Quality control alternative test procedures

The accuracy of an analysis can be determined by several procedures. One common method is to analyze a known sample, such as a standard solution or a quality control check standard solution that may be available commercially, or a laboratory-prepared standard solution made from a neat compound, and to compare the test results with the true values (values expected theoretically). Such samples must be subjected to all analytical steps, including sample extraction, digestion, or concentration, similar to regular samples. Alternatively, accuracy may be estimated from the recovery of a known standard solution spiked or added into the sample in which a known amount of the same substance that is to be tested is added to an aliquot of the sample, usually as a solution, prior to the analysis. The concentration of the analyte in the spiked solution of the sample is then measured. The percent spike recovery is then calculated. A correction for the bias in the analytical procedure can then be made, based on the percent spike recovery. However, in most routine analysis such bias correction is not required. Percent spike recovery may then be calculated as follows ... 

Even standard methods of test often allow alternative test pieces or procedures and these may not yield equivalent results. Hence, it is important to clearly define which procedure has been followed and, when a standard has been used, to identify any deviations made from the set procedure. Test procedure requires careful attention to detail as small, apparently innocent changes can produce significant deviation in results. Equally, it is essential that that test conditions are accurately set and maintained, which is really just one aspect of quality control considered in more detail in Section 7. 

The force required to propagate a starter tear in the specimen is recorded in the course of the standard test procedures. The tear. strength is expressed either as the maximum value (alternatively the average value) of the tearing force recorded or as the maximum force (alternatively the average force) per specimen thickness (in kN/m). The information from these standard tests, while useful for quality control and acceptance testing, does not produce a fundamental material property suitable for design applications. 

Note that alternative procedures for defining internal quality control limits may also be acceptable. Limits may be based, for example, on proficiency testing, such as 1 ng or 15% of the mean (whichever is greater). These should be clearly defined. 

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