Multicollector inductively coupled mass spectrometry (MC-ICP-MS) has become the workhorse of isotope ratio mass spectrometry since its introduction 25 years ago. Over this time, there have been dramatic improvements in the resolving power of the technology with the introduction of smaller slit sizes and energy filters. However, there remain a number of isobaric interferences that cannot be resolved by mass-to-charge (m/z) resolution alone. Collision/reaction cell (CRC) technology can be advantageous in MC-ICP-MS as it enables reactions of specific elements with the reacting gas, thereby creating a mass difference between the element of interest and the isobaric interference.
This setup enabled the successful separation of 40Ca and 41K from 40Ar+ and 40Ar1H+. However, for other isotope systems, this technology has its limitations because all ions enter the CRC. This means that a large number of secondary reactions can occur, resulting in losses in accuracy and precision of the isotopic ratio.
A pre-cell mass filter eradicates the issue of secondary reactions in the CRC. The Thermo ScientificTM
Proteus MC-ICP-MS/MS combines the front half of a Thermo Scientific iCAPTMRQ with a Thermo Scientific NeptuneTM MC-ICP-MS. The pre-cell quadrupole of the iCAP RQ is used to isolate a single mass or a small window of masses into the CRC. Proteus successfully demonstrated precisions for 40Ca/44Ca  and 41K/39K . More excitingly, Proteus has also pioneered in-situ isotope analysis, allowing Rb-Sr dating of geological samples [3, 4] and Ti isotopes in chondrite samples  via LA-ICP-MS.
However, Proteus has two drawbacks. First, the sensitivity of the instrument is low. Second, the instrument bias does not follow an exponential mass bias law, which is crucial for applications that use internal normalization such as Sr, Nd, and Hf. Both of these limitations stem from the use of a quadrupole pre-cell mass filter. To combat these drawbacks, the Thermo ScientificTM
Vienna MC-ICP-MS/MS uses a double Wien filter. The Wien filter is essentially two electromagnets with a slit in the middle. The first magnet disperses the ions based on m/z ratio, the slit allows only a selected mass range to pass through, and the second magnet recombines the selected masses into a single ion beam before entering the CRC.
Unlike Proteus, Vienna MC-ICP-MS/MS does not limit the sensitivity and follows exponential mass bias, resulting in accurate and reproducible isotope ratios . The instrument showed the potential for systems such as Ca and K isotopes, as well as Rb-Sr . But where the pre-filter technology really sets itself apart from a CRC-only approach, is in applications such as Si isotopes, where molecular interferences (associated with the Ar sample gas) obscure the 29Si signal. Without a pre-cell filter, the CRC greatly amplifies the interferent peak, whereas with the pre-cell filter the interferent peak is greatly reduced and the sensitivity of Si doubles .
In summary, the recent development of pre-cell mass filters has greatly enhanced the range of applications possible to measure using MC-CRC-ICP-MS. Collision/reaction cells allow isobaric interferences to be removed, but without the pre-cell mass filter, the reaction chemistry in the cell is unpredictable and can result in inaccuracies in isotope ratio analysis. The double Wien filter of the Vienna MC-ICP-MS/MS provides both the ability to remove isobaric interferences and high sensitivity providing the gold standard for MC-ICP-MS isotope analysis.
1. Luu, T.-H. et al. High Precision Ca Isotope Measurements by Collision Cell MC-ICPMS. Goldschmidt Abstr. 2088 (2019)
2. Morgan, L. E. et al. High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K. J. **bleep**. At. Spectrom.33, 175–186 (2018).
3. Bevan, D. et al. Detrital K-Feldspar Geochronology by Collision Cell MC-ICPMS/MS. Goldschmidt Abstr. (2020).
4. Bevan, D. et al. In situ Rb–Sr dating by collision cell, multicollection inductively-coupled plasma mass-spectrometry with pre-cell mass-filter, (CC-MC-ICPMS/MS). J. **bleep**. At. Spectrom.36, 917–931 (2021)
5. Pfeifer, M. et al. In situ Titanium Isotope Measurements in Meteorites Using the Collision Cell MC-ICPMS, Proteus. Goldschmidt Abstr. (2019).
6. Craig, G. et al. Project Vienna: A novel pre-cell mass filtre for a collision/reaction cell MC-ICPMS/MS. **bleep**.Chem. (2021)