Spectrophotometer operating variables

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. 

Use of an infrared spectrophotometer in good operating condition does not in itself assure accurate results. The validity of the spectra also depends on the sample-handling techniques, which will be discussed in Chapter 7, and on the selection of proper instrument operating variables, which is a primary concern of this chapter. To aid in understanding the operating variables and their interrelations we shall first examine the basis of operation of one type of widely used infrared spectrophotometer. This will be followed by a discussion of the operating variables and their qualitative and quantitative interdependence. The remainder of the chapter will describe the components and features of infrared spectrophotometers. 

The schematic optical diagram in Figure 2-2 provides an additional basis for the discussion of operating variables (Section 2.2). This diagram demonstrates the salient features of the general type of doublebeam spectrophotometers found in most analytical laboratories. Details and information relative to other types of instrument systems will be presented in Section 2.3B. 

The features in this category allow the analyst to manage the fundamental variables in a way that is most advantageous to the problem at hand. The interdependence of fundamental operating variables of a spectrophotometer was discussed in Section 2.2. The controls which more directly affect resolution, photometric accuracy, and scanning speed follow. 

Despite seven decades of technical and scientific progress, the original Hammett method has not become obsolete. The colorimeter has been replaced by modern spectrophotometers that can be operated at selected wavelengths extending the spectra beyond visible into the ultraviolet region of the electromagnetic spectrum. The experimental variable, which is wavelength-dependent, is the optical density D. D is related to the concentration by the Beer-Lambert law [Eq. (1.23)]. 

Over and above the fundamental specifications discussed here, there is an extensive list of other specifications which accompany many spectrophotometers. These cover such matters as range of variables, chart presentation, special features, operating controls, sources, detectors, and fundamental systems data. 

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