Instrument Mass fractionation

For precise measurement of isotopic composition by mass spectrometry, it is also common to use either a natural, known isotopic ratio to correct for instrumental mass fractionation (e g., internal normalization) or to add a tracer for this purpose. For example for natural uranium samples, one can use the natural U/ U of 137.88 to correct for fractionation. Alternatively, one can use an added double spike of ratio -unity... 

Accuracy for all thorium measurements by TIMS is limited by the absence of an appropriate normalization isotope ratio for internal correction of instrumental mass fractionation. However, external mass fractionation correction factors may be obtained via analysis of suitable thorium standards, such as the UC-Santa Cruz and IRMM standards (Raptis et al. 1998) for °Th/ Th, and these corrections are usually small but significant (< few %o/amu). For very high precision analysis, the inability to perform an internal mass fractionation correction is probably the major limitation of all of the methods for thorium isotope analysis discussed above. For this reason, MC-ICPMS techniques where various methods for external mass fractionation correction are available, provide improved accuracy and precision for Th isotope determinations (Luo et al. 1997 Pietruszka et al. 2002). 

However, thorium has only two naturally occurring long-lived isotopes, and all Th measurements by TIMS are limited by the absence of a well-constrained isotope ratio that can be used for internal normalization purposes to correct for instrumental mass fractionation. In this regard, one of the most important advantages of MC-ICPMS over MC-TIMS is the ability to admix two elements with overlapping mass ranges and use the... 

Instrumental Mass Fractionation U N/A <0.3%/AMU Time- dependent <0.3%/AMU Time- dependent <0.5%/AMU Time- independent <1%/AMU Time- independent <0.5%/AMU Time- independent <1%/AMU Time- independent ... 

Analysis of terrestrial samples demonstrated that the tuning of the ion extraction system, and consequently the amount of instrumental mass fractionation, was very sensitive to charge build-up on the sample, the position of the primary beam relative to the spectrometer optic axis, and the position of the sample relative to the extraction lens. To optimize reproducible tuning of the extraction system from sample to sample, we developed the following criteria (1) resistance of the sample (Au-coated) to ground less than 106 fi (2) alignment of the primary beam to within 10 pm using... 

The data plot along a line of dope 0.5, reflecting instrumental mass fractionation of 7%famu. The proper correction for is indicated by the terrestrial plagioclase... 

Iron. Fe has 4 isotopes of which the heaviest Fe has a very small abimdance of about 0.3%. The precision of thermal ionization mass spectrometers is around 10 s on this isotope and there is only a hint in some normal inclusions for an excess in 5 Fe (VoUcening and Papanastassiou 1989). Recent ICPMS measurements at the 2 s precision level display normal isotopic compositions for Fe in planetary materials but no Allende inclusion was reported in this study (Kehm et al. 2003). If excesses of similar magnitude to Ca, Ti, Cr were present they would not be clearly resolved in agreement with the observations. When Fe and Fe are used to correct for instrumental mass fractionation, Fe exhibits normal abundances, suggesting all three isotopes are present in solar relative abundances. 

Mass bias, or the instrumental mass fractionation, is the variable transmission of the ion beam into the mass spectrometer. A variety of phenomena create conditions that lead to variable transmission of ion beams. For modem instmments, the transmission in the flight tube and the efficiency of ion conversion to electrons at the collector are almost quantitative. Most fractionation processes, therefore, take place within the source, namely in the area where the analyte is introduced into the mass spectrometer and ionized, or at the interface between the source and the mass analyzer. 

The majority of instrumental mass fractionation that takes place during TIMS occurs on or around the filaments used to produce ions. As a simple example, the ionization of Ca at the surface of a Ta filament may be represented by the formal reactions ... 

Figure 2. Plot of the measured ["Fe/ Fe] .. j/[= Fe/ Fe], j, versus the P Fe/ Fe] ,ypTe7 Fe]t j, of a representative TIMS analysis of an equal atom Fe standard (e.g., Ab Fe k Ab Fe k Ab Te Ab Fe Beard and Johnson 1999). Lines show the linear and exponential mass fractionation curves. The TIMS instrumental mass fractionation is best approximated using an exponential law for the first 2/3 to 3/4 of the data taken in a typical analysis. Inset shows the P Fe/ Fe] j yp Fe/ Fe] j, from the beginning to the end of the run. Modified from Beard and Johnson (1999).

