Thermal ionization mass spectrometry collectors

J. L. Mann and W. R. Kelly. Measurement of Sulfur Isotope Composition (834S) by Multiple-Collector Thermal Ionization Mass Spectrometry Using a 33S-36S Double Spike. Rapid Commun. Mass Spectrom., 19(2005) 3429-3441. 

Thermal-ionization mass spectrometers (TIMS) combine a hot-filament source with a magnetic-sector mass spectrometer. The mass spectrometers are operated at low to moderate mass-resolving power. A large number of elements can be measured with thermal ionization mass spectrometry. Special care is taken to purify the samples using ion exchange columns. Samples are loaded onto the filaments along with an emitter, and a typical run may take several hours. Modem systems have multiple collectors so that several isotopes can be measured simultaneously. High-precision measurements are done with Faraday cup detectors, but low-abundance isotopes can be measured on electron multipliers. Modem machines are capable of precisions of 0.1 to 0.01 permit. 

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. 

Table 1 Comparison of Isotopic Compositions Measured by Multiple-Collector Inductively Coupled Plasma Mass Spectrometry and Those Measured by Thermal Ionization Mass Spectrometry... 

Figure 10 High-precision U isotopic compositions of NIST SRM 960, as measured by thermal ionization mass spectrometry (TIMS) and multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). (From Halliday et al., 1998b.)...

In cases which require extreme accuracy and precision, isotope dilution mass spectrometry (ID-MS) may be used to measure Pb concentrations. This consists of an addition to the sample of a solution of well-known Pb concentration and isotopic composition ( spike ) followed by determination of the isotopic composition of the spiked sample using mass spectrometry, Q-ICP-MS, ICP-SMS, multi-collector ICP-MS (MC-ICP-MS), or thermal ionization mass spectrometry (TIMS). 

Although several different mass spectrometric methods have been deployed to determine enriched stable isotopes in human studies of nutrient mineral metabolism, thermal ionization mass spectrometry (TIMS) and particularly ICP-MS are now used almost exclusively. ICP-MS is rapid, very sensitive, and sample preparation and introduction is often simplified. Furthermore, ICP-MS can be coupled directly to separation techniques such as size-exclusion chromatography (SEC), high-performance liquid chromatography (HPLC), or CE so that speciation, the determination of the chemical form of particular elements, may also be studied. The two major drawbacks of ICP-MS, low precision relative to TIMS and interference from polyatomic ions in the argon plasma, have largely been overcome by new generations of instruments equipped with multiple collectors and collision/reaction cells, respectively. 

Burger, S., Ricipnti, L.R., Bostick, D.A. et al. (2009). Isotope ratio analysis of actinides, fission prodncts, and geolocators by high-efficiency multi-collector thermal ionization mass spectrometry, Int. J. Mass Spectrom. 286, 70-82. 

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. 

Thermal ionization mass spectrometry (TIMS) suffers from time-dependent mass bias, referred to as mass fractionation, as a result of the finite amount of sample on the source filament and the more efficient thermal ionization of the lighter isotope. Mass bias correction is more crucial with multi-collector (MC)-ICP-MS as the latter suffers significantly larger bias and, as noted earher, it may not necessarily be constant over extended periods of time. Therefore, rigorous correction methods are required. Over the last few decades, several different mass bias correction methods have been successfully used for the determination of isotope amount ratios, as illustrated by Albarede et al. [16]. 

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). 

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. 

Gunther D, Heimich CA (1999) Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier. J Anal At Spectrom 14 1363-1368 Habfast K (1998) Fractionation correction and multiple collectors in thermal ionization isotope ratio mass spectrometry. Inti J Mass Spectrom 176 133-148... 

Habfast K (1998) Fractionation correction and multiple collectors in thermal ionization isotope ratio mass spectrometry. Int J Mass Spec 176 133-148... 

Advances in TIMS-techniques and the introduction of multiple collector-ICP-MS (MC-ICP-MS) techniques have enabled the research on natural variations of a wide range of transition and heavy metal systems for the first time, which so far could not have been measured with the necessary precision. The advent of MC-ICP-MS has improved the precision on isotope measurements to about 40 ppm on elements such as Zn, Cu, Fe, Cr, Mo, and Tl. The technique combines the strength of the ICP technique (high ionization efficiency for nearly all elements) with the high precision of thermal ion source mass spectrometry equipped with an array of Faraday collectors. The uptake of elements from solution and ionization in a plasma allows correction for instrument-dependent mass fractionations by addition of external spikes or the comparison of standards with samples under identical operating conditions. All MC-ICP-MS instruments need Ar as the plasma support gas, in a similar manner to that commonly used in conventional ICP-MS. Mass interferences are thus an inherent feature of this technique, which may be circumvented by using desolvating nebulisers. 

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... 

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