Isotopes stable, standard reference materials

Table 9.1 A selection from the Table of Light Stable Isotope Standard Reference Materials avail-...

Tests on glass standard reference materials (SRM) from the National Institute of Standards and Technology (NIST) show that a stable signal is obtained when each isotope is measured using the peak jumping mode and one point per peak with a dwell time of 18,000 ps. The quadrupole mass spectrometer scans the entire mass range three times per replicate and accumulates 9 replicates for a total acquisition time of about 1 minute. For this application, fifty-three isotopes were selected (Table I). 

In 2005, De Laeter discussed the role of isotope reference materials for the analysis of non-traditionaT stable isotopes. At present, no isotopically certified reference materials exist for a large number of elements, including Cu, Zn, Mo and Cd, and it is important that this situation be rectified as soon as practicable. Before the isotopically certified reference materials become available for selected elements, suitable reference materials can be created as a standard if sufficient and reliable isotope data have been obtained by interlaboratory comparisons. For example, the Hf/ Hf isotope ratio was measured using hafnium oxide from Johnson Matthey Chemicals, JMC-475, for hafnium isotope ratio measurements with different multi-collector mass spectrometers (ICP-MS and TIMS) as summarized in Table 8.1. However, no isotope SRM is certified for the element Mo either. Mo isotope analysis is relevant, for example, for studying the isotope fractionation of molybdenum during chemical processes or the isotope variation of molybdenum in nature as the result of the predicted double (3 decay of Zr or 18.26-28 spectroscopically pure sample from Johnson Mattey Specpure is proposed as a laboratory standard reference material if sufficient and reliable isotope data are collected via an interlaboratory comparison. 

Procedures for the analysis of some organochlorines in environmental samples (e.g., chlorinated dioxins - EPA Method 1613) [68] also require the use of surrogate standards (usually 13C-labeled) and certified or standard reference materials (CRMs or SRMs). At present there are no stable isotope labeled reference materials and no reference materials have yet been certified for PCA... 

Further extension of the possibilities of the method is the introduction of isotope dilution to MS. It is applicable to elements with at least two stable isotopes or to monoisotopic elements having a long lived radioactive isotope (e.g. Th). IDA-ICP-MS has been applied to analysis of various standard reference materials sediments, waters etc. (Barnes, 1996). Analytical precision of 2% has been reached and even down to 1 % in the case of flow injection IDA-ICP-MS and determination of Cu, Cd and Pb in river water. 

Recently, LC/MS has made significant contributions in the analysis of sterols and steryl esters. Takatsu and Nishi (1993) have employed discharge-assisted LC/TS/MS for the determination of total serum cholesterol. The method incorporates stable isotope dilution using [3,4- C] cholesterol as an internal standard. [MM — H20] ions were monitored by the SIM method. Satisfactory agreement between the analytical result and the certified value of the National Institute of Standards and Technology standard reference material was obtained with a relative standard deviation of 0.6%. The method does not require sterol derivatization. Yang et al. (1992) used FAB/MS to identify cholesteryl sulphate m/z 465) as the [M — H] ion recovered from the appropriate TLC fraction. 

High precision in stable isotope mass spectrometry is achieved through repeated measurements of a reference gas, and inter-laboratory compatibility is maintained through calibration against standard reference materials. Analytical precision for most modem mass spectrometers is about 0.1%o for both 6 C and... 

Gonfiantini, R., W. Stichler, and K. Rozanski. 1995. Standards and intercomparison materials distributed by the International Atomic Energy Agency for stable isotope measurements. In Reference and Intercomparison Materials for Stable Isotopes of Light Elements. International Atomic Energy Agency, Vienna, IAEA-TECDOC-825, pp. 13-29. 

To facilitate the d values being discussed in an international forum it is necessary to cross-calibrate the data obtained from individual laboratories. This is accomplished by knowing the 6 value of each laboratory s reference material relative to an internationally accepted standard. The standards used for stable isotopic measurements are given in Table 2. In the case of carbon, where the standard is Pee Dee be-lemnite (PDB),... 

Aggarwal et al. recently implemented methods for the determination of isotope ratios and concentrations of different elements in biological specimens. Thermally stable and volatile chelates were identified for a number of trace elements Ni [43], Cr [45], Pt [46], Cu [33], Se [47], Co [48], Cd [49], Pb [44], Hg [50], and Te [51]. Memory effect was evaluated and found to be insignificant. Precision and accuracy in the measurement of isotope ratios was evaluated. Finally, the isotope dilution GC-MS method was validated by analyzing NIST reference materials (e.g., urine, semm), by comparing the results with ETAAS and by using the standard addition approach. 

Guidelines for selecting laboratory standards and a review of a strategy to have common referencing procedures between laboratories for isotopic measurements have been reported [4], Stable isotope reference materials (whether they be certified/primary standards or laboratory standards) should comply with a number of criteria. These criteria include ... 

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