Isotope-dilution mass spectrometry certified materials

At present, isotope-dilution mass spectrometry provides the best method to certify iron concentration in the recommended deep-water reference material (an expected iron concentration of approximately 0.7 nM) and to obtain an information value for iron in the recommended surface water reference material (an expected iron concentration of approximately 50 pM or less). 

The fundamentals and several applications of isotope dilution mass spectrometry requiring accurate isotope ratio measurements are reviewed by Heumann.50,51 Today isotope dilution mass spectrometry (IDMS) is recognized as a primary measurement method, by means of which accurate results with sufficiently small uncertainties can be achieved and therefore it has been used in certifying the composition of reference materials. A requirement of isotope dilution analysis in mass spectrometry is to achieve equilibration of spike and sample so that very careful sample preparation steps, especially in solid mass spectrometry, are necessary when a homogeneous sample spike mixture is to be prepared. 

Isotope dilution is usually the most accurate and most sensitive quantification technique when sample size is limited. It has in recent years even been used in certifying the compositions of reference materials [2]. The use of isotope dilution mass spectrometry in the standard reference materials program of the National... 

Primary Reference Material PRM a reference material certified with (a) primary method(s). In practice such materials are mainly pure substances, manufactured materials, rarely matrix materials as already explained above. PRMs of matrix materials are very limited in number. NIST has available some CRMs certified with thermal ionisation-isotope dilution mass spectrometry (TMDMS) which has been classified by the Comite Consultatif sur la Quantite de Matiere (CCQM) of BIPM as primary method. PRMs also exist for gas analysis. The Netherlands Meet Instituut (NMI) in Delft, The Netherlands has available several primary gas RMs prepared by gravimetry... 

The use of a definitive analytical method can also be used to establish standard reference materials. Definitive methods are ones that can produce exacting quantitative data without the need to compare measurements to a calibration standard. The gravimetric analysis method is a definitive technique. Isotope-dilution mass spectrometry, which is extensively used by NIST and other agencies producing certified standard reference materials, is also considered to be a definitive method of analysis. As discussed in Chapter 7, isotope dilution quantitation can be effectively used with ICP-MS. Therefore, a laboratory with ICP-MS instrumentation can produce reference materials in specific sample matrices for selected elements by using the isotope dilution technique.These standard reference materials still must be considered secondary standards, because they are usually not traceable to existing certified standards. 

The primary and immediate need is for a trace metal reference material, but a certified reference material would provide even greater benefits. A technique based on isotope dilution with detection by inductively-coupled plasma mass spectrometry (ICP-MS) (Wu and Boyle, 1998) most clearly meets the traceability criteria required for a certified reference material. Although useful for iron and several other metals, isotope dilution is not possible for monoisotopic elements like cobalt, so other techniques must also be used. Indeed, it is advisable that several techniques be used to certify a trace metal reference material. 

Following the classification of the analytical methods given by ISO 32 [3] two major type of calibration materials can be certified. For relative methods such as all spectrometric ones, pure substances are necessary. They can be certified for the stoichiometry and degree of purity but also for isotopic composition. The latter case is a prerequisite for measurements of radioactive materials and for stable isotope mass spectrometry (isotope dilution TIMS or ICP-MS). For comparative methods, pure substances and mixtures of substances are necessary, as well as matrix materials for which the element to be determined is perfectly known and also the major compounds that produce a matrix influence on the signal (e.g. alloys, gases). 

Yoshinaga, J., Morita, M., and Edmonds, J. S. (1999). Determination of copper, zinc, cadmium and lead in a fish otolith certified reference material by isotope dilution inductively coupled plasma mass spectrometry using ofi hne solvent extnction.J.Anal.At. Spectrom. 14(10), 1589. 

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