Biological samples, trace element determination

Taylor, V., Jackson, B., Chen, C. Mercury speciation and total trace element determination of low-biomass biological samples. Anal. Bioanal. Chem. 392, 1283-1290 (2008)... 

Miller-Ihli, N.J. Trace element determinations in foods and biological samples using inductively coupled plasma atomic emission spectrometry and flame atomic absorption spectrometry. J. Agric. Food Chem. 44, 2675-2679 (1996)... 

Table 19.1. Examples of trace element determination in biological samples by isotope dilution in combination with substoichiometric isolation.

A variety of small devices has been designed to meet specific functions not effectively served by commercially available freeze-dryers. Such devices were preceded by a number of customized dryers that were developed for various analytical purposes. Thus, Nakaguchi et al. [10] designed a new drying apparatus equipped with a device for trapping evaporated substances that was used to condition biological samples prior to determining trace elements. 

Suggested sample treatment methods which could be satisfactorily employed for specific human biological material prior to trace element determinations by different instrumental techniques are outlined below ... 

Baumann, H. (1992). Solid sampling with inductively coupled plasma mass spectrometry-a survey, Fres. Z. Anal. Chem. 342,907-916 Behne,D. and Matamba, P.A.(1975). Drying and ashing of biological samples in the trace element determination by NAA, Fres. Z. Anal. Chem. 274,195-197 Belcher, R. Macdonald,A.M.G. and West, T.S. (1958). The determination of metals in organic compounds by the closed flask method, Talanta 1, 408-410 Berman, E. (1980). Toxic metals and their analysis, Heyden, London... 

Labeled compounds offer many advantages for food science. The extreme sensitivity of radioactivity measurement enables one to determine and characterize biologically important trace elements and compounds (e.g., pesticides and pesticide residues) in food samples. 

TABLE 4. Trace Element Determination by GC-MS in Biological Samples... 

The demand for analytical procedures with ever-increasing detection power is especially acute in the context of biologically relevant trace elements because of the ubiquitous concentrations of these materials in all natural matrices. It is the environmental concentrations that effectively establish lowest levels of the corresponding elements that are subject to determination in any biotic matrix. In most cases these levels are in the range > 0,1 ng/g, and thus within a region that could today be regarded as practically accessible—at least in principle. Exceptions include the concentrations of certain elements in Antarctic or Arctic ice samples, for example, or samples from research involving ultrapure sub-... 

In order to study the chemical species of trace elements in biological samples, the elemental contents in the effluents will be determined after separation. The multielement character of NAA, as well as its high sensitivity for many elements, is ideal for the task. Since the column effluents are liquid, the fractions are usually freeze dried, then subjected to NAA. 

Baker, S. A., Bradshaw, D. K., and Miller-Ihli, N. J. (1999).Trace element determinations in food and biological samples using inductively coupled plasma mass spectrometry. At. Spectrosc. 20(5), 167. 

Two methods were examined for digestion of biological samples prior to trace element analysis. In the first one a nitric acid-hydrogen peroxide-hydrofluoric acid mixture was used in an open system, and in the second one nitric acid in a closed Teflon bomb. The latter method was superior for Ge determination, however, germanium was lost whenever hydrogen fluoride had to be added for disolving sihcious material. End analysis by ICP-AES was used for Ge concentrations in the Xg/g range13. 

Wet oxidation of samples of biological origin with a mixture of nitric acid and perchloric acid is a common procedure that may be problematic in certain cases. An alternative procedure for sample preparation is irradiation with a strong UV source. Acidified samples of hair (0.1 g) containing about 2 pg Pb and other trace elements, were irradiated for 3 h with a 500 W source a buffer (pH 5.5) and a small amount of catechol violet (9) were added, and the complex of the dye with the trace elements was determined polarographically RSD ca 5% for Pb95. 

The potential for the employment of plasma emission spectrometry is enormous and it is finding use in almost every field where trace element analysis is carried out. Some seventy elements, including most metals and some non-metals, such as phosphorus and carbon, may be determined individually or in parallel. As many as thirty or more elements may be determined on the same sample. Table 8.4 is illustrative of elements which may be analysed and compares detection limits for plasma emission with those for ICP-MS and atomic absorption. Rocks, soils, waters and biological tissue are typical of samples to which the method may be applied. In geochemistry, and in quality control of potable waters and pollution studies in general, the multi-element capability and wide (105) dynamic range of the method are of great value. Plasma emission spectrometry is well established as a routine method of analysis in these areas. 

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). 

Probably the most effective use of XRF and TXRF continues to be in the analysis of samples of biological origin. For instance, TXRF has been used without a significant amount of sample preparation to determine the metal cofactors in enzyme complexes [86]. The protein content in a number of enzymes has been deduced through a TXRF of the sulfur content of the component methionine and cysteine [87]. It was found that for enzymes with low molecular weights and minor amounts of buffer components that a reliable determination of sulfur was possible. In other works, TXRF was used to determine trace elements in serum and homogenized brain samples [88], selenium and other trace elements in serum and urine [89], lead in whole human blood [90], and the Zn/Cu ratio in serum as a means to aid cancer diagnosis [91]. 

The possibility of contaminating the sample with the elements under investigation during the various steps is a major risk in trace analysis. The precautions required in order to minimize this problem are similar to those already identified when dealing with biological materials used for total element determinations. Heydom Ver-sieck et al. Stoeppler Behne and Aitio et al. have diam ed the extent... 

It is not possible to prescribe specific pretreatment procedures here because these can only be decided upon when the system and the purpose of the experiments has been properly defined. However, a wealth of information exist in various biochemical reference books on how to isolate various biological compounds. The recommended techniques and methods could be used as part of the trace element speciation protocol often after slight modification, taking into consideration the following points First, the trace element blank levels have to be low, less than 10% of the total concentration in the sample. Second, the regents used should not interfere with subsequent analytical determinations. Third, the experimental conditions should not deviate markedly from those found in vivo, especially the pH and ionic strength of the medium. 

In addition to ICP-MS for the multi-element analysis of aqueous solutions, LA-ICP-MS allows the direct determination of trace elements in biological samples and due to this feature it is a well suited analytical technique for microlocal analysis with spatial resolution. In 1995, Outridge el al.lg reported on the performance of an LA-ICP-MS analysis for studying incremental biological structures as archives of trace element accumulation. The use of LA-ICP-MS for several biological (and environmental) applications is reviewed by Durrant and Ward.19 Selected examples for determination of trace elements and species in biological samples are summarized in Table 9.25. 

Table 9.25 Determination of trace elements and species in biological samples.

Elemental speciation is the identification and quantification of the chemical form of an element. Traditional analytical techniques for trace elemental analysis have focused on determining the concentration of a particular element within a sample. However, knowledge of total element concentration may not provide sufficient information to determine toxicity since the toxicity of many elements is dependent upon their chemical forms. The oxidation state of the element as well as the organic substituents attached to it may have a dramatic effect on the biological properties observed. As a result, researchers have endeavored to develop new and better analytical techniques that are capable of performing elemental speciation. 

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