Multiple-collector magnetic mass analyze

Micromass has applied this new hexapole technology to a fast scanning magnetic sector multiple-collector instrument (Fig. 8.6). The source is at ground potential so most of the lens system and analyzer float at —6 kV. The ions are extracted into the hexapole through a sample cone with a 1.1-mm orifice, a 0.8-mm orifice skimmer cone, and finally a 2-mm orifice transfer lens held at —400 V. The hexapole is inclined to prevent line-of-sight transmission and damage to the detectors from the source. A lens system transfers the ions into the mass analyzer. 

Special mass spectrometry systems are built for isotope ratio measurements. Most isotope ratio mass spectrometers consist of a single-focusing magnetic sector instrument and a multiple Faraday cup detection system. Because the Faraday cup exhibits a stable response, it is an ideal detector for isotope ratio measurements. Simultaneous collection of relevant ion beams from all isotopes provides high-precision isotopic measurements. A three-Faraday cup detection system is shown in Figure 7.9. The multicup assembly is placed at the focal plane of the mass analyzer and can be used for the simultaneous detection of each isotopic form of the analyte species (e.g., m/z 44, 45, and 46 from CO2). Commercial instruments with up to nine collectors are available. 

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). 

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