a resolution of 1 cm-1 if 4 data points are collected within each spectral interval of 1 cm-1 .

In order to acquire a spectrum at higher, an increased number of data points is needed, requiring a longer stroke of the moving mirror.

For higher resolution instruments an aperture is needed in order to improve parallelism within interferometer.

FT-IR Terms and Definitions

Apodization - a mathematical operation to reduce unwanted oscillation and noise contributions from the interferogram and to avoid aberrations coming from the “finite” nature of real (non theoretical interferograms). Common apodization functions include Beer-Norton, Cosine and Happ-Genzel.

Apodization

FT-IR Terms and Definitions

Scan mode - Either single beam or ratio. Single beam can be a scan of the background (no sample) or the sample. Ratio mode always implies the sample spectrum divided by, or ratioed against, the single beam background.