Thermal ionization magnetic sector

Neither quadrupole nor magnetic sector machines with just a single ion detector, known as a collector, have adequate precision (better than 0.01%) for most isotope ratio studies. For isotope ratios, multiple collectors are used so that measurements of the different isotopes can be done at the same time. Multiple collectors are commonly employed with the ICP-MS discussed above ( MC-ICP-MS ) and on thermal ionization magnetic sector mass spectrometers (TIMS). TIMS uses a wire filament on which the purified element of interest is heated to ionize the sample ( thermal ionization ), instead of using plasma. 

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. 

In general, ions generated by thermal ionization possess low initial energies (0.1-0.2eV), therefore mostly single magnetic sector field mass spectrometers are used for ion separation. 

Magnetic sector thermal Ionization mass spectrometry Is capable of far better precision and accuracy than the other two methods of analysis. A recent comparative study of NAA and Tl/MS zinc analysis In human blood samples demonstrated the marked difference In precision between the two methods (12). The standard deviation of zinc ratio determinations was 0.13% for TI/MS and 8.3% for NAA. It Is likely that, while maximum attainable precision for both methods Is better than this (see Table III), these levels of precision are the best which can be achieved In biological samples. The excellent precision of TI/MS Is offset by several disadvantages. Analysis Is slow, requires considerable skill and experience, and Is expensive. Extensive sample preparation Is required to obtain pure samples pure enough for accurate analysis. However, the high degree of precision attainable by this method make It the only choice when excellent precision and accuracy (e.g. within 1%) are required. 

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. 

Instrumentation Isotopic measurements were carried out with a DP-102 magnetic sector mass spectrometer (DuPont Instrxments) operated in the electron-impact ionization mode. Because the chelate was thermally unstable and decomposed on a GO column, direct probe introduction was used a capillary tube containing the chelate was placed in the sample cup at the end of the probe. The temperature of the ion-source chamber was set at HO C and the probe heater was turned off. 

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). 

Installing additional accessories in front of the ion source can render analytes amenable to ionization and subsequent mass spectrometric analysis. On-line sample treatment is especially important when analyzing liquid-phase, complex, and/or concentrated samples. For example a thermal vaporizer was used to enable analysis of liquid samples by a process mass spectrometer designed for gas analysis [196], This system has been successfully implemented in the monitoring of an esterification reaction [197]. The obtained data were in a good agreement with those recorded by in-line mid-infrared spectrometry. The setup incorporated a magnetic sector analyzer with two detectors an electron multiplier detector... 

Thermal ionization mass spectrometers (TIMS) with magnetic sector are thus the basic instruments for Pu and U isotopic analysis in safeguards inspection samples. The performance of commercial instruments has improved tremendously, particularly in the last 30 years, in terms of vacuum capability, design of the ionization source and detector assembly, stability of electrical supplies, sensitivity and linearity of ion current amplifiers, and includes full automatization of the measurement and data reduction. 

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