Flame emission instrumentation requirements

Use of Molecular Emission from Cool Flames Molecular Emission Cavity Analysis Instrumental Requirements in AFS... 

Interference arises when the intensity of the signal from the required substance is modified by another substance, although the signals of the two substances are adequately resolved. Thus, the presence of phosphate reduces the flame emission of calcium by forming thermally stable calcium phosphate. The instrumental conditions can sometimes be altered to reduce the effect, or the interfering material removed by pretreatment of the sample. Standard solutions are usually chosen to resemble samples as closely as possible in the hope that any interference will occur equally in both. 

In clinical analysis, flame AAS is very useful for serum analysis. Ca and Mg can be determined directly in serum samples after a 1 50 dilution, even with microaliquots of 20-50 pL [314]. In the case of Ca, La3+ or Sr2+ are added so as to avoid phosphate interferences. Na and K are usually determined in the flame emission mode, which can be realized with almost any flame AAS instrument. The burner head is often turned to shorten the optical path so as to avoid self-reversal. For the direct determination of Fe, Zn and Cu, flame AAS can also be used but with a lower sample dilution. Determination of trace elements such as Al, Cr, Co, Mo and V with flame AAS often requires a pre-concentration stage, but in serum and other body fluids as well as in various other biological matrices some of these elements can be determined directly with furnace AAS. This also applies to toxic elements such as Ni, Cd and Pb, which often must be determined when screening for work place exposure. When aiming towards the direct determination of the latter elements in blood, urine or serum, matrix modification has found wide acceptance in working practices that are now legally accepted for work place surveillance, etc. This applies e.g. for the determination of Pb in whole blood [315] as well as for the determination of Ni in urine (see e.g. Ref. [316]). 

This expression is only valid for low concentrations in the absence of selfabsorption or of ionization. As previously suggested, to conduct an analysis by flame emission, the response of the instrument requires a calibration with a series of standards. 

Some atomic absorption instruments are also designed to be capable of flame emission work. When scanning flame emission is contemplated for qualitative analysis, the monochromator must distinguish between a multiplicity of lines. For this purpose, resolution becomes important. For scanning flame emission, a monochromator of better resolution than 2A. is useful, though by no means always required. 

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. 

In 1955 Walsh established the foundations of modern analytical atomic absorption spectroscopy. In the same year Alkemade and Milatz also published a paper suggesting similar procedures. The work of Walsh, however, was much more detailed, since he examined the theory of the method, the basic principles involved, the instrumentation requirements, and its advantages over flame emission. 

Flame atomic absorption was until recently the most widely used techniques for trace metal analysis, reflecting its ease of use and relative freedom from interferences. Although now superceded in many laboratories by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, flame atomic absorption spectrometry still is a very valid option for many applications. The sample, usually in solution, is sprayed into the flame following the generation of an aerosol by means of a nebulizer. The theory of atomic absorption spectrometry (AAS) and details of the basic instrumentation required are described in a previous article. This article briefly reviews the nature of the flames employed in AAS, the specific requirements of the instrumentation for use with flame AAS, and the atomization processes that take place within the flame. An overview is given of possible interferences and various modifications that may provide some practical advantage over conventional flame cells. Finally, a number of application notes for common matrices are given. 

Flame OES can be performed using most modern atomic absorption spectrometers (discussed in Chapter 6). No external lamp is needed since the flame serves as both the atomization source and the excitation source. A schematic diagram of a flame emission spectrometer based on a singlebeam atomic absorption spectrometer is shown in Figure 7.2. For measurement of the alkali metals in clinical samples such as serum or urine, only a low-resolution filter photometer is needed because of the simplicity of the spectra. The filter photometer is discussed in Section 7.I.I.2. Both instruments require a burner assembly, a flame, a wavelength selection device, and a detector. 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

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. 

Manganese in aqueous solution may be analyzed by several instrumental techniques including flame and furnace AA, ICP, ICP-MS, x-ray fluorescence and neutron activation. For atomic absorption and emission spectrometric determination the measurement may be done at the wavelengths 279.5, 257.61 or 294.92 nm respectively. The metal or its insoluble compounds must be digested with nitric acid alone or in combination with another acid. Soluble salts may be dissolved in water and the aqueous solution analyzed. X-ray methods may be applied for non-destructive determination of the metal. The detection limits in these methods are higher than those obtained by the AA or ICP methods. ICP-MS is the most sensitive technique. Several colorimetric methods also are known, but such measurements require that the manganese salts be aqueous. These methods are susceptible to interference. 

Figure 21-24 Flame, furnace, and inductively coupled plasma emission and inductively coupled plasma—mass spectrometry detection limils (ng/g = ppb) with instruments from GBC Scientific Equipment, Australia. [Flame, furnace. ICP from R. J. Gill. Am. Lab. November 1993, 24F. ICP-MS from T. T. Nham, Am. Lab. August 1998. 17A Data for Ct Br, and l are from reference 14.] Accurate quantitative analysis requires concentrations 10-100 times greater than the detection limit.

At the present time, the majority of elemental determinations conducted by FES are performed using instruments designed primarily for AAS. The only modification required is the incorporation of an amplifier capable of measuring the unmodulated emission signals from the flame, a standard feature on almost all AAS instruments. 

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