Detector, atomic spectrometer dispersion

As most commonly used, the technique employs a pulsed laser and a focusing lens to generate a plasma that vaporizes a small amount of a sample. A portion of the plasma light is collected and directed to a spectrometer. The spectrometer disperses the light emitted by excited atoms, ions, and simple molecules in the plasma, a detector records the emission signals, and electronics take over to digitize and display the results. 

There have been a number of reviews in the literature on the identification of metal species by LC/AAS (40-42) but to successfully utilize the combination, both the LC and the spectrometer system have to be optimized and this has also been the subject of a number of publications (43-45). It has been claimed (44) that the poor sensitivity obtained from the LC/AAS system relative, to that obtained from the atomic spectrometer alone, was due to the dispersion that takes place in the column. Although substantially true, this misunderstanding arises from the fact that the spectroseopist views the chromatograph as just another sampling device and not as a separation system. The point of interfacing a liquid chromatograph with an atomic spectrometer is to achieve a separation before detection and consequently, the important dispersion characteristics are not those that occur in the column but those that occur in the interfaces between the detector and the spectrometer and in the spectrometer itself. 

Color Plate 23 Polychromator for Inductively Coupled Plasma Atomic Emission Spectrometer with One Detector for Each Element (Section 21-4) Light emitted by a sample in the plasma enters the polychromator at the right and is dispersed into its component wavelengths by grating at the bottom of the diagram. Each different emission wavelength (shown schematically by colored lines) is diffracted at a different angle and directed to a different photomultiplier detector on the focal curve. Each detector sees only one preselected element, and all elements are measured simultaneously. [Courtesy TJA Solutions, Franklin, MA.J... 

Image Devices. Although he never assembled an actual TV-spectrometer, Margoshes was the first to recognize the potential of TV-type detectors in analytical atomic spectroscopy. In a series of reports (50,51,52) he speculated on the advantages of using an SEC tube [vide infra] to detect radiation dispersed by an echelle spectrograph. These reports and the recent availability of various solid-state array detectors have prompted numerous... 

If the flame background emission intensity is reduced considerably by use of an inert gas-sheathed (separated) flame, then an interference filter may be used rather than a monochromator, to give a non-dispersive atomic fluorescence spectrometer as illustrated in Figure 14.36-38 Noise levels are often further reduced by employing a solar blind photomultiplier as a detector of fluorescence emission at UV wavelengths. Such detectors do not respond to visible light. The excitation source is generally placed at 90° to the monochromator or detector. Surface-silvered or quartz mirrors and lenses are often used to increase the amount of fluorescence emission seen by the detector. 

The general construction of an atomic absorption spectrometer, which need not be at all complicated, is shown schematically in Fig. 1. The most important components are the light source (A), which emits the characteristic narrow-line spectrum of the element of interest an absorption cell or atom reservoir in which the atoms of the sample to be analysed are formed by thermal molecular dissociation, most commonly by a flame (B) a monochromator (C) for the spectral dispersion of the light into its component wavelengths with an exit slit of variable width to permit selection and isolation of the analytical wavelength a photomultiplier detector (D) whose function it is to convert photons of light into an electrical signal which may be amplified (E) and eventually displayed to the operator on the instruments readout, (F). 

The flame atomic absorption spectrometer is inherently a flow-through detector, with which the sample solutions are continuously fed into the nebulizer-burner system through suction. Despite the relatively large volume of the spray chamber (usually about 1(X) ml) in comparison to the spectrometric flow-cell, the detector was shown to have very little contribution to the dispersion of the injected sample in comparison to other components of the FI system [11]. With careful optimization, as little as 50-80 //I sample may be injected to achieve 80-95% of the steady state signal obtained by conventional sample introduction (see Fig. 2.14). 

Table 3.4 compares detection limits with secondary fluorescers to the results with the RMF method and 15-kV broadband excitation [16,17]. Four different fluorescence analyzers were tested (units A, B, C, and D), and the results were corrected for differences in performance for the energy-dispersive spectrometers employed on each unit. Unit A used a chromium anode tube, and unit B used a tungsten anode tube. Unit A was a commercial, general-purpose instrument. Unit B was specifically designed for atmospheric aerosol analysis, where closer coupling between the tube, fluorescer, sample, and detector could be employed with some sacrifice of insensitivity to specimen-positioning errors. Table 3.5 lists the x-ray tube operating conditions required for Table 3.4. For medium- to high-atomic-number elements, the secondary fluorescer method provides detection limits equivalent to the RMF element, but requires much higher x-ray tube power. For light elements.

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