Atomic spectroscopy techniques

Element Wavelength most suitable for Other lines Flames suitable for  

The emission line in bold type indicates the method of choice. 

Whereas flame emission photometry relies on the excitation of atoms and the subsequent emission of radiation, atomic absorption spectrophotometry relies on the absorption of radiation by non-excited atoms. Because the proportion of the latter is considerably greater than that of the excited atoms, the potential sensitivity of the technique is also much greater. 

The sample is converted into an aerosol in an atomizer. It then passes through an expansion chamber to allow a fall in the gas pressure and the larger droplets to settle out before passing to the burner, where the solvent evaporates instantly, the atoms remaining as a finely distributed gas. Atoms in the sample that are bound in molecules should be decomposed at the flame temperature so rapidly that the same effect is achieved. In practice only a small proportion of the sample (approximately 5%) is effectively atomized because the drop size of the remaining 95% is so large that the water is never effectively stripped away. In low temperature flames, for instance, only one sodium atom in about 60000 is excited but despite this apparently low efficiency the technique is very sensitive. 

Any of the monochromating systems described for absorptiometers may be used, although the cheaper models of flame photometer usually employ filter systems. In these cases interference from other elements at wave- 

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Practically all classical methods of atomic spectroscopy are strongly influenced by interferences and matrix effects. Actually, very few analytical techniques are completely free of interferences. However, with atomic spectroscopy techniques, most of the common interferences have been studied and documented. Interferences are classified conveniently into four categories chemical, physical, background (scattering, absorption) and spectral. There are virtually no spectral interferences in FAAS some form of background correction is required. Matrix effects are more serious. Also GFAAS shows virtually no spectral interferences, but... 

The following block schemas show the essential instrumental features of the various atomic spectroscopy techniques. Clearly, there are many similarities between these techniques. The subsequent discussions will describe the instrumental components of these techniques. 

A number of organometallic compounds show promise for LCEC study, and a few have already been examined in detail (especially mercury alkyls) [9]. More highly conjugated organic compounds such as a,a-unsaturated ketones and imines are occasionally good candidates, but at this time UV detectors frequently outperform electrochemical detectors for such systems. At this writing there have been only a few reported LCEC studies of metal ions or metal complexes. Perhaps the major reason for this is that very little modern LC has been carried out on them using any detector. It is difficult to compete with atomic spectroscopy techniques for the determination of most elements, but as speciation becomes more important, it seems likely that more LCEC methods will be developed for metal complexes. 

TABLE 3.1. Instrumental LoDs in pg 1 From Guide to Atomic Spectroscopy Techniques and Applications, Perkin-Elmer, 2000 ... 

As with all atomic spectroscopy techniques, ICP-MS also suffers from a number of interferences. 

Figure 1.5 Three types of atomic spectroscopy techniques shown diagrammatically. (Reproduced by kind permission copyright 1999-2008, all rights reserved, PerkinElmer, Inc.)...

The selectivity and sensitivity offered by atomic spectroscopy techniques can be used for direct and indirect determination of metals in a range of pharmaceutical preparations and compounds. Metals can be present in pharmaceutical preparations as a main ingredient, impurities, or as preservatives which can be prepared for analysis using non-destructive (direct or solvent dilution) or destructive methods (microwave acid digestion, bomb combustion, extraction, etc.) and the metal of interest measured against standards of the metal prepared in the same solvents as the sample. Methods associated with some pharmaceutical products are already described in the international pharmacopoeias and must be used in order to comply with regulations associated with these products, e.g titration techniques are carried out according to methods that are the same for all pharmaceutical products. 

In most cases the determination of organometallic complexes by atomic spectroscopy techniques is the only acceptable method because the analysis is selective, accurate and precise. Analysis of these complex salts may only involve a simple dilution in a solvent or destruction methods depending on the matrix it is formulated into. The presence of some sample matrices containing organometallic complexes can be severely restricted by the matrix material to achieve accurate detection and quantification of these salts. 

Analysis of initial, intermediate and final stages of most reactions involving metals used as catalysts, activators, etc., needs to be monitored at each stage to ensure that the process in which the metal salt is used is effective. In certain reactions it may be necessary to carry out analysis to determine if the metals have been effectively removed, if the process so requires. All metal catalysts can be readily monitored using atomic spectroscopy techniques after appropriate sample preparations. 

