Performance of the double-Wien filter of the Neoma MC-ICPMS/MS with an application to copper stable isotope compositions

Abstract

The new Neoma MC-ICPMS/MS is equipped with a prefiltering system consisting of a double-Wien filter and a collision/reaction cell whose performances are challenged using different combinations of magnetic and electrostatic field values and adjustable slit apertures. The results show an asymmetrical attenuation of transmission relative to the chosen axial mass-to-charge value, with higher efficiency at removing low masses than high masses, even when magnetic induction is minimal. The resulting asymmetry of the bandpass window is fully predictable by theoretical calculations of ion trajectories in a Wien filter, either as a function of the magnetic field value or that of the aperture of the adjustable slit. With an axial mass-to-charge value set at ¹²⁰Sn, the vertical deviation for a magnetic field value at 100% will be approximately 3 mm and 4 mm for a variation of mass-to-charge of ±20%, respectively. In these conditions, Ar was already barely detectable with the lowest magnetic field value (10%) and a fully open adjustable slit, while Pb is quantitatively transmitted. We then use the prefiltering system to remove on-line the ⁴⁰Ar²³Na⁺ compound that produces an isobaric interference with ⁶³Cu, hampering high-precision measurement of Cu stable isotope composition (⁶⁵Cu/⁶³Cu). While Na is not transmitted thanks to the double-Wien filter, ⁴⁰Ar²³Na⁺ still interferes with ⁶³Cu, demonstrating that this argide is produced in the plasma source but not in the reaction cell. Helium in the collision/reaction cell is necessary to remove the ⁴⁰Ar²³Na⁺ interference. The MS/MS technology of the Neoma allows for the correction of the ⁴⁰Ar²³Na⁺ interference up to a Na/Cu ratio of 10, where other classic MC-ICPMS already show an offset of the Cu stable isotope composition for a Na/Cu ratio of 1. The inability to correct the Cu stable isotope composition with a Na/Cu ratio higher than 10 suggests that the Na-based interference is no longer spectral and becomes linked to the matrix. We next measure the Cu stable isotope composition in eight certified reference materials prepared with a simple automated single step ion-chromatography procedure to purify Cu. The results show a very good agreement with previously reported values. The overall results suggest that the MS/MS technology of the Neoma MC-ICPMS allows efficient on-line isolation of analytes, therefore reducing potential spectral and matrix interferences to permit much better resolved and controlled subsequent effects in the collision/reaction cell.