Certification, NIST approach

There are many similarities in the certification approaches used by NIST and BCR as well as major differences. Both the NIST and BCR approaches are based on results from multiple independent methods. For the NIST SRMs the measurements are generally all performed at NIST with emphasis on the independence of the methods, whereas the BCR relies on results from a large number of laboratories that may use different methods. The certification approaches used by NIST and BCR are described below using as an example the measurement of PAHs in an environmental matrix. 

The three historical approaches to certification mentioned above were recently expanded to identily seven modes that are used at NIST for value assignment for chemical composition (May et al. 2000). These seven modes and the resulting values are summarized in Table 3.13. The basic principles of value assignment remain unchanged however, these modes now provide a well-defined link between the process used for value assignment and the definition of the assigned value (i.e. certified, reference, or information value). The terms described above provide a clear indication of the level of confidence that NIST has in the accuracy of the assigned value. The definition of a certified value implies that NIST must be involved in the measurement process for the value to be classified as a NIST certified value (see modes 1-3 in Table 3.13). Thus, modes 4 and 7, which do not involve NIST measure-... 

The first two approaches using definitive or primary or reference methods within one single laboratory require that in this laboratory everything is done to eliminate sources of systematic errors. Experience has demonstrated that it is very difficult to achieve 100% certainty and that within the laboratory a systematic bias does not remain. An additional confirmation through an — even limited — interlaboratory study is therefore advisable. Such an approach is used by NIST the single laboratory certification complies with the demand of US law that results and certificates must be NIST traceable . 

Many LA-ICPMS papers from the field of archaeometry have reported the use of this calibration approach, non-matrix-matched standards plus an internal reference, which is also typical in other fields (e.g., geology, environmental sciences). As calibration standards, the NIST series of glasses SRM 610-617 are very popular [10,38,47,56,57,62,66,69,71,75-82], because these glasses are known to be very homogeneous, and reference values for approximately 30 elements can be found in the certification report, and even more values can be easily obtained from the literature [83-85]. 

A proposed protocol submitted to the NIST committee for consideration is described below. Most, if not aU, elements of the protocol are expected to become requirements for future certification as a viable CWA detection device. Techniques described in this protocol serve as approaches that laboratories may follow. While suggested methodologies may not be the only or the best that are available, they have been thoroughly employed and have demonstrated highly satisfactory results for many years. Any deviation from these procedures must be thoroughly proven valid before use. That is the procedure must not cause artifacts. The ultimate objective is to evaluate the test item with the following questions in mind ... 

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