Spectral isolation

Zone (or region) in flame which is focussed into the entrance-slit of spectral-isolation-unit. 

First, the experimental results were compared with atomic-code calculations that assume a steady state in order to derive time- and space-averaged electron temperatures of the bulk plasma. In this calculation, Thot is assumed to be 50 keV and a to be 1%. C2H3C1 plasma with an ion density of 9 x 1022 cm 3 or Al plasma with a density of 6 x 1022/cm3 was used. Results are shown in Fig. 10.4a for Cl. Due to difficulties with spectrally isolating the Cl9+ O-like Ka line from C1+ Cl8+ lines, the intensity ratios are taken with respect... 

Vibrational spectroscopy has been used in the past as an indicator of protein structural motifs. Most of the work utilized IR spectroscopy (see, for example, Refs. 118-128), but Raman spectroscopy has also been demonstrated to be extremely useful (129,130). Amide modes are vibrational eigenmodes localized on the peptide backbone, whose frequencies and intensities are related to the structure of the protein. The protein secondary structures must be the main factors determining the force fields and hence the spectra of the amide bands. In particular the amide I band (1600-1700 cm-1), which mainly involves the C=0-stretching motion of the peptide backbone, is ideal for infrared spectroscopy since it has an large transition dipole moment and is spectrally isolated... 

A laser is a radiation source which produces a very high spectral radiance in a small spectral range at a fixed wavelength. A laser combines a radiation source with spectral isolation of its radiation - two important components of a spectrometer. The word laser is an acronym which stands for light amplification by stimulated emission of radiation. The essential elements of a laser are an active medium a pumping process to produce a population inversion and a suitable geometry or optical feedback elements (Moore et al., 1993). Most lasers are essentially Fabry-Perot interferometers whose cavities contain... 

The measurement of absorbance requires three basic components (1) an optical source, (2) a means of spectral isolation, (3) and a detector (see Chapter 3). 

In automated systems spectral isolation is commonly achieved with interference filters. Such filters are now quite inexpensive, and only a few are necessary in any one instrument, because only a fimited number of wavelengths is required for analysis of a large number of absorbing species. Typical interference filters have peak transmission of 30% to... 

Fluorometry is widely used for automated immunoassay. It is approximately 1000 times more sensitive than comparable absorbance spectrophotometry, but background interference caused by fluorescence of native serum can create a major problem. This interference is minimized by careful design of the filters used for spectral isolation, by the selection of a fluorophore with an emission spectrum distinct from those of interfering compounds, or by using time- or phase-resolved fluorometry (see Chapter 3). 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

Instrumentation for diode laser based AAS is now commercially available and the method certainly will expand as diode lasers penetrating further into the UV range become available, especially because of their analytical figures of merit that have been discussed and also because of their price. In diode laser AAS the use of monochromators for spectral isolation of the analyte lines becomes completely superfluous and correction for non-element specific absorption no longer requires techniques such as Zeeman-effect background correction atomic absorption or the use of broad band sources such as deuterium lamps. 

Chloroplast cytochromes can readily be characterized quantitatively by optical spectroscopy, as they have characteristic absorption spectra as well as distinct absorption-difference spectra. The thylakoids in higher plants (e.g., spinach) contain three cytochromes Cyt b559, in either of two forms associated with PS II, and Cytb and Cyt/in the Cyt b complex. Although the a-absorption bands of all three cytochromes lie close to one another in the 550-560 nm region, it is possible to spectrally isolate the individual cytochromes by imposing carefully chosen redox potentials, as the redox-potential values of the different types of chromophores are sufficiently different from one another. Some of the optical and redox properties of the three spinach cytochromes are shown in the table accompanying the spectral properties in Fig. 8. 

Commercial AAS instrumentation may be purchased with fixed shts or with variable slits. Fixed mechanical slit widths are available so that the resolution and sensitivity are acceptable for most analytical purposes at lower cost than instruments with variable sht widths. Variable slit widths are desirable for maximum flexibility, especially if samples are varied and complex. Instruments that have both flame and graphite furnace atomizers often have separate sets of slits of different heights for each atomizer. The furnace slits are usually shorter to avoid having emission from the small diameter incandescent furnace reach the detector. In general, the analyst should use the widest slit widths that minimize the stray light that reaches the detector while spectrally isolating a single resonance line for the analyte from the HCL. 

Laser Fluorescence Detector. A helium-cadmium laser (Model 4240B, Llconlx, Sunnyvale, CA) was (diosen as the excitation source because of Its stability and convenient wavelengths (325 and 442 nm). The UV laser radiation (325 nm, 5-10 mH cw) was Isolated with a dielectric mirror and was focused on the miniaturized flowcell with a quartz lens. Sample fluorescence, collected perpendicular to and coplanar with the excitation beam, was spectrally Isolated by appropriate Interference filters and then focused on a photomultiplier tube (Centronlc Model Q 4249 B, Bailey Instruments Co., Inc., Saddle Brook, NJ). The resulting photocurrent was amplified with a plcoammeter (Model 480, Kelthley Instruments,... 

