Flame techniques, trace analysis

Gas Chromatography. Gas chromatography is a technique utili2ed for separating volatile substances (or those that can be made volatile) between two phases, one of which is a gas. Purge-and-trap methods are frequently used for trace analysis. Various detectors have been employed in trace analysis, the most commonly used being flame ioni2ation and electron capture detectors. 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

Different analytical techniques such as ICP-OES (optical emission spectrometry with inductively coupled plasma source), XRF (X-ray fluorescence analysis), AAS (atomic absorption spectrometry) with graphite furnace and flame GF-AAS and FAAS, NAA (neutron activation analysis) and others, are employed for the trace analysis of environmental samples. The main features of selected atomic spectrometric techniques (ICP-MS, ICP-OES and AAS) are summarized in Table 9.20.1 The detection ranges and LODs of selected analytical techniques for trace analysis on environmental samples are summarized in Figure 9.15.1... 

GC has been used extensively for the separation and determination of volatile organic molecules, and most aspects of this application area are fully documented in monographs on this technique. In the inorganic trace analysis area, however, fewer species possess the required volatility, and applications tend to be limited to the separation of volatile species of low molecular weight (such as methyl derivatives of As, Se, Sn, Hg) and the separation of semi-volatile organo-metals, metal halides, metal hydrides, metal carbonyls and metal chelates. For organo-metal species, the type of detection system required varies with the nature of the analyte, and the options include electron capture detection, flame photometric detection (sometimes ICP), AAS and MS. 

Four techniques based on mass spectrometry are widely used for multi-elemental trace analysis of inorganic compounds in a wide range of sample types. These techniques are thermal ionization (TI), spark source (SS), glow discharge (GD) and inductively coupled plasma (ICP) mass spectrometry. In these techniques, atomization and ionization of the analysed sample are accomplished by volatilization from a heated surface, attack by electrical discharge, rare-gas ion sputtering and vaporization in a hot flame produced by inductive coupling. 

The detection limits obtained by flame and graphite furnace AAS and the concentration levels of the elements in seawater are summarized in Table 2. In general, graphite furnace AAS provides better sensitivities for many elements than the flame technique. Even so, AAS sensitivity is insufficient for the direct determination of most ultra-trace elements. Furthermore, concentrated salts and undissolved particulates cause severe interferences with the determination of trace elements by AAS. Therefore, it is necessary to concentrate the analytes before the determination, and, if possible, to separate the analytes from dissolved major constituents and particulates. Solvent extraction, coprecipitation and ion-exchange techniques are the most widely used techniques for the preconcentration of seawater. In the following sections, these techniques will be reviewed. It should be noted here that the efficiencies of the recovery of the analytes as well as the contamination from reagents and solvents must be carefully examined when the preconcentration techniques are applied. Chakrabarti et al. [10] have summarized the work on the application of preconcentration techniques to marine analysis by AAS. Hence, only some representative applications will be introduced hereafter. 

The few articles currently available regarding trace analysis without preconcentration, use in general the graphite furnace technique [102,120, 138] with sample sizes of the order of microliters, and deal with the elements Sb [47, 83], Pb and Bi [48-50], As, Sb, Bi, Sn, Cd, Pb [10, 57, 116] as well as Al, Cr, Sn [6, 62], Co, and Mg [104]. Alkaline earths can be determined directly with the flame method [122, 147], Further techniques of atomic absorption by flame use concentration methods, for example for the determination of small concentrations of tin [17], Te [26], Co, Pb, and Bi [104], and W [106]. From the analytical viewpoint, it is only useful to remove the iron matrix. The extraction of the elements to be determined from the matrix always carries with it the danger of losses and therefore results showing concentrations that are too low. 

Flame atomic absorption can also be used for trace analysis when the iron matrix is extracted [39, 147]. When this extraction is combined with the Hoesch injection technique [3, 130], trace analysis can also be performed [31, 99, 142]. By using less than IOOjliI of sample solution, the improvement compared to conventional techniques is at least a factor of 10 [13, 14]. 

In general, for trace analysis, very small dilutions are preferred [54], in which case only small solution quantities are available for atomic absorption. These can be brought into the flame either with the Hoesch injection technique [3, 13, 14, 31, 99, 130, 142] or with the boat technique [76] or they can be determined in the furnace atomiser [75, 90, 91, 120] so that even such elements as La in ores [109] or the rare earths generally [36, 37] as well as Pb [48, 50], Bi [49], As, Sb, and Sn [116] can be determined. 

