Quality control encompasses all activities used to bring a system into statistical control. The most important facet of quality control is a set of written directives de- scribing all relevant laboratory-specific, technique-specific, sample-specific, method-specific, and protocol-specific operations. Good laboratory practices (GLPs) describe the general laboratory operations that need to be followed in any analysis. These practices include properly recording data and maintaining records, using chain-of-custody forms for samples that are submitted for analysis, specifying and purifying chemical reagents, preparing commonly used reagents, cleaning and calibrating glassware, training laboratory personnel, and maintaining the laboratory facilities and general laboratory equipment.
Good measurement practices (GMPs) describe operations specific to a tech- nique. In general, GMPs provide instructions for maintaining, calibrating, and using the equipment and instrumentation that form the basis for a specific tech- nique. For example, a GMP for a titration describes how to calibrate a buret (if necessary), how to fill a buret with the titrant, the correct way to read the volume of titrant in the buret, and the correct way to dispense the titrant.
The operations that need to be performed when analyzing a specific analyte in a specific matrix are defined by a standard operations procedure (SOP). The SOP describes all steps taken during the analysis, including: how the sample is processed in the laboratory, the analyte’s separation from potential interferents, how the method is standardized, how the analytical signal is measured, how the data are transformed into the desired result, and the quality assessment tools that will be used to maintain quality control. If the laboratory is responsible for sampling, then the SOP will also state how the sample is to be collected and preserved and the na- ture of any prelaboratory processing. A SOP may be developed and used by a single laboratory, or it may be a standard procedure approved by an organization such as the American Society for Testing and Materials or the Federal Food and Drug Ad- ministration. A typical SOP is provided in the following example.
Although an SOP provides a written procedure, it is not necessary to follow the procedure exactly as long as any modifications are identified. On the other hand, a protocol for a specific purpose (PSP), which is the most detailed of the written quality control directives, must be followed exactly if the results of the analysis are to be accepted. In many cases the required elements of a PSP are established by the agency sponsoring the analysis. For example, labs working under contract with the Environmental Protection Agency must develop a PSP that addresses such items as sampling and sample custody, frequency of calibration, schedules for the preventive maintenance of equipment and instrumentation, and management of the quality assurance program.
Two additional aspects of a quality control program deserve mention. The first is the physical inspection of samples, measurements and results by the individuals responsible for collecting and analyzing the samples.1 For example, sediment sam- ples might be screened during collection, and samples containing “foreign objects,” such as pieces of metal, be discarded without being analyzed. Samples that are dis- carded can then be replaced with additional samples. When a sudden change in the performance of an instrument is observed, the analyst may choose to repeat those measurements that might be adversely influenced. The analyst may also decide to reject a result and reanalyze the sample when the result is clearly unreasonable. By identifying samples, measurements, and results that may be subject to gross errors, inspection helps control the quality of an analysis.
A final component of a quality control program is the certification of an ana- lyst’s competence to perform the analysis for which he or she is responsible.7 Before an analyst is allowed to perform a new analytical method, he or she may be required to successfully analyze an independent check sample with acceptable accuracy and precision. The check sample should be similar in composition to samples that the analyst will routinely encounter, with a concentration that is 5 to 50 times that of the method’s detection limit.