Spectrophotometers double-beam-in-time

Fig. 2 (A) Block diagram of a double-beam-in-time spectrophotometer (single-detector system with a beam-splitter) a chopper motor can be used in place of the mirror. (B) Block diagram of a double-beam-in-space spectrophotometer.

Figure 25-20c illustrates a double-beam-in-time spectrophotometer. Here the beams are separated in time by a rotating sector mirror that directs the entire beam through the reference cell and then through the sample cell. The pulses of radiation... 

With single-beam designs, the array dark current is measured and stored in conipulcr memory. Next, the specirum of the source is obtained and stored in memory after dark-current subtraction. Finally, the raw spectrum of the sample is obtained and. after dark-current subtraction, the sample values are divided by the source values at each wavelength to give absorbances. Multichannel instruments can also be configured as double-beam-in-time spectrophotometers. 

Figure 8.43 Schematic optical diagram of a double beam-in-time spectrophotometer with double monochromation (Cary Model 17D). (Courtesy of Varian Associates, Inc.)...

Block diagram for a double-beam in-time scanning spectrophotometer with photo of a typical instrument. 

Figure 25-20 Instrument designs for UV/visible photometers or spectrophotometers. In (a), a single-beam instrument is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector. In (b), a double-beam-in-space instrument is shown. Here, radiation from the filter or monochromator is split into two beams that simultaneously pass through the reference and sample cells before striking two matched photodetectors. In the double-beam-in-time instrument (c), the beam is alternately sent through reference and sample cells before striking a single photodetector. Only a matter of milliseconds separates the beams as they pass through the two cells.

Figure 7.14. Schematic diagram of two types of double-beam spectrophotometers. A The double-beam-in-space configuration. B The double-beam-in-time configuration.

Two identical reaction solutions were prepared, one at T,(= 25.000 °C) in the sample compartment of a double-beam spectrophotometer, the other at T2( = 27.170 °C) in the reference beam. A direct recording of AAbs = Absi - Abs2 was made as a function of time while the difference in reaction temperature was maintained to 0.0001 °C. Evaluate kffk and AW1 for the run shown note this calculation is possible with an arbitrary time axis. 

Another method for the determination of salicylic acid in aspirin powder by second derivative ultra-violet spectrometry was done by Kitamura et al. (28). A differentiator with electronic derivative circuit and incorporating three circuits with differencial time constants of 27, 82 and 22 was connected to a double beam spectrophotometer derivative spectra were recorded with a slit width of 1 mm and scanning speed f 120 nm min 1. The second derivative spectrum of salicylic acid showed a trough at 292 nm and a satellite peak at 316 nm, in the presence of large amounts of aspirin, the trough disappeared, but the peak was unaffected. The relationship between the height of this peak and the concentration of salicylic acid was rectilinear for 1 to 10 //g ml 1. 

Most commercially available atomic absorption spectrophotometers have damping built in. Single beam instruments can usefully employ damping time constants up to about 20 seconds. Double beam instruments, with their greater freedom from drift, can benefit from damping time constants as high as two minutes. 

Alternatively, the sample and reference may be compared many times a second, as in double-beam instruments. The light from the source, after passing through the monochromator, is split into two separate beams—one for the sample and the other for the reference. Figure 7.14 shows two types of double-beam spectrophotometers. The measurement of sample and reference absorption may be separated in space, as in Figure 7.14A this, however, requires two detectors which must be perfectly matched. Alternatively, the sample and reference measurement may be separated in time as in Figure 7.14B this technique makes use of a rapidly rotating mirror or... 

Since this is a book concerned primarily with applications, no further details are given concerning instrumentation. The reader is referred to Alpert et al. (1970), in which are discussed an optical diagram of a double-beam spectrophotometer operating variables (resolution, photometric accuracy) components of infrared spectrophotometers (sources, types of photometers, dispersing elements, detectors, amplifiers, and recorders) special operating features, such as optimization of scan time and available instruments and their specifications. The books by Martin (1966), Conn and Avery (1960), and Potts (1963), and the chapter by Herscher (1966) are also recommended for details on some of these topics. 

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