Instrumental mass bias

In contrast to thermal ionization methods, where the tracer added must be of the same element as the analyte, tracers of different elemental composition but similar ionization efficiency can be utilized for inductively coupled plasma mass spectrometry (ICPMS) analysis. Hence, for ICPMS work, uranium can be added to thorium or radium samples as a way of correcting for instrumental mass bias (e g., Luo et al. 1997 Stirling et al. 2001 Pietruszka et al. 2002). The only drawback of this approach is that small inter-element (e g., U vs. Th) biases may be present during ionization or detection that need to be considered and evaluated (e.g., Pietruszka et al. 2002). 

In principle, the three isotope method may be widely applied to new isotope systems such as Mg, Ca, Cr, Fe, Zn, Se, and Mo. Unlike isotopic analysis of purified oxygen, however, isotopic analysis of metals that have been separated from complex matrices commonly involves measurement of several isotopic ratios to monitor potential isobars, evaluate the internal consistency of the data through comparison with mass-dependent fractionation relations (e.g., Eqn. 8 above), or use in double-spike corrections for instrumental mass bias (Chapter 4 Albarede and Beard 2004). For experimental data that reflect partial isotopic exchange, their isotopic compositions will not lie along a mass-dependent fractionation line, but will instead lie along a line at high angle to a mass-dependent relation (Fig. 10), which will limit the use of multiple isotopic ratios for isobar corrections, data quality checks, and double-spike corrections. 

Typically, the relative instrumental mass-dependent fractionation is on the order of 1 to 5%o per mass unit, but this fractionation is variable during the course of the analysis, as well as variable from hlament to hlament. The degree of instrumental mass bias can be minimized by use of a double or triple hlament ionization source, as compared to a single hlament source. In the double and triple hlament source the temperature of the evaporation hlament... 

Rigorous correction for instrumental mass bias is required if the precision of an isotope ratio measurement needs to be greater than l%o per mass unit. This concept is well illustrated by the definitive Ca isotope work of Russell et al. (1978), which used a double-spike approach. Prior to the Ca isotope investigation of Russell et al. (1978), natural mass-dependent Ca... 

Matrix effects are typically divided into spectral (isobaric) and non-spectral types. The spectral or isobaric effects include 1) elemental isobaric interferences such as Cr at " Fe, 2) molecular interferences such as Ca O at Fe and Ar N at Fe, 3) double charge interferences such as Ca at Mg. Non-spectral matrix effects are largely associated with changes in the sensitivity of an analyte due to the presence of other elements (Olivares and Houk 1986). Changes in sensitivity correspond to a change in instrumental mass bias, and therefore non-spectral matrix effects can have a significant impact on the accuracy of isotope measurements. 

Figure 12. Plot of the Fe/ Fe ratio of an Fe standard, analyzed at 200 to 600 ppb concentrations, relative to the average Fe/ Fe of bracketing 400 ppb an Fe standard, versus the measured Fe volts (10 fl resistor). The measured Fe isotope composition varies relative to Fe concentration, which reflects differences in instrumental mass bias as a function of concentration. Data were taken over a 24 hour period using the University of Wisconsin-Madison Micromass IsoProbe.

Several effects, like instrumental mass bias, isobaric interferences, instrumental background, contamination of the solution introduction system and the sampler and skimmer cone and lens... 

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. 

Matrix effects (termed instrumental mass bias) also present problems in isotope ratio analysis as the measured isotope ratio is almost always light isotope enriched relative to the accepted ratio, and the degree of this enrichment depends on the matrix composition. Although mass bias variations between different mineral compositions are relatively minor for most elements (a few percentage points at most), the high accuracy required in isotope analysis (often 1% or better) demands careful calibration. For isotope ratio measurements in essentially isochemical... 

The ICP-SFMS instrument was tuned using a I ng mf uranium standard solution prior to analysis. Sensitivity was about 2 x 10 cps for 1 ng g" solution. Concentrations of plutonium isotopes and " Am were calculated as a function of Pu/ Pu, Pu/ Pu, Pu/ Pu and Am/ Am ratios according to the isotope dilution method. All raw data were corrected for instrumental mass bias using linear correction. NdFs micro coprecipitated alpha sources were counted by a PIPS type alpha Si detector with a surface... 

Accurate and precise measurements of isotope ratios can also be compromised by matrix effects. Some elements have isotopes of the same mass (e.g. Cr and Fe), so they must be separated from one another with care prior to analysis. Sample matrix can also have non-isobaric effects. These are largely associated with changes in the sensitivity of an anal3de due to the presence of other elements. Changes in sensitivity result in a change in instrumental mass bias (Fig. 16) for this reason, it is important to ensure that the sample matrix is the same as that of the standard. Preferably, the anal3fie should be completely separated... 

Instrumental mass bias It occurs because mass spectrometers and their associated ion optics do not transmit ions of different mass equally. In other words, if a sample composed of two isotopes with an exactly 1 1 molar ratio is analyzed, a 1 1 isotope amount ratio will not necessarily be observed. This so-called mass bias depends on mass and the type of mass spectrometer used, but generally tends to be greatest at low mass, and decreases with increasing mass. Even very small mass-biases can have deleterious effects on the accuracy of ID-MS, so a correction must often be made, usually in one of two ways as follows. 

The advantage of this approach for mass bias correction is that any temporal changes in the instrumental mass bias during the measurement process are accounted for. However, matrix-induced mass bias cannot be fidly compensated with this type of mass bias correction and the corrected isotope amount ratios can show a dependence on the sample concentration [56]. Hence a near-perfect matrix separation, and dose matching of analyte and calibrant concentrations are required to yield accurate isotope amount ratios. 

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). 

Equations (8.1) and (8.2) are not corrected for instrumental mass bias (mass discrimination) and blank values. In the literature, one can therefore find much... 

Barling, J. and Weis, D. (2008) Influence of non-spectral matrix effects on the accuracy of Pb isotope ratio measurement by MC-ICP-MS implications for the external normalization method of instrumental mass bias correction. J. Anal. At. Spearom., 23 (7), 1017-1025. 

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. 

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