Introduction

This book has been produced as a deliverable under the UK's Valid Analytical Measurement (VAM) Programme. VAM is a programme of work sponsored by the Department of Trade and Industry as part of the UK's National Measurement System. The book provides guidance and information on the application of certified reference materials (CRMs) in the context of how they can best be used to achieve valid analytical measurements and thereby improve quality in the analytical laboratory.

The main applications of CRMs in analytical chemistry, such as instrument calibration, method validation, checking laboratory performance, internal quality control and uncertainty estimation are described. Worked examples are used to illustrate key issues and to lead the analyst through the most common calculations and statistical tests employed. The first two chapters provide general information on CRMs and how they are produced, whilst in Chapter 3 the statistics relating to the use of CRMs are explained in an easy to understand manner. Chapters 4–6 describe the main applications of CRMs, and are written so that they may be read independently of each other.

1.1 Certified Reference Materials and the VAM Principles

All chemical measurements, whether qualitative or quantitative, depend upon and are ultimately traceable to, a CRM or a standard material of some sort. Qualitative measurements of identity based, for example, on gas chromatographic retention times or spectroscopic properties, require a reliable, authentic, reference material to calibrate the particular instrument or test used. Quantitative determinations have the additional requirement that the instrument is calibrated with an accurately known amount of the reference material concerned.

The key role of CRMs in analytical chemistry is recognised in the VAM Principles, a set of six statements, shown in the box, that set out the essential practical requirements for achieving valid analytical measurements.

The testing of methods and equipment referred to in VAM Principle 2 is most effectively accomplished by the use of appropriate CRMs. For example, the accuracy of the wavelength scale of UV-visible HPLC detectors can be verified by the use of a CRM comprising a solution that has absorption peaks with well characterised reference values for the wavelengths of maximum absorbance. Likewise, the accuracy of an entire analytical method, such as the determination of the proximate constituents in foods, could be checked by the use of a CRM comprising a food-type matrix with well characterised reference values for constituents such as protein (nitrogen), moisture, fat, fibre and ash.

VAM Principle 5 emphasises the importance of comparability between analytical data produced in different laboratories and locations. One effective means of achieving this is to ensure that all analytical data are traceable to reliable CRMs. A laboratory can check the repeatability of its data by setting up internal quality control (IQC) procedures to provide evidence of day-to-day consistency in its results, but a laboratory relying exclusively on IQC procedures could conceivably be producing consistent, but biased results. The use of CRMs as measurement benchmarks can provide the essential reference or anchor points against which analysts can achieve comparability of their measurements. When several laboratories can achieve the same analytical result (within the uncertainty specified) for a given CRM, they will have demonstrated the comparability of their measurements.

1.2 Definitions and Hierarchy of Reference Materials

Although this book mainly covers the uses of CRMs, they are essentially a subset of materials that are generically known as reference materials. Within the scope of the generic term there are several classifications, and the following international definitions for some of these have been published by the International Organisation for Standardisation (ISO):

• Primary Standard: Standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity.

• Secondary Standard: Standard whose value is assigned by comparison with a primary standard of the same quantity.

• Certified Reference Material (CRM): Reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure which establishes its traceability to an accurate realisation of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence.

• Reference Material (RM): Material or substance one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials.

These various types of reference materials can be conveniently arranged, as shown in Figure 1.1, according to their relative metrological positions (essentially, the uncertainty associated with their measurement value) between the SI base units and routine test sample measurements.

It should be noted that the position of a particular reference material in the hierarchy is not necessarily an indication of its suitability for a particular purpose. For example, in the determination of trace copper in a soil, a matrix CRM comprising a soil with certified levels of copper would be of greater value than a primary standard consisting of ultra-pure copper.

Primary standards represent the top-tier of chemical standards and, in principle, provide a means of establishing the traceability of analytical data to the SI measurement units, such as the kilogram, the metre and the mole. The concept of primary standards in analytical chemistry has been in existence for many years, and a detailed system for establishing the traceability of a range of pure chemicals and their solutions to a primary standard of ultra-pure silver was published in 1950. Subsequently, the characteristics of primary standards were defined in more detail, and now a number of pure chemicals are recognised as primary standards.

The essential characteristics of a primary standard are that it should be:

• readily available commercially

• of high chemical purity 100% [+ or -] 0.02%

• of high stability

• homogeneous

• non-hygroscopic and non-efflorescent

• readily soluble

• of high equivalent weight (to minimise weighing errors)

• able to undergo accurate stoichiometric reaction in titration

In addition, variations in isotopic abundance should not materially affect the molecular weight. A...