Analysts atoms

GFAAS = graphite furnace (flameless) atomic absorption spectroscopy MCAAS = micro-cup atomic spectroscopy DCOP-AES = direct current plasma-atomic emission spectroscopy HFP-AES = high frequency piasma-torch-atomic emission spectroscopy NAA - neutron activation analyst-, atomic absorption spectroscopy AAS - atomic absorption spectrophotometer XES = X-ray energy spectrometry and SEM - scanning electron microscopy. 

Equation 15 is not used direcdy for quantitation of aes data due to the same limitations discussed above for xps. Particularly troubling for aes is the inabihty to determine B for a given matrix. Thus, the analyst is left with comparing aes data from an unknown with those of known materials in an attempt to estimate relative atom ratios. This proceeds along the same lines as for xps and allows quantitation of aes data to a precision of typically better than ca 20%. 

The relative error is the absolute error divided by the true value it is usually expressed in terms of percentage or in parts per thousand. The true or absolute value of a quantity cannot be established experimentally, so that the observed result must be compared with the most probable value. With pure substances the quantity will ultimately depend upon the relative atomic mass of the constituent elements. Determinations of the relative atomic mass have been made with the utmost care, and the accuracy obtained usually far exceeds that attained in ordinary quantitative analysis the analyst must accordingly accept their reliability. With natural or industrial products, we must accept provisionally the results obtained by analysts of repute using carefully tested methods. If several analysts determine the same constituent in the same sample by different methods, the most probable value, which is usually the average, can be deduced from their results. In both cases, the establishment of the most probable value involves the application of statistical methods and the concept of precision. 

In Table I are listed comprehensive citations of published methods for analyses of trace metals In body fluids and other clinical specimens by means of electrothermal atomic absorption spectrometry. Readers are cautioned that many of the early methods that are cited In Table I have become outmoded, owing to Improvements In Instrumentation for electrothermal atomic absorption spectrometry. All of the published methods need to be critically evaluated In the prospective analyst s laboratory before they can be confidently employed for diagnostic measurements of trace metals In body fluids. Despite these caveats, the author believes that Table I should be helpful as a guide to the growing literature on clinical and biological applications of electrothermal atomic absorption spectrometry. 

Kamel, H., Brown, D.H., Ottaway, J.M. and Smith, W.E. (1977) Determination of gold in separate protein fractions of blood serum by carbon furnace atomic-absorption spectrometry. Analyst, 102, 645-663. 

Reference methods are generally arrived at by consensus and fairly extensive testing by a number of laboratories. For example, the flame atomic absorption method for Ca in serum developed under the leadership of the agency fondly remembered as NBS, now NIST (Cali et al. 1972), was established after several inter-laboratory comparison exercises. The results were evaluated after each exercise and the procedure was changed as necessary. After five exercises, it was felt that the state-of-the-art had been reached, with the reference method being capable of measuring Ca in serum with an accuracy of 2% of the true value determined by IDMS (note that attainment of high accuracy and precision is not only a matter of the method, but is a function of both the method and analyst expertise). 

Backmank S, Karlsson RW (1979) Determination of lead, bismuth, zinc, silver and antimony in steel and nickel-base alloys by atomic-absorption spectrometry using direct atomization of solid samples in a graphite furnace. Analyst 104 1017-1029. 

Munoz O, Velez D, Montoro R (1999) Optimization of the solubilization, extraction and determination of inorganic arsenic [As(III) i- As(V)] in seafood products by acid digestion, solvent extraction and hydride generation atomic absorption spectrometry. Analyst 124 601-607. 

The introduction of fast-atom bombardment (FAB) half a decade later extended the analyst s biomarker repertoire to intact polar lipids, desorbed... 

A quantification of the repertoire of analytical chemistry is shown in Fig. 1.6. The field of operation covers over 30 orders of magnitude and more when the amount of lots is included. On the other hand, the relative amounts (contents, concentrations) with which the analyst has to do covers 20 orders and more because single atom detection has become reality now. 

Methods for determining metals in seawater have been published by the Standing Committee of Analysts (i.e., the blue book series, HMSO, London) they are not reproduced in this book, as they are available elsewhere. These methods are based on chelation of the metals with an organic reagent, followed by atomic absorption spectroscopy. 

The concentration of copper in a sample may be determined by using an iodometric titration or by atomic absorption spectrometry. In each of the following examples, calculate the cost of the assay (assume that the charge for the analyst s time is 50 per hour) ... 