Corrections for instrumentally-produced mass fractionation that preserve natural mass dependent fractionation can be approached in one of two ways a double-spike method, which allows for rigorous calculation of instrumental mass fractionation (e.g., Dodson 1963 Compston and Oversby 1969 Eugster et al. 1969 Gale 1970 Hamelin et al. 1985 Galer 1999 see section Double-spike analysis ), or an empirical adjustment, based on comparison with isotopic analysis of standards (Dixon et al. 1993 Taylor et al. 1992 1993). The empirical approach assumes that standards and samples fractionate to the same degree during isotopic analysis, requiring carefully controlled analysis conditions. Such approaches are commonly used for Pb isotope work. However, it is important to stress that the precision and accuracy of isotope ratios determined on unknown samples may be very difficult to evaluate because each filament load in a TIMS analysis is different. 

Sample-standard comparison is more applicable in MC-ICP-MS, in which instrument mass fractionation is fundamentally a steady state phenomenon (Marechal et al. 1999). This method has been used successfully for some non-traditional stable isotopes, particularly involving Fe, in which analyses of samples are bracketed by standards to cope with systematic instrumental drift (e.g., Zhu et al. 2002 Beard et al. 2003). However, other methods have been used for Mo stable isotope work published to date because of concerns about non-systematic changes in instrument mass fractionation, particularly arising from differences in matrices, between samples and standards. Such concerns are more acute for Mo than for Fe and many other elements because Mo is a trace constituent of most samples, increasing the challenge of rigorous, high-yield sample purification. 

Element spike. An additional benefit of MC-ICP-MS is the ability to use an element spike approach. In this method, the isotopic composition of a different element, added to the sample, is to monitor variations in instrument mass fractionation (Longerich et al. 1987 Marechal et al. 1999). This method is possible with MC-ICP-MS because the relative... 

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). 

Large instrumental mass fractionations have been observed during measurements of oxygen isotope ratios on Fe-Mg-Ca garnets in SIMS.83 Part of this fractionation depends on crystal structure and mineral composition. 

Mass spectrometers are fundamentally designed to determine isotope ratios. For example, the Pb isotopic composition of a U- and Th-bearing phosphate can be measured directly using a thermal ionization mass spectrometer provided the instrumental mass fractionation is known or can be assessed in the course of the analysis. Determining absolute abundances of isotopes requires a technique called isotope dilution in which a pre-determined quantity of a spike of known but exotic isotope composition is added to... 

Lead has four stable isotopes, ° Pb, Pb, Pb, and ° Pb, but three of these are radiogenic, such that there is no invariant isotope that can be used for internal correction of instrumental mass fractionation. This poses a significant analytical challenge for precise Pb isotopic analysis. This problem is most readily overcome with MC-ICP-MS, because the Pb isotope ratio measurements can utilize the ratio of added T1 for external normalization of the Pb isotope ratio data (see also Chapter 5). The simplicity of this procedure is the main reason why MC-ICP-MS Pb isotopic analysis is now commonly used in geo- and cosmochemistry. This approach needs to be applied with care, however, and it has been argued that Pb isotopic analysis that utilizes the double spike methodolc (either in conjunction with TIMS or MC-ICP-MS) is typically superior in accuracy and precision [86, 87]. 

Understanding of the Instrument Mass Fractionation (IMF) effect. This results in small but measurable deviations from true isotope ratios as discussed in Section 3.3.1.2.3. 

Analysis-induced mass fractionation is encapsulated in what is referred to as the Instrument Mass Fractionation (IMF). Because this is matrix dependent, this can only be assessed through comparison of known isotope compositions of some standard reference material with the isotopic composition derived via SIMS analysis of the above-mentioned reference. The fact that a matrix-dependent IMF is observed leads to the realization that examination into the IMF can provide the much-needed information on the fundamentals of the secondary ion formation/survival process. 

Processing methodologies used in deriving isotope fractions require that the Instruments Mass Fractionation be corrected for. 

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