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Double-beam spectrometer

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  425-426

2026-11-12

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Double-beam spectrometer

Double-beam spectrometers are designed to compensate for instrument drifts and to eliminate the need to determine I₀ and I_t in separate measurements. The layout of a typical double beam spectrometer is illustrated in Fig. 1. Dispersive instruments in the IR have the sample located next to the light source before the monochromator, unlike their UV-visible counterparts. In a double-beam instrument, IR light from the source, typically an electrically conducting element such as a Globar (an electrically-heated silicon carbide rod) maintained at about 1000 K, equally illuminates two mirrors M₁ and M₂. The light from mirror M₁ acts as the reference beam; the light from M₂ is the sample beam. Mirrors M₃ and M₄ send the light beams to mirrors on a mechanical chopper. The mechanical chopper is a rotating disc carrying mirrors that alternately reflect the reference and sample beams into the monochromator. After passage through the monochromator, monochromatic light from the sample and reference beams is detected alternately by the single detector as the wave length drive changes the wavelength that passes to the detector. In the reference-beam chopper period the detector registers I0 and in the sample beam chopper period It is measured. The alternating signals are amplified and their ratio is calculated to give I_t /I₀=T or log (I₀ /I_t) = A. The preferred detector, certainly by the 1980s, was deuterated triglycine sulfate (TGS). This is a pyroelectric detector with an electrical resistance very sensitive to heat (IR intensity). All measurements can be made simply with air in the reference beam. Placing a cell with solvent in the reference beam enables compensation for unwanted absorption (e.g. from solvents in which The sample is dissolved). The measurement cell pathlengths in the reference and sample beams must be identical. A computer interfaced with the spectrometer enables post-measurement subtraction of the reference spectrum from the sample spectrum. The amount of polychromatic IR light that hits the detector from the natural, black-body, room-temperature radiation of the cell compart ment walls can be 10-fold or more than the desired monochromatic IR radiation from the reference or the sample. This background radiation is stray light that severely affects the accuracy of the I₀/I_t value. Locating the sample before the monochromator ensures that the ‘chopped’ signal selected by the detector system is related almost exclusively to light derived from either the sample or reference beams. Artificial reduction of absorbance values is thereby greatly reduced. However, locating a sample close to an IR source can cause deleterious heating effects.

Figure 1 Double-beam dispersive IR spectrometer.