Mass discrimination correction procedures

There are two different procedures to correct for mass discrimination, namely external correction and internal correction. External correction is based on the use of a matrix-matched standard of known isotopic composition [98,103], whereas internal correction is usuily based on the monitoring of an isotope ratio for another element (present in the sample or added) of very similar mass and known isotopic composition [36] for instance, an element with (practically) constant isotopic composition in nature. In the case of Pb isotopic analysis, this element is typically Tl. In some cases, another isotope ratio of the target element itself can... 

In all these applications, isotope ratio data are produced, which are interpreted on an absolute or relative basis and which have an impact on our daily life, whether this is in science (e.g., age of an artifact), in society (e.g., provenance of food), or in public safety (e.g., neutron shielding in nuclear power plants). To ensure that these data are reliable and accurate, some specific requirements have to be fulfilled. The main requirement is that all these measurement results are comparable, which means that the corresponding results can be compared and differences between the measurement results can be used to draw further conclusions. This is only possible if the measurement results are traceable to the same reference [25]. This in turn can only be realized by applying isotopic reference materials (IRMs) for correction for bias and for validation of the analytical procedure. Whereas in earlier days only experts in mass spectrometry were able to deliver reproducible isotope ratio data, nowadays many laboratories, some of which may even have never been involved with mass spectrometry before, produce isotope ratio data using inductively coupled plasma mass spectrometry (ICP-MS). Especially for such users, IRMs are indispensable to permit proper method validation and reliable results. The rapid development and the broad availability of ICP-MS instrumentation have also led to an expansion of the research area and new elements are under investigation for their isotopic variations. In this context, all users require IRMs to correct for instrumental mass discrimination or at least to allow isotope ratio data to be related to a commonly accepted basis. 

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). 

External normalization is the most commonly used procedure for correction for instrumental mass bias in isotopic analysis by MC-ICP-MS [27, 35]. This method makes use of the observation that the mass discrimination associated with plasma source mass spectrometry is, to first order, primarily a function of mass, such that elements with similar masses display a nearly identical mass bias. Using a solution containing a mixture of two such elements, the mass discrimination observed for an admixed element of known isotopic composition can be used to determine the unknown isotopic composition of the second (analyte) element [27, 35]. 

In concluding this section, it is pertinent to take note of a special kind of isotopic fractionation ubiquitous, often quite severe, and arguably the most important source of fractionation that must be taken into consideration in noble gas geochemistry. This fractionation arises in mass spectrometric analysis contributory effects can and do arise in gas extraction and transport through the vacuum system, in the ion source (especially when a source magnet is used), in beam transmission, and in ion collection and detection (especially when an electron multiplier is used). As noted in Section 1.3, sample data are corrected for instrumental (and procedural) discrimination, which is calibrated by analysis of some standard gas (usually air). This is a roundabout and imperfect near-equivalent to the 8 value convention, which is the norm in stable isotope geochemistry (O, C, H, S, N, etc.). The reproducibility of instrumental discrimination inferred from repeated calibration analysis is usually quite satisfactory, but seldom is any care taken to try to match operating conditions in samples and calibration analyses. It is thus a matter of faith - undoubtedly quite... 

In the mass spectrometric method it is usually most convenient to convert both the substrate and product to the same chemical species for isotopic analysis. This procedure, furthermore, eliminates the difficult corrections which would have to be applied for isotopic discrimination in the mass spectrometer. If the substrate or products contain the isotopes A and A9 in groups or substituents other than the reaction center, there may be complications from isotopic homogeneity. This has been pointed out previously.8 Recently Yankwich and Promislow70 have shown that there is a 1 per cent difference in the C18 content of the methyl and aldehyde carbons of acetaldehyde derived from the air oxidation of propane... 

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