Technique for Commonly Determined Elements

Generally a new flame spectrometer arrives with a fairly full set of instructions on how to set the instrument up and the key parameters to use for each element that may be determined. The latter may be in a hard-copy cook-book of instructions, or stored on a computer disk for rapid availability of information. Top-of-the-range instruments may even set the instrumental conditions automatically to those specified. In theory, then, there should be no need for this chapter at all. However, in practice, such manufacturers guides often tacitly make simplifying assumptions about the sorts of samples to be analysed, and rarely tell you what to do if the instrument can t meet your needs directly. The purpose of this chapter, then, is to provide a useful guide to what can and cannot be achieved by flame spectrometric methods for each of the commonly determined elements of environmental interest. It is also intended to provide cautionary advice whenever such advice is necessary. 

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To summarise, FAAS is very easy to use. Interferences are known and can be controlled. Extensive application information is also readily available. Its precision makes it an excellent technique for the determination of a number of commonly analysed elements at higher concentration in polluted soil samples. Its main drawback is its speed in relation to multi-element techniques such as ICP-AES and ICP-MS. Where direct-aspiration flame atomic absorption technique does not provide adequate sensitivity, reference is made to specialised techniques (in addition to graphite furnace procedure) such as the gaseous-hydride method for arsenic, antimony and selenium and the cold-vapour technique for mercury. 

The most common technique for the determination of mercury in environmental samples is cold vapour atomic absorption spectrometry (CV-AAS) due to its simplicity and sensitivity. The flameless procedure was investigated by Hatch and Ott (1968) with a view to simplifying the apparatus required and improving the sensitivity of the method. The method is based on the unique properties of mercury. Elemental mercury has an appreciable vapour pressure at ambient temperature and the vapour is stable and monatomic. Mercury can easily be reduced to metal from its compounds. The mercury vapour may be introduced into a stream of an inert gas and measured by atomic absorption or atomic fluorescence of the cold vapour without the need of atomiser devices. 

It was fortunate that the return of the first lunar samples in 1969 took place following ten years of development and refinement of analytical techniques for the determination of the rare earths. Accordingly, from the outset of lunar studies accurate and reliable values at the parts per million level wo-e available and the progress of the investigations was not hampered, as has commonly been the case in trace element studies, by data of questionable and uncertain quality. General reviews may be found in works by Taylor (1975,1982). 

A reasonably common technique for the determination of many isotopes is mass spectrometry. This technique is very sensitive and isotope specific. It is especially suitable for heavy elements like the actinides, where isobaric disturbances are few. Three different t5q3es of mass spectrometers have been used for the determination of radionuclides in the environment. These types are the thermal ionization mass spectrometer (TIMS), the inductively coupled plasma-mass spectrometer (ICP-MS), and the accelerator mass spectrometer (AMS). The AMS is used mainly for the determination of geologic age or for the study of radionuclide production in the atmosphere. Thermal ionization mass spectrometry is a very sensitive technique with very low detection limits however, TIMS... 

Principles and Characteristics Combustion analysis is used primarily to determine C, H, N, O, S, P, and halogens in a variety of organic and inorganic materials (gas, liquid or solid) at trace to per cent level, e.g. for the determination of organic-bound halogens in epoxy moulding resins, halogenated hydrocarbons, brominated resins, phosphorous in flame-retardant materials, etc. Sample quantities are dependent upon the concentration level of the analyte. A precise assay can usually be obtained with a few mg of material. Combustions are performed under controlled conditions, usually in the presence of catalysts. Oxidative combustions are most common. The element of interest is converted into a reaction product, which is then determined by techniques such as GC, IC, ion-selective electrode, titrime-try, or colorimetric measurement. Various combustion techniques are commonly used. 

For routine determinations of metals and metalloids, laboratories use the following common trace element analysis techniques ... 

Mass spectrometric techniques are based on the measurement or counting of ions produced at high temperatures. An ion can be identified on the basis of its mass-to-charge ratio (m/z), characteristic of a certain isotope. In addition, quantification is based on the dependence between the number of ions and the concentration of a given isotope in the sample. Mass spectrometers consist of an ion source, a mass analyzer, and an ion detector. The ion source is typically the basis for the different types of mass spectrometric techniques. Plasmas are the most common ion sources for Mass spectrometric elemental determinations, and it is mass spectrometry (MS) using this ion source that will now be described. Complete details of this technique can be found in published monographs.29,30... 

It is obvious, therefore, that 14 MeV neutron activation analysis can not compete with thermal neutron activation analysis as a technique for trace element analysis. In simple matrices, however, the rapid and non-destructive nature of the technique recommends its use for routine analysis of large numbers of samples for elemental abundances at the one milligram level, or above. It is unfortunate that the element carbon can not be determined by this technique. The nuclear reaction 12C(n, 2n)1 C which would be of great analytical importance is endoergic to the extent of nearly 19 MeV. This reaction is obviously not energetically possible using the 14.7 MeV neutrons produced by the 2H(3H,w)4He reaction commonly employed in most neutron generators. 

Before the development of efficient chromatographic separation techniques and selective and sensitive detectors, analytical methods for the determination of specific analytes in environmental samples were very limited. Those methods depended on highly selective chemical reactions that are relatively rare and difficult to discover, or on very selective physical measurements such as atomic absorption or emission techniques for elemental analytes. Therefore only a relatively few analytical methods for the most common and amenable organic and inorganic compounds or... 

Recent reviews have appeared on the analysis of glasses and related materials using AES and AAS [1—3]. It is the purpose of this chapter to present to the laboratory technician with experience in AES and AAS some selected methods for determining the elements common to such substances. Since AES and AAS methods are based upon comparisons with standards, it is desirable to have reasonable estimates of the elemental concentrations in a sample before beginning an analysis. Sometimes this information is obtainable from batch compositions. Frequently, the composition of the material is unknown. A commonly used technique for obtaining semiquantitative estimates is optical emission spectroscopy, for which a procedure has been previously published [ 2]. 

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