IR Spectrometers

A dispersive IR spectrometer uses a monochromator containing a rapidly rotating beam chopper and diffraction grating to separate radiation into different frequencies. The beam chopper alternates sample and reference beams toward the grating, which varies the wavelength of radiation reaching the thermocouple detector. This process records the spectrum in the frequency domain as the grating rotates.

In an FTIR spectrometer, the beam splitter is a mirror positioned at 45° to incoming radiation that separates the beam into deflected and undeflected radiations. These separate beams travel to fixed and moving mirrors, then recombine at the splitter. The recombination creates an interferogram containing both constructive and destructive interference patterns due to path length differences.

Constructive interference occurs when the peaks of two waves align, increasing the amplitude of the combined wave. Destructive interference happens when one wave's peak aligns with another's trough, canceling each other and reducing amplitude. Both interference patterns are present in the interferogram produced by the FTIR beam splitter and mirrors.

The thermocouple detector measures the intensity ratio between the sample and reference beams after they pass through the monochromator. This ratio is then amplified and recorded to create the final spectrum. The detector's measurement allows the spectrometer to quantify how much infrared radiation the sample absorbs at each wavelength.

The detector in an FTIR spectrometer captures the interferogram modified by the sample, which contains a wide range of wavelengths and frequencies combined together. A computer then applies the Fourier transform operation to mathematically separate and extract the individual absorption frequencies from this complex interferogram data.

FTIR spectrometers are preferred because they operate faster and provide greater sensitivity compared to dispersive IR spectrometers. The FTIR design allows simultaneous measurement of all wavelengths through the interferogram, whereas dispersive spectrometers measure frequencies sequentially as the diffraction grating rotates.

In a dispersive IR spectrometer, infrared radiation from a hot wire is divided by mirrors into two parallel equal-intensity beams. One beam passes through the sample while the other serves as a reference. The thermocouple detector compares their intensities after passing through the monochromator, enabling measurement of sample absorption.