Table 1 shows the detection limits of atomic absorption spectrophotometry for various metals. In general, flame atomic absorption spectrophotometry is quantitative in the lower parts-per-million levels and is readily automated for routine, high-volume samples. The other three techniques are used primarily for trace analysis and are quantitative to the lower parts-per-million levels for many elements. 

On-line solid-phase extraction (SPE) by ion-exchange and preconcentration using FIA techniques are becoming increasingly popular for trace analysis of heavy metals both by flame AAS and by hydride generation and cold vapor AAS45-49. 

The review by Tblg (1987) (Extreme trace analysis of the elements - the state of the art today and tomorrow) is an insightful review by an experienced trace analyst concentrating on atomic spectrometric methods including AAS, OES, XRF, MS with many variants of excitation. A table is provided comparing the capability of determinative methods listing the method, the specific technique, limit of determination, matrix effects, multielement determination, and speciation analysis. Methods compared include AAS (flame, furnace, HG), ZAAS (furnace), OES-DCP(PN), OES-ICP(PN), OES-MIP(PN, ETV, HG), OES-HC (volatilization), FANES, AES (laser ICP/ICP, HC/... 

Light Absorption Spectrometry Light absorption spectrometry (molecular absorption) (LAS) has several names, and includes techniques such as UV/VIS (visible) spectrometry, colorimetry, flame molecular absorption, reflectance spectrometry, turbidimetry, nephelometry, ring oven technique, ion test paper and spot tests. Its colorful history and principles may be found in the older, classical books on analytical chemistry. Upor et al. (1985) have an entire volume on photometric methods in inorganic trace analysis in the respected Comprehensive Analytical Chemistry series covering interference separation and analyte concentration, preparation of samples, factors... 

Spectrophotometric techniques have been the basis of many coal analysis methods. One of the most widely used techniques for analysis of trace elements is atomic absorption spectrometry, in which the standards and samples are aspirated into a flame. A hollow cathode lamp provides a source of radiation that is characteristic of the element of interest and the absorption of characteristic energy by the atoms of a particular element. X-ray fluorescence is also employed as a quantitative technique for trace element determination and depends on election of orbital electrons from atoms of the element when the sample is irradiated by an x-ray source. 

For trace analysis, the main ceramic elements of interest are Zn, Pb, Cu, Bi, Sb, Sn, Ag, As, Mn, Cr, Se, and Hg. Many of these are environmentally important. In certain cases the detection limits of flame AAS are inadequate, so that hydride generation for antimony, selenium, arsenic and bismuth, cold vapor for mercury, and graphite furnace AAS for lead and cadmium are required. A variation of AAS is atomic fluorescence, and this is used to achieve the detection limits needed for Hg and Se in environmental samples. Microwave digestion techniques for sample preparation are becoming more common, where, unlike fusion, there is no risk of loss of volatile elements from unfired samples and fewer reagents are... 

The detection and determination of traces of cobalt is of concern in such diverse areas as soflds, plants, fertilizers (qv), stainless and other steels for nuclear energy equipment (see Steel), high purity fissile materials (U, Th), refractory metals (Ta, Nb, Mo, and W), and semiconductors (qv). Useful techniques are spectrophotometry, polarography, emission spectrography, flame photometry, x-ray fluorescence, activation analysis, tracers, and mass spectrography, chromatography, and ion exchange (19) (see Analytical TffiTHODS Spectroscopy, optical Trace and residue analysis). 

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. 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

Cadmium in acidified aqueous solution may be analyzed at trace levels by various instrumental techniques such as flame and furnace atomic absorption, and ICP emission spectrophotometry. Cadmium in solid matrices is extracted into aqueous phase by digestion with nitric acid prior to analysis. A much lower detection level may be obtained by ICP-mass spectrometry. Other instrumental techniques to analyze this metal include neutron activation analysis and anodic stripping voltammetry. Cadmium also may be measured in aqueous matrices by colorimetry. Cadmium ions react with dithizone to form a pink-red color that can be extracted with chloroform. The absorbance of the solution is measured by a spectrophotometer and the concentration is determined from a standard calibration curve (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, DC American Public Health Association). The metal in the solid phase may be determined nondestructively by x-ray fluorescence or diffraction techniques. 

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