Isotope-dilution mass spectrometry laboratory method

Table 2 Comparison of Cholesterol Reference Method Laborato- tute, NIST National Institute of Standards and Technology (US ry Network (CRMLN) traceability model to the International Or- NMI), IDMS isotope dilution mass spectrometry, ACL accredited ganization for Standardization (ISO) model. BIPM International calibration laboratory, AK Abell-Kendall Bureau of Weights and Measures, NMI National Metrology Insti-...

Atomic Absorption Spectrometry Methods The National Committee for Clinical Laboratory Standards (NCCLS) has approved a method using atomic absorption spectrophotometry (AAS) as a reference method for measuring total serum calcium. This method has been compared with isotope dilution-mass spectrometry (ID-MS), the definitive method for total serum calcium developed by the National Institute of Standards and Technology. The reference method is reported to have an accuracy of 100 2%, compared with 100 0.2% for ID-MS. Although AAS can provide better accuracy and precision for total serum calcium than the widely used photometric methods, it is used by only a few laboratories. It should continue to be used for validating new total calcium methods. 

Definitive method method with a high scientific status applied in a laboratory of high proven quality [10]. In brief it means that only negligible systematic errors may remain compared to the precision and trueness required for the final result. Isotope Dilution Mass Spectrometry (IDMS), in its thermo-ionisation mode, for the determination... 

If this is a requirement, the test material must be prepared or analyzed so that the quantity value is assigned. In interlaboratory comparisons of elements in water organized by IRMM under the International Measurement Evaluation Program (IMEP), the test materials were prepared by gravimetry or analyzed by a primary method such as isotope dilution mass spectrometry by an independent laboratory. 

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. 

A stable isotope dilution assay using mass spectrometry to measure insulin, proinsulin, and C-peptide has been developed. The difference in mass among the three analytes allows specific measurement of each protein. Comparison of patient samples revealed that most, but not all, results were higher by immunoassay than mass spectrometry. Thus immunoassays may overestimate insulin, particularly at low concentrations. The high protein concentration in the serum requires extraction of proteins (e.g., by immunoaffinity) and purification by high-performance liquid chromatography (HPLC) before quantification by mass spectrometry. This method is not suitable for routine laboratory analysis. 

A number of methods which cannot be classified conveniently either as optical or electroanalytical suggest themselves on the basis of sensitivity or other characteristics. Mass spectrometry is extremely sensitive and of general applicability. The results may be very good, especially when the method is combined with stable isotope dilution. However, sample preparation is difficult, and the method is both time consuming and expensive. Relatively few laboratories have the necessary equipment, and hence it is unlikely that much work will be done by this means. Catalytic reaction methods are also extremely sensitive but very limited in scope. Most chemical methods of analysis do not have the necessary sensitivity and particularly in low level work, are subject to errors by adventitious contamination. 

Thienpont, L.M. Van Nieuwenhove, B. Reinuer, H. De Leenheer, A.P. Determination of reference method values by isotope dilution-gas chromatography-mass spectrometry A five years experience of two European reference laboratories. Eur. J. Clin. Chem. Clin. Biochem. 1996, 34, 853-860. 

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