Thermal ionization mass spectrometry, lead

R. Doucelance and G. Manhes. Reevaluation of Precise Lead Isotope Measurements by Thermal Ionization Mass Spectrometry Comparison with Determinations by Plasma Source Mass Spectrometry. Chem. Geol, 176(2001) 361-377. 

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. 

Lead curse tablets from Roman Carthage contain variable amounts of very small metallic inclusions. Electron microprobe analysis confirmed these metallic inclusions were bronze, brass, and a Sn-Sb alloy. This was interpreted as possible evidence of lead metal recycling. Six samples were chosen to represent a range of tablets containing the minimum to the maximum number of inclusions. Thermal ionization mass spectrometry of the Pb isotopes in the curse tablets appear to define a mixing line, with the tablets containing the least number of inclusions plotting closest to the Tunisian lead ore isotope ratios. 

A.D. were found in old mines, and sizable slag deposits suggest at least some level of lead exploitation during Roman times (2). By the careful selection of Roman lead artifacts likely to have been manufactured in Carthage, and the use of lead isotope analysis, it may be possible to confirm the use of Tunisian lead ores during the Roman period. In this preliminary study, analysis of 22 curse tablets by electron microprobe analysis (EMPA) and six tablets by thermal ionization mass spectrometry (TIMS) suggest that these artifacts might be used to better understand Roman lead use and trade. 

So far, the only available technique capable of sufficiently accurate measurement of lead isotope compositions, both for isotope geochemistry and for archaeological applications, is thermal ionization mass spectrometry. Any well-equipped laboratory can routinely measure lead isotope ratios with... 

The most common method of determining the U-Pb date of an accessory mineral is the determination of uranium and lead isotopic abundances via isotope dilution and thermal ionization mass spectrometry (ID-TIMS). Most measurements in the 1970s and 1980s required that an aliquot of the dissolved mineral solution be made, with one portion being spiked with an enriched U and ° Pb tracer solution, and the... 

Modern thermal ionization mass spectrometry (TIMS) is now sufficiently sensitive and precise to measure individual selenium-isotope abundances (e.g., Se/ Se) in solid samples or residues so that it can be used to study environmental cycling/distri-butions (Johnson era/., 1999). Microbial reduction of selenate leads to isotopically lighter selenite, i.e., the reduction has a Se/ Se fractionation factor, , of about — 5.5%o (Johnson et al., 1999). INAA has been used to determine different selenium isotopes, especially Se in plant tracer studies. 

Thermal ionization mass spectrometry (TIMS) has been the technique of choice in most lead and strontium isotope ratio studies. TIMS is a bulk technique, where samples are powdered and concentrated prior to analysis. This bulk technique has two negative features. First, processing of samples is time-consuming and requires additional laboratory equipment. Second, the samples must be homogeneous. This aspect of TIMS is problematic in samples where small-scale compositional vari-... 

Table 1. Lead contamination blanks for serum collection, processing, and analysis by isotope dilution thermal ionization mass spectrometry in the WIGS Trace Metal/Mass Spectrometry facihty. University of California, Santa Cruz. Lead values are based upon repeated measurements.

Thermal Ionization Mass Spectrometry (TIMS). TIMS continues to be the standard method by which all other lead concentration measurements are evaluated (NRC 1993). TIMS is considered the definitive method for accuracy in elemental analyses using isotope dilution mass spectrometry (IDMS), and it provides unprecedented precision, sensitivity, and detection limits for lead concentration analyses (Heumann 1988). It also has been the method used to demonstrate that many previously reported lead concentrations in environmental and biological samples were orders of magnitude higher than the true values due to sample contamination and analytical inaccuracies (e.g., Ericson et al. 1979 Everson and Patterson 1980 Flegal and Coale 1989 Patterson 1965 Settle and Patterson 1980). 

Bingol et al ° have demonstrated, especially by means of electrospray ionization mass spectrometry (ESI-MS), that polymerization of vinylphos-phonates is mainly dominated by transfer reactions the predominant transfer occurs by intramolecular hydrogen transfer of phosphonate ester groups, which in consequence creates a P-O-C bond in the main chain (Scheme 3.2). Moreover P-O-C bonds are more thermally labile compared to phosphonate, and thus lead to chain scission reactions. 

Spark source (SSMS) and thermal emission (TEMS) mass spectrometry are used to determine ppb to ppm quantities of elements in energy sources such as coal, fuel oil, and gasoline. Toxic metals—cadmium, mercury, lead, and zinc— may be determined by SSMS with an estimated precision of 5%, and metals which ionize thermally may be determined by TEMS with an estimated precision of 1% using the isotope dilution technique. An environmental study of the trace element balance from a coal-fired steam plant was done by SSMS using isotope dilution to determine the toxic metals and a general scan technique for 15 other elements using chemically determined iron as an internal standard. In addition, isotope dilution procedures for the analysis of lead in gasoline and uranium in coal and fly ash by TEMS are presented. 

Caution must be exercised when interpreting mass spectral eliminations in the manner discussed for one of the most common artifacts in mass spectrometry is sample decomposition, particularly by elimination of neutral molecules of those substances listed in Table 2.5 prior to volatilization and ionization. The high temperatures required to vaporize some materials are great enough to promote thermal elimination, which leads in most cases to mixtures of olefins. The occurrence of thermal elimination would certainly produce ambiguous results but is detectable by a number of methods. Low electron energies ( 10 to 15 eV) usually produce less... 

In early mass spectrometry applications of lasers, the sample was irradiated directly by a laser beam to desorb intact sample-related ions [27]. In this direct mode, termed laser desorption/ionization (LDI), the extent of energy transfer is, however, difficult to control and often leads to excessive thermal degradation. Also, not all compounds absorb radiation at the laser wavelength and thus are not amenable to LDI. Only those compounds that have mass below 1000 Da can be analyzed by LDI. Analytical sensitivity is also poor. A key contribution of LDI experiments is the observation that the desorption efficiency of amino acids and peptides that absorb the laser fight beam is greater than those without the chromophore [28]. IR lasers (e.g., an Nd YAG laser at 1.06 p m and a pulsed CO2 laser at 10.6 pm) and UV lasers (frequency-quadrapled Nd YAG laser at 266 nm) have aU been used. The detection of malaria parasites in blood by LDI with an N2 laser has been demonstrated [29]. 

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