Optical systems single-beam optics

Double-beam AA spectrophotometers are still marketed by instrument manufacturers. A double-beam system compensates for changes in lamp intensity and may require less frequent re-zeroing than a single-beam instrument. These considerations had more merit some years ago when hollow cathode lamps suffered from some instability. It should be noted, however, that the optical... 

It would be logical to arrange the components so far described in a straight line optically, and indeed this is done in the most successful instruments producing the so-called single beam system represented in Fig. 11. 

Forward optic spectrophotometers are either singlebeam or double-beam spectrophotometers. The singlebeam instruments can be either very simple or expensive depending on the sophistication desired or needed. Simple single-beam instruments have poor stability and excessive drift. These advantages are eliminated in systems equipped with a high-resolution monochromator with adjustable slits, controlled by microprocessors for rapid data acquisition and evaluation of data. 

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. 

FIGURE 13-16 Single-beam photometer (a) and double-beam photometer for flow analysis (b). In the single-beam system, the reference cell is first placed in the light path and later replaced by the sample cell, tn the double-beam system (b). a liber optic splits the beam into two branches. One passes through the sample cell and the other through the reference cell. Two matched photodiodes are used in this double-beam-in-space anangement. 

There are two main advantages of double-beam operation over single-beam operation. Very rapid monitoring of sample and reference helps to eliminate errors due to drift in source intensity, electronic instability, and any changes in the optical system. Also, double-beam operation lends itself to automation—the spectra can be recorded by a strip-chart recorder. 

The double-beam system is used extensively for spectroscopic absorption studies. The individual components of the system have the same function as in the single-beam system, with one very important difference. The radiation from the source is split into two beams of approximately equal intensity using a beam splitter, shown in Fig. 2.28. One beam is termed the reference beam, the second beam, which passes through the sample, is called the sample beam. The two beams are then recombined and pass through the monochromator and slit systems to the detector. This is illustrated schematically in Fig. 2.28. In this schematic, there is a cell in the reference beam that would be identical to the cell used to hold the sample. The reference cell may be empty or it may contain the solvent used to dilute the sample, for example. This particular arrangement showing the monochromator after the sample is typical of a dispersive IR double-beam spectrophotometer. There are many commercial variations in the optical layout of double-beam systems. 

List the components of a single-beam optical system for absorption spectroscopy. List the components of single-beam optical system for emission spectroscopy. 

Spectrometers are instmments that provide information about the intensity of light absorbed or transmitted as a function of wavelength. Both single-beam and the doublebeam optical systems (see the schematics in Chapter 2) are used in molecular absorption spectroscopy. Single-beam systems and their disadvantages were discussed in Chapter 2. Most commercial instmments for absorption spectrometry are double-beam systems, so these will be reviewed. 

Fig. 12.13 Optical system for a single-beam atomic absorption spectrophotometer.

Optical systems corresponding to single-beam or double-beam spectrometers can be distinguished in... 

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