The written directives of a quality control program are a necessary, but not a suffi- cient, condition for obtaining and maintaining an analysis in a state of statistical control. Although quality control directives explain how an analysis should be properly conducted, they do not indicate whether the system is under statistical control. This is the role of quality assessment, which is the second component of a quality assurance program.
The goals of quality assessment are to determine when a system has reached a state of statistical control; to detect when the system has moved out of statistical control; and, if possible, to suggest why a loss of statistical control has occurred so that corrective ac- tions can be taken. For convenience, the methods of quality assessment are divided into two categories: internal methods that are coordinated within the laboratory and exter- nal methods for which an outside agency or individual is responsible.
The most useful methods for quality assessment are those that are coordinated by the laboratory and that provide the analyst with immediate feedback about the sys- tem’s state of statistical control. Internal methods of quality assessment included in this section are the analysis of duplicate samples, the analysis of blanks, the analysis of standard samples, and spike recoveries.
An effective method for determining the precision of an analysis is to analyze duplicate samples. In most cases the duplicate samples are taken from a single gross sample (also called a split sample), although in some cases the duplicates must be independently collected gross samples. The results from the duplicate samples, X1 and X2, are evaluated by determining the difference, d, or the relative difference, (d)r, between the samples
and comparing the results with accepted values, such as those shown in Table 15.1 for the analysis of waters and wastewaters. Alternatively, the results for a set of n duplicates are combined to estimate the standard deviation for the analysis
where di is the difference between the ith pair of duplicates. The degrees of freedom for the standard deviation is the same as the number of duplicate samples. If dupli- cate samples from several sources are combined, then the precision of the measure- ment process must be approximately the same for each. The precision obtained is then compared with the precision needed to accept the results of the analysis.
The use of a blank was introduced as a means of correcting the measured signal for contributions from sources other than the analyte. The most common blank is a method, or reagent blank, in which an analyte-free sample, usually distilled water, is carried through the analysis using the same reagents, glassware, and instrumentation. Method blanks are used to identify and correct systematic errors due to impurities in the reagents and con- tamination in the glassware and instrumentation. At a minimum, method blanks should be analyzed whenever new reagents are used, although a more frequent analysis provides an ongoing monitoring of the purity of the reagents. A new method blank should also be run whenever a sample with a high concentration of the analyte is analyzed, because any residual carryover of the analyte may contami- nate the glassware or instrumentation.
When samples are collected in the field, the method blank may be augmented with field and trip blanks. A field blank is an analyte-free sample carried from the laboratory to the sampling site. At the sampling site the blank is transferred to a clean sample container, exposing it to the local environment, preserved, and trans- ported back to the laboratory for analysis. Field blanks are used to identify and correct systematic errors due to sampling, transport, and analysis. Trip blanks are analyte-free samples carried from the laboratory to the sampling site and returned to the laboratory without being opened. A trip blank is used to identify and correct systematic errors due to cross-contamination of volatile organic compounds during transport, handling, storage, and analysis.
The analysis of a standard containing a known concentra- tion of analyte also can be used to monitor a system’s state of statistical control. Ide- ally, a standard reference material (SRM) should be used, provided that the matrix of the SRM is similar to that of the samples being analyzed. A variety of appropriate SRMs are available from the National Institute of Standards and Technology (NIST). If a suitable SRM is not available, then an independently prepared synthetic sample can be used if it is prepared from reagents of known purity. At a minimum, a standardization of the method is verified by periodically analyzing one of the cali- bration standards. In all cases, the analyte’s experimentally determined concentra- tion in the standard must fall within predetermined limits if the system is to be con- sidered under statistical control.
One of the most important quality assessment tools is the recov- ery of a known addition, or spike, of analyte to a method blank, field blank, or sam- ple. To determine a spike recovery, the blank or sample is split into two portions, and a known amount of a standard solution of the analyte is added to one portion. The concentration of the analyte is determined for both the spiked, F, and unspiked portions, I, and the percent recovery, %R, is calculated as
where A is the concentration of the analyte added to the spiked portion.
Spike recoveries on method blanks and field blanks are used to evaluate the general performance of an analytical procedure. The concentration of analyte added to the blank should be between 5 and 50 times the method’s detection limit. Sys- tematic errors occurring during sampling and transport will result in an unaccept- able recovery for the field blank, but not for the method blank. Systematic errors occurring in the laboratory, however, will affect the recoveries for both the field and method blanks.
Spike recoveries for samples are used to detect systematic errors due to the sample matrix or the stability of the sample after its collection. Ideally, samples should be spiked in the field at a concentration between 1 and 10 times the expected concentration of the analyte or 5 to 50 times the method’s detection limit, whichever is larger. If the recovery for a field spike is unacceptable, then a sample is spiked in the laboratory and analyzed immediately. If the recovery for the labora- tory spike is acceptable, then the poor recovery for the field spike may be due to the sample’s deterioration during storage. When the recovery for the laboratory spike also is unacceptable, the most probable cause is a matrix-dependent relationship be- tween the analytical signal and the concentration of the analyte. In this case the samples should be analyzed by the method of standard additions. Typical limits for acceptable spike recoveries for the analysis of waters and wastewaters are shown in Table 15.1.
Internal methods of quality assessment should always be viewed with some level of skepticism because of the potential for bias in their execution and interpretation. For this reason, external methods of quality assessment also play an important role in quality assurance programs. One external method of quality assessment is the certification of a laboratory by a sponsoring agency. Certification is based on the successful analysis of a set of proficiency standards prepared by the sponsoring agency. For example, laboratories involved in environmental analyses may be re- quired to analyze standard samples prepared by the Environmental Protection Agency. A second example of an external method of quality assessment is the volun- tary participation of the laboratory in a collaborative test 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 sam- ples 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.