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A survey of methods of analysis for minerals in feedstuffs ^{1, 2}

Pacific Agri-Food Research Centre–Summerland, Agriculture and Agri-Food Canada, Summerland, BC V0H 1Z0 Canada

	Abstract

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A wide range of minerals occurs in feedstuffs as naturally occurring and purposely added elements, as well as by adventitious contamination. These mineral elements can generally be classified as nutritionally essential major elements, nutritionally essential minor and trace elements, and those regarded as toxic or with an essential/toxic duality. A survey is presented of methods used for the determination of major, minor, and trace elements in feedstuffs and related biological materials. Currently available methods include the following: atomic absorption spectrometry, atomic emission spectrometry, mass spectrometry, neutron activation analysis, x-ray emission spectrometry, molecular light absorption spectrometry, molecular fluorometry, electrochemistry, Kjeldahl method (nitrogen), combustion elemental analysis, volumetry, ion chromatography, and gravimetry. Available reference, routine, official, unofficial, and recommended methods are reviewed as a basis for recommendations of methods most suitable for feedstuffs.

Key Words: Analytical Methods • Feeds • Minerals • Recommended Methods • Reference Methods • Trace Elements

	Introduction

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A survey is presented regarding methods (official/recommended) used for the determination of inorganic analytes (major, minor, and trace elements) in feedstuffs, plants, and related biological materials. Currently available reference, routine, official, and recommended methods are reviewed. Challenges posed by analytical endeavors in general, as well as specific challenges posed by some difficult-to-measure elemental analytes or particular matrices are discussed. Finally, recommendations are made regarding the most suitable methods for determining the more usual elements in feedstuffs.

Reliable measurements are mandatory for legal compliance with regulations, long-term monitoring and baseline studies, standardization of laboratories over time and location, and for research (Cali and Marsh 1983 and references therein; Ihnat 1988a). In agriculture/food science, accurate data on the chemical composition of raw agricultural products, foods, and feeds are needed to assess effects of farm management practices, changes in crop culture and bioengineering, and processing on the nutrient and toxic chemical content of retail food and feed products. Elemental concentration information is required to establish the necessity of a nutrient or the toxicology of a toxicant. The generation of data on the nutrient content of agricultural products and foods forms the basis for 1) estimating nutrient intakes of populations to determine the roles of nutrients in health and disease; 2) research in nutrient requirements and metabolism to identify adequate, inadequate, or marginal intakes by farm animals and human consumers; 3) establishing nutrient dietary requirements; 4) accumulating baseline concentration data to investigate the effects of various methods for food processing on nutrient levels; and 5) compliance with legal nutritional labeling requirements for consumer protection and nutrition education. Toxicant chemical composition is used to assess effects of farm management practices, crop culture, and food processing on chemical content and implications for human and animal health.

	Elements and Materials Considered

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Elements

In this paper, the term minerals has been expanded to include all inorganic, elemental analytes, including those denoted as macro- and microminerals, and thus includes all major, minor, and trace elements of nutritional, toxicological, and environmental importance.

The occurrence of elements in foods is a function of the biological roles played by the elements in the structure and physiology of the food tissue, and adventitious contamination during growth, processing, and preparation (Ihnat 1982a). Twenty-six of the naturally occurring elements are known to be essential for life. A wide range of minerals occur in feedstuffs as naturally occurring and purposely added elements, as well as by adventitious contamination. Mineral elements can generally be classified as nutritionally essential major elements, such as Ca, Cl, K, Mg, N, Na, P, and S; nutritionally essential minor and trace elements, such as B, Br, Fe, I, and Si; and those regarded as toxic or with an essential/toxic duality: As, Cd, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, Pb, Pd, Se, Sn, Tl, V, and Zn. At excessive levels, even nutritionally essential elements may exhibit toxicity. Some idea of the elemental content to be expected in feedstuffs (with perhaps the exception of premixes and other specialty products) can be surmised from a listing of estimated typical ranges of some of the more important elements in 12 classes of foods (similar to feed materials) presented by Ihnat (1982a).

