Thermal ionization mass instrument

Kim Israel, a technician at LANL, Chemistry Division Bioassay Program, runs a thermal ionization mass spectrometry (TIMS) instrument. 

The relatively small mass differences for most of the elements discussed in this volume requires very high-precision analytical methods, and these are reviewed in Chapter 4 by Albarede and Beard (2004), where it is shown that precisions of 0.05 to 0.2 per mil (%o) are attainable for many isotopic systems. Isotopic analysis may be done using a variety of mass spectrometers, including so-called gas source and solid source mass spectrometers (also referred to as isotope ratio and thermal ionization mass spectrometers, respectively), and, importantly, MC-ICP-MS. Future advancements in instrumentation will include improvement in in situ isotopic analyses using ion microprobes (secondary ion mass spectrometry). Even a small increase in precision is likely to be critical for isotopic analysis of the intermediate- to high-mass elements where, for example, an increase in precision from 0.2 to 0.05%o could result in an increase in signal to noise ratio from 10 to 40. 

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. 

Traditionally thermal ionization mass spectrometry was the instrument of choice for the isotopic analysis of metals because thermal ionization produced an ion beam with a very small kinetic energy spread ( 0.5 eV). Therefore only a magnetic mass analyzer is needed to resolve one isotope from another. Moreover, ionization of unwanted material, such as atmospheric contaminates, hydrocarbons from pump oil, or production of doubly ionized particles is almost non existent, thus background counts are minimized and signal-to-noise ratio is maximized. 

Ireland TR, Fahey AJ, Zinner EK (1991) Hibonite-bearing microspherules a new type of refractory inclusions with large isotopic anomalies. Geochim Cosmochim Acta 55 367-379 Johnson CM, Beard BL (1999) Correction of instrumentally produced mass fractionation during isotopic analysis of Fe by thermal ionization mass spectrometry. Int J Mass Spect 193 87-99 Jungck MHA, Shimamura T, Lugmair GW (1984) Calcium isotope variations in Allende. Geochim Cosmochim Acta 48 2651-2658... 

Johnson, C.M. and Beard, B.L., 1999. Correction of instrumentally produced mass fractionation during isotopic analysis of Fe by thermal ionization mass spectrometry. International J. Mass Spectrometry, 193 87-99. 

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. 

The ideal internal standard is the same element as the analyte because it has similar mass, ionization energy, and chemical properties. Therefore, isotope dilution based calibration provides high accuracy as long as isotope equilibration is attained and the measured isotopes are free of spectral overlaps [192,193]. Standards do not need to be matrix-matched. Quadrupole-based ICP-MS instruments can typically provide isotope ratio precision of 0.1% to 0.5%. Much better isotope ratio precision can be obtained by using simultaneous MS detection, such as a multicollector-based instrument or perhaps time-of-flight MS. In comparison to thermal ionization mass spectrometry, ICP-MS provides much higher sample throughput and simpler, faster sample preparation. 

The analysis of stable Isotopes requires extensive sample preparation and sophisticated, expensive Instruments. In our work we use a thermal ionization mass spectrometer. Thermal ionization mass spectrometers cost approximately 300,000. In contrast, analysis of the radioisotope requires little sample preparation and analysis can be done quickly and easily using a gamma counter. 

The mass spectrometer we now use for zinc analysis, in the laboratory of Maynard Michel of Lawrence Berkeley Laboratory, is a thermal ionization mass spectrometer, a single direction focusing instrument with a 12" radius magnetic sector, double filament, rhenium ionizing source and electron multiplier detector. In addition, have done some preliminary work for Fe and Cu analysis with an automated TI/MS which speeds analysis considerably with excellent precision. We hope to be able to develop methods to use this automated Instrument for zinc analysis as well. 

TIMS has been used for many years as the benchmark technique especially for uranium isotope analysis. Instrumental improvements have enabled ICP-MS to approach the accuracy and precision obtained by TIMS in measuring data. In addition, due to time consuming sample preparation steps and the need for a large volume of urine, the method has been replaced by the more powerful ICP-MS in many laboratories. An interlaboratory analytical exercise on the determination of natural and depleted uranium in urine was carried out by different ICP-MS instruments, by thermal ionization mass spectrometry (TIMS) and instrumental neutron activation analysis. TIMS has also been employed to determine fg quantities of Pu and °Pu in bioassay samples (such as human urine and artificial urine), ° in an interlaboratory comparison for the analysis of the Pu and Pu/ °Pu atomic ratios in synthetic urine by TIMS and AMS as reported in reference. ... 

In thermal ionization mass spectrometry (TI-MS), solid, inorganic compounds may be volatilized from a heated surface. TI-MS is the most precise method for the measurement of isotopic ratios of minerals and has been used to analyze 58pe in fecal samples collected from a human study (H). The major drawbacks of this technique are the costly instrument and the slow sample through-put. Conventional mass spectrometry produces ions by electron bombardment of the vapor of volatile compoimds. This is called electron-impact ionization mass spectrometry (EI-MS). Analysis of iron by EI-MS requires derivitization to volatile forms before introduction into the mass spectrometer. A method has been developed for the synthesis of volatile iron-acetylacetone chelates from iron in blood serxm (1 ). A tetraphenylporphyrin chelate has also been synthesized and used in an absorption study in which 54pe and 57pe were given orally (16). 

Today, thermal ionization mass spectrometry is preferably used for the isotope analyses of inorganic solid samples, and electron impact instruments are preferably applied for the analyses of low-molecular gases. Therefore, Fig. 9 gives a summary of the possibilities for the isotope ratio determination in the periodic table of the elements corresponding to these two ionization methods. A position which is not marked in Fig. 9 represents an element with not more than one stable or long-lived radioactive isotope. In these cases an isotope ratio measurement by mass spectrometry is usually not possible. For the isotope ratio determination of those elements which are marked by a black bar, at least one long-lived radioactive isotope has to be used. Except uranium, these elements are only monoisotopic in nature (Be, Al, Mn, Nb, I, Cs, Bi, Th), or they are synthetic elements (Tc, Np, Pu, Am, Cm). 

Samples weighing 2-5 mg are then dissolved in 5-molar nitric acid. The strontium fraction is purified using ion-specific resin and eluted with nitric acid followed by water. This solution is loaded onto a titanium filament for placement in the instrument (Fig. 4.20). Isotopic compositions are obtained on the strontium fraction thermal ionization mass spectrometer (TIMS). This is a single focusing, magnetic sector instrument equipped with multiple Faraday collectors. Strontium is placed on a thin filament and measured. Sr/ Sr ratios are corrected for mass fractionation using an exponential mass fractionation law. Sr/ Sr ratios are reported relative to a value of 0.710250 for the NIST 987 standard (e.g., if the Sr/ Sr ratios for the standards analyzed with the samples average 0.710260, a value of 0.000010 is subtracted from the ratio for each sample). 

Thermal ionization mass spectrometer (TIMS) A scientific instrument for measur-... 

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