Spike isotopic

Encinar JR, Rodriguez-Gonzalez P, Fernandez JR, Alonso JIG, Diez S, Bayona JM, Sanz-Medel A. Evaluation of accelerated solvent extraction for butyltin speciation in PACS-2 CRM using double-spike isotope dilution-GC/ICPMS. Anal. Chem. 2002 74 5237-5242. 

In principle, double-spike techniques represent the most suitable approach to determine the isotope composition of elements with four isotopes or more (Fe, Zn). In most cases, these techniques involve the addition of an isotope which is usually minor in natural samples, such as Zn or Fe, implying that the risk introduced by memory effects on these spike isotopes must be carefully weighed against the added gain in precision from using the double spike. Such a risk is clearly more present with MC-ICP-MS than with TIMS. 

The limitations discussed above also apply approximately to measurements of mass dependent Ca isotope effects. The additional problem is to separate mass dependent fractionation in nature from mass dependent fractionation in the mass spectrometer. The maximum observed natural fractionation is about +0.1% per mass unit, whereas instrumental fractionation is about +0.5% per mass unit (for TIMS and much larger for ICPMS). The separation is accomplished with the use of a double spike (Russell et al. 1978b). The approach is illustrated here using the methods of Skulan et al. (1997), but other researchers have used slightly different algorithms and double spike isotopes (Zhu and MacDougall 1998 Heuser et al. 2002 Schmitt et al. 2003a). 

A double spike technique is essential for TIMS analyses of Se and Cr, and may also be useful in MC-ICP-MS analysis. Briefly, two spike isotopes with a known ratio are added to each sample, and the measured ratio of the spike isotopes is used to determine and correct for instrumental bias. Examples of Se and Cr double spikes currently in use are given in Table 1. The fact that small amounts of the spike isotopes are present in the samples and small amormts of nominally unspiked isotopes are found in the spikes is not a problem, as the measurements allow highly precise mathematical separation of spike from samples. Algorithms for such calculations are described by Albarede and Beard (2004) and, specifically for Se, by Johnson etal. (1999). 

Isotopic double spike. The most rigorous approach is to use an isotopic double spike , in which samples are doped with a known quantity of spike Mo which consists of two isotopes in a known ratio (Wetherill 1964 Siebert et al. 2001). These spike isotopes serve as an internal standard to monitor mass fractionation of the sample subsequent to spiking. The fundamental advantage over the element spike is that the spike isotopes follow exactly the same fractionation behavior as the isotopes of interest. A disadvantage is the need to carefully prepare and calibrate the double spike. 

The best precision is obtained for isotope ratios near unity (unless the element to be determined is near the detection limit, when the ratio of spike isotope to natural isotope should be between 3 and 10) so that noise contributes only to the uncertainty of natural isotope measurement. Errors also become large when the isotope ratio in the spiked sample approaches the ratio of the isotopes in the spike (overspiking), or the ratio of the isotopes in the sample (underspiking), the two situations being illustrated in Fig. 5.11. The accuracy and precision of the isotope dilution analysis ultimately depend on the accuracy and precision of the isotope ratio measurement, so all the precautions that apply to isotope ratio analysis also apply in this case. 

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. 

Note that isotope i is usually that of high abundance in the sample and k the isotope of high abundance in the spike, but using the reverse definition is algebraically equivalent. Note that the equation simplifies if the spike isotope is not present in the sample. 

In the laboratory, the isotope dilution procedure involves adding a known amount of spike of known isotopic composition to a known amount of sample of known isotopic composition the mixture of spike and sample is equilibrated the ratio of the sample isotope to the spike isotope is then measured and the resulting R is inserted into the equation. For replicate analyses, this is the only parameter... 

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. ... 

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). 

This factor can be calculated for each specific combination of the natural (analyte) isotope and the enriched (spike) isotope abundances for the element of interest. The description of error propagation plots (see below) includes a number of examples showing how this factor varies as a function of the isotope amount ratios in the spiked sample. The isotope proportions in the unmixed natural and isotopically enriched materials heavily influence the shape of these plots. Thus for... 

A number of error propagation plots have been calculated to illustrate how the error propagation factor may be minimised by adjustment of the amounts of the analyte and spike isotopes. Details of the chosen isotope systems and their lUPAC abundances are given in Table 4 with the spike isotope listed first). 

The detection limit of a linear calibration is defined as three times the standard uncertainty of the concentration of the blank. This definition is, however, not an ideal representation of the detection limit in ID-MS. A formulation for the detection limit for ID-MS is available [45], where the ID-MS detection limit is described as a function of the enrichment of the isotopic spike and of the uncertainties in the measurement of the spiked isotope. It states that when the spike is not enriched isotopically, the detection limit is infinite and unusable. When the spike is enriched in either isotope, the ID-MS measurement uncertainties approach the linear calibration detection limit. 

In ID, the natural isotopic abundance ratio of Cd is altered in the sample by spiking it with an exact and known amount of Cd-emiched isotope (the so-called spike , with a different isotopic abvmdance ratio than natural cadmium). The reference isotope is usually the isotope of highest natural abundance ( " Cd), while the spike isotope is one of the lesser abundant natural isotopes (normally Cd, Cd, or Cd). As a result of the spiking process, the measurement by ICP-MS of the new isotope ratio (e.g., " Cd/ Cd) and its comparison with the natural isotope ratio offers the original Cd concentration in the sample. If the isotope dilution is performed online in an LC- or CZE-ICP-MS experiment, quantification of Cd in each of the isolated species can be accurately achieved by integration of each chromatographic/electrophore-tic peak after transformation of the data into mass flow by means of the ID equation. 

In ID-MS a spike solution, containing the isotopi-cally enriched analog of the analyte, is added to the sample containing the natural abundance analyte. In order to measure the isotope ratio two isotopes (for inorganic ID-MS) or isotopomers (for organic ID-MS) are chosen, the sample (A) and the spike (B). The sample isotope, or isotopomer, is usually most abundant in the sample and the spike isotope, or isotopomer, is most abundant in the spike solution. 

It should be noted here that there are some fundamental differences between organic and inorganic ID-MS. In organic ID-MS, the isotopically enriched analog is an organic molecule that has been labeled with (usually) either H, or on a nonlabile site. In order to prevent interference from low abundance isotopomers multiple labeling is usually performed so that the mass of the labeled spike differs from the analyte by at least 3 mass units. The spike isotopes have extremely low abundances in nature (Table 1), so the labeled spike isotopomer is of extremely low abundance in the sample. Conversely,... 

In inorganic IDMS, the isotopically enriched analog is not an array of individual labeled molecules, but rather a mixture of individual isotopes that contribute directly to the enrichment of the array, but in different proportion to the sample (Table 3 and Figure 1). Ideally, the sample isotope should be of low abundance in the spike, and the spike isotope should be of low abundance in the sample, but this is very often not the case (Table 3). The isotope ratio is measured by ratioing the signal strengths of the spike and sample isotopes at their corresponding masses. 

The sample containing the analyte is spiked with the isotopically enriched analog. The sample isotope (A) and the spike isotope (B) are ratioed to give an isotope amount ratio (Rb) in the sample/spike blend. This is a ratio of the number of moles of isotope A to the number of moles of isotope B, originating from both the sample and spike, and is given by eqn [1] ... 

By using the terminology in eqn [2], where the subscript y denotes the enriched spike material and the subscript x denotes the natural isotopic abundance analyte in the sample, the isotope amount fractions can be expressed in the form of eqn [4]. If the spike isotope (B) is used as the reference isotope then the isotope amount fraction of isotope A to isotope B in the spike (Aspite) can be expressed as (eqn [5])... 

Thus, the isotope amount ratio, R, of the sample to spike isotopes in the sample/spike blend can also be written as... 

A further simplification can be made if the spike isotope is of extremely low natural abvmdance and there is very little of the spike isotopomer in the sample. In this case, R tends toward infinity, so eqn [13] can be simplified to... 

For inorganic ID-MS, error propagation plots can be used to calculate the optimum analyte-spike isotope amount ratio for the minimization of errors during the measurement of the isotope amount ratio. This ratio can be calculated for a particular isotope pair using eqn [16], examples of which are shown in Figure 2. In practice, the isotope amount ratio should lie between 1 4 and 4 1 to give a reasonable signal for each isotope, and it is probably simplest to use an isotope amount ratio as close to unity as possible ... 

Once the sample has been spiked, equilibrated, and prepared in a way suitable for introduction into the instrument, the sample-spike isotope amovmt ratio (A B) must be measured. The observed isotope amount ratio must be corrected for the effects of spectroscopic interferences, mass fractionation in the sample introduction system and/or ion source, and mass bias in the mass spectrometer and associated ion optics. 

Even when the sample contains as an analyte, the use of as a tracer is not precluded. In this case the techniques of isotope dilution are called a spiked/unspiked analysis. The sample is subdivided quantitatively into two or more aliquots. One of these aliquots is traced with a known amount of (the spiked fraction), while a second aliquot is un traced (the unspiked fraction) since the results of the analysis of the unspiked fraction enter the determination only through an isotope ratio, the aliquot need not be quantitative. A purified uranium fraction is recovered from each sample, and they are then analyzed by mass spectrometry. The unspiked isotope ratio, ( U/ U)atom,u> the spiked isotope ratio (238u/233u) om.S, combined with the mass of in the tracer aliquot, M, can be used to determine the mass of in the spiked aliquot, M238 ... 

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