Isotope ratio data, standardization

The technique of internal normalization is commonly apphed in both MC-ICP-MS and TIMS for the precise correction of the instrumental mass bias (see also Chapter 5) that is encountered during the analysis of radiogenic isotopic compositions [33, 34]. The ICP ion source of MC-ICP-MS, however, also features two characteristics that play an important role for isotopic analysis, where internal normalization cannot be applied. First, an ICP source operates at steady state and therefore mass fractionation is not primarily a time-dependent process, as in TIMS where the measured isotopic compositions change with time due to the progressive evaporation of a sample from the filament. The steady-state operation of an ICP ion source is beneficial for the correction of instrumental mass bias by external standardization, where the isotope ratio data obtained for a sample are referenced to the values obtained for bracketing analyses of an isotopic standard [27, 35]. Hence, this procedure is commonly termed standard-sample bracketing. 

The steady-state operation of the ICP source is beneficial for the correction of instrumental mass bias by standard-sample bracketing, where the raw (measured and uncorrected) isotope ratio data of a sample are referenced to the results obtained for an isotopic standard, which is preferentially analyzed before and after each sample [27, 35]. This technique is similar to the standardization method commonly used in gas source isotope ratio mass spectrometry. To account best for drifts in instrumental mass bias, which can be particularly severe for light elements such as Li and B, data collection often utilizes multiple but short analytical measurements for samples that are each bracketed by standard analyses. Switching between samples and standards can be very rapid, if long washout protocols are not required, and mass spectrometric measurements of about 5 min or less have been used to optimize the precision of Li and Mg isotope ratio measurements by MC-ICP-MS [111, 112]. 

Figure 10.10 Comparison of Cd isotope ratio data obtained using different normalization procedures for repeated measurements of two terrestrial rocks samples and the Allende CVS chondrite (from [38]). (a) Comparison of results obtained by standard-sample bracketing and by external normalization (based on added Ag) using the empirical method of Marechal et al. [37], The bracketing approach yields inaccurate results for some samples, due to matrix-induced changes in instrumental mass bias, (b) Comparison of results obtained by external normalization using Ag using either the exponential law in combination with standard-sample bracketing or the empirical technique of Marechal et al. [37]. Both methods yield similar results for all samples. The isotopic data are shown as eCd/amu values, which denote the variation in Cd isotopic composition relative to a terrestrial standard and normalized to a mass difference of lu [38, 113].

Figure 10.12 Diagram comparing Cd and Zn isotope compositions for different classes of carbonaceous and ordinary chondrites. The Cd and Zn isotope ratio data are shown in 8-notation to denote isotopic variation in Cd/ °Cd and Zn/ Zn, respectively, relative to a terrestrial standard reference material in permil. For both Cd and Zn, pronounced isotope fractionation is apparent for the ordinary chondrites (and particularly unequilibrated ordinary chondrites of petrological type 3), whereas such effects are much more limited for the carbonaceous chondrites. This suggests that nebular processes only generate small Zn and Cd isotope fractionations, whereas large isotope effects are produced as a result of partial evaporation/condensation during parent body metamorphism [134]. Data from [38, 105, 127, 134).

Usually, 10 to 20 measurements are made of the isotope ratio for one substance. Sometimes, one or more of these measurements appears to be sufficiently different from the mean value that the question arises as to whether or not it should be included in the set at all. Several statistical criteria are available for reaching an objective assessment of the reliability of the apparently rogue result (Figure 48.10). Such odd results are often called outliers, and ignoring them gives a more precise mean value (lower standard deviation). It is not advisable to remove such data more than once in any one set of measurements. 

Natural abundance data are nearly always reported as delta values, 5 in units of per mil ( mil = 1000), written %o. This is a relative measurement made against a laboratory s own reference material, a working standard , calibrated against an international standard. Delta values are calculated from measured isotope ratio as ... 

Also, since ratios of isotopic ratios are commonly used to report isotope effect data (see Equations 7.17 and 7.18 for example), the actual numerical value of the abundance ratio of the standard is not important since it drops out from the final equation expressing the isotope effect ... 

The vast majority of isotopic data are reported in delta notation , where the isotopic composition is cast as the deviation of an isotopic ratio relative to the same ratio in a standard (e.g., O Neil 1986a) ... 

The Ca isotope ratios of meteoritic samples are of interest because they can give information on early solar system processes and because meteorites represent the materials from which the Earth accreted and hence relate to the expected values for the bulk Earth. Russell et al. (1978b) made the first measurements of stable Ca isotope variations in meteorites. They formd variations of about +l%o for the Ca/ Ca ratio in samples from six different meteorites. Although some of these samples were spiked after having separated the Ca with an ion exchange column and hence may contain artifacts, it is clear from their data that bulk meteorites have some variability in 8 Ca and that the average value is quite close to the terrestrial standard. No data on bulk meteorites have been reported since the Russell et al. (1978b) measurements, and since their one measurement of an ordinary chondrite had a poor Ca column yield, there exist no reliable measurements that can be used to verify the composition of typical chondritic meteorites. 

