Abstract
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.
Highlights
Quantification of very small gas amounts by a quadrupole mass spectrometer (QMS) requires a very careful in situ calibration procedure of the instrument at the lowest range of partial pressures
The system consisted of a main spherical calibration chamber of V 0.040 m3, two turbo-molecular pumps, extractor type ion gauge (EXTR), calibrated spinning rotor gauge (SRG), two quadrupole mass spectrometers, QMS 1 (PrismaPlus QMG 220 F1 with yttriated iridium filament) and QMS 2 (PrismaPlus QMG 220 M2 with tungsten filament), and two inlets for calibration gases
We introduce a relative measure of the argon impact: the enhancement factor (EF)
Summary
Quantification of very small gas amounts by a quadrupole mass spectrometer (QMS) requires a very careful in situ calibration procedure of the instrument at the lowest range of partial pressures. In most relevant publications on recommended practices it is assumed that at low partial pressures, the achieved sensitivity for a particular pure gas is constant over a wide pressure range, which enables accurate and trusted quantification of gas mixtures.[1,2] The effect, which is described as interference is an important source of error only at relatively high ion currents when the space charge repulsion within the ionization cell causes deviations from constant sensitivity.[3,4] Basic limitations in quantification of small gas amounts are set by physical limitations.[5] The explicit argon impact on hydrogen sensitivity has been observed in hydrogen-argon mixtures, but not explained in our previous work.[6] To get an interpretation, and at least partial explanation, more systematic work had to be performed on another ultrahigh vacuum (UHV) system equipped with two QMSs with different ion source designs
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