Double-beam optics

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

Atomic absorption spectrophotometers (Figure 10.37) are designed using either the single-beam or double-beam optics described earlier for molecular absorption spectrophotometers (see Figures 10.25 and 10.26). There are, however, several important differences that are considered in this section. 

FIGURE 9.11 An illustration of a double-beam optical path. See text for explanation. 

This specific type of the double-beam optical-null recording spectrophotometer is termed so because it critically balances out by the help of optical means the differential between the two beams. 

Double beam optical spectrometers (scanning type)... 

Smaller values of are obtained for interferometers operated in a double-beam mode, since the moveable mirror must be left stationary for a fraction of the cycle time to allow the detector to stabilize each time the beam is switched from the sample to the reference position. With an optical null grating spectrometer the chopper is used not only to modulate the beam but also to alternate the beam between sample and reference channels. Thus, it takes approximately the same time to measure a transmittance spectrum using a double beam optical null spectrometer as it takes to measure a single-beam spectrum with the same S/R. Hence, for this type of spectrometer may be assigned a value of 2. 

FIG. 1 Schematic diagram of a double-beam, optical null, infrared spectrometer. 

Early UV-Vis and IR spectrophotometers, back in the 1950s, were big clunkers that usually had double-beam monochrometers to compensate for opti-cal drift and electronic noise They were slow and only moderately sensitive. Improvements in optical and electronic technology have reduced the necessity for double-beam optical systems that reduce, the energy of the transmitted beam. Modem single-beam instruments are smaller faster, more sensitive, and more economical than the older versions. But double-beam instruments still provide the optimal stability and the choice depends on your need. All modem dispersive IR instmments are single beam. 

Fourier Transform Infrared Spectroscopy.—New Fourier transform (FT) spectrometers recently described in the literature include a double-beam optically compensated spectrometer suitable for weak i.r. absorptions (0.5—1%) and a... 

The 970A employs true double beam optics tor maximum stability. Refererx e cells are available which make it unnecessary to pre-scanyour mobile phase. 

Figure 2.30 Radiation intensity reaching the detector using double-beam optics and a rotating disk beam splitter, (a) No absorption by the sample (b) 50% absorption by the sample.

Double-beam optics permit continuous realtime background subtraction... 

All of the flicker noises can be effectively eliminated by the use of double-beam optics in conjunction with a background correction system such as Zeeman splitting or a well-aligned (or wavelength-modulated) continuum source. Thus the ultimate limiting noise in atomic absorption is source shot noise, which can be reduced (relative to total source intensity or I, ) by increasing the source intensity, up to the point of optical saturation. 

Figure 8. Double-beam optics for sample and reference spectra collection. (Digilab FTS Systems.)...

Detection systems employed with hydride generation approaches are conventional AA spectrometers, usually fitted with intense electrodeless discharge or hollow cathode lamp sources. Quartz tube cells are of suitable dimensions to be compatible with the optical systems of all modem spectrometers. Background correction is usually achieved in double-beam optics using deuterium sources, and Zeeman-effect background correction can be implemented when the graphite furnace is used as the atomization cell. 

Spectrometers are instruments that provide information about the intensity of light absorbed or transmitted as a function of wavelength. Both single-beam and double-beam optical systems (see the schanatics in Chapter 2) are used in molecular absorption spectroscopy. Single-beam systems... 

Instruments with single and double beam optics are available. The double beam optical arrangement Is more or less similar to that of double beam apparatus in absorption spectrophotometry. 

Indirect fluorescence detection also applies to anion analysis [80,81]. Mho and Yeung [80] utilized salicylate as fluorescent eluant ions. Owing to their structural similarities to phthalate and benzoate, these ions exhibit good elution properties. Salicylate shows a strong absorption at 325 nm and a fairly efficient emission at 420 nm. An HeCd laser with double-beam optics that emits polarized UV light with a wavelength of 325 nm at a power of about 7 mW was used as an excitation... 

Answer. This idea that single-beam instruments are better for quantitative analysis arises from the fact that a single-beam instrument measures beam intensity directly, while in a double-beam optical null instrument it is the movement of a comb that is amplified as a measurement of intensity. The comb must be made linear throughout the range in which it is used, otherwise it will not give a linear relation between the intensity and the comb movement, and this will lead to errors in intensity measurements. 

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