Mass spectrometers multi-collector

The observed range of natural variations of 5 Ca is about 4 to 5%o in terrestrial materials and up to 50%o in high temperature condensate minerals in carbonaceous chondrites. The typical reproducibility of measurements is about +0.15%o. Broader application of Ca isotope measurements in geochemistry may be possible, particularly if the reproducibility can be improved to 0.05%o to 0.03%o. There is hope that this can be achieved either with inductively coupled plasma source mass spectrometry (Halicz et al. 1999) or with a new generation of multi-collector thermal ionization mass spectrometers (Heuser et al. 2002). 

Fletcher IR, Maggi AL, Rosman KJR, McNaughton NJ (1997b) Isotopic abundance measurements of K and Ca using a wide-dispersion multi-collector mass spectrometer and low-fractionation ionisation techniques. Int J Mass Spectrom Ion Proc 163(1-2) 1-17... 

Multi-collection mass spectrometers can analyze isotope ratios in a static mode to eliminate the errors from beam instability. However, the static multi-collection method depends on the extent to which the collectors (e.g., Faraday cups) are identical and to the extent to which the gain of each collector is stable. An alternative approach is to use the so-called dynamic multi-collector mode, to cancel out beam instability, detector bias, and performing a power-law mass fractionation correction. The following descriptions are modified from the Finnigan MAT 262 Operating manual (Finnigan, 1992). 

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. 

Halliday, A.N., Christensen, J.N., Jones, C.E., Walder, A.J., and Freedman, P.A. (1993) Precise measurement of parent/daughter ratios and time with an ICP magnetic sector multi collector mass spectrometer equipped with a wide flight tube. Trans. Amer. Geophys. Un. 74, 626-627. 

In double-focusing sector field ICP-MS spectrometers with single ion detection, the best abundance sensitivity with a mass difference of two is observed at medium mass resolution m/Am = 4400 (9 x 10 ). At low mass resolution mlAm = 400) an abnndance sensitivity for °Sr determination of 2 x 10 was measured in the author s laboratory. In quadrupole ICP-MS with and without a hexapole coUision cell, an abundance sensitivity of 6 x 10 and 6 x 10 , respectively, for determination in the presence of abundant with high ion intensity was determined by Boulyga et al. ° A significant improvement in abundance sensitivity for the determination of the isotope ratio has been obtained by multi-collector ICP-MS (Nu Plasma... 

In 2005, De Laeter discussed the role of isotope reference materials for the analysis of non-traditionaT stable isotopes. At present, no isotopically certified reference materials exist for a large number of elements, including Cu, Zn, Mo and Cd, and it is important that this situation be rectified as soon as practicable. Before the isotopically certified reference materials become available for selected elements, suitable reference materials can be created as a standard if sufficient and reliable isotope data have been obtained by interlaboratory comparisons. For example, the Hf/ Hf isotope ratio was measured using hafnium oxide from Johnson Matthey Chemicals, JMC-475, for hafnium isotope ratio measurements with different multi-collector mass spectrometers (ICP-MS and TIMS) as summarized in Table 8.1. However, no isotope SRM is certified for the element Mo either. Mo isotope analysis is relevant, for example, for studying the isotope fractionation of molybdenum during chemical processes or the isotope variation of molybdenum in nature as the result of the predicted double (3 decay of Zr or 18.26-28 spectroscopically pure sample from Johnson Mattey Specpure is proposed as a laboratory standard reference material if sufficient and reliable isotope data are collected via an interlaboratory comparison. 

FIGURE 17.15. A schematic diagram of a multi-collector mass spectrometer. Figure from Smith (2000), pg. 14. 

Fiedler R (1995) Total evaporation measurements experience with multi-collector instruments and a thermal ionization quadrupole mass spectrometer. Int J Mass Spectrom Ion Processes 146-147 91-97 Fiedler R, Donohue D (1988) Pocket sensitivity calibration of multicollector mass spectrometers. In Symposium on trace dement analysis by mass spectrometry, Regensburg, 30 Sep 1987—2 Oct 1987 Fresenius Z Anal Chem 331(2) 209—213 Filippov VI, Moritz W, Terentjev AA, Vasiliev AA, Yakimov SS (2007) IEEE Sens J 7(2) 192-196 Fisher D (1997) History of the International Atomic Energy Agency, the first forty years, STI/PUB/1032, IAEA, Vienna... 