Materials

Materials of interest in this paper are, to a first approximation, plants, plant-based materials, and feedstuffs. Because feedstuffs can also include products of animal origin, we can logically extend the material covered to include "biological materials" in general. Thus, analytical methodologies developed for a wide range of biological materials are of interest to the feed and plant analyst and are worth considering and evaluating. This range of biological materials hence encompasses foods, feeds, plants, animal tissues and fluids, agricultural and clinical materials, and related materials, such as water and organic wastes, and is characterized by a wide variety of chemical compositions that influence the performance of chemical analytical measurements. Major constituents of these materials having impacts on processing and determinative steps of the methods are fat, protein, carbohydrate, fiber, and ash. Many of the great variety of over 2,000 feedstuffs available for animal feeding are by-products of food production for humans and include roughages, pasture and range grasses, high-energy concentrates, protein sources, minerals, vitamins, and additives (Kellems and Church, 2002).

	Analytical Methods

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General Methods Available for Determination of Elements in Biological Materials

For the measurement of minerals in biological materials, a wide range of analytical methods is available, ranging from classical, through currently and commonly utilized instrument-based methods, to highly specialized definitive methods. In general, the typical method is composed of three steps: 1) sample pretreatment and treatment, 2) analyte preconcentration or isolation/separation, and 3) its quantitative measurement. Methods of analysis are not equal and in fact can differ substantially with respect to the many attributes describing method performance. In selecting a particular technique, consideration of the following technical and operational characteristics can be useful (Dabeka and Ihnat, 1987): multielement capability; detection limit; sample preparation requirements and hence analytical throughput; spectral, physical, and chemical interferences, precision and accuracy; operational expertise and cost, with weighting and judgmental factors at the discretion of the analyst.

Specific Methods Available for Determination of Elements in Feedstuffs, Plants, and Related Materials

Methods may be labeled "official" (methods with legal or paralegal status from agencies such as AOAC, ISO, and national standards organizations), "quasi-official," recommended, reference, standardized, validated, routine, field, and so on. They come from official agency methodology manuals, with methods subjected to interlaboratory collaborative validation; official methods designed and approved by committee consensus (not subjected to collaborative study); other methodology manuals; textbooks; reviews; and single methods in scientific papers.

A gamut of agencies is active, to various degrees, in analytical method development. One of the most active, widely recognized, and respected is AOAC International (Gaithersburg, MD). This organization has developed and continues to develop analytical methods for inorganic and organic analytes in food and related materials; methods are published every 5 yr (with annual updates) in Official Methods of Analysis of AOAC International (e.g., Horwitz, 2000). An important trademark of the AOAC is the unfailing incorporation of the important, experimental procedure of interlaboratory, collaborative study (now under an ISO/IUPAC/AOAC harmonized protocol) to validate methods of analysis, which yields valid, widely regarded methods with legal standing (AOAC, 1995; Horwitz, 2000, Appendix D). A number of other organizations and agencies are also developing methods via experimental and/or consensual approaches and are publishing compendia of methods (Table 1).

	Analytical Challenges

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The heterogeneity of data quality is a characteristic of analytical information published in the literature. The problem facing analysts a century ago of unacceptably large variation in analytical data persists today. Even though increasing attention is being paid to data quality and some improvement may be evident, contemporary analyses dealing with ultratrace, trace, and even major concentrations of analytes are still plagued by uncertainty and error in the reported results (Ihnat, 1988a). With reference to the analysis of standard rocks and minerals, Abbey (1980) clearly and forcefully observed that "given a highly incoherent set of results for the determination of each constituent of a proposed reference sample, the originator is faced with the difficult problem of estimating the ‘true’ concentration. No known test can prove the validity of a concentration value derived from a mass of incoherent data." Invariably, results from the many reported interlaboratory investigations exhibit divergence, scatter, and disagreement.

Sources of Errors

The general lack of agreement among laboratories of the outcome of analytical work stems from the multitude of factors influencing the validity and reliability of the final numerical results. These factors can be broadly categorized as presampling, sampling, sample manipulation, and measurement factors—with each category containing subdivisions. Other important considerations, such as contamination control, data quality control, and the analyst, transcend the above boundaries. A general listing of possible sources of errors in the chemical analysis of biological materials is presented in Ihnat (1988a,b). Overall, the role of the analyst and quality control impacts all facets of analysis.

Shortcomings of Official/Recommended Methods of Analysis

Official methods of analysis from associations, especially those from organizations developing methods by the all-important scheme of collaborative study, have successes but there are grounds for improvement. Firstly, progress is sometimes slow in the development of official or recommended methods of analysis due to the proliferation of interest in new analytes, lower legal or reporting concentration limits, new materials, analyte/material combinations, and increased difficulty in attracting volunteer efforts (much work is carried out on a voluntary basis). Current AOAC activity to establish electronically available methods, at various levels of "official acceptability," for faster delivery of information appears to be a worthwhile effort and is complementary to the production of print and CD-ROM versions of Official Methods of Analysis. Secondly, variants of the same fundamental method for the same element have been developed for various commodities and reappear under different headings. In addition, methods that are or can be considered to have multielement capability have been developed for only one element or a small number of elements.

Difficult Elements and Materials

Analytical challenges are posed by general and specific situations and by difficult-to-process materials and difficult-to-measure elements. There are 40 elements for which at least one method is reported in Official Methods of Analysis: Ag, Al, As, B, Ba, Be, Br, C, Ca, Cd, Cl, Co, Cr, Cu, F, Fe, H, Hg, I, K, Mg, Mn, Mo, N, Na, Ni, P, Pb, S, Sb, Se, Si, Sn, Sr, Th, Ti, Tl, U, V, and Zn. A small number of these elements—the more common ones, such as Ca, Cu, Fe, K, N, P, Pb, and Zn—have the luxury of having many similar methods for different commodities. About half of these elements, however, such as Be, C, Cr, H, Sb, Si, Sn, Ti, Tl, and U, can be denoted as being less common, with each having only one to five reported methods.

Several of the less commonly reported elements can also be classified as elements with methodological insufficiencies (i.e., difficult to determine). Analytical difficulties can be due to the following: incomplete extraction from the matrix by dissolution techniques used; the requirement for analyte separation from the matrix by special treatment/processing, such as separation/concentration by solvent extraction, coprecipitation; and the usual problems of working at trace levels (incomplete recovery, contamination). Elements with methodological difficulties include As, Be, Cd, Pb, Mo, S, Si, Sn, Ti, Tl, and V.

Problems can also be posed by the nature of the material, presenting difficulties to decomposition and dissolution procedures utilized. Although many biological materials, including plants and feedstuffs, with nominal matrix compositions, are amenable to satisfactory processing by the common wet digestion and dry ashing procedures, those with large amounts of fat, inorganic additives, refractory granules, and naturally occurring siliceous matter may offer problems (Ihnat, 1982b). This siliceous material is likely to occlude or adsorb a fraction of the element sought, giving rise to systematic errors.

The Role of the Analyst

The analyst plays a significant and at times overlooked central role in the scheme of analysis. According to some experienced practitioners in the science and art of analysis (S. Abbey, personal communication), it matters little how the analysis was performed but rather who did it; all that is required for proper analysis is the selection of a proper analyst—all else falls into place. Analyst training, experience, familiarity with the problem at hand, skill, attitude, motivation, and judgment are prerequisites with which satisfactory solution of analytical problems is possible. Although critical practitioners of analytical chemistry (trained and experienced analysts with a healthy scepticism regarding data quality) have a keen realization about the need for appropriately reliable data generated by trained analysts, this is not universally the case among the population of analysts and "casual analysts" (part-time, contract, temporary, summer, insufficiently experienced, and the like). Accurate determinations require a high degree of knowledge, expertise, and judgment in choosing an appropriate method and suitable sampling, storage, manipulation, and contamination and quality control procedures.

Impact of New Instrumentation and Techniques

The development and proliferation of advanced, sophisticated, automated, computer-controlled analytical instrumentation has greatly increased analytical laboratory capability, but subtle pitfalls face the unwary analyst. Numbers generated by such instrumentation are not beyond criticism. One must also not be led to believe that instrumentation will solve all of the complex analytical problems and that superficial training is all that is required in operating the "black boxes." Instrumental data are only as good as the knowledge and experience of the operator and the care exercised in operating the instrument.

