Measuring Isotope Ratios with Single-Collector ICP-MS

Measuring Isotope Ratios with Single-Collector ICP-MS... 

In double-focusing sector field ICP-MS spectrometers with single ion detection, the best abundance sensitivity with a mass difference of two is observed at medium mass resolution m/Am = 4400 (9 x 10 ). At low mass resolution mlAm = 400) an abnndance sensitivity for °Sr determination of 2 x 10 was measured in the author s laboratory. In quadrupole ICP-MS with and without a hexapole coUision cell, an abundance sensitivity of 6 x 10 and 6 x 10 , respectively, for determination in the presence of abundant with high ion intensity was determined by Boulyga et al. ° A significant improvement in abundance sensitivity for the determination of the isotope ratio has been obtained by multi-collector ICP-MS (Nu Plasma... 

Internal correction has the advantage of simultaneous (MC instmments) or quasi-simultaneous (single-collector instruments) correction, but in this case obviously the measured isotope ratio cannot be corrected by using the same isotope ratio. In ICP-MS the mass discrimination for a limited mass range (< 10 mass units) only depends on the mass difference. Other dependencies have not yet been observed. Thus the mass discrimination for a certain isotope ratio in the first approximation can be calculated from a different isotope ratio close to the target one. In practice another isotope ratio of the same element with a constant universal value is used or the sample... 

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. 

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