Interferences isobaric, mass spectrometry

An alternative to the bridge technique was recently reported for thorium analysis in silicate rocks for which both Th and Th are measured on a single lon-counting detector (Rubin 2001). With careful chemistry and mass spectrometry, °Th/ Th ratios of igneous rocks can be measured with this technique with a precision that is similar to the bridge method. The disadvantage of this technique is that °Th ion-count rates are extremely low (around 10 cps) with normal silicate thorium ratios and are therefore subject to perturbations from background variation and low-level isobaric interferences in normal samples. 

Resonance-ionization mass-spectrometry is still in the development stage in terms of its application to cosmochemistry. The Charisma instrument, which is operated by Argonne National Laboratories, uses multiple lasers to resonantly ionize only the elements of interest, which are then analyzed with a time-of-flight mass spectrometer. The Charisma instrument has made isotopic measurements of molybdenum, zirconium, strontium, ruthenium, barium and other elements in presolar grains. These measurements are made possible by the high ionization efficiency of the RIMS technique and its ability to completely eliminate isobaric interferences. Work is now underway to build a RIMS instrument that can be operated by an individual investigator in a university laboratory. If this succeeds, RIMS will play an increasing role in analysis of extraterrestrial materials. 

Operation of quadrupole mass filters operated in higher zones of stability is described with an emphasis on the potential of elemental mass spectrometry by Du et al.19 These quadrupole mass filters allow a mass resolution in the second stability region of up to 9000 and make it possible to resolve a multitude of isobaric interferences. 

Am with a half-life of ti = 432 a has to be determined in waste and environmental samples by mass spectrometry. The determination of 238Pu (ti 88 a) at the trace level is difficult in the presence of uranium due to isobaric interference with 238 U+ in mass spectra measured by ICP-MS or LA-ICP-MS. Therefore the application of a sensitive and selective technique such as RIMS or AMS is advantageous. The determination of 238Pu for plutonium isotope analysis in irradiated... 

A major topic in isotope mass spectrometry is the determination of the half-lives of long-lived radionuclides. De Bievre and Verbruggen34 determined the half-life of 241 Pu for 3-decay in the isobaric radionuclide 241 Am on material from Oak Ridge that had initially been about 93% isotopically enriched. Due to the isobaric interference of 241 Pu and 241 Am radionuclides during mass spectrometric measurements by TIMS, Am had to be removed by chemical separation immediately (less than 48 h) prior to measurements as described in reference 34. On the basis of all the measurements performed over an extended period of more than 20 years and after considering the possible effects of systematic errors during these measurements, a half-life for the 3 decay of 241 Pu of (ti/2 = 14.290 0.006 a) was reported.34... 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

In inductively coupled plasma-mass spectrometry, isobaric interference occurs between species with the same mass and charge. Interference can be eliminated if the mass spectral resolution is sufficiently great or by dissociating an interfering polyatomic species with a collision cell. When laser ablation is used to sample a solid, matrix-matched standards are often necessary for quantitative analysis. 

Figure 4.21. Nominal mass isobaric interferences for the GSH adduct of nefazodone when tandem quadmpole mass spectrometry is used.

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