Duplicates, blind

Agency. A second example of an external method of quality assessment is the voluntary participation of the laboratory in a collaborative test (Chapter 14) sponsored by a professional organization such as the Association of Official Analytical Chemists. Finally, individuals contracting with a laboratory can perform their own external quality assessment by submitting blind duplicate samples and blind standard samples to the laboratory for analysis. If the results for the quality assessment samples are unacceptable, then there is good reason to consider the results suspect for other samples provided by the laboratory. 

Measurement laboratories are often sent blind duplicates of material for analysis (i.e., the measurement laboratory is not told they are duplicates). Why ... 

Staff become bored performing repetitive tasks, whereas a reliable laboratory robot will perform procedures uniformly, eliminating human error. Automation permits the frequent use of replicates, standards and controls to verify precision, so the analyst and customer can have complete confidence in the results. Taylor et al. [14] have suggested that the number of blind duplicates sent to control laboratories by operating staff can be as high as 70% of total work load. Reliability and traceability are important benefits of... 

Precision of the analysis was calculated using three in-house replicates, and two blind duplicates submitted by the authors. Accuracy was determined using ACME s in-house reference material, DS2 (HMTRI, 1997) (Table 16.1). 

Table XI. Sensory Heat Ratings for Blind Duplicate Samples of Red Pepper3...

In this collaborative study fourteen laboratories analyzed six different commodities containing aflatoxin B, at two concentration levels as blind duplicates. Two standards (15 and 50 ng B,/g) were provided, and collaborators were asked to report their results as <15 or >15 ppb. Laboratories with microtiter well readers were asked to determine aflatoxin concentrations both visually and spectrophotometrically. In this kit aflatoxin B,-antibodies are coated onto plastic microtiter wells. The aflatoxin-containing sample is extracted with Me0H-H,0 (55+45). The extract is defatted with hexane, and the MeOH extract mixed with the aflatoxin-enzyme conjugate and added to the well of the antibody-coated microtiter plate. The aflatoxin in the extract and the aflatoxin-enzyme complex compete for the antibody binding sites. The enzyme substrate(ABTS) and HjOj solution are then added, the reaction leading to a colored product in the presence of enzyme. The intensity of color is determined visually or spectrophotometrically at 580nm. 

Thirteen laboratories participated in this study each one received the complete protocol including precise instructions about the preparation and storage of each reagent, test samples to calibrate the procedure before beginning the interlaboratory study, and five blind duplicate samples. The relatively low number of samples is due to the fact that only two concentrations (2000 and 5000 units/g) were tested in this study. 

Mean values accumulated for all concentrations and all participants per test kit and food matrix) RSOj, variation of duplicate analysis of the same sample, same test kit, and analysis by the same laboratory RSDjy, variation of a duplicate analysis of blind duplicate samples employing the same test kit by the same... 

Obvious questions are how many replicates should be included in the design and also how many analytical runs should be completed. Eurachem guidance suggests a minimum of 10 replicates for recovery (accuracy) and precision. A collaborative smdy design is recommended to include a minimum of five materials (matrices) at three concentrations, in blind duplicate, which means that each participating laboratory produces 10 results for each concentration. However, when one considers the recommendation that at least six different sources of matrix should be used in validation for methods for veterinary drug residues in foods and that the validation should include analyses conducted on multiple days, with inclusion of other variables, such as analyst, equipment, and reagents, it is obvious that 10 replicates will prove insufficient to provide the necessary data for a suitable assessment of... 

For quality-assurance purposes, about 8 percent of all samples were submitted for immunoassay analysis as blind duplicates. In addition, about 25 percent of the samples (mostly from the pre- and post-application periods) were analyzed in a second laboratory by immunoassay analysis. The procedure used by the second laboratory was identical to that described above except that a differential photometer (Artel, Inc., Windham, Maine) was used instead of the Milton Roy spectrophotometer. These quality-assurance samples provided a means to evaluate the variation in results within a laboratory and between laboratories. 

During the three sampling periods, blind duplicates of 28 samples were submitted for immunoassay analyses by a single laboratory as a quality-assurance measure. Results for the samples are summarized in table II. The maximum difference between duplicates was 0.5 ug/L and 86 percent of the samples had absolute differences of 0.14 ug/L or less. These results indicate that within-laboratory variation of the method is relatively small. 

Table II. Comparison of immunoassay results obtained on blind duplicate samples by a single laboratory (within lab) and results on samples analyzed by two different laboratories (between labs)...

Table II. Results of Visual Method for ImnunoChemical Detection of Zearal K e in Blind Duplicate Sanples...

Table IV. Results of Imnunochemical Determinatic l >ectroEhotoinetric Method of Zearaloxxie in Blind Duplicate Sanples of Ocnoi, Mieat, and Feed...

RMS = root mean square noise of the instrument. SEC = standard error of calibration for each constituent. SEA = standard error of analysis or the difference between analysis values from the same samples analyzed by NIR and the reference laboratory. SED = standard error of a difference. Factory SED = standard error of the difference between the same samples analyzed by the master and slave instrument at different times. H = standardized H statistic. SEE = standard error of the laboratory reference values. (This statistic can be either the difference between duplicates in one laboratory or the difference between the same samples analyzed by two different laboratories.) M SEE = standard error of blind duplicates in the master reference laboratory. 

Note 9—The following precision data were developed in a 1991 interlaboratory cooperative test program. Participants analyzed sample sets comprised of blind duplicates of 14 types of hydrocarbons and hydrocarbon-oxygenate blends. The oxygenate content (MTBE, ethanol, and methanol) ranged fnm 0 to IS % by volume nominal and the vapor pressure ranged from 14 to 100 kPa (2 to IS psi) nominal. A total of 60 laboratories participated. Some participants performed more than one test method, using separate sample sets for each. Twenty-six samfdes sets were tested by Test Method D 4953, 13 by this test method, and 27 by... 

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