A method for measurement of 36Ar without H35Cl interference

- Measured 36Ar/40Ar using a multicollector mass spectrometer with MRP = 6000.
- The 36Ar peak is difficult to edge-resolve from H35Cl (m/dm = 3938).
- Target position on 36Ar is aligned with the 50% point on side of 40Ar.
- This verifies the mass during actual analysis; 40Ar measured in separate step.
- Small corrections are described for HCl tail and imperfect peak shape.

Most noble gas analyses are made in static mode when instrument volume is minimized to maintain adequate sensitivity. This makes the building of large instruments to obtain high resolving power impracticable. A method is presented which makes improved use of the available resolving power to remove isobaric interferences, which may be used on multicollector instruments. By arranging that the target mass position on a minor isotope (e.g. 36Ar), from which the interference must be removed, coincides with the approximately 50% point on the side of a major isotope (e.g. 40Ar), it is possible both to set the mass accurately and to verify the mass position and stability during measurements. The peak top of the major isotope is measured in a separate mass step. Calibration measurements are necessary, using different relative amounts of target/interference, to assess residual tailing to the measurement position and also the relative efficiency at the extreme edge of the target peak. The method is demonstrated by using it to obtain 36Ar measurements free of H35Cl. With samples containing 4 × 10− 15 to 3 × 10− 14 mol of 40Ar, 36Ar/40Ar was measured, without HCl interference, to a 1σ precision of 0.5%, only slightly worse than counting statistics. This is potentially useful for 40Ar/39Ar dating, where 36Ar is used to correct for trapped air, and may be particularly significant for smaller or younger samples.