Proper Usage of Reference Materials

Since the early 1970 s there has been a growing belief that chemical measurements must not only be done correctly, but that data, the product of the measurement process, must be seen to be accurate, precise, and reliable. Analytical data have become another manufactured product and like all manufactured products, the customers demand that Quality Assurance (QA) must be built in. 

There is an abundance of references defining and describing the role played by QA, Quality Control (QC) and Total Quality Management (TQM) in a modem commercial analytical laboratory. The role played by reference materials (RMs) and certified reference materials (CRMs) in the pursuit of analytical measurement accuracy is also well documented. 

It has become an accepted wisdom that the use of RMs or CRMs will help to improve the accuracy and precision of an analytical process. This belief has led to a rapid growth in the use of RMs and CRMs in commercial laboratories. The authors and many analysts the world over support this view, but also recognize that in far too many cases inexperience and carelessness conspire together with the result that error accumulates and often unreliable data are produced. 

In this Chapter we highlight the practical considerations that must be understood by all users of RMs and CRMs we look at some of the issues of traceability and make the CRM user aware of the uncertainty budgets that need to be considered with the use of CRMs. No attempt will be made to advise CRM users on the proper use of statistics in the analytical measurement process and no statistical approaches on the establishment of measurement uncertainty will be given. There are a number of good texts on the subject which should be consulted. These are listed in the Further Reading section of the references at the end of this Chapter and include Miller and Miller (1993) and Taylor s work for NIST (Taylor 1985). 

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Jenks PJ and Zeislee R (2001) Proper usage of reference materials. In Stoeppler, M, Wolf WR and Jenks PJ, eds. Reference materials for chemical analysis - certification, availability and proper usage, pp. 236-255. Wiley-VCH, Weinheim. 

Certificatioii and availability of reference materials are dealt with elsewhere in this book but in keeping with the title it is appropriate to discuss the proper usage of RMs for quality control and quality assessment. 

Standard, white reflectance n. In general usage, may refer to a physical white standard which is nearly a perfect white diffuser, such as pressed barium sulfate of high purity, pressed magnesium oxide or freshly smoked magnesium oxide. Other less perfectly diffusing or reflective materials, such as white ceramics, may be calibrated in reference to the perfect diffuser or to one of the near-perfect materials, and used in place of the material relative to which it is cahbrated. Such a white reference standard should properly be called a secondary reference standard. 

One further point might be made for clarity. As we have seen, dielectrophoresis is the translational motion evoked by a nonuniform electric field. In the case of some solid materials and in certain semisolid ones (e.g., liquid crystals) there is seen still another mechanical response of a neutral body to an electric field, that of a distortion. This is electrostriction, and refers to the distortional response or strain resulting from an imposed electrical stress. Electrostrictive strains are used in sound transducers, for example. Historically speaking the two effects, translational (dielectrophoresis) and distortional (electrostriction), where both at times referred to as electrostriction with resultant confusion. Modem usage has tended to restrict the term electrostriction to the discussion of distortional strain that has been induced electrically. For the sake of brevity, we shall frequently use the abbreviation DEP response as that referred to properly as dielectrophoresis. One can, of course, couple a moment arm to the dielectrophoretic force (e.g., DEP force) producing a torsion, and possibly a realignment of the body in the field. 

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