	Recommendations and Concluding Remarks

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General Analytical Recommendations

Recommendations presented here are applicable to chemical analysis in general, providing the analyst with some practical information on evaluating and ensuring accuracy (Ihnat 1988a,c; Dabeka and Ihnat, 1987).

1. Goals of projects need be thoroughly defined and integrated with analytical requirements; the cooperation of professional analysts in all phases of the study is strongly recommended.
   
2. Considerable care and expertise are required in the application of analytical methods, supported by appropriate laboratory, managerial, and administrative environments.
   
3. The laboratory manager, analytical chemist, and analytical technologist must be familiar with concepts underlying the measurement system and accuracy/reliability.
   
4. Inclusion of laboratory quality assurance and quality control systems should result in improved results.
   
5. Education and training, including continuing in-house training, of professional and technical staff members will provide the competence required in demanding analytical endeavors.
   
6. Selection of analytical methods should be done considering the acceptable validation level required for the task at hand; applicability to materials and elements; availability to the facility; and their feasibility with respect to personnel competency.
   
7. In method development and testing, emphasis should be placed on verification using recovery studies, certified reference materials, and confirmation of results by independent methods of analysis.
   
8. Participation in national and international quality assurance programs is recommended to update analysts with useful analytical methods, confirm the accuracy and precision of methods, and detect problems.
   
9. Suitable diagnostic and operational tests should be included in instrumental analytical protocols. Software, programs for analytical calculations, those specifically developed for the particular instrument, and stand-alone programs such as spreadsheets must be demonstrated to be operating and performing correctly.
   

General Recommendations on Incorporation of Reference Materials

One convenient, cost-effective, and vital component for measuring method and analyst performance, for assessing and maintaining analytical data quality, and for achieving compatibility and transferring accuracy among laboratories is the incorporation into the scheme of analysis of appropriate, compositionally similar reference materials for chemical composition, certified for concentrations of analytes of interest. Routine usage of reference materials is highly recommended for monitoring the aggregate of all steps, such as sample treatment, manipulation, analyte measurement, and data calculation, subsequent to the point at which the reference materials is introduced into the scheme of analysis (e.g., Ihnat 1988a,c). A protocol for reference material use in plant analysis, including step-by-step instructions on selection, utilization, performance interpretation, and corrective action has been published by Ihnat (1998).

Recommended Specific Methods for Feedstuffs, Plants and Related Materials

There are a large number of useful "official" and other methods in the scientific literature from various organizations and individuals. With a vast variety of approaches to sample treatment and analyte determinations, the situation can be confusing. It is beyond the scope of this paper to offer comprehensive recommendations beyond referring the analyst to available sources (Tables 1 and 2) and giving a very small listing of examples of recommended methods specifically geared to plants and feedstuffs in the category of "official, reference, recommended" methods (Table 3). The progress of the eCAM (electronic compilation of analytical methods) project under development by AOAC International should be followed and considered because it may provide a timely database of information on methods of analysis from many sources.

	Implications

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This paper presents a brief overview of currently available reference methods for the analysis of feedstuffs for inorganic elemental composition. Challenges posed by analytical endeavors in general, as well as specific challenges posed by some difficult-to-measure analytes or particular matrices are discussed, and recommendations are made of some for the most suitable methods for the determination of the more usual elements in feedstuffs. Information is also provided on the following items with the aim of sensitizing the analyst to be proactive in "searching for the truth": the general state of the quality of analytical data, shortcomings of official/reference methods, the role of the analyst (often overlooked), and the impact of new instrumentation. The central thrust is the presentation of 1) general analytical recommendations; 2) examples of the scope of available official and other methodologies; and 3) sampling of specific recommended official, recommended, or reference methods—all of which guide the plant/feed analyst in the further search for appropriate methodologies.

	Footnotes

 
¹ Contribution No. 2184 from Pacific Agri-Food Research Centre.

² Correspondence—phone: 250-494-6411; fax: 250-494-0755; E-mail: ihnatm{at}agr.gc.ca.

Received for publication July 12, 2002. Accepted for publication September 2, 2003.

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