Double-beam spectrophotometers resolution

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. 

Spectral resolution is a measure of the separation between two adjacent wavelengths. The two adjacent peaks are said to be resolved when the minimum absorbance between the two peaks is 80%i or lower of the maximum absorbance. Most double-beam spectrophotometers have resolution below 0.5, typically 0.1. However, many diode array and rapid scan spectrophoto-metric detectors have resolution between 1 and 2 nm. 

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. 

The monochromator generates a single-beam spectrum where the vertical coordinate is radiation energy as a function of wavelength or wavenumber reaching the detector. In double-beam spectrophotometers, the monochromator alternately measures source radiation intensity with an without sample absorption for each wavelength or wavenumber resolution element of the spectrum. From this, the instrument generates a percent transmission spectrum. 

Measured extinction spectra for aqueous suspensions of polystyrene spheres—the light scatterer s old friend—are shown in Fig. 11.19. Water is transparent only between about 0.2 and 1.3 jam, which limits measurements to this interval. These curves were obtained with a Cary 14R spectrophotometer, a commonly available double-beam instrument which automatically adjusts for changing light intensity during a wavelength scan and plots a continuous, high-resolution curve of optical density. To reproduce the fine structure faithfully, the curves were traced exactly as they were plotted by the instru-... 

IR Analysis. IR absorption spectra were determined on neat samples of shale oils to furnish data for estimates of various olefinic types of compounds. Samples were run on a high-resolution, double-beam grating spectrophotometer at 0.1-mm path length between KBr plates. Quantitative measurements were made using the cut and weigh method with baselines drawn from point to point of minimum absorption. 

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