Chromium in the crystalline form is a steel-gray, lustrous, hard metal characterized by an atomic weight of 51.996, an atomic number of 24, a density of 7.14 g/cm3, a melting point of 1857°C, and a boiling point of 2672 C. Four chromium isotopes occur naturally Cr-50 (4.3%), -52 (83.8%), -53 (9.6%), and -54 (2.4%), and seven are man-made. Elemental chromium is very stable but is not usually found pure in nature. Chromium can exist in oxidation states ranging from -2 to +6, but is most frequently found in the environment in the trivalent (+3) and hexavalent (+6) oxidation states. The +3 and +6 forms are the most important because the +2, +4, and +5 forms are unstable and are rapidly converted to +3, which in turn is oxidized to +6 (Towill et al. 1978 Langard and Norseth 1979 Ecological Analysts 1981 USPHS 1993). 

Spectral interferences are not common in atomic absorption but can occur. An element with an absorption line sufficiently close to the one of the test element that it overlaps would cause a positive interference. Fassel et a/.20) have discussed the problems of spectral interference. This type of interference, especially in biological samples, occurs only rarely, but the analyst should be aware of it. It is more serious if a continuous source is used. Molecular absorption is a more common spectral interference and occurs when a molecular absorption band overlaps with the atomic absorption line. For example, the CaOH species absorbs in the region of the barium 5535.5 A line. A 1 % calcium solution gives an absorption equivalent to what is expected from about 75 ppm barium21). 

Sheppard, B. S., Heitkemper, D. T., and Gaston, C. M. (1994). Microwave digestion for the determination of arsenic, cadmium and lead in seafood products by inductively coupled plasma-atomic emission and mass spectrometry. Analyst 119 1683-1686. 

Tamba, M. G., del M., Falciani, R., Lopez, T. D., and Coedo, A. G. (1994). One-step microwave digestion procedures for the determination of aluminium in steels and iron ores by inductively coupled plasma atomic emission spectrometry. Analyst 119 2081-2085. 

The sources of acetylene, nitrous oxide, and sometimes air are usually steel cylinders of the compressed gases purchased from specialty gas or welders gas suppliers. Thus, several compressed gas cylinders are usually found next to atomic absorption instrumentation and the analyst becomes involved in replacing empty cylinders with full ones periodically. Safety issues relating to storage, transportation, and use of these cylinders will be addressed in Section 9.3.7. The acetylene required for atomic absorption is a purer grade of acetylene than that which welders use. 

We have spoken frequently in this chapter about sensitivity and detection limit in reference to advantages and disadvantages of the various techniques. Sensitivity and detection limit have specific definitions in atomic absorption. Sensitivity is defined as the concentration of an element that will produce an absorption of 1% (absorptivity percent transmittance of 99%). It is the smallest concentration that can be determined with a reasonable degree of precision. Detection limit is the concentration that gives a readout level that is double the electrical noise level inherent in the baseline. It is a qualitative parameter in the sense that it is the minimum concentration that can be detected, but not precisely determined, like a blip that is barely seen compared to the electrical noise on the baseline. It would tell the analyst that the element is present, but not necessarily at a precisely determinable concentration level. A comparison of detection limits for several elements for the more popular techniques is given in Table 9.2. 

Chattaraj S, Das A. 1991. Indirect determination of free cyanide in industrial waste effluent by atomic absorption spectrometry. Analyst (London) 116(7) 739-741. 

K. Stewart, and J. Taggert, analysts. Analysis for Li by induction-coupled plasma spectroscopy, S. Wilson, analyst. Analysis for Na, K, and Sr by atomic-absorption spectroscopy, D. D. Eberl, analyst. 

Part of this chain is formed by the analyst in his/her laboratory (the "end user"), while part of it may be formed between NIST and the vendors. For example, a laboratory analyst can purchase a primary standard acid (which a vendor can certify as traceable to an SRM) for solution standardization and then base a number of secondary standardizations, such as acids and bases, on that one primary standard. Similarly, an analyst can purchase an atomic absorption reference standard (which a vendor can again certify as being traceable to an SRM) and then make one or more dilutions of this reference standard before creating the final series for the standard curve. 

Van Loon, J. C., and Parlssis, C. M., Scheme of silicate analysis based on the lithium metaborate fusion followed by atomic absorption spectrophotometry. The Analyst, 1969, 1057-1062. 

A. Lopez-Molinero, O. Mendoza, A. Callizo, P. Chamorro and J. R. Castillo, Chemical vapor generation for sample introduction into inductively coupled plasma atomic emission spectroscopy vaporisation of antimony(III) with bromide. Analyst, 127(10), 2002, 1386-1391. 

I. Arambarri, R. Garcia and E. Millan, Optimisation of tin determination in aqua regia-HF extracts from sediments by electrothermal atomic absorption spectrometry using experimental design. Analyst, 125(11), 2000, 2084-2088. 

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