Since spectra produced by flames are much simpler (fewer lines) than those produced by arc and spark emission, simple devices for spectral isolation could be used. Developments in Europe in the 1930 s led to simpler burners and made use of colored glass filters for spectral isolation. Read-out systems composed only of a photocell connected directly to a galvanometer were used. Such instruments, many still in use, were adequate for simple liquid samples for the determination of the alkali metals. 

Since the early developments in instrumentation in this country and the success of flame methods for the alkali metals there have been improvements in instrumentation that permit flame excitation to be used for other elements. These improvements have dealt primarily with the use of more efficient aspirators, hotter flames, monochromators (gratings or prisms) for spectral isolation, photomultipliers for increased sensitivity, and recorders for reading spectral line intensities. Today some 45-50 elements can be determined by use of flame excitation procedures. 

The interference filter can provide better spectral isolation than a color filter. The bandwidth at one-half transmittance may be as low as 100 A. The principle of the interference filter is shown in Figure 3-3. The filter itself is composed of two parallel, half-silvered (semitransparent) surfaces / and J, carefully spaced by a transparent material. The half-silvered surfaces are separated by one-half of the desired wavelength. 

Frequencies harmonically related to the fundamental frequency of the interference filter also will be reinforced. A filter with maximum transmittance at 6000 A also will transmit at 3000 and 2000 A, corresponding to a path length for CD plus DE of 2X and 3X, respectively. If these harmonically related frequencies are a source of difficulty in the use of the interference filter, they can be removed by combining the interference filter with a color filter with peak transmittance at the same wavelength as the fundamental of the interference filter. Such a combination will provide the better spectral isolation of the interference filter with removal of harmonically related transmission peaks. 

Interference filters provide the best wavelength selectivity of any filters available. It is not possible to provide the necessary resolving power required for more complex spectral isolation. The filters therefore are primarily useful for simple systems where passage of a spectral band will meet spectral isolation requirements. 

To use atomic spectra for analytical purposes, regardless of the application, certain basic instrumentation is required. Included are a spectral isolation device—filter, prism, or grating a slit to permit the radiation to strike the monochromator as a narrow beam a device to allow observation of the spectrum and the necessary optics, lenses, mirrors, etc. to collect and focus the incident light beam. These devices, assembled into one instrument, constitute the monochromator. Some lenses and mirrors, an optical bench, and the instrumentation for observing the spectrum are or may be external to the basic monochromator. 

For some purposes, the necessary spectral isolation may require only the isolation of a band of frequencies. For such cases a filter may be used. Most situations, however, require a spectral resolution much better than can be achieved with a filter. A prism or a grating is necessary for such applications. There are numerous methods and optical arrangements used in monochromators. They each have advantages and disadvantages and many are designed for a particular use. The following discussion describes the more common monochromators, their characteristics, and some of their applications. 

Flame excitation methods, coupled with simple read-out devices, provided high sensitivity and high reliability for the determination of the alkali metals in simple liquid systems. Further development of burners and aspirators, higher flame temperatures, better spectral isolation using gratings or prisms, and more sensitive detection and read-out devices has increased the list of elements that can be detected by flame excitation to between 50 and 60. 

The processes listed above occur in an environment that includes fuel and oxidant molecules and the products of the reactions between the fuel and the oxidant. The temperature of the flame, which is primarily responsible for the occurrence of these processes, is determined by several factors, including (1) type of fuel and oxidant, (2) the fuel-to-oxidant ratio, (3) type of solvent, (4) amount of solvent entering the flame, (5) type of burner, and (6) the region in the flame that is focused onto the entrance slit of the spectral isolation unit. 

Direct spectral line interference occurs when the spectral line energy of two or more elements reaches the detector circuit. One type of spectral line interference involves spectral line overlap. This occurs because spectral lines have a finite linewidth. If the spectral energies of two lines overlap, the result is spectral interference regardless of the resolving power of the spectral isolation system of the spectrometer. At high flame temperatures, when... 

Spectral line interferences also can occur when spectral lines of two or more elements are close but do not produce an actual overlap of their energy envelopes. This type of interference is especially troublesome when the spectral isolation device is a filter. With a filter, lines separated by as much as 50-100 A may be passed through the filter to the detecting circuit, thus producing an incorrect read-out signal. As the resolution of the spectral isolation system increases, such interference possibilities decrease. They cannot be eliminated entirely, however, because of the finite width of the spectral isolation system and the finite slit widths required in such systems. 

Spectral line interferences frequently can be minimized by using any one of a variety of procedures, depending on the particular situation. One important instrumental parameter that can be used to minimize spectral line interferences is the resolution of the spectral isolation device. Filter instruments are of little use in this respect except for very simple analytical samples since they may have a band pass of as much as 800-900 A. Interference filters give better resolution (100-200 A) and thus may be used in applications where colored filters are not acceptable. 

Reference should be made to Chapters 3 and 4 regarding the theory and performance of spectral isolation devices and monochromator design. The discussion here will be limited to specific applications for atomic absorption... 

In some instances where the spectral isolation demands are minimal, simple interference filters can be used in place of a dispersive-type instrument. In these cases, the term filter photometer rather than spectrometer is employed. 

Bandpass Interference or Tpk-maximum transmittance of filter X-, Spectral isolation... 

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