Extreme trace analysis

Selection of digestion vessels the size of the digestion vessels depends on the sample volume. Standard vessels are 25, 40, SO, 100 and ISO ml glass. For extreme trace analysis, quartz and Teflon vessels are available. Vessels can also be supplied with dust covers and reflux extensions. 

Inorganic trace analysis requires more than cursory attention to sample collection and treatment techniques since contamination is not only possible but often unavoidable. A good discussion of problems encountered in extreme trace analysis has been published by Tolg (99). The following section discusses some innovations in atmospheric and water sampling procedures. 

When the graphite furnace is used for extreme-trace analysis, relative standard deviations are in general above 10 relative percent. The solid sample analysis described in section II.B.l, using a steel chip and the rectangular cuvette, leads to an RSD of 13 relative percent in the determination of a... 

In time, the use of flames as atom reservoirs for atomic absorption spectrometry was also transformed into an analytical methodology, as a result of the work of Walsh [2], Flame atomic absorption spectrometry became a standard tool of the routine analytical laboratory. Because of the work of L vov and of Massmann, the graphite furnace became popular as an atom reservoir for atomic absorption and gave rise to the widespread use of furnace atomic absorption spectrometry, as offered by many manufacturers and used in analytical laboratories, especially for extreme trace analysis. However, in atomic absorption spectrometry, which is essentially a single-element method, developments due to the multitude of atomic reservoirs and also of primary sources available, is far from the end of its development. Lasers will be shown to give new impetus to atomic absorption work and also to make... 

Nash et al. (2000) (Methodologies for determination of antimony in terrestrial environmental samples). The review by Tolg (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 spec-trometric methods including AAS, OES, XRE, 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, ZAAS, OES-DCP, OES-ICP, OES-MIP, OES-HC, EANES, AES, XRS, MS, NAA, voltammetry, LAS and fluorimetry. 

Tolg G (1987) Extreme trace analysis of the elements - the state of the art today and tomorrow. Analyst (London) 112 365-376. 

The principal challenge facing the trace analyst is that diminishing concentration leads to a rapid increase in systematic error [16], [.52], Extreme trace analysis with respect to the elements is therefore subject to large systematic deviations from true content, even though results obtained with a particular method may be quite reproducible. 

A special place is reserved for methods of activation analysis, involving slow and fast neutrons, charged particles, or photon.s, applied either directly or in combination with some type of radiochemical separation (Section 1.6.13). These methods quickly became almost indispen.sable, especially in extreme trace analysis of the ele-... 

Exceptionally low limits of detectability can be achieved with automated on-line systems. Here the complete eluate is analyzed rather than simply an aliquot, which lowers the risk of evaporative losses and sample contamination. Automated sample preparation through solid-phase extraction is becoming increasingly important in extreme trace analysis, as in the determination of PCDDs in tissue or in the blood of uncontaminated persons [219]. 

For nanofractions (contents below ng/g) the term extreme trace analysis has become generally accepted. 

The extreme sensitivity and high resolving power of trace analysis techniques encourage the quest for single atom detection. 

Limits of detection become a problem in capillary electrophoresis because the amounts of analyte that can be loaded into a capillary are extremely small. In a 20 p.m capillary, for example, there is 0.03 P-L/cm capillary length. This is 1/100 to 1/1000 of the volume typically loaded onto polyacrylamide or agarose gels. For trace analysis, a very small number of molecules may actually exist in the capillary after loading. To detect these small amounts of components, some on-line detectors have been developed which use conductivity, laser Doppler effects, or narrowly focused lasers (qv) to detect either absorbance or duorescence (47,48). The conductivity detector claims detection limits down to lO molecules. The laser absorbance detector has been used to measure some of the components in a single human cell (see Trace AND RESIDUE ANALYSIS). 

The usefulness of x-ray emission spectrography in trace analysis is clearly foreshadowed in the wrork of Laby,10 von Hamos,11,12 and Engstrom.12,13 The method cannot reach into the micromicrogram range with any assurance when ordinary equipment is used, nor can it reveal chemical constitution—-both objectives that are often within reach of the classical microchemical methods that are growing continually more powerful as the result of work such as that being done by Yoe and his collaborators.14 To be sure, special equipment to be mentioned in Chapter 9 does permit analysis of extremely small samples by x-ray emission spectrography. 

Because of their high sensitivity, fluorescence detectors are particularly useful in trace analysis when either tire sample size is small or the analyte concentration is extremely low. Although fluorescence detectors can become markedly nonlinear at concentrations where absorption detectors are still linear in response, their linear dynamic range is more than adequate for most trace analysis applications. Unfortunately, fluorometric detectors are often susceptible to background fluorescence and quenching effects that can plague all fluorescence measurements. 

When one is deciding what column geometry is optimal for trace analysis with unlimited sample volume, two additional points should be evaluated. First, to what extent does the analysis require accurate and reproducible injections Strict performance specifications may eliminate microbore columns from consideration. The accuracy and reproducibility of injection systems that deliver 0.1-, 0.2-, and 0.5-/xL samples have not been adequately characterized. Second, if the analyte of interest requires postcolumn derivatization, construction of a postcolumn reaction system that is compatible with the exceedingly small band volumes characteristic of microbore columns may be extremely difficult, but not impossible. Apffel et al. (28) developed and evaluated both packed-bed and open tubular postcolumn reactors for use with 1-mm i.d. analytical columns. Catecholamines were postcolumn derivatized with o-phthal-aldehyde and detected spectrofluorometrically. The 5-/zm particle... 

For trace analysis (analysis of species at ppm and lower levels), impurities in reagent chemicals must be extremely low. For this purpose, we use very-high-purity, expensive grades of acids such as trace metal grade" HN03 or HC1 to dissolve samples. We must pay careful attention to reagents and vessels whose impurity levels could be greater than the quantity of analyte we seek to measure. 

Contamination is a problem one always faces in trace analysis. Mercury is ubiquitous in many laboratories, as is fluoride. Lead is present in dust, particularly in laboratories located close to heavy automobile traffic. Extreme caution must always be exercised as contamination on the trace level may come from unexpected sources. Table I shows that using manganese steel in the jaws used to crush the coal increased the manganese content of the crushed coal more than twofold. 

Synchrotron storage rings, for instance, are able to provide an extremely high flux of nearly monochromatic X-radiation on a small sample area. They could form the basis of XRF set-ups and enhance other microana-lytical methods to provide accurate determinations. In the future they could serve as a reference method for elemental trace analysis on the microscopical level (with the quality of the random number generator, a non-SI concept, as the prime source of error). 

---