Special considerations in trace element analysis of foods and biological materials.
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Animals↗
Biomedical subjects
W Horwitz
Publications and source records attributed to W Horwitz.
Interlaboratory variability in trace element analysis.
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Food Analysis↗
Commonly used methods of analysis for tin in foods.
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Colorimetry↗
IUPAC gas chromatographic method for determination of fatty acid composition: collaborative study.
An international collaborative study of IUPAC methods II.D.19 and II.D.25 for preparation and GLC analysis of fatty acid methyl esters was begun in 1976. The IUPAC methodology, applicable to animal and vegetable oils and fats and fatty acids from all sources, contains special instructions for preparation and analysis of methyl esters of fatty acids containing 4 or more carbon atoms (analysis of milk fat). Twenty-three collaborators participated in the analysis of 5 known mixtures, 4 vegetable oils, 1 fish oil, and 2 butterfats. Several blind duplicate samples were included. The experimental data were subjected to statistical analysis to examine intra- and interlaboratory variation. Reproducibility and accuracy data for the higher fatty acid (14:0-22:1) mixtures and fish and vegetable oils were satisfactory and were in good agreement with results from an AOCS Smalley Committee check sample program involving analysis of the same samples. Typical coefficients of variation (%) at various concentrations were 15 (2% level), 8.5 (5% level), 7 (10% level), and 3 (50% level). Low recoveries and poor reproducibility were characteristics of results obtained for butyric acid in the butterfat and related known mixtures. A coefficient of variation of about 19% was found for analysis of butyric acid in butterfat, vs coefficients of variation in the range of 4-13% for similar levels of other components in butterfat and other samples. The IUPAC methodology for GLC analysis of fats and oils other than milk fats has been adopted by the AOAC as official first action to replace the current GLC method, 28.063-28.067.
Animals↗
The establishment of official analytical methodology.
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Food Analysis↗
The variability of AOAC methods of analysis as used in analytical pharmaceutical chemistry.
Pharmaceutical analytical chemistry, which ordinarily deals with the analysis of formulations containing from 0.1 to 100% of active ingredient, uses methods with a reproducibility (between-laboratory variability) of about 2.5% and a repeatability (within-laboratory variability) of about half that amount. The best between-laboratory precision attainable appears to be about 1.0% and within-laboratory precision, about 0.5%. On the basis of the results available, automated methods do not appear to be any more precise than manual methods, although the studies show fewer outlying data points. Replicates (preferably blind ones) should always be conducted in a collaborative interlaboratory study in order to obtain the important information as to whether efforts should be concentrated on improving the method itself or on the performance of laboratories and analysis in applying it.
Chemistry, Pharmaceutical↗
Problems of sampling and analytical methods.
The need for acceptable, reliable, and practical methods of analysis, chemical, physical, and biological, by the Food Standards Program of the Food and Agriculture Organization/World Health Organization is leading to cooperation by those organizations with the resources and experience to supply them. Methods should be shown to be workable and practicable and then validated in a properly designed international collaborative study for the efficient utilization of the time and effort of participating organizations and laboratories. The methods to be subjected to interlaboratory collaborative study should be clearly written so that the method itself is being tested without unauthorized variations. Satisfactory reference standard materials are often an essential part of the method. Uniform, international methods which have been developed through interlaboratory collaborative studies are applicable to the areas of microbiology and toxicology, as well as chemistry.
Food Analysis↗
A review of sampling plans and collaboratively studied methods of analysis for aflatoxins.
Aflatoxins are the only food contaminants being monitored routinely on an international scale with methods operating at the order of magnitude of 10 mug/kg. At this concentration level, methods of analysis which can achieve coefficients of variation of 30-40% with recoveries of 70% or greater in interlaboratory collaborative studies can be considered eligible for referee status. In most cases, sample reproducibility is the variable limiting the reliability of methods of analysis. The inherent uncertainty of the identity of chromatographically separated entities requires the application of confirmatory tests to verify that the characteristic measured results from the presence of aflatoxin. The methods are also inoperable without a verification of the identity, purity, and concentration of the reference standards used. Screening methods which reliably eliminate negative samples from further consideration are indispensible for the practical operation of monitoring programs.
Aflatoxins↗
Performance characteristics of methods of analysis used for regulatory purposes. Part II. Pesticide formulations.
The precision parameters of the method-performance (collaborative) studies published in the AOAC Journal from 1915 through 1990 for pesticide formulations have been recalculated on a uniform basis by the International Union of Pure and Applied Chemistry 1987 protocol. About 93% of the 953 accepted assays, which are predominantly gravimetric (G), volumetric (V), and gas (GC) and liquid (LC) chromatographic methods, exhibit relative standard deviations among laboratories (RSDR) that are generally less than 2 times the values predicted from the Horwitz equation: RSDR (%) = 2 exp (1-0.5 log C), where C is the concentration expressed as a decimal fraction. UV, VIS, and IR spectrophotometric (S) methods are somewhat poorer, with about 80% of the reported RSDR values less than twice the predicted RSDR value. The precision parameters of pesticide formulations analyzed by the older methods (G, V, GC) are equivalent to those previously found for drug preparations in the same concentration range; the precision parameters of pesticide formulations analyzed by LC and S are somewhat poorer. Overall, however, the precision parameters of pesticide formulations are generally independent of analyte, method, and matrix, and are primarily a function of concentration. The method-acceptability decisions of the AOAC for pesticide formulations during the past 75 years can be approximated retrospectively by using a criterion for RSDR that is less than 2 times the RSDR calculated from the Horwitz equation.
