Argon impact on the quantification accuracy of a QMS in gas mixtures with nitrogen or carbon monoxide at very low partial pressures

Quantification accuracy of nitrogen and carbon monoxide in argon used as the matrix gas was studied by quadrupole mass spectrometers (QMS) at very low partial pressures when no interference between ions was expected. The effect of mutual ion interactions within the ionization cell is well recognized, but it arises at much higher pressures. Our measurements were performed in an UHV system designed for pressure gauge calibrations. In two in situ calibrated QMSs with different ion source designs, the argon leak rate noticeably increased the QMS signal of a trace gas: at low leak rates, nitrogen was increased for a factor of up to ∼3, and carbon monoxide signal for a factor of up to ∼2.8 at the first QMS, while this effect was much smaller in the other QMS. The anomaly was not observed at high leak rates of trace gases. The underlying physical process seems to be the argon-assisted charge transfer to nitrogen and carbon monoxide molecules adsorbed on the quadrupole rods in the filter section. Despite the fact that this physical effect is manifested as an instrumental error, the accuracy of quantification of unknown gas mixtures containing these gases can be substantially improved after numerical correction factors for particular QMS device have been determined.