A new paper by James documents an isobaric interference on 4He, which affects in-situ U-Th-He measurements on young and U-Th-poor samples. This interference varies with trap current and filament voltage but can be corrected for by monitoring double-charged carbon.
Reference: Schwanethal, J., 2015, Minimising 12C3+ interference on 4He+ measurements in a noble gas mass spectrometer, J. Anal. At. Spectrom. DOI: 10.1039/C4JA00422A