Improved three-filament thermal ionization mass spectrometry ion source for isotope ratio determination of two samples.

Rationale: In order to make a single-filament thermal ionization mass spectrometry (TIMS) instrument more versatile and efficient in element isotope analysis, a multifilament ion source has to be employed. In the currently used three-filament ion sources, the same sample must be loaded on all the evaporator filaments, because of the possibility of cross-contamination.

Methods: The elimination of cross-contamination in a TIMS three-filament ion source was achieved by installation of a wide ionizer filament (ribbon shape) perpendicular to the extracting slit plane between two parallel evaporators. In such a configuration, the ionizer filament serves also as a screen separating two evaporator filaments on which two different samples can be loaded.

Results: The lack of cross-contamination (on a measurement uncertainty 1σ level of 0.3‰) has been demonstrated by measurements of the isotope ratios of a potassium chloride sample ( K/ K = 13.7801 ± 0.0023) versus a spike ( K/ K = 1.5670 ± 0.0003). The reproducibility of the isotope ratios for potassium K/ K and lithium Li/ Li is about 600 ppm (1σ).

Conclusions: The small change of the ionizer filament configuration significantly improves the functionality of the ion source. The proposed modification enables us to perform alternating isotope analyses under the same working conditions for two different samples (e.g. studied sample and standard).