Databases, Factual↗
Precision parameters of methods of analysis required for nutrition labeling. Part I. Major nutrients.
Major components of foods and feeds are fat, protein, and carbohydrates. Fat and protein are determined by direct measurements that are interpreted as the quantity of the constituent. Carbohydrates are usually calculated by difference. For this calculation, values for moisture/solids, ash, and "fiber" are also needed. The readily available collaborative studies for the determination of these major components are reviewed in an attempt to assign precision parameters to validated methods of analysis. When a number of studies for the same analyte, in the same food, by the same method are available, it is seen that the precision parameters among laboratories (standard deviations, SR; relative standard deviations, RSDR) and the ISO maximum tolerable difference functions (repeatability value, r; reproducibility value, R) are not characterized by any conventional distribution. The precision data are best summarized as a median or average parameter and the interval containing the centermost 90% of reported values. Typically, the precision of methods of analysis can be expressed as a function of concentration only, independent of analyte, matrix, and method. The average RSDR value from each collaborative data set can then be used as the numerator in a ratio containing, as the denominator, the value calculated from the Horwitz equation: RSDR = 2 exp (1 - 0.5 log C) where C is the concentration as a decimal fraction. A series of ratios consistently above 1, and especially above 2, probably indicates that a method is unacceptable with respect to precision. By this criterion, only the protein (Kjeldahl) determination is unqualifiedly acceptable with a 90% interval for RSDR of 1 to 3% at C values above about 0.01 (1 g/100 g). Fat, moisture/solids, and ash are acceptable down to limiting concentrations in the region of 1 to 5 g/100 g, if a test portion large enough to provide at least 50 mg of weighable residue or volatiles is specified. Measurements of individual carbohydrates and fiber-related analytes have unexpectedly poor precisions among laboratories. The variability, although high, may still be suitable for nutrition labeling. Reliability of analyses for the control of labeling of the primary nutrients must be achieved through quality assurance programs that require strict adherence to the directions of empirical methods and the use of suitable reference materials for absolute methods.
Databases, Bibliographic↗
Precision parameters of standard methods of analysis for dairy products.
The available collaborative studies for standard methods of analysis for various constituents of milk and milk products were examined in an attempt to assign specific repeatability and reproducibility precision parameters to these methods. The different collaborative assays for the primary constituents (moisture/solids, fat, protein), the nutritionally important elements (calcium, sodium, potassium, phosphorus), and miscellaneous analytes/physical constants (ash, lactose, salt, freezing point) produced different estimates of the precision parameters for the same method. A suitable summary of the precision estimates from collaborative studies is given by the reproducibility relative standard deviation, RSDg, which is relatively constant within a product and permits comparisons across products. An estimate of the variation of RSDR for an analyte from a number of collaborative studies is presented in terms of the median and 90% interval (the range of the centermost 90% of values). These estimates are only informative when a substantial number of independent studies are available for pooling the independent estimates to form a distribution of RSDR values. The RSDR for the determination of the primary constituents of milk and milk products is characterized by a median RSDR of 1% and a 90% interval of 0.3-3%, with RSDR estimates occasionally occurring below 0.3% and above 4%. These overall estimates appear to be independent of analyte, matrix, and method and apply to concentrations of primary constituents that range from about 2 to 80%. The repeatability relative standard deviation, RSDr, is unstable, although it tends to converge to about 0.5-0.7 X RSDR. Too few collaborative assays are available to characterize RSDR for the determination of certain other constituents (acidity, ash, lactose, salt, and the nutritionally important elements) unless RSDR values for different analytes, methods, and matrixes are pooled on the basis of similar analyte concentrations. When pooled, the RSDR values are generally better than predicted from the Horwitz equation, RSDR (%) = 2 exp (1-0.5 log10C), where C is the concentration expressed as a decimal fraction; all but one of 661 RSDR values are within the upper empirical limit of twice this curve.
Animals↗
Determination of nitrogen content in milk by the Kjeldahl method using copper sulfate: interlaboratory study.
Copper sulfate was substituted for mercury as the catalyst in the International Dairy Federation (IDF) Standard 20A:1986 method for the determination of nitrogen content in milk. The substitution was supported by results obtained in an interlaboratory study by 24 laboratories in 12 countries. Each laboratory analyzed 12 test samples of milk as blind duplicates in a double split level design with high, medium, and low nitrogen concentrations. The method protocol requires the concurrent analyses of an ammonium salt solution and a tryptophan solution as internal quality control standards with a minimum nitrogen recovery between 99 and 100% for the former and at least 98% for the latter. The repeatability and reproducibility relative standard deviations are 0.5 and 1%, respectively, for the range 0.35-0.70 g N/100 g. The performance of the laboratories that did not meet the required quality control specifications was clearly poorer than that of those that did meet the specifications.
