Interferences atomic fluorescence

Raman spectroscopy has enjoyed a dramatic improvement during the last few years the interference by fluorescence of impurities is virtually eliminated. Up-to-date near-infrared Raman spectrometers now meet most demands for a modern analytical instrument concerning applicability, analytical information and convenience. In spite of its potential abilities, Raman spectroscopy has until recently not been extensively used for real-life polymer/additive-related problem solving, but does hold promise. Resonance Raman spectroscopy exhibits very high selectivity. Further improvements in spectropho-tometric measurement detection limits are also closely related to advances in laser technology. Apart from Raman spectroscopy, areas in which the laser is proving indispensable include molecular and fluorescence spectroscopy. The major use of lasers in analytical atomic... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Why are spectral interferences less important in atomic absorption spectroscopy and atomic fluorescence spectroscopy than atomic emission spectroscopy ... 

In some flame AFS systems, interference filters and solar blind photomultipliers have been used to reduce the background, but usually a conventional monochromator is used. As in AAS, the source signal is modulated so that the atomic fluorescence can be distinguished from atomic emission. 

The determination of organic selenium compounds is done preferably by GC coupled to element-or molecule-specific detectors, such as GC-AED or molecular mass spectrometric detection (GC-MS).240 In this case, ICP-MS detection does not yield the improvement in sensitivity otherwise seen, which is due to spectral interferences. Dietz et al.241 have compared the analytical figures of merit of three detector systems for GC (AED, atomic fluorescence spectroscopy (AFS), and ICP-MS), arriving at the conclusion that GC-AED is the most sensitive and most practical... 

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. 

C25. Cresser, M. 8., and West, T. S., Some interference studies in atomic fluorescence spectroscopy with a continuum source. Spectrochim. Acta, Part B 25, 61-68 (1970). 

Because the glow-discharge creates a cloud of sputtered atoms in a low-pressure, rare-gas environment that is an excellent quenching medium, it is an effective source for atomic fluorescence. In addition, the inert gas acting as diluent poses few chemical interferences and the elemental absorption lines are relatively narrow. It should be noted that GD-AF has not been used to the same extent as other GD techniques, however. 

In atomic fluorescence spectrometry spectral interferences are low as the fluorescence spectra are not line rich. 

Because the atomic fluorescence is measured at a right angle to the source, spectral interferences are minimal and a simple cutoff filter may often be used to isolate the emission line. The intensity of the fluorescence is directly proportional to the analyte concentration. As the analyte concentration within the flame becomes large, self-absorption of resonance fluorescence becomes significant, as it does in flame emission spectroscopy. Under these conditions, the linearity of the instrumental response breaks down and a calibration curve must be used or the analyte solutions diluted accordingly. 

In-line filtration without a filtering element is also feasible. To this end, a three-dimensional reactor [299], also called a knitted or knotted reactor (see 6.2.3.4), can be used, as emphasised in the landmark article reporting the flow injection determination of lead in blood and bovine liver by flame atomic absorption spectrometry [300]. The analyte was co-precipitated the complex formed was retained on the inner walls of a knitted reactor and then released by isobutyl methyl ketone and transported to the detector. Interference from iron(III) at high concentrations was circumvented, sensitivity was markedly improved and precise results were obtained. This innovation was recently exploited to remove organic selenium and determine the speciation of inorganic selenium in a flow-injection system with atomic fluorescence spectrometric detection [301]. 

FI. Wu, Y. Jin, Y.-Q. Shi, S.-P. Bi, On-line organoselenium interference removal for inorganic selenium species by flow injection coprecipitation preconcentration coupled with hydride generation atomic fluorescence spectrometry, Talanta 71 (2007) 1762. 

A dispersive system for atomic fluorescence measurements cotisisis of a modulated source, an alomi/.er (flame or nonflame), a monochromator or an interference filter system, a detector, and a signal processor and readoul. Wilh the exception of the source, most of these components are similar to those discussed in earlier parts of this chapter. 

Interferences encountered in atomic fluorescence spectroscopy are generally of the same type and of about the same magnitude as those found in atomic absorption speclrosciipy."... 

The atomic absorption method for determining the concentration of metallic elements has now gained wide acceptance. Instrumentation is relatively inexpensive and simple to use. Analytical interferences are less prevalent than with most other techniques means of recognizing and combating the interferences that do exist are described. The article discusses the basic principles of atomic absorption and also describes the fundamental design and modern improvements in the major components of instrumentation hollow-cathode lamps, burners, photometers, and monochromators. Atomic absorption is compared with some of its rival techniques, principally flame emission and atomic fluorescence. New methods of sampling and the distinction between sensitivity and detection limit are discussed briefly. Detection limits for 65 elements are tabulated. 

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

In flame spectroscopy, a solution is aspirated into a flame and the inorganic compounds thermally dissociated into atomic vapor. There are three types of flame spectroscopy atomic absorption, atomic emission, and atomic fluorescence. The first two techniques will be emphasized because commercial instruments are widely available for these, whereas atomic fluorescence is used more for specific applications and as a research tool. Many of the points made with respect to flame chemistry, interferences, and so forth apply also to atomic fluorescence. Various types of atomic fluorescence and the specific instrumentation required are considered at the end of the chapter. 

The basic instrumentation for atomic-fluorescence spectroscopy is shown in Figure 10.13. The source is placed at right angles to the monochromator so that its radiation (except for scattered radiation) does not enter the monochromator. The source is chopped to produce an AC signal and minimize flame-emission interference. As in molecular fluorescence (Chap. 9), the intensity of atomic fluorescence is directly proportional to the intensity of the light impinging on the sample from the source. 

Spectral Interferences. Relatively few examples of actual spectral interferences have been reported in atomic absorption or atomic fluorescence spectrometry. This means that the possibility that a resonance line emitted from a line-like radiation source may overlap with an absorption line of another element present in the atomizer is very small. 

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