Multiple isotope dilution analysis

P. Rodriguez-Gonzalez, M. Monperrus, J. I. Garcia Alonso, D. Amouroux and O. F. X. Donard, Comparison of different numerical approaches for multiple spiking species-specific isotope dilution analysis exemplified by the determination of butyltin species in sediments, J. Anal. At. Spectrom., 22, 2007, 1373-1382. 

Uncontrolled species transformations during analysis form another source of error. For methylmercury determinations in sediments it was demonstrated that errors of up to 80% resulted from the formation of the compound from inorganic mercury during separation and analysis [28, 29], For the study of possible species transformations during analysis multiple isotope dilution could be used as a diagnostic tool for identifying the error and bias inherent in specific methods of storage, sample preparation and measurement [30, 31]. 

As discussed in Chapter 2, many elements have multiple stable isotopes of varying abundance. The abundances for stable isotopes of the elements are listed in Appendix 2. The ratio of two stable isotopes of a given element can be determined by independently measuring their ion currents.The isotope ratio is then computed by dividing one m/z isotope ion current by the other m/z isotope ion current. If no isobaric mass spectral interferences are present, and the sample has a natural isotopic abundance for the element in question (i.e., no fractionation or radiogenic processes have affected the abundances), the calculated ratio should be close to the theoretical value computed from the Appendix 2 table entries. This ability to perform isotope ratio measurements provides an opportunity to carry out isotope dilution analysis. 

Most mass spectrometers measure one m/z value at a time. A single channel ion detector is used for these instruments, either an electron multiplier or a Faraday cup. TOF, ion trap, and FTICR mass spectrometers have the ability to extract ions with many m/z values simultaneously, so simultaneous detection of these ions is desirable. One approach to multiple ion detection has been to use multiple detectors. Multiple detectors are also used for high-resolution magnetic sector MS instruments designed for very precise isotope ratio determination and for quantitative analysis using isotope dilution. Instruments with... 

Isotope dilution mass spectrometry (cf. (Heumann, 1992 Yu et al., 2002)) has two main requirements. The first is that the element being analyzed must have more than one isotope. The second is to have a well-characterized and pure tracer solution that has a significantly different isotopic composition from the element under analysis. In practice, a known amount of the tracer is added to the sample, which is then treated by any necessary chemical separations before being inserted into the mass spectrometer. The tracer must be isotopically equilibrated with the sample by forcing them into a common valence state, as discussed in Section 4.7. For elements with multiple valence states (such as uranium or plutonium) this is a crucial requirement. Failure to achieve isotopic equilibration will lead to erroneous results. Sample quantitation by isotope dilution can be determined by use of the following general equation ... 

Quantitative determinations will be precise only if full corrections for losses occurring during extractions and possible detector instabilities are made. The mass spectrometer measuring the mass of molecules, the use of auxins labeled with stable isotopes provides almost ideal IS for accurate quantifications. The stable isotope dilution method [22] consists in measuring by Multiple Ion Monitoring (MIM) the ratio of normal to heavy isotopes added at the beginning of the analysis. The absolute amount of auxin in plant extract is then obtained [see 19]. 

Speciation Analysis Simultaneous Determination of Multiple Species of the Same Element At the beginning of the development of species-specific isotope dilution, only a single spike compound was used [77]. Later, species-spedfic IGP-IDMS has also been used for the determination of more than one spedes of the same element, using different spike spedes labeled with the same isotope. Examples include the determination of mono- and dimethylmercury (MMM and DMM) and of the three butyltin compounds TBT, DBT, and MBT [80-83]. Various spike isotopes, usually of medium natural abundance, such as Hg or ° Hg and Sn, Sn, or Sn, have been applied. The reason for selecting this type of spike... 

IEC continues to have numerous applications to the detection and quantification of various inorganic ions.1 1 This is particularly true in water analysis.5-14 Inorganic ions in a variety of other sample types, such as food and beverages,1518 rocks,19-23 biological fluids, (blood, urine, etc.),24-31 pharmaceutical substances,32 33 concentrated acids,34 alcohols,35 and cleanroom air36 have also been analyzed by IEC. IEC has also been employed in isotopic separation of ions,37 including the production of radioisotopes for therapeutic purposes.3839 Typical IEC sample matrices are complex, and may contain substances that interfere with measurement of the ion(s) of interest. The low detection limits required for many IEC separations demand simple extraction procedures and small volumes to avoid over-dilution. Careful choice and manipulation of the eluent(s) may be needed to achieve the desired specificity, especially when multiple ions are to be determined in a single sample. 

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