Multichannel spectrometer

Another approach to multielemental analysis is to use a multichannel instrument that allows for the simultaneous monitoring of many analytes. A simple design for a multichannel spectrometer consists of a standard diffraction grating and 48-60 separate exit slits and detectors positioned in a semicircular array around the diffraction grating at positions corresponding to the desired wavelengths (Figure 10.50). 

Prior to the use of plasma excitation, arc and spark sources were used on multichannel spectrometers, the so-called direct-reading instruments. 

Table 8.40 compares the main characteristics of WDXRF and EDXRF. Multidispersive XRF combines the benefits of the WDXRF technique for routine elemental analysis with the complete flexibility offered by EDXRF for nonroutine analysis. Clearly, modem XRF instrumentation is rather varied, ranging from simple benchtop EDXRF equipped with a low-power X-ray tube and high-resolution proportional counter for some key elements, to 4 kW simultaneous multichannel spectrometers with 28 fixed element channels for... 

Multichannel spectrometer, 202 Multicomponent analysis, 213 Multiwavelength linear reg., 213... 

Application of laser Raman multichannel spectroscopy to a kinetic investigation in the liquid phase has been reported by Crunelle-Cras and Merlin (1977). A multichannel spectrometer with photoelectric image devices, interfaced with a data aquisition and handling system, permits the study of fast processes, exemplified by the fast oxidation of the sulfite ion by the bromate ion in an acidic medium. 

Due to the development of fast observation methods in vibrational spectroscopy, even faster reactions can now be investigated. It is possible to record an IR spectrum in a fraction of a second. For Raman spectroscopy pulsed lasers and multichannel spectrometers are used (for further information see Sec. 6.6). 

MIO. Mavrodineanu, R., and Hughes, R. C., A multichannel spectrometer for simultaneous atomic absorption and flame emission analysis. Appl. Opt. 7, 1281-1285 (1968). 

Spectrometer designs vary in their sensitivity to flicker noise as will be discussed in Chapters 8 and 9. For example, a multichannel spectrometer is fairly immune to flicker noise because all wavelengths are monitored in parallel, while a scanning system is very sensitive to flicker noise depending on the flicker frequency. Interferometers used in FT-Raman are sensitive to flicker noise when the sampling frequency is comparable to the flicker frequncy. 

The analyte shot noise limit illustrates the fundamental Importance of multichannel detection for improving SNR. Equations (4.15) and (4.16) are valid for each resolution element in the system in a single or multichannel spectrometer (but not for a multiplex spectrometer). As noted in Section 3.4 and Eq. (3.9), a spectrometer that can monitor Nk resolution elements simultaneously increases the measurement time for each resolution element by a factor of Nr over the time required for a single-channel system. Since Im = NrIs,... 

In effect, each resolution element of a multichannel spectrometer collects signal for a time tM, while the single-channel systems collect signal for s(= m/Nr). If La and C are equal for both the single and multichannel spectrometers, Eq. (4.18) yields the relative SNRs ... 

The large improvement in SNR available with multichannel spectrometers is illustrated in Figure 4.8. Note that the higher SNR was achieved for a shorter measurement time and lower laser power. Although (4.18) states the often quoted Nr multichannel advantage, it should be emphasized that all other experimental parameters were assumed equal. This is rarely the case (and was not in Fig. 4.8), but there is nevertheless a major gain in SNR for multichannel spectrometers. As additional warning deals with multiplex as... 

Figure 4.8. Spectra of glassy carbon (a hard form of sp carbon) with single-channel and multichannel spectrometers. The multichannel system (Nr = 512) achieved higher SNR with shorter measurement time and lower laser power.

Raman spectra of monolayers were first obtained on roughened silver surfaces that exhibit strong field enhancement, since the scanning/photomultiplier tube spectrometers of the time needed the gain in sensitivity of a factor of to provide useful spectra (3,4). Multichannel spectrometers permitted spectrum acquisition without field enhancement about 8 years later (12), for the reasons discussed in Section 13.2. Surface Raman without field enhancement is conceptually simpler, so it will be discussed first. Section 13.5 describes the requirements and additional benefits when field enhancement occurs. 

Figure 25-21 Diagram of a multichannel spectrometer based on a grating spectrograph with a photodiode array detector.

Fig. 15-6 Multichannel spectrometer, curved-crystal type, with relative arrangement of x-ray tube, sample, and one analyzing channel. (The tube shown is of the end-window type the face of the target is inclined to the tube axis and the x-rays produced escape trough a window in the end of the tube. Other spectrometers of this kind use a side-window x-ray tube.)...

Multichannel spectrometers may contain, in addition to the fixed channels, a scanning channel in which a moving crystal and counter can sequentially scan the entire spectrum, just as in a single-channel instrument. This feature is useful for qualitative analysis or for quantitative determination of an element for which a fixed channel has not been preset. 

Multichannel spectrometers are made by several manufacturers. Figure 15-7 shows one example. 

Multichannel spectrometers which would have a large number of measurement channels and allow the simultaneous determination of a large number of elements, as is done in atomic emission spectrometry, have as yet not found a way into AAS. However, work over a number of years with high-intensity continuous sources and... 

In a system for coherent forward scattering, the radiation of a primary source is led through the atom reservoir (a flame or a furnace), across which a magnetic field is applied. When the atom reservoir is placed between crossed polarizers scattered signals for the atomic species occur on a zero-background. When a line source such as a hollow cathode lamp or a laser is used, determinations of the respective elements can be performed. In the case of a continuous source, such as a xenon lamp, and a multichannel spectrometer simultaneous multielement determinations can also be performed. The method is known as coherent forward scattering atomic spectrometry [309, 310]. This approach has become particularly interesting since flexible multichannel diode array spectrometers have became available. 

For superconductor materials and precursors high-precision analysis is very important. To this aim flame atomic absorption can be applied, provided the precision is optimized, as is possible, for example, by using internal standardization and multichannel spectrometers. It should, however, be remembered that to control the stoichiometric composition, in addition the determination of non-metals such... 

For most chemiluminescence reactions, a multichannel spectrometer is essential to obtain spectra information because with discrete sampling (injection of the final reagent to initiate the reaction), the CL reaction lasts only at most a few seconds. Some chemiluminescence reactions proceed for minutes or even hours, but still there are usually significant intensity changes over a minute such that a distorted spectrum would be... 

A notable multichannel spectrometer which does not use an IPDA is the SIDS (Spatial Imaging Detector System) spectrometer... 

Spectrometer 1 is based on a linear self-scanned image sensor, capable of providing a 10 ms per spectrum, scan time. This spectrometer was borne by the S-310-8 rocket, which was launched from Kagoshima Space Center (131°04 45"E, 31°15 00"N) at 17 47 JST (Japan Standard time, 135°E) on February 2, 1980. The spectrometer, that measured the NIR absorption of atmospheric constituents such as and H 0, was according to our knowledge the first rocket-borne multichannel spectrometer capable of measuring spectra with an altitude resolution better than 2 km. 

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