Animals↗
Sampling and preparation of sample for chemical examination.
Sampling and methods for reducing a laboratory sample to a test sample are discussed, with particular emphasis on sampling peanuts for aflatoxin analysis as a practical example. The only way to control the total error in the analysis of this heterogeneous product is to take and to analyze many and large samples.
Aflatoxins↗
Performance characteristics of methods of analysis used for regulatory purposes. I. Drug dosage forms. F. Gravimetric and titrimetric methods.
The original gravimetric and titrimetric methods approved by AOAC for the analysis of pharmaceutical preparations, particularly during the period 1915-1950, show precision, recovery, and outlier parameters approximately the same as those exhibited by the previously reviewed instrumental methods that are currently used. Fifty-nine published collaborative studies utilized gravimetric methods and 85 used titrimetric. The studies of the gravimetric methods encompassed 47 analytes, 95 dosage forms, and 136 assays; the corresponding figures for the titrimetric studies are 72, 112, and 152. An average of approximately 7 laboratories participated per study. The line of best fit of the relative standard deviation between-laboratories (RSDR) plotted against the negative logarithm of the fractional concentration, C, extends from 1.2 and 1.0% for the gravimetric and titrimetric methods, respectively, at 100% concentration to 2.2 and 2.8% at 1.0% concentration. Below this concentration the precision of the titrimetric methods degenerates faster than that of the gravimetric methods. Above about 0.1% concentration the gravimetric and titrimetric methods are somewhat more precise than the instrumental methods in current use for drug analysis. The difference, however, is not statistically significant and the general equation, RSDR = 2 exp(1-0.5 log C), is also applicable to gravimetric and titrimetric methods above a concentration level of about C = 0.001 (0.1%).
Chemical Phenomena↗
Harmonization of collaborative study protocols.
The 2 major protocols for the design, conduct, and interpretation of collaborative analytical studies--those from AOAC and the International Organization for Standardization--are already fairly well harmonized. The statistical models are identical and the outlier tests are essentially the same. The major differences are in symbols and terminology and in the specification of the minimum number of laboratories and replicates.
Models, Theoretical↗
Performance characteristics of methods of analysis used for regulatory purposes. I. Drug dosage forms. D. High pressure liquid chromatographic methods.
Precision parameters of high pressure liquid chromatographic methods approved by AOAC for the analysis of drug dosage forms were recalculated on a consistent statistical basis, using the computer program "FDACHEMIST." Eleven collaborative studies of 12 compounds in 66 dosage forms analyzed by an average of 9 laboratories per study, with a total of 1150 determinations, were reviewed. For the approved methods and methods awaiting approval (9 studies, 11 compounds, 54 dosage forms, and 959 determinations), the average repeatability relative standard deviation (within-laboratory; RSDo) was 1.0%; reproducibility relative standard deviation (among-laboratories, including within-; RSDx) was 2.5%; the ratio RSDo/RSDx was an unusually low 0.40, with an average outlier rate of 0.6% of the reported values. The line of best fit for RSDx plotted against - log concentration increases with decreasing concentration, extending approximately from RSDx = 2% at 100% concentration to RSDx = 3.6% at 0.01% concentration, a change in RSDx of about 0.4% for each 10-fold decrease in concentration, independent of analyte and matrix.
Chromatography, High Pressure Liquid↗
Determination of phosphorus in processed cheese: collaborative study.
A second interlaboratory collaborative study of the determination of phosphorus in processed cheese products by the molybdenum blue method verifies that this method is prone to producing a laboratory-induced systematic error. It would be useless to continue to make minor modifications in the details of the method, which will improve only the within-laboratory precision, until an accuracy control of the final measurement step is incorporated into the method.
Cheese↗
Performance characteristics of methods of analysis used for regulatory purposes. I. Drug dosage forms. C. Automated methods.
For analysis of drug dosage forms, precision measures of AOAC approved automated methods, usually containing a spectrophotometric or fluorometric measurement step, were recalculated on a consistent statistical basis, using a computer program "FDACHEMIST." Ten collaborative studies of 14 compounds in 38 materials, consisting of various dosage forms, usually in 10 replications by an average of 7 laboratories, with a total of 2461 determinations, were reviewed. The average relative standard deviations within-laboratory (RSDo) and among-laboratories (RSDx) were 1.1 and 1.9%, respectively, and the ratio of RSDo/RSDx was 0.57, with an average outlier rate of 0.57% of the reported values. The line of best fit for RSDx plotted against - log concentration increases slightly with decreasing concentration, extending from an RSDx of about 1.6% at 100% concentration to an RSDx of 2.2% at 0.1% concentration, a change in RSDx of about 0.2% for a 10-fold decrease in concentration, independent of analyte and matrix.
Autoanalysis↗