Spectrophotometers dispersive

These can provide more spectral information than photometers or conventional dispersive spectrophotometers but are much more expensive and generally less sensitive (Figure 4.34(a) and p. 355). 

ASTM E387-04 Standard test method for estimating stray radiant power ratio of dispersive spectrophotometers by the opaque filter method. 

IR spectroscopy is one of the oldest spectroscopic measurements used to identify and quantify materials in on-line or near-line industrial and environmental applications. Traditionally, for analyses in the mid-IR, the technologies used for the measurement have been limited to fixed wavelength NDIR filter-based methods and scanning methods based on either grating or dispersive spectrophotometers or interferometer-based FTIR instruments. The last two methods have tended to be used more for instruments that are resident in the laboratory, whereas filter instruments have been used mainly for process, field-based and specialist applications, such as combustion gas monitoring. 

UVIVisible Photometers and Dispersive Spectrophotometers These detectors respond to UV/visible absorbing species in the range 190 to 700 nm and their response is linear with concentration, obeying the Beer-Lambert law (p. 355). They are not appreciably flow or temperature sensitive, have a wide linear range and good but variable sensitivity. 

Infra-red spectra in the vapour phase are rarely determined with dispersive spectrophotometers unless the substance is a gas at room temperature. Special long-path, airtight gas cells are required. The technique has become more widely used with the advent of Fourier transform infra-red spectrophotometers coupled directly to gas chromato- graphs. 

FIGURE 13-23 Optical diagrafTi of the Vanan Cary 300 double-dispersing spectrophotometer. The instrument is essentially identical to that shown in Figure 13-22. except lhat a second monochromalor is added immediately after the source. (Varian Inc., Palo Alto, CA.)... 

As an alternative, the electrode potential can be modulated quickly and a step-scan (S ) FTIR spectrometer or a dispersive spectrophotometer is used to measure spectra (Section 4.9). 

As discussed in Section 2.2, the maximum sensitivity in IRRAS is achieved with p-polarized radiation at grazing angles of incidence. With a single reflection, the method can detect CO adsorbed on Ir at 0.002-monolayer (ML) coverage [90]. A SNR on the order of 1000 can be reached, even with a conventional doublebeam dispersive spectrophotometer [91]. This is crucial for studies of catalysis... 

Dispersive spectrophotometers described so far scan through a spectrum one wavelength at a time. A diode array spectrophotometer records the entire spectrum at once. The entire spectrum of a compound emerging from a chromatography column can be recorded in a fraction of a second by a photodiode array spectrophotometer. At the heart of rapid spectroscopy is a photodiode array such as the one in Figure 19-11, which contains 1 024 individual semiconductor detector elements (diodes) in a row. 

A dispersive spectrophotometer spreads light from the source into its component wavelengths and then measures the absorption of one narrow band of wavelengths at a time. 

What characteristic makes a photodiode array spectrophotometer suitable for measuring the spectrum of a compound as it emerges from a chromatography column and a dispersive spectrophotometer not suitable What is the disadvantage of the photodiode array spectrophotometer ... 

Until the 1980s, the most widely used instruments for IR measurements were dispersive spectrophotometers. Now, however, this type of instrument has been largely displaced for mid- and far-lR measurements by Fourier transform spectrometers because of their speed, reliability, signal-to-noise advantage, and convenience. Dispersive spectrometers are still used in the near-lR where they are often extensions of UV-visible instruments, but many dedicated near-IR instruments are of the Fourier transform-IR (FTIR) type. 

IR transducers are of three general types (1) pyroelectric transducers, (2) photoconducting transducers, and (3) thermal transducers. The first is found in photometers, some FTIR spectrometers, and dispersive spectrophotometers. Photoconducting transducers arc found in many FTIR instruments. Thermal detectors are found in older dispersive instruments but are too slow to be used in FTIR spectrometers. 

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