EVALUATION OF DUPLICATE SAMPLES REGIONAL SEDIMENT SURVEYS , BRITISH COLUMBIA
INTRODUCTION The ability to discriminate real trends related to geological and geochemical causes from those that result from spurious factors such as sampling and analytical errors is of paramount importance in the suI:cess of geochemical data interpretation. Since the estimate of reproducibility (precision) allows us t o quantify the amount of variation due t o sampling and laboratory analysis it is an integral part of the evaluetion of geochemical data and should be conducted prior to carrying out any detailed interpreation. As part of our continuing study of rapid but thorough evaluation procedures for multi-element stream-sediment data (for example, Matysek. et a/., 1981) we designed a systematic computer-oriented method of evaluating the quality of geochemical survey data based on field-site duplicates. It irlcorporates a bias test, an analysis of variance technique, and the Thompson and Howarth (1976) approach to qua'ytifying precision. This detailed procedure utilizes the type and quality of data incorporated in various regional geochemical programs undertaken by the British Columbia Ministry of Energy, Mines and Petloleum Resources (the Ministry), but can be adapted easily for other programs, GENERAL METHODOLOGY steps: In brief, our general approach t o evaluation of the quality of geochemical data sets involves the following (1) Extraction of a t least 50 independent, field-site duplicate pairs from a geochemical data set f cmr use in subsequent data analysis. (2) Determination of the degree of systematic bias between duplicate pairs based upon the number of cases in which the first observation is greater than the duplicate. (3) Evaluation of the duplicated geochemical data set in terms of metal variability a t the regional, between-sample site and a t the local, within-sample site, sampling, and analvtical levels by a two-factor analysis of variance technique. (4) Quantification of the within-sample site variability by estimating precision utilizing the Thsmpson and Howarth (19761 method. D A T A Data obtained from the three most recent regional geochemical surveys undertaken Iby the Ministry (1981 I were evaluated using this method. The base data consist o f analyses of stream sediments and water samples collected a t an average density of one sample per 17.3 square kilometres over NTS map-sheet areas 92H, 921, and 92J. Silt samples were field dried and the minus 80-mesh (177 microns) fraction retained for subsequent analyses. The samples were analyzed for zinc, copper, lead, nickel, cobalt, silver, manganese, iron, molybdenum, arsenic, and antimony by the atomic absorption …
A new approach to the estimation of analytical precision
Abstract Methods used for the estimation of analytical precision commonly suffer from two deficiencies which give rise to misleading results. These are: (1) the methods take no account of changes in absolute or relative error over the concentration range, and (2) they tend for other reasons to produce optimistically-biassed results. The difficulties can be avoided by the correct use of duplicate determinations. One method presented allows precision parameters to be estimated. The other gives rise to a simple control chart for use in geochemical analysis.
Duplicate analysis in geochemical practice. Part I. Theoretical approach and estimation of analytical reproducibility
Duplicate analytical results can be used to give rapid and realistic estimates of precision in analytical systems. In particular the effects of varying concentration of analyte on the variance of the measurement can be taken into account. When only small numbers of duplicate observations are available, the precision can be rapidly tested against an empirical standard level by use of a special control chart. Some common data-recording practices, however, can lead to erroneous estimates of detection limit, irrespective of the estimation procedure employed.