Pereira de Oliveira O, Sarkis JES, Ponzevera E, Alonso A, De Bolle W, Quetel C (2008) Evaluating the accuracy of uranium isotope amount ratio measurements performed by a quadrupole and a multi-collector magnetic sector inductively coupled plasma mass spectrometers for nuclear safeguards. In Atalante 2008 nuclear fuel cycle for a sustainable future, Montpellier, 19-23 May... 

The first commercially available multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) setup was the Plasma 54, introduced in 1992 by VG Elemental. This mass spectrometer incorporated the detector platform from the Sector 54 thermal ionization mass spectrometer and included an electrostatic analyzer before the entrance to the magnetic sector. This instmment featured seven Faraday cups and a Daly detector. The Daly detector [7] incorporates an A1 knob maintained at +25 kV together with a scintillator screen and photomultiplier (Figure 3.2). Incoming ions are accelerated to the A1 knob and large numbers of secondary electrons are produced as result of the impact of these ions on the aluminum surface. These electrons are then accelerated towards the scintillator. 

The corabination of an inductively coupled plasma ion source and a magnetic sector-based mass spectrometer equipped with a multi-collector (MC) array [multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)] offers precise and reliable isotope ratio data for many solid elements. In fact, MC-ICP-MS provides data, the trueness (accuracy) and precision of which is similar to, or, in some cases, even superior to, that achieved by thermal ionization mass spectrometry (TIMS), considered the benchmark technique for isotope ratio measurements of most solid elements [1], The basic strength of ICP-MS lies in the ion source, which achieves extremely high ionization efficiency for almost all elements [2, 3]. Consequently, MC-ICP-MS is likely to become the method of choice for many geochemists, because it is a versatile, user-friendly, and efficient method for the isotopic analysis of trace elements [4-8], The ICP ion source also accepts dry sample aerosols generated by laser ablation [9-16], The combination of laser ablation (LA) with ICP-MS is now widely accepted as a sensitive analytical tool for the elemental and isotopic analysis of solid samples. 

For the determination of isotope ratios, the precision of TOF-ICP-MS has been studied in a preliminary comparison with other mass spectrometer systems [643]. Typical isotope ratio precisions of 0.05% were obtained, thus overtaking sector field mass spectrometry with sequential detection, for which values of 0.1-0.3% for Cu/ Cu in Antarctic snow samples have been reported [644]. Similar results were obtained by Becker et al. [645] for Mg and Ca in biological samples (0.4-0.5%). In principle, the features of TOF-ICP-MS may be superior to those of sequential sector field or quadrupole mass spectrometry however, true parallel detection of the signals, as is possible with multi-collector systems or array detector mass spectrometry, may be the definitive solution, as shown by Hirata et al. [646]. Here, the use of detectors which allow true parallel measurement of the signals within the relevant mass range, just as the CCDs do for optical atomic spectrometry, may be the ultimate solution and bring about the final breakthrough for ICP-MS isotope ratio measurements as is required in isotope dilution mass spectrometry. 

The Isoprobe MC-ICP-MS (GV instruments, Wythenshawe, UK), a multi-collector device, is equipped with a pressurized hexapole collision cell to reduce the energy spread of the incoming ion before it enters the magnetic sector. Since it is a prerequisite that all ions must be accelerated by the same potential, all ions exit the hexapole at < 1 V potential and are then accelerated. The optional wide aperture retarding potential (WARP) filter is applied to enhance abundance sensitivity in this device. The WARP filter was originally developed for thermal ionization mass spectrometers in an attempt to improve the abundance sensitivity by two orders of magnitude. The principle of the WARP filter is that it excludes those ions that are not at the full accelerating potential. Any ions that have collided with residual gas molecules in the analyser are not transmitted by the WARP filter. 

Nakai, S., Fukuda, S., and Nakada, S. (2001) Thorium isotopic measurements on silicate rock samples with a multi-collector inductively coupled plasma mass spectrometer. Analyst, 126,1707-10. 

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