Reliable Measurement of Major, Minor, and Trace Elemental Nutrients

[1] Harnly, J. M., O'Haver, T. C., Golden, B., and Wolf, W. R., Anal. Chem. 51, 2007 (1979). [2] Miller-Ihli, N. J., Anal. Chem. 57, 2892 (1985). [31 Harnly, J. M., Miller-Ihli, N. J., and O'Haver, T. C., Spectrocim. Acta 39fl, 305 (1984). (41 Harnly, J. M., and O'Haver, T. C., Anal. Chem. 53, 1291 (1981). [5] Kingston, H. M., and Jassie, L. B., Anal. Chem. 58, 2534 (1986). [61 Miller-lhli, N. J., J. Anal. At. Spectrom., in press.


Introduction
A number of major, minor, and trace elements function as important nutrients in foodstuffs of crop and animal origin. These elements occur in a broad range of concentrations in an exceedingly vast array of foods available commercially and home-prepared. In disciplines such as regulatory compliance, product development, quality and safety, and research, reasonably reliable analytical information is mandatory for conclusions and decisions of impact. Although analytical scientists recognize the need for valid data, solid analytical information is at times elusive not only in the area of determination of chemical elements at low (trace) levels [1][2][3][4][5] but also when present as major constituents [6,7].
Of the many analytical techniques available for determination of inorganic nutrient constituents in biological matrices, those based on atomic spectrometry (AAS) are convenient and widely applied. The thrust of current research is to define impacts of a number of parameters on the performance of flame AAS (FAAS)-based methods of analysis of soil and biological materials to lead to reference methods for the measurement of major, minor and trace levels of elements of nutritional and toxicological pertinence. Some of these parameters as well as the excellent performance possible with well-applied versions of FAAS procedures are discussed.

High Reliability Flame Atomic Absorption Spectrometry
In spite of wide acceptance of the technique and the proliferation of published articles on its application, very few AAS-based methods have reached official method or reference method status [8]. Requirements for analytical data of the highest reliability (high precision and accuracy) in the author's research dealing with levels of elemental nutrients and toxicants in soils and biological tissues and with official and reference versions of procedures based on AAS have led to detailed investigations of a number of parameters bearing on method performance. Of the parameters listed in table 1, those incorporating appropriate standard and procedural reagent blanks. Two types of working standard solutions were prepared with one set (regular standard) containing the eight analytes at concentrations required for analysis. The second set (matrix-matched) contained the analytes at approximate concentrations occurring in the plant materials being analyzed and, in addition, contained three other major constituents, Al, S and P present in the samples to match the analytes and matrices of the analytical samples. The scheme of analysis is depicted in figure 1.
Development and application of a high reliability version of an analytical procedure requires familiarity with all factors having a possible bearing on the precision and systematic errors of the overall procedure. The effect of calibration expressed as differences in elemental concentrations measured using regular and matrix-matched standard solutions is a function of element and sample type. Differences in concentrations ranged from 0.2% to 10.2% depending on element and matrix, indicating that for measurements of the highest reliability, systematic error arising from calibration can be reduced by using reasonable close matrix-matched standard solutions [9]. Effects of the two sample decomposition procedures leading to acid-soluble and total concentrations showed that error in analytical data, without considering any contribution from the residue, ranged from 0.04% for Mg in spinach to 42% for Na in orchard leaves for the three materials certified at the time of the study [9,10]. For high reliability analysis, decomposition technique must be assessed and contributions of residues must be incorporated either by separate decompositions or one step procedures [10,11].
That meticulous attention to detail can lead to a high precision and high accuracy procedure is demonstrated by information in tables 2 and 3, and figure 2. Figure 2,  that consideration of appropriate details and careful work can lead to a truly high fidelity analytical procedure based on flame atomic spectrometry.   Accurate determination of trace elements in foods is a formidable challenge for even the most capable analyst. The complex and disparate matrices of foods, coupled with the often very low concentrations of nutritive or toxic elements, combine to push the capabilities of existing trace element analytical methods to their limits.
Inductively coupled plasma atomic emission spectrometry (ICP-AES) is now a widely used technique for food analysis. Na, P, Ca, Mg, Fe, Zn, Cu, Mn and Sr can be readily determined in most foods by ICP-AES. However, several other important elements such as Al, Mo, Ni, Cr, Pb, Se, As, Cd, Co, V and Hg cannot be easily determined in most foods by ICP-AES unless extensive chemical procedures are used to concentrate these elements