Minimising 12C3+ interference on 4He+ measurements in a noble gas mass spectrometer

Abstract

Measurements of helium isotopes used for geochronological studies are subject to interferences from ¹²C³⁺ and HD⁺. The ¹²C³⁺ interference was studied on a Nu Instruments Noblesse mass spectrometer which has sufficient mass resolution to measure the interference on the ⁴He⁺ peak due to ¹²C³⁺ directly, by setting the magnet at the relevant peak shoulders. Helium sensitivity varies with source trap current. Increasing the source trap current increases the electron fluence, and hence the probability of ionisation, within the source. The source trap current on the Noblesse is typically held at ∼400 μA. Increasing the trap current to 550 μA increases sensitivity to ⁴He⁺ approximately two-fold, but ¹²C³⁺ increases approximately 17 times. The ⁴He⁺ and ¹²C³⁺ peak shoulders at m/z = 4 are narrow, despite the high mass resolution, and consequently any instrument drift can cause the peak positions to be lost. However, it is shown that measurement of ¹²C²⁺ at m/z = 6 can be used to correct for this interference, allowing the intensity of the broad flat top of m/z = 4 to be used.