Isotope amount ratio determination

Single-Collector Versus Multi-Collector ICP-MS It was the combination of more than one detector with sector field ICP-MS instrumentation that allowed 

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 Bias Correction and Drift Effects Instrumental mass discrimination and drift effects are two influence quantities having a major impact on the uncertainty budget if not properly taken into account. Instrumental mass bias affecting the isotope amount ratio result is considerably larger for ICP-MS than for TIMS. This in turn calls for careful validation of the mass bias correction by applying and comparing the effects of different correction procedures (see below). 

Changes in the cone surface condition during a series of measurements can cause drift effects either through shift in the mass calibration or from changes in the signal intensities. A decrease in intensity will decrease the precision of the isotope amount ratio and possibly increase the uncertainty of the mass bias correction factor. 

In any case, it is of paramount importance to separate the analyte from the matrix during sample preparation, as molecular and isobaric interferences can often be avoided in this way, mass bias becomes more stable and similar for samples and standards, and drift effects during ICP-MS analysis can be mitigated. 

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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... 

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. 

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]. 

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... 

The observed calcium/phosphate ratio of 4.5 at the intercept of the calcium and phosphate retention curves that should minimize the sum of the urine calcium plus urine phosphate losses was difficult to believe in view of both the known Ca/P ratio of bone and the amounts we were adding to these solutions. This disparity between the optimal ratio determined experimentally and what we had assumed this ratio should be on the basis of known body composition is partially reconciled by the experiment of Sutton and Barltrop. They fed preterm infants stable Ca46 and observed that up to 20% of the isotope absorbed was subsequently excreted in the stool. Our infants also were undoubtedly having unmeasured calcium losses from the bile, pancreatic juice and succus entericus secreted into their intestine... 

Finally the temperature dependence of the primary isotope effects was determined. Here the traditional expectations of Chart 3 were fully met the results translate into AH/AD = 1.1 0.1, aD — aH = 0.8 kcal/mol. Thus the amount of tunneling present, adequate to produce the observed exaltation of secondary isotope effects, violations of the Swain-Schaad relationship, and violations of the Rule of the Geometric Mean in the neighborhood of room temperature, does not lead to anomalies in either the ratio of isotopic pre-exponential factors nor the isotopic activation energy difference over the temperature range studied (approximately 0-40 °C). As will be seen later, the temperature dependence of isotope effects for reactions that include tunneling is in general a complex, unresolved issue. 

If the isotope abundance (isotope amount fraction) of the sample is not known this must be determined experimentally via an isotopic abundance run. Otherwise, the only information required is the weight of the enriched isotopic analogue added (and hence the elemental concentration of the spike solution), the weight of the sample and the measured ratios. Modem analytical balances are capable of routinely weighing to 1 part in 50 000, so that errors arising from the weights of the sample and spike should be minimal unless very small aliquots are taken. 

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