Principle double-beam

Because process mixtures are complex, specialized detectors may substitute for separation efficiency. One specialized detector is the array amperometric detector, which allows selective detection of electrochemically active compounds.23 Electrochemical array detectors are discussed in greater detail in Chapter 5. Many pharmaceutical compounds are chiral, so a detector capable of determining optical purity would be extremely useful in monitoring synthetic reactions. A double-beam circular dichroism detector using a laser as the source was used for the selective detection of chiral cobalt compounds.24 The double-beam, single-source construction reduces the limitations of flicker noise. Chemiluminescence of an ozonized mixture was used as the principle for a sulfur-selective detector used to analyze pesticides, proteins, and blood thiols from rat plasma.25 Chemiluminescence using bis (2,4, 6-trichlorophenyl) oxalate was used for the selective detection of catalytically reduced nitrated polycyclic aromatic hydrocarbons from diesel exhaust.26... 

Many instruments utilize a double beam principle in that radiation absorbed or emitted by the sample is automatically compared with that associated with a blank or standard. This facilitates the recording of data and corrects for matrix effects and instrumental noise and drift. Instrumentation for the generation of radiation is varied and often peculiar to one particular technique. It will be discussed separately in the relevant sections. Components (b) and (c), however, are broadly similar for most techniques and will be discussed more fully below. 

The double beam is particularly useful in qualitative analysis and most IR spectrophotometers operate on the double beam principle. A simplified double beam IR spectrophotometer has been given in the following figure with its essential units and functions. 

Figure 10.9—Schematic diagram of various infrared spectrometers, a) Single beam model its principle is still used for measurements at a single wavelength b) double beam model c) single beam Fourier transform instrument. Contrary to UV/VIS spectrometers, the sample is placed immediately after the light source. Since photon energy in this range is insufficient to break chemical bonds and degrade the sample, it can be permanently exposed to the full radiation of the source.

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

Many commercial visible-UV spectrophotometers are suitable for this experiment. These instruments range from simple single-beam devices such as the Spectronic model 20 to high-performance double-beam scanning spectrophotometers such as various Varian-Cary models. The components and operational principles of these instruments are... 

Recording spectrophotometers in the infrared, visible and ultraviolet regions necessarily employ the double-beam principle of operation (figures 9.1(a) and (b) and p. 274), but for quantitative measurements at fixed... 

In passing, we note that parametric four-wave mixing processes could in principle contribute to the effects described above. Thus, in the single-beam case, the CARS process co + co cOi + CO2 single center could also result in the synergistic excitation of two chemically different centers. Equally in the double-beam case, the four-wave interaction coi +o 2-> (o + (o followed by absorption of the frequency co, could contribute to the excitation of a pair of neighboring molecules of the same species. However, both of these four-wave interactions will be relatively ineffectual unless (one of) the emission frequencies is stimulated by an additional source moreover, the processes described here are not associated with the wave-vector matching characteristics of CARS and related phenomena. 

Dispersive spectrometers. For a long time, mid-IR spectrometers were constructed on the principle of the double beam design. This fairly complex optical assembly, which remains in use for the UV/Vis, yields progressively over real time the... 

Internal Standard Instruments. In some instruments a double-beam principle is employed. An internal standard element, such as lithium, is added to a constant concentration to aU unknown and standard solutions. Dual optical paths are employed and the internal standard emission, after... 

Thus, any successful kinetic analysis requires an optimal choice of the spectroscopic detection method, whereby special methods (e.g. pre-separation by chromatography) or modem approaches (e.g. interferometry) must also be considered. Therefore in the following sections different approaches in spectroscopy are presented, some combined irradiation and measurement devices introduced, and different set-ups classified with respect to their principle of operation (sequential, multiplex, single and double beam equipment). In addition some special devices are presented which allow an automated examination even of complex photochemical reactions (in part superimposed by thermal reactions) at a highly sophisticated level using various combinations of modem equipment and supplying data for multicomponent analysis. 

If a double-beam spectrometer is used to analyze a multicomponent mixture, then it is possible to eliminate the spectrum of one or more of the components by putting the same amount of the component in the reference beam. In principle, one may cancel the spectrum of each component separately and successively, making it possible to analyze the mixture without overlapping bands. In practice, this is usually limited to three or fewer components. This procedure is useful if small amounts of impurities are to be determined. 

The following schematic diagram shows a different 90 scattered light photometer by the same company, working on the double-beam principle. 

Figure 3 Principle of construction of atomic absorption spectrometers. (A) Single-beam spectrometer with electrically modulated lamp radiation (B) double-beam spectrometer with reflection and splitting of the primary radiation by a rotating, partially mirrored quartz disk (chopper). 1 - radiation source, 2 -sample cell (atomizer), 3 - monochromator, 4 - detector, 5 -electronics and readout (by permission of Wiley-VCH from Welz B and Sperling M (1999) Atomic Absorption Spectrometry, 3rd, completely revised edition. Weinheim Wiley-VCH).

The use of infrared spectroscopy for the characterization of polymer blends is extensive (Olabisi et al. 1979 Coleman and Painter 1984 Utracki 1989 He et al. 2004 and references therein Coleman et al. 1991, 2006). The applicability, fundamental aspects, as well as principles of experimentation using infrared dispersive double-beam spectrophotometer (IR) or computerized Fourier transform interferometers (FTIR) were well described (e.g., Klopffer 1984). 

There are many variations of the principle of integrating spheres for both the monobeam mode and the double-beam mode. An example of the latter is given in Fig. 4-52. 

Figure 11.54 Principle of a double-beam infrared spectrometer...

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