The first requirement can be easily fulfilled by the preconcentration of the analyte before the analysis. Preconcentration has been applied to sample preparation for flame atomic absorption (25) and, more recently, for ICP (79,80) spectroscopy. However, preconcentration is not completely satisfactory, because of the increased analysis time (which may be critical in clinical analysis) and the increased chance of contamination or sample loss. Most important, however, a larger initial sample size is necessary. The apparent solution is a more sensitive technique. Table 2 lists concentrations of various metals in whole blood or serum (81,82) in comparison to limits of detection for the various atomic spectroscopy techniques. In many cases, especially for the toxic heavy metals, only flameless atomic absorption using a graphite furnace can provide the necessary sensitivity and accommodate a sample of only a few microliters (Table 1). The determination of therapeutic gold in urine and serum (83,84), chromium in serum (85), skin (86) and liver (87), copper in semen (88), arsenic in urine (89), manganese in animal tissues (90), and lead in blood (91) are but a few examples in analyses which have utilized the flameless atomic absorption technique. 

ICP-AES is the best overall multi-element atomic spectroscopy technique for soil metal analysis, with excellent sample throughput and very wide... 

The fact that lithium thickener still dominates the grease market means that any atomic spectroscopy technique to be considered should include the capability of determining the concentration of this important analyte. The complex nature of the grease sample matrix makes it difficult to get into a form that is compatible with ICP-AES. However, the elemental composition may be effectively measured if a suitable digestion technique can be developed. 

Gravimetry (for sulfur), titrimetric (chloride), fluo-rimetry (Se, Al, F), colorimetric methods, or atomic spectroscopy are the traditional classical chemical methods for the determination of individual elements. Atomic spectroscopy techniques have a much higher sensitivity and specificity and provide a complete profile of elements in a food/feed. Emission... 

Most mining companies use XRF for the analysis of their process streams and monitor the separation of metals from the ore. XRF is faster than any wet or atomic spectroscopy technique since it can measure the sample as a solid. Trace elements in soil and sediment can be measured to collect geological, agricultural, and environmental data both in the lab and in the field, using portable or benchtop EDXRF analyzers. 

Measurements of the solar atmosphere by atomic spectroscopy techniques reveal that the outermost region—the solar corona from which CMEs are expelled from the sun—has a much lower density and a much higher temperature than the underlying surface (the photosphere). The... 

Detection limits achievable by various atomic spectroscopy techniques are reviewed in Table 11.1. 

The major strengths of atomic spectroscopy techniques over other methods is that they are relatively inexpensive, and they provide outstanding flexibility in terms of automation and multielement analysis capabilities (almost the whole Periodic Table). These advantages, coupled with high precision and accuracy, make atomic spectoscopy a preferred method of analysis. 

Table 1 provides side-by-side comparison of the most important characteristic features for the technqiues in Fig. 2. A graphical representation of element detections for different atomic spectroscopy techniques is given in Fig. 3. 

Most of these techniques have either limited applicability or suffer from inconsistent precision and accuracy and therefore have not been adopted as routine approaches. Laser ablation is probably one of the most promising methods in the above list, with high potential to provide an alternative sample introduction route for the different atomic spectroscopy techniques. 

FIGURE 20.2 Typical detection limit ranges for major atomic spectroscopy techniques. 

Graphite furnace applications are well-documented, though not as complete as flame AA. It has exceptional detection limit capabilities but with a limited analytical working range. Sample throughput is less than that of other atomic spectroscopy techniques. Operator skill requirements are much more extensive than for flame AA. 

This is the most widely used multielement atomic spectroscopy technique, with excellent sample throughput and very wide analytical range. Operator skill requirements are somewhere between those of flame AA and ETA. ICP-OES is now a mature technique, which means that sufficient applications literature is available. 

Revised chapter on comparing ICP-MS with other atomic spectroscopy techniques to reflect the most recent developments... 

Comparison of ICP-MS with Other Atomic Spectroscopy Techniques ... 

Spectroscopic determination of atomic species can only be carried out in the gas phase, where the individual atoms or ions are well separated. Consequently, the first step in the process is atomization, where the sample is volatilized (heated to the gas phase) and decomposed to produce an atomic gas. The differences between the various atomic spectroscopy techniques available, largely lie in the different ways of doing this. The most widely used method is flame atomization, where the sample is decomposed in a flame (a sophisticated version of the common flame test), but other common methods (Table 5.2) are... 

Table 5.2 Some of the atomic spectroscopy techniques commonly available and their acronyms...

Lewen N, Schenkenberger M, Raglione T and Mathew S (1997) The application of several atomic spectroscopy techniques in a pharmaceutical analytical research and development laboratory. Spectroscopy 12 14-23. 

The numbers of papers focusing on the determination of inorganic UV filters is very scarce, perhaps due to the fact that only two compounds, Ti02 and ZnO, are currently used as UV filters. Atomic spectroscopy techniques, such as atomic absorption spectromety (AAS) (Mason, 1980), inductive coupled plasma atomic emission spectrometry (ICP-AES) (Salvador et al, 2000) and X-ray fluorescence spectrometry (XRFS) (Kawauchi et al, 1996) have been used for titanium oxide determination, whereas to our knowledge zinc oxide has only been determined by AAS (Salvador et al, 2000). 

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