Single-beam system

For large telescope apertures, Na LGS offer improve sampling of the atmospheric turbulence due to their much higher altitude. Single beam systems are now being developed for and deployed on 8-10 m class telescopes. Since resonant backscattering from the mesospheric Na layer is the method chosen for most LGS projects, we will concentrate mostly on this technique. 

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. 

An atomic absorption instrument contains the same basic components as an instrument designed for molecular absorption measurements, as shown in Figure 28-16 for a single-beam system. Both single- and double-beam instruments are offered by numerous manufacturers. The range of sophistication and the cost (upward from a few thousand dollars) are both substantial. 

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. 

Single-beam systems require a background spectrum and a spectrum of the sample plus background. The ratio of the two spectra is found by dividing the two ordinates (that is, the two intensities) at small frequency increments over the entire range scanned. A plot of these ratios against the frequencies at which each ratio was obtained is the spectrum of the sample. Almost all quantitative and qualitative analysis today is done on double-beam instruments. 

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. 

However, FTIR is a single-beam system and both air and solvent contribute to the signal, so corrections must be made in several steps. 

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. 

The spectrometer system for AAS can be configured as a single-beam system, as shown in Fig. 6.8, as a double-beam system, shown in Fig. 6.14, or as a pseudo-double-beam system, which will not be discussed. (See the reference by Beaty and Kerber for a description of this system.) Note that in AAS the sample cell is placed in front of the monochromator, unlike UV/VIS spectrometers for molecular absorption or spectrophotometry, where the sample is placed after the monochromator. 

A single-beam system is cheaper and less expensive than a double-beam system, but cannot compensate for instrumental variations during analysis. In a double-beam system, part of the light from the radiation source is diverted around the sample cell (flame or furnace atomizer) to create a reference beam, as shown in Figure 6.14. The reference beam monitors the intensity of the radiation source and electronic variations (noise, drift) in the source. The signal monitored by the detector is the ratio of the sample and reference beams. This makes it possible to correct for any variations that affect both beams, such as short-term changes in lamp intensity due to voltage... 

In the single-beam instrument, photometric accuracy to a great extent is dependent on the precise linearity of the electronic amplifiers, as well as on the stability of these amplifiers between the time that the 100 % level is established, the time that the zero line is checked, and the time during which the spectrum is obtained. Another factor is the presence of atmospheric absorption bands or solvent bands in the vicinity of the band undergoing a quantitative measurement. This is one of the inherent limitations in the accuracy of single-beam systems which is subject to some control by the analyst. 

In electronic ratio-recording systems as well as single-beam systems, a slide-wire is used in the output recording system. Therefore, the... 

Single-Beam Systems. The earliest work in infrared spectroscopy utilized single-beam systems. Although still prevalent in some applications, primarily related to infrared physical measurements, these systems play only an infrequent role in analytical chemistry. 

In contemporary single-beam systems the radiation is interrupted at a frequency compatible with the detector (e.g., 13 Hz ) to utilize ac amplifier systems. These have many advantages over the dc amplifiers which would otherwise be required, including increased stability and freedom from drift. Nevertheless, the stability requirements of the source and the amplifiers in a single-beam system exceed those of the double-beam systems to be described below. 

In order to measure transmittance bands in a single-beam system it is necessary to scan the region at least twice—with and without the sample cell. To ensure that amplifiers and source are adequately stable, it is preferable to record the background scanned without the sample... 

The subsequent use of the rectified signal depends on the photometric system employed. In single-beam systems it is fed directly to a potentiometric recorder. However, in a double-beam optical null system the rectified signal is remodulated, this time at the line frequency (e.g., 60 Hz). The object of this is to obtain an ac signal which can be amplified to sufficient power to drive a servomotor, which then positions the optical attenuator to establish a null signal. Line-frequency remod-... 

---