Isotope dilution enriched spike

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. 

The isotope dilution method consists of mixing a natural sample with an artificial spike and measuring the isotopic ratios of the mixture using mass spectrometry, providing very precise quantitative determination of the concentrations of elements in trace quantities. A spike is a solution that contains a known concentration of the element, artificially enriched in one of its minor isotopes. For example, natural Rb samples have 27.84% and 72.16% Rb (Fig. 11.lA). Rb spikes are made by artificially enrich the minor isotope Rb. And a solution with 90% Rb and 10% Rb (Fig. 11.IB) is a Rb spike. Of course, a solution with 99.99% Rb and 0.01% Rb is a better Rb spike. When the known quantities (mass) of the sample and spike are mixed, the resulting isotopic compositions can be used to calculate the concentration of the element in the sample. 

A direct result of the ability to measure isotope ratios with ICP-MS is the technique known as isotope dilution analysis. This is done by spiking the sample to be analysed with a known concentration of an enriched isotopic standard, and the isotope ratio is measured by mass spectrometry. The observed isotope ratio (RJ of the two chosen isotopes can then be used in the isotope dilution equation (Eqn. 5.7) to calculate the concentration of the element in the sample ... 

As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

An excellent possibility for quantifying analytical data in inorganic mass spectrometry is isotope dilution analysis (IDA) based on more precise isotope ratio measurements. IDA uses highly enriched isotope spikes of analytes of known concentration for calibration and is the method of choice if a high accuracy for element concentrations is required. The principles and applications of this method will be discussed below. 

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... 

Clough and co-workers reported on the uncertainty contribution using single and double isotopi-cally enriched spikes in the isotope dilution approach employing a multiple ion collector ICP-MS combined with an HPLC and with cold vapour generation (CV) to form HPLC-CV-MC-ICP-MS for the determination of MeHg in fish tissue.8 Two fish tissue reference materials DORM-2 (NRC, Canada) and BCR 464 were employed in these studies. The amount of each certified reference... 

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... 

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... 

Isotope dilution is applicable to any element for which an enriched isotope is available. Figure 1.1 of Chapter 1 indicates which elements are amenable to isotope dilution in most cases the natural element has at least two stable isotopes, but this is not necessarily the case. For example, 232Th, though radioactive (half-life of 1.4 X 1010 years), is present in the earth s crust 230Th (half-life of 7.5 X 104 years), an isotope present in nature at such low levels as to be negligible for most applications, is used as a spike for isotope dilution purposes in the author s laboratory. Another common example is the use of 233U (a synthetic isotope) as a spike for uranium analyses. The only elements not amenable to the technique are those, like cobalt and arsenic, that have only one stable isotope and all of whose radioactive isotopes have half-lives so short as to preclude their use. 

Isotope Ratio Measurements The analysis of milk samples for the accurate determination of its constituent elements has also been done by isotope dilution analysis [34, 102, 103]. For that purpose, an enriched isotope of the element or elements to be determined is spiked to the sample, altering the natural isotopic composition of the element to be analyzed. By measuring the altered... 

In past years, on line chromatographic coupling techniques such as HPLC and CE coupled to ICP-MS with the isotope dilution technique have been used for element quantification in speciation analysis. An interesting application of the isotope dilution technique in medical research was proposed recently by Prange and co-workers, who added highly enriched " S, Cu, Zn and Cd spikes to the interface of the CE-ICP-MS system. The authors separated isoforms of metallothionein (e.g., of rabbit liver) by capillary electrophoresis and quantified S, Cd, Cu and Zn concentrations in isoforms by ICP-SFMS using the isotope dilution technique. A new selenized yeast reference material (SELM-1) for methionine, selenomethionine (SeMet) and total selenium content has also certified by an intercomparison exercise. ... 

---