Isotope instrumental mass discrimination

Using the three measured ratios, Ca/ Ca, Ca/ " Ca and Ca/ " Ca, three unknowns can be solved for the tracer/sample ratio, the mass discrimination, and the sample Ca/ Ca ratio (see also Johnson and Beard 1999 Heuser et al. 2002). Solution of the equations is done iteratively. It is assumed that the isotopic composition of the Ca- Ca tracer is known perfectly, based on a separate measurement of the pure spike solution. Initially it is also assumed that the sample calcium has a normal Ca isotopic composition (equivalent to the isotope ratios listed in Table 1). The Ca/ Ca ratio of the tracer is determined based on the results of the mass spectrometry on the tracer-sample mixture, by calculating the effect of removing the sample Ca. This yields a Ca/ Ca ratio for the tracer, which is in general different from that previously determined for the tracer. This difference is attributed to mass discrimination in the spectrometer ion source and is used to calculate a first approximation to the parameter p which describes the instrumental mass discrimination (see below). The first-approximation p is used to correct the measured isotope ratios for mass discrimination, and then a first-approximation tracer/sample ratio and a first-approximation sample CeJ Ca... 

The tracer-subtraction procedure adds negligible uncertainty to the measured CaJ Ca ratios. However, it is in fact essentially impossible to entirely eliminate the effects of instrumental mass discrimination for the measurements of either the Ca- Ca mixed tracer or for the standard Ca isotope ratios. Hence, it is necessary to have a standard material with an agreed-upon value of 5 Ca. At the time of writing of this article there is no such standard. 

To maximize analytical precision and reproducibility, Marshall and DePaolo (1982) chose n = 42, and hence report all data in terms of Ca/ Ca ratios normalized to RJa/ Ca = 0.31221. This choice allows one to use an isotope ratio spanning two mass units ( RJa/ Ca) to make a correction (for instrumental mass discrimination) to another isotope ratio spanning two mass units ( Ca/ Ca). The only other likely choice is to use Ca/ Ca (i.e., n = 44), which spans four mass units and hence would have twice as large a correction for instrumental mass discrimination. 

The instrumental mass discrimination effects between MC-ICP-MS (VG Axiom and Isoprobe from Micromass) and MC-TIMS (VG Sector 54, Micromass) were compared for 8UB measurements.82 Measurements by MC-ICP-MS yielded more precise isotope ratios than those obtained by TIMS (one magnitude better compared to negative ion TIMS) despite showing the highest mass bias. However, the mass discrimination effects appear to be unaffected for 8nB values.82... 

By using collisional damping, the isotope ratio precision obtained can be improved to values of approximately 0.05% RSD (under optimum conditions). This gain in isotope precision, however, comes at the cost of a more pronounced instrumental mass discrimination [100-103], caused by preferential collisional losses of the lighter nuclide (Figure 2.25). 

NIST SRM 915a CaC03 isotopic reference material. The data are in good agreement with mass fractionation and there is no evidence of spectral interferences from isobaric nuclides, polyatomic ions, and/or doubly chained ions affecting the results. The variability is the result of the drift in the extent of instrumental mass discrimination exhibited by the MC-ICP-MS instrument used. Reproduced from [14],... 

Correction of Instrumental Mass Discrimination for Isotope Ratio Determination with Multi-Collector Inductively Coupled Plasma Mass Spectrometry... 

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). 

Internal Versus External Correction for Mass Bias If more than two isotopes of an element are available and one isotope ratio can be considered as constant, internal mass bias correction can be used. Let us consider an element with three isotopes, X, Y, and Z. If the X/Z ratio can be considered as constant in nature, the extent of instrumental mass discrimination can be determined by comparing the experimental value for X/Z with Xref/Zref and the correction factor thus obtained can be... 

Strategy. Also, possible coelution of matrix components needs to be taken into account as this might cause spectral interferences or affect the instrumental mass discrimination. Finally, the sample preparation steps should not alter either the original isotope ratio or the original speciation of the sample [26]. 

Although MC-ICP-MS permits isotope ratios to be measured with very high precision (down to 0.002% RSD), it needs to be realized that the raw data show a substantially larger bias (1% per mass unit order of magnitude at mid-mass) with respect to the corresponding true value. This bias is caused by preferential transmission of the heavier isotope during the extraction process, and obviously this instrumental mass discrimination needs to be corrected for. The methods used for this purpose are discussed into detail in Chapter 5. 

Figure 9. Sketch of the double spike Zn- Zn method. The surface is constructed by drawing an infinite number of straight-lines through the point representing the spike composition (supposed to be known with no error) and each point of the mass fractionation line going through the point representing the measured mixture. One of these straightlines, which is to be determined from the calculations, is the sample-spike mixing line (stippled line). Each determination of the Zn isotope composition of a sample involves only one run for the mixture of the sample with the spike. Since all natural samples plot on the same mass fractionation line, any reference composition will adequately determine isotope composition of the sample, note that, since the instrumental bias is not linear with mass, the mass discrimination lines are curved.

Figure 1. Schematic representation of the calcium mass spectrum in (a) natural materials, (b) a Ca- Ca tracer solution used for separating natural mass dependent isotopic fractionation from mass discrimination caused by thermal ionization, and (c) a typical mixture of natiwal calcium and tocer calcium used for analysis. The tracer solution has roughly equal amounts of Ca and Ca. In (c) the relative isotopic abundances are shown with an expanded scale. Note that in the mixed sample, masses 42 and 48 are predominantly from the tracer solution, and masses 40 and 44 are almost entirely from natural calcium. This situation enables the instrumental fractionation to be gauged from the Ca/ Ca ratio, and the natural fractionation to be gauged from the sample Ca/ Ca ratio.

All isotope ratio measurements have to be corrected for instrumental mass bias by normalising to an invariant isotope of the same element (internal correction) or, whenever the internal approach cannot be applied, to a well-characterised isotope standard material (external correction). However, the external correction method requires the mass discrimination of an element being identical for the sample and the standard, which is not always the case. A large benefit of the hyphenated chromatography-ICP-MS system is that all measurements of standards and real samples can be carried out with exactly the same matrix - the eluent of the HPLC system. 

Most instruments show some mass discrimination which tends to lower the observed abundance of the heavier isotope a few per cent. 

Sr). Over the past 30 years, lead and strontium isotope ratios have been measured with thermal ionization mass spectrometry (TIMS). Elemental salts are deposited on a filament heated to produce ionized particles, which are then sent into a mass spectrometer where they are detected by multiple Faraday cups arrayed such that ions of several masses are collected simultaneously. TIMS is capable of high precision isotope discrimination, but the instruments tend to be large and expensive, and extensive sample preparation is required prior to sample introduction. Newer ICP-MS-based technologies like multi-collector ICP-MS (especially laser ablation) circumvent some of the sample preparation issues while exploiting the precision of simultaneous mass discrimination, but they are still limited by the number and configuration of ion collectors. 

Velocity of the secondary positive ion also has an effect on its survivability. A high-velocity ion is more likely to escape the surface without being neutralized than is a slower one. This effect is illustrated by the fact that in isotopic measurements higher-mass ions have a lower probability of detection. This isotopic mass effect, usually called mass discrimination, can be compounded by instrument design effects. 

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