Note fiiat many labs analyze at least fiiree isotopes, and many data tables typically report both Fe/ Fe and Fe/ Fe isotope ratios. Conversion between igneous rock baselines and IRMM-014 baselines can be done using file IRMM-014 standard. 

The results can be reported in the conventional delta notation of stable isotopes, which is the relative deviation in parts per one thousand of a given isotopic ratio in a sample with respect to the same ratio in a standard sample. Reporting results in the epsilon notation (in parts per 10,000 as in Zhu et al. 2000, 2002) may seem legitimate, but so far stable isotope data have overwhelmingly been reported in per mil and competing notations are a source of confusion. 

Fitzsimons et al. (2000) have reviewed the factors that influence the precision of SIMS stable isotope data. All sample analyses must be calibrated for instrumental mass fractionation using SIMS analyses of a standard material. Under favorable circumstances, precision can reach a few tenths of a per mill. The latest version of ion-microprobe is the Cameca-lMS-1280 type, allowing further reduction in sample and spot size and achieving precise analysis of isotope ratios at the 0. %o level (Page et al. 2007). 

Schoenberg et al. (2008) presented the first set of Cr isotope data for rocks and Cr(ll) rich ores. Mantle derived rocks and chromite ores from layered intrusions have a uniform Cr/ Cr isotope ratio very close to the certified Cr standard NIST SRM 979. The Cr isotope composition of hydrothermal lead chromates is substantially heavier (S Cr from 0.6 to 1.0%o) than the rocks from which the chromium was leached. 

A gas chromatograph coupled to a MC-ICP-MS for the precise determination of isotope ratios as part of the speciation application has been described, for example, for the elements S, Pb, Hg and Sb.2 Transient signals of sulfur isotope ratios (32S/34S) have been measured in an isotopic gas standard (PIGS 2010, IRMM) to determine SF6 using GC-MC-ICP-MS (Isoprobe, Micromass, UK) with a hexapole collision cell.42 For data evaluation of chromatographic peaks, peak integration limits were defined by the determination of a uniform isotope ratio zone inside... 

Furthermore, isotope standard reference materials are required in different chemical forms for a multitude of elements in order to calibrate and evaluate isotope ratio measurements by mass spectrometry. In addition, reliable analytical data required in science and routine work will only be obtained by improving the quality assurance of the measurement procedures and also using interlaboratory comparisons of analytical results in round robin tests. 

Work by Catanzaro et al. in 1968 (6) led to a new analytical procedure permitting the measurement of isotopic ratios to about 0.05% (95% L.E.) this resulted in the availability of three standard reference materials, so that results could be placed on an absolute basis. This procedure, still the most precise and accurate one available, requires about 1 mg of lead for an analysis. A second procedure (7) has been developed which utilizes silica gel as an ionization enhancer. This method permits the measurement of isotopic ratios to about 0.1% (95% L.E.), but it requires only 0.1 /xg of lead per analysis. In addition, the instrumentation and data handling have been vastly improved so that many samples can be studied quickly and conveniently. 

The results of the second interlaboratory study of PCN analytical methods using environmental matrices, undertaken by the US National Institute of Standards and Technology (NIST), should provide an indication of the comparability of published environmental PCN data and show where additional method enhancements are needed. Further method development efforts in the analysis of PCNs, and other complex mixtures are likely to focus on improving efficiencies by optimizing run times and separation. Examples may include time-of-flight mass spectrometry and multidimensional GC. New methods, such as isotope ratio mass spectrometry, may contribute to further source apportionment of complex mixtures [88]. 

In the isotope dilution method, labeled analogs of analytes are first added onto the standard solutions of the analytes as well as to the samples/sample extracts. Before the analysis, a calibration curve is prepared plotting relative response (RR) against concentrations using a minimum of five data points. Relative response of a compound is measured from the isotope ratios of the compound and its labeled analog as follows ... 

The practical performance of a calibration thus is identical in all four cases of ion interference. One determines the isotope ratio mu/m+ "for a number of standards with a known mole ratio X/Y and performs a linear regression on these data. If there is interference by the unlabeled compound, the inverse mole ratio Y/X is used as a variable and if both unlabeled and labeled compound interfere, Y/(X + IY) is used as variable. In the latter case, the introduction of the experimental factor l, however, will be an important source of error propagation, especially if qj p. (Jonckheere, 1982 Jonck-heere et al., 1982). 

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