William Horwitz

Bio: William Horwitz is an academic researcher from Food and Drug Administration. The author has contributed to research in topics: Fatty acid & Sampling (statistics). The author has an hindex of 19, co-authored 58 publications receiving 4525 citations.

The variability of AOAC methods of analysis as used in analytical pharmaceutical chemistry.

Abstract: 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.

The Horwitz ratio (HorRat): A useful index of method performance with respect to precision.

Abstract: The Horwitz ratio (HorRat) is a normalized performance parameter indicating the acceptability of methods of analysis with respect to among-laboratory precision (reproducibility). It is the ratio of the observed relative standard deviation among laboratories calculated from the actual performance data, RSDR (%), to the corresponding predicted relative standard deviation calculated from the Horwitz equation PRSDR (%) = 2C(-0.15), where C is the concentration found or added, expressed as a mass fraction. It is more or less independent of analyte, matrix, method, and time of publication (as a surrogate for the state of the art of analytical chemistry). It is now one of the acceptability criteria for many of the recently adopted chemical methods of analysis of AOAC INTERNATIONAL, the European Union, and other European organizations dealing with food analysis (e.g., European Committee for Standardization and Nordic Analytical Committee). The origin and applications of the formula are described. Consistent deviations from the ratio on the low side (values 2) may indicate inhomogeneity of the test samples, need for further method optimization or training, operating below the limit of determination, or an unsatisfactory method.

Quality assurance in the analysis of foods for trace constituents

Abstract: An examination of the results of over 50 interlaboratory collaborative studies conducted by the AOAC on various commodities for numerous analytes shows a relationship between the mean coefficient of variation (CV), expressed as powers of 2, with the mean concentration measured, expressed as powers of 10, independent of the determinative method. Some typical CV values are: drug formulations at a 1% concentration, 2%; sulfonamides in feeds at a 0.01% concentration, 4%; pesticide residues and toxic elements, 10(-6) (1 ppm), 16%; and aflatoxins 10(-8) (10 ppb), 32%. Analytical work at trace levels must be constantly monitored through analysis of reference materials and surrogate samples, and by independent replication.

Determination of Reference Values for NIST SRM 610–617 Glasses Following ISO Guidelines

Abstract: We present new reference values for the NIST SRM 610–617 glasses following ISO guidelines and the International Association of Geoanalysts’ protocol. Uncertainties at the 95% confidence level (CL) have been determined for bulk- and micro-analytical purposes. In contrast to former compilation procedures, this approach delivers data that consider present-day requirements of data quality. New analytical data and the nearly complete data set of the GeoReM database were used for this study. Data quality was checked by the application of the Horwitz function and by a careful investigation of analytical procedures. We have determined quantitatively possible element inhomogeneities using different test portion masses of 1, 0.1 and 0.02 μg. Although avoiding the rim region of the glass wafers, we found moderate inhomogeneities of several chalcophile/siderophile elements and gross inhomogeneities of Ni, Se, Pd and Pt at small test portion masses. The extent of inhomogeneity was included in the determination of uncertainties. While the new reference values agree with the NIST certified values with the one exception of Mn in SRM 610, they typically differ by as much as 10% from the Pearce et al. (1997) values in current use. In a few cases (P, S, Cl, Ta, Re) the discrepancies are even higher. Nous presentons des nouvelles valeurs de reference pour les verres NIST SRM 610–617 en suivant les recommandations de l’ISO et le protocole de l’IAG. Les incertitudes au niveau de confiance de 95% ont ete determinees a des fins d’analyse totale et de micro-analyse. Contrairement aux procedures de compilation precedentes, cette approche fournit des donnees qui tiennent compte des exigences actuelles dans la qualite des donnees. De nouvelles donnees analytiques et le jeu de donnees presque complet de la base de donnees GeoReM ont ete utilises pour cette etude. La qualite des donnees a ete verifiee par l’application de la fonction de Horwitz et par un examen minutieux des procedures analytiques. Nous avons determine quantitativement les possibles inhomogeneites d’element en utilisant des prises d’essai de masses differentes correspondant a 1, 0.1 et 0.02 μg. Bien que nous ayons evite les zones de bordure des disques de verre, nous avons trouve des inhomogeneites moderees pour plusieurs elements chalcophiles/siderophiles et des inhomogeneites flagrantes de Ni, Se, Pd et Pt pour les prises d’essai de petites masses. La mesure d’inhomogeneite a ete incluse dans la determination des incertitudes. Alors que les nouvelles valeurs de reference sont en accord avec les valeurs NIST certifiees a la seule exception du Mn dans SRM 610, elles sont generalement differentes, avec des ecarts de pres de 10%, des valeurs de Pearce et al. (1997) qui sont d’un usage courant. Dans quelques cas (P, S, Cl, Ta, Re), les ecarts sont encore plus eleves.

Sampling, handling and analyzing plant tissue samples.

Abstract: Plant analysis (sometimes referred to as leaf analysis) is the determination of the total elemental content of a specified plant part. The emphasis in this chapter will be on the determination of those elements required for plant growth. Interpretation is normally based on the use of a "critical value" or "sufficiency range" (Smith, 1962) comparison between the elemental concentration found and a known norm (Goodall & Gregory, 1947; Chapman, 1966; Reuter & Robinson, 1986; Adriano, 1986; Martin-Prevel et al., 1987). An alternative method of interpretation is Diagnosis and Recommendation Integrated System (ORIS), which interprets the ratios of elements (N/P, K/Ca, and K/Mg) as indicators of elemental status (Beaufils, 1973; Sumner, 1977, 1982). Most growers primarily use a plant analysis for diagnosing suspected elemental insufficiencies, while its most significant, yet little used application, is for evaluating the soil/plant elemental status. This is partially reflected in the relatively few plant tissue samples assayed for growers, about 500 000, in the USA each year (Jones, 1985). Tissue testing, an elemental assay of extracted cell sap by means of quick chemical tests in the field, seems to be gaining an interest at levels equal to that observed several decades ago. A plant analysis is carried out in a series of steps as shown in Fig. 15-1. The results obtained are no better than the care taken in collecting, handling, preparing, and analyzing the collected tissue. An error made in one of these steps can result in an erroneous interpretation leading to recommendations that may be either unnecessary, costly, or even damaging to the crop. Therefore, it is important for those employing either a plant analysis or tissue test to follow the proper sampling, preparation, and analysis procedures. This chapter deals with the procedures required to successfully conduct a plant analysis or tissue test.

Electrochemical Biosensors: Recommended Definitions and Classification

Abstract: Republication or reproduction of this report or its storage and/or dissemination by electronic means is permittedwithout the need for formal IUPAC permission on condition that an acknowledgement, with full reference to thesource along with use of the copyright symbol q, the name IUPAC and the year of publication are prominentlyvisible. Publication of a translation into another language is subject to the additional condition of prior approvalfrom the relevant IUPAC National Adhering Organization.

Joint fao/who food standards programme

Abstract: 3. Those comments were considered and a new Draft Codex Strategic Plan was prepared by the Executive Committee Member from North America, with assistance from the Members from Europe and the Southwest Pacific. The revised draft was considered by the 67 th Session of the Executive Committee and the 35 th Session of the Codex Alimentarius Commission. The 67 th Session of the Executive Committee 1 agreed to establish a subcommittee, chaired by the CAC Vice-Chairperson Dr.Samuel Godefroy (Canada), with a mandate to: