Determination of Isotope Amount Ratios

Description RMs certified only for isotope amount ratios relative to a specific standard, so-called 5-values, are also isotopic reference materials, but less specified. These materials will be characterized as delta reference materials (5-RMs). 

Each ion current, lots, measured with an ICP-MS instrument is biased as a result of mass discrimination, variations in detector gain and detector efficiency, and other effects. These other effects, such as background, detector dead time effects (counting devices), interferences, and matrix effects, will not be discussed here, because they vary strongly depending on the type of mass spectrometer. Discussions on these topics can be found in Chapters 2 and 3 and in the literature [38-42]. An observed isotope amount ratio, or in other words an ion current ratio, Robs.i [Eq- (6.2)], calculated for two isotopes a and b is therefore also biased. This means that every measured or observed isotope amount ratio is biased or, to put it bluntly, these observed isotope amount ratios are wrong, unless they have been corrected for all effects mentioned above. 

Assuming that no bias from interferences or drift effects occurs and that gain and efficiency factors of the detectors [e.g., Faraday cups in multi-collector (MC)-ICP-MS] have been determined and applied correctly, the ion current will be biased only by mass discrimination. To correct for this instrumental mass discrimination, a correction factor, the so-called JC-factor, has to be determined, using Eq. (6.3)  

True values are unknowable by definition (see Section 6.2) and therefore require a replacement for practical work. The value which is closest to the true isotope amount ratio is the certified isotope amount ratio of an IRM, which in turn means that any correction for mass discrimination requires an IRM with known isotopic composition. With the ion current ratio and the certified isotope amount ratio of the IRM, Rcert,i, the fC-factor can be calculated according to Eq. (6.4)  

With the use of IRMs, these false assumptions can be avoided and traceable isotope amount ratios can be generated. At present, however, only for a limited number of elements is an IRM available, as can be seen from Tables 6.1 and 6.2. Natural-like IRMs (Table 6.1) can be used to correct for mass discrimination, whereas enriched IRMs (Table 6.2) are mainly used in tracer studies and isotope dilution experiments. The enriched IRMs offer sufficient quality, but the number of elements covered is too low. This has been discussed at several meetings with the summarized outcome that several enriched isotopes have been exhausted 

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Thermal ionization mass spectrometry (TIMS) suffers from time-dependent mass bias, referred to as mass fractionation, as a result of the finite amount of sample on the source filament and the more efficient thermal ionization of the lighter isotope. Mass bias correction is more crucial with multi-collector (MC)-ICP-MS as the latter suffers significantly larger bias and, as noted earher, it may not necessarily be constant over extended periods of time. Therefore, rigorous correction methods are required. Over the last few decades, several different mass bias correction methods have been successfully used for the determination of isotope amount ratios, as illustrated by Albarede et al. [16]. 

Similarly, this raises questions regarding the determination of isotope amount ratios of neodymium or osmium - is it adequate to use the ratio for nonradiogenic isotopes for the mass bias correction of other isotope ratios Are they indeed invariant in Nature The 2009 lUPAC report on Atomic Weights addressed this question rather explicitly by abrogating the conventional notation of Standard Atomic Weights in favor of the interval of the Standard Atomic Weights [57]. Furthermore, concerning the mass bias correction, it is now established that mass bias correction factors are not identical for all isotopes of the same element. Hence the limitations of traditional mass bias correction methods must be accepted. 

At present, the determination of isotope amount ratios is possible with reproducibilities of the order of 0.001% [47, 86], and future isotope research surely will go below this limit. This automatically means that the requirements for IRMs are of the same order or even higher. IRMs with uncertainties around 0.001% for the isotope amount ratio are currently not available. A representative selection of IRMs with uncertainties covering the existing range is presented in Table 6.4. The range... 

Internal Standard (IS) The internal standard (IS) is a compound added in a fixed, known amount to every quantitation sample to serve as an internal control for the analysis. Most commonly, the IS is used to normalize response through determination of peak area ratio as described above. The ideal IS will track with the analyte(s) through the extraction, chromatography, and mass spectrometry to account for variable recovery, minor spills, and changes in response over time. Stable-isotope versions of the analytes are ideal IS for LC-MS quantitation, but in many cases structural analogs exhibit sufficiently similar chemistry to be useful in this role (Jemal et al., 2003 Wieling, 2002 Stokvis et al., 2005). 

R results from the signals of the isotopes, hs(l) and hs(2) as a rule are the natural abundances. GA is the absolute amount of standard added, hA( 1) and hA(2) are known from the isotopic composition. Isotope dilution in ICP-MS has been applied in studies on Pb (see e.g. Ref. [519]). Also tracer experiments for Fe in biological systems (see e.g. [520]) have been described. The precision achievable in the determination of isotope ratios for abundances which differ by a factor of less than 10 is in the lower percent range. 

Ratio of stable isotopes may be measured in a mass spectrometer it is advisable to use relatively simple molecules (for example from degradation of one under study) so that the parent peaks are isolated from other possible ones. For example COj and C02 have parent masses at 44 and 45 and the only nearby peaks are those from small amounts of O and 0 in the oxygen. A complex organic molecule will normally yield many smaller peaks immediately around the parent due to normal deuterium, C and 0 and determination of isotopic enrichments is only possible in uncomplicated mass spectra. Results good to 0.1-0.2% are easily obtained but simultaneous collection of ions from the two peaks of interest using a dual collector instrument gives precision to 0.01%. 

Using appropriate ions of the natural analyte and the spike, the isotope amount ratios for the spiked sample and the spiked calibration standard are determined. It is suggested that alternating measurements of the isotope amount ratio are made on these two solutions (repeated measurement of the calibration blend allows mass bias correction to be performed for inorganic IDMS (see Section 3 10), repeating each five times. The mean value of the five measurements will minimise the effects of any instrument drift. An improved estimate of the natural analyte concentration in the sample can then be calculated from the data. 

The procedure described in Section 3.8.4 is illustrated using the determination of p,p -DDE in 2,2,4-trimethylpentane. This example shows how the sample and calibration solutions may be prepared so that the natural and isotopically labelled analogue concentrations and their isotope amount ratios are as close to being identical as possible. Additionally, to obtain high accuracy the measured isotopic ion abundance ratios should be as close to unity as possible. For the highest accuracy to be achieved, all solutions should be prepared gravimetrically except where identified below. Conventional volumetric techniques will limit the accuracy attainable by this IDMS method. The symbols used in this example should be read in conjunction with Equation 11 (Annex 3) which was used for the calculation of results. 

A recent development is species-specific IDA, whereby organometallic compounds or elemental species can be chromatographically separated, introduced into the ICP-MS instrument, and the isotope amount ratio of the elemental moiety can be determined for the purposes of IDA. An example of this is the speciation of methylmercury in environmental samples, where the sample is spiked with an analog of methylmercury enriched in The resulting... 

This book is not intended to cover important branches of mass spectrometry that provide accurate and precise quantitative measurements of relative concentrations, e.g., of variations in isotopic ratios of an element by isotope ratio mass spectrometry (IRMS) and accelerator mass spectrometry (AMS). Rather, this book is mainly concerned with determinations of absolute amount of substance (see Chapter 1 for a definition and explanation), particularly for compounds present at trace levels in complex matrices. (The only exception is the inclusion of a brief description of methods used to determine differences in levels of proteins in living cells or organisms subjected to different stimuli, e.g., disease state vs normal state). 

A stock solution was prepared by dissolving the enriched isotope. Diluted solutions were prepared from this stock solution on a weight basis. The isotopic composition of this solution was determined experimentally by GC-MS analysis of the chelate. The internal standard solution was calibrated by reverse isotope dilution GC-MS using the atomic absorption primary standard. Weighed amounts of the primary standard solution were mixed with weighed amounts of the enriched isotope solution. Chelates were prepared from the spiked samples (as described below) and were used for mass spectrometric determination of isotope ratios. Concentration of the trace metal in the spike solution was calculated using the experimentally determined isotope ratios, the weights of the standard and... 

In addition to the apphcations mentioned above, isotopic variations can in general be relied on to assess whether two samples are identical or of the same origin or not. However, it is only possible to determine indisputably that they are not identical, that is, when obtaining significantly different values for the isotope amount ratios - often referred to in short as isotope ratios - for the two samples. If both samples show the same isotope amount ratio(s), one can only assume that they are identical with a certain probability, because they could have been affected... 

These are rather general requirements and need to be specified further (see below). Isotope amount ratios are used for different applications, such as the determination of elemental contents, determination of isotopic variations for the interpretation of specific incidents and processes in bio- or geochemistry, and characterization of materials. Interference corrections are also based on isotope amount ratios calculated from the lUPAC-tabulated isotope amount fractions [57]. 

Single-collector instruments also prove very usefid for mass content determinations via isotope dilution, as carefiil estimation of all quantities that influence the uncertainty budget demonstrates that the precision on the isotope amount ratio is typically not the dominant factor for high-precision measurements. Often, the accuracy of a mass content measurement will hardly improve through the use of MC-ICP-MS instruments as other influence quantities, such as uncontrolled spectral interferences and sample inhomogeneities, typically deserve more attention. 

Mass spectrometry is one of the most versatile and powerful tools in chemical analysis because of its capacity to discriminate between atoms of different masses. When a sample containing a mixture of isotopes is introduced into a mass spectrometer, the ratio of the p>eaks observed reflects the ratio of the p>er-cent isotopic abimdances. This ratio provides an internal standard from which the amount of a certain isotope present in a sample can be determined. This is accomplished by deliberately introducing a known quantity of a particular isotope into the sample to be analyzed. A comparison of the new isotope ratio to the first ratio allows the determination of the amount of the isotope present in the original sample. 

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... 

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