Instant Notes: Analytical Chemistry

permits location of the area of interest using visible illumination, and which

may then be altered so that the IR (or R) spectrum of that area is obtained by

reflectance or transmission.

Dispersion, or separation, of the IR radiation is generally done by IR diffrac-

tion gratings, or by Fourier transformation. Resolution for routine IR and NIR

spectra of 2-4 cm-^1 is adequate, but to resolve vibration-rotation bands, a resolu-

tion of better than 1 cm-^1 is needed. For Raman spectra, the introduction of

Fourier transform techniques has been most valuable (Topic E3).

Detectors for IR radiation relate to the source, heat sensors being suitable.

Older instruments used thermocouple detectors. Pyroelectric detectors, which

change electrical properties when exposed to IR radiation, especially doped

triglycine sulfate, are often used.

Semiconductors, such as mercury cadmium telluride (MCT) have a more

sensitive response in the mid IR.

E10 – Infrared and Raman spectrometry: principles and instrumentation 241

Sample

Sample

ATR crystal

IR

q

To detector

IR

Sample

(a)

(b)

Fig. 5. (a) ATR sampling system for infrared spectrometry. (b) Diffuse reflectance sampling

for infrared spectrometry.