Double-beam instruments 23 Gratings

Because a CCD is a two-dimensional array of pixels, it is possible to create a double-beam instrument with a single detector. Coupled in an optical design with an imaging grating, the output of two fibers may be imaged simulfaneously and separately onto the CCD and both spectra readout by the electronics. 

Until the early 1980s, most IR spectrometer systems were double-beam dispersive grating spectrometers, similar in operation to the double-beam system for UV/VIS spectroscopy described in Chapter 2. These instruments have been replaced almost entirely by FTIR spectrometers because of the advantages in speed, signal-to-noise ratio, and precision in determining spectral frequency that can be obtained from a modern multiplex instrument. There are NIR instruments that are part of double-beam dispersive UV /VIS/NIR systems, but many NIR instruments are stand-alone grating instruments. 

The use of a double-beam instrument will help in the problem of water-vapor absorption and zero drift. In addition, the instrument can be gas-purged or evacuated. In Model 301, the instrument is gas-purged, and the source, reststrahlen plates, and gratings can be changed without breaking the entire purge. 

The design of a typical double-beam instrument that allows for relative background correction is schematically shown in Fig. 6 and consists of a radiant source, such as a hydrogen-deuterium lamp for ultraviolet and a tungsten lamp for visible radiations a filter, prism, or grating monochromator for wavelength selection sample and reference cells and a photocathode, usually associated with a photomultiplier for detection. 

Modern instruments isolate a narrow wavelength range of the spectrum for measurements. Those that use filters for this purpose are referred to as filter photometers those that use prisms or gratings are called spectrophotometers. Spectrophotometers are classified as being either single or double-beam. 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

Figure 9- 1.1b is a schematic of a typical double-beam-intime instrument. The beam from the hollow-cathode source is split by a mirrored chopper, one half passing through Ihe tlame and the other half around it. The two beams are then recombined by a half-silvered mirror and passed into a Czerny-Turner grating monochromator a photomultiplier tube serves as Ihe transducer. The output from the latter is the input to a lock-in amplifier that is synchronized with the chopper drive. The ratio between the reference and sample signal is then amplified and fed to the readout, which may be a digital meter or a computer. 

I igure 13-21 shows construction details of a typical, relatively inexpensive, manual, double-beam ultraviolet-visible spectrophotometer In this instrument, the radiation is dispersed by a concave grating, W hich also focuses the beam on a rotating sector mirror. The instrument design is similar to that shown in Tig ure 13-13c. 

Figure 13-22 shows the optical design of the Varian Cary 100, a more sophisticated, double-beam-in-timo recording spectrophotometer. This instrument employs a 30 X. 3.S mm plane grating having 120()lines/mm. Us range is from 190 to 9(X1 nm. Randwidths of 0.2 to... 

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