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P&B, IDE, AND IQE Defined for Ideal Water-Testing

ASTM Committee D19 on Water has produced three standard practices that fully characterize the performance of analytical methods at the interlaboratory level. Developed for water-testing methods, these standards have broad applicability where an increased level of the analyte results in an increased response. The standards can be applied using data from a single interlaboratory round-robin study, provided the selected concentrations span the trace-level and working ranges of the method. They explicitly incorporate a common phenomenon: an increased level of the analyte results in an increased standard deviation of the measurements.

Created by large task forces of over 200 chemists, laboratory technicians, and water-industry specialists in Subcommittee D19.02 on General Specifications, Technical Resources, and Statistical Methods, the standards are:

D 2777, Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water, aids development of round-robin-study designs, evaluation of data and rejection of outliers, and calculation of precision and bias over the working range of a method.

With a few exceptions, the requirements of D 2777 must be satisfied for approval of new D19 test methods; therefore D 2777 is referred to as the “Method Validation” or the “Precision and Bias” standard practice. Released in 1998, it provides extensive guidance to planners of interlaboratory studies. Procedures for selection of test concentrations, management of participating laboratories, role and responsibilities of a study supervisor, and Youden-pair study designs are included. Also provided are application of Grubbs’ method and lab-ranking to screen data for outliers, and formulas for predicting the precision and bias of a method.

D 6091, Standard Practice for 99%/95% Interlaboratory Detection Estimation (IDE) for Analytical Methods with Negligible Calibration Error Practice, calculates the lowest level of reliable detection. Released in 1997, D 6091 depends on the interlaboratory-study approach of D 2777. Within a population of laboratories, the IDE is computed to be the lowest concentration at which one can be 90 percent confident to achieve detection 95 per cent of the time, and no detection for 99 percent of measured blanks.

D 6091 provides guidance on inclusion of concentration levels that are consistent with determination of detection, for example, the addition of a few low-concentration samples to the historically working-range samples of D 2777. The IDE document describes additional analysis of the study data, after outlier removal, to compute the IDE. An innovation is the choice and use of models that relate the interlaboratory standard deviation (ILSD) to concentration. To generate an IDE value that reflects data-quality objectives, the user is able to select alternatives for the confidence level, the true-detection rate, and the non-detection rate for blanks.

Nearing publication, D 6512, Standard Practice for an Interlaboratory Quantitation Estimate (IQE) covers calculation of the lowest level of reliable quantitation. The IQE is computed to be the lowest concentration with a marginally acceptable relative standard deviation (RSD = 30%, 20%, or 10%; all, some, or none of these values may be achievable for a given test method) among a population of laboratories. D 6512 uses the data and the interlaboratory-study approach of D 2777, and involves modeling the interlaboratory standard deviation, as in D 6091. The user can evaluate the suitability of methods by comparing each method’s IQE values to data-quality objectives.
Collectively, these standards provide a new, technically sound means for method-performance characterization. D 2777 forces the refinement of a method to the point that multiple laboratories can perform the test and get comparable results. This precision-and-bias document also characterizes the behavior of a method within its working range, which might span two or more orders of magnitude in spike concentrations. Consistent with recent ISO quantitation and detection concepts in “Nomenclature in Evaluation of Analytical Methods, Including Detection and Quantification Capabilities,” Pure and Applied Chemistry, Vol. 67, No. 10, 1995, by L. Currie, the IQE and the IDE standard practices allow determination of a method’s lower levels of usefulness. The IQE can be treated as the lowest level at which a measurement value might be recorded for routine use, and the IDE can be seen as the lowest level at which one can confidently achieve detection.

ASTM standards are available by calling Customer Service (610/832-9585) or through the Web site (www. astm.org).

For further technical information on D 6091 (IDE) or D 6512 (IQE), contact D19 member David Coleman, Alcoa Technical Center, 100 Technical Dr., Alcoa Center, PA 15069 (724/337-5913). For further D 2777 (P&B) information, contact member Lynn Vanatta, Air Liquide, Box 650311, M/S 301, Dallas, TX 75265 (). Committee D19 meets June 24-28 in Tucson, Ariz. For meeting or membership details, contact manager Bruce Noe, ASTM (610/832-9719). //