Spike isotopic enrichment

Figure 8.6 Error multiplication factors for IDMS determination of the polyisotopic elements boron, chlorine, and bromine and of the monoisotopic element iodine for specified spike isotope enrichments as a function of the analyte/spike atomic ratio.

Isotope dilution mass spectrometry (IDMS) can be applied with most of the ionisation methods used in mass spectrometry to determine isotope ratios with greater or lesser accuracy. For calibration by means of isotope dilution, an exactly known amount of a spike solution, enriched in an isotope of the element(s) to be determined, is added to an exactly known amount of sample. After isotopic equilibration, the isotope ratio for the mixture is determined mass spectrometrically. The attraction of IDMS is its potential simplicity it relies only on the measurement of ratios. The... 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

A single spike is enriched in one minor isotope whereas a double spike is enriched in two minor isotopes (Fig. 11.3). Double spike method needs two runs. The first run is to measure the un-spiked sample and the second run is to measure the spiked mixture. Double spike method not only provides the concentrations of the sample but also corrects the mass fractionation in mass spectrometers (and thus giving the true isotopic ratios). 

For application in the isotope dilution technique and for tracer experiments using isotope enriched spikes, Merck (Darmstadt, Germany) have launched the production of new sets of isotopically enriched materials, such as 53Cr or mCd enriched spike calibration solutions, which were characterized by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium).52... 

Figure 8.3 Transient signals of 63 Cu and 65Cu in protein spots of Alzheimer s brain samples measured by LA-ICP-MS (isotopic-enriched 65Cu spike solution was doped to the gel after 2D gel electrophoresis). (J. Su. Becker et al, Int. ). Mass Spectrom., 242, 135 (2005). Reproduced by permission of The Royal Society of Chemistry.)...

The principle of isotope dilution analysis is surprisingly simple. It relies on the intentional alteration of the isotope abundance of an endogenous element in a given sample by the addition of a known amount of an enriched isotope of the same element (spike). Therefore, the element to be analysed must have, at least, two stable isotopes that can be measured free of spectral interferences in a mass spectrometer. This principle is illustrated in Figure 1.11 for an element containing two different isotopes, a and b. As can be observed, the a isotope is the most abundant one in the sample whereas the spike is isotopically enriched in the b isotope. It is clear that the abundances of the two isotopes... 

For low metal concentrations, any suitable extractant may be used to concentrate the metal after equilibration with the enriched spike. High extraction efficiencies are not required since at this point the analysis depends on establishing a ratio between the enriched spike isotope and one of the major isotopes of the metal being sought. 

Figure 2 shows a mass spectrum of cadmium spiked with enriched 106Cd. The solid line at position 106 represents the 106Cd spike, and the dashed lines represent the relative abundance for the other cadmium isotopes. The dashed line at juxtaposition at 106 is the relative abundance of 106Cd as it occurs in nature. Table I shows the IBM 1130 computer-programmed output for a typical isotope dilution analysis. The program... 

Table 4 Bias Corrected Isotope Ratio Measurements in the Unspiked Sample, Isotopically Enriched Spike, and Spiked Sample for Three Oil Leachate Samples as Determined by Glow Discharge Mass Spectroscopy Solution Residue Method...

A combined use of single and multiple isotope enriched spike solution for the detection of butyl tin compounds in sediments by GC-ICP-MS has been evaluated in order to validate an analytical procedure proposed for routine determination." A mixture of mono-, di- and tributyl tin (MET, DBT and TBT, respectively) eiuiched in Sn and a triple spike solution containing each butyl tin species eiuiched with a different tin isotopes ( Sn and Sn) were employed to develop the analytical technique. This demoustrated the advautage provided by the combined use of single and multiple spike solutious to validate a methodology for the determination of butyl tin content by GC-ICP-MS after a quantitative solid-Uquid extraction procedure for species in unknown sediment." ... 

Isotope ratio measurements were employed for the quantification of analytical data using the isotope dilution strategy. For example, isotope dilution analysis was developed by Sanz-Medel s group for the determination of selenomethionine in Se enriched yeast material by HPLC-ICP-MS using a Se-enriched selenomethionine spike obtained by growing yeast on a Se rich culture medium. For Cr(III)/Cr(VI) determination in yeast, Caruso et al employed the double spike species specific isotope dilution technique measured by HPLC-ICP-MS. The isotope pattern deconvolution approach applied in this work delivers a more intuitive and elegant solution to an otherwise complex data analysis without the need for iterative calculations as widely practised in double spike isotope dilution. The results are in exact agreement with the conventional isotope dilution calculations. ... 

A conventional mass spectrometer was used to measure ion abtmdance ratios of the diligand fragments [Fe(6511702)2] which were formed during electron-impact ionization. Sample isotopic enrichment levels were obtained from standard curves that related ion abundance ratios to enrichment levels. Tracer concentration was calculated from the values for total iron content and enrichment level. The relative standard deviation for the ion abundance measurement was less than 2%. Recovery of tracers from spiked fecal samples ranged from 90% to 104%. The method was used to analyze samples collected from a human study. Iron availability from breakfast meals was determined in 6 yo mg women by giving 7 mg of in apple juice on one... 

Performing chemistry with a very low bias to determine the exact amount of spike Ny would, however, be very difficult and require a large (and hence costly) amount of isotopically enriched material. The solution to this can be another isotope dilution i.e. assaying the spike Y (in solution for example) with accurately assayed material Z of... 

Spike material has to be an isotopically enriched material, is hence available to limited amounts only and/or can be expensive. Reverse isotope dilution can be used to perform its assay because it can be performed on minute amounts or very small concentrations (ug.mr ). 

The analyte isotope in organic IDMS is usually C, H or N. Any of the inorganic isotopes can be used as the analyte isotope in inorganic IDMS. The choice will depend on considerations such as which isotope is available as the enriched isotopic analogue (this is used as the spike isotope) and the detection limit required (which limits the use of lower abundance isotopes). 

Figure 4 The three main components in the IDMS procedure (a) natural standard, (b) sample and (c) isotopically enriched spike...

This step is required if the spike solution of the isotopically enriched analogue only has an indicative concentration value. The spike solution is blended with a gravimetrically prepared solution of the pure reference isotope of known concentration. The concentration of the enriched isotope in the spike solution can then be calculated from a first reverse IDMS run of this blended solution. The revised value of the concentration can then be used to prepare a second blended solution. The first blend can be used as a mass bias standard, with the revised value for the concentration, and run alternately with the second blended spike solution. This second iteration should give an acceptable value for the spike concentration but if necessary a third iteration can be carried out. The run sequence for characterising the second blend is shown in Figure 6. 

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