Quantification of hydrogen in a gas mixture of noble gases

Quantification of hydrogen fraction in the gas mixture with inert gases kept in a small enclosure of the gas surge arrester (GSA) is a challenging task. Hydrogen greatly influences device properties, but as an omnipresent gas it represents also the background of any mass spectrometer. Hydrogen fraction in a particular GSA was quantified after its puncture in an evacuated batch inlet and subsequent introduction to a pumped chamber housing a quadrupole mass spectrometer (QMS). Its calibration was performed by an innovative in-situ calibration procedure which should yield high accuracy. In the first stage, a pure gas (Ar, Ne, H2) contained in a calibrated volume was set by a leak valve to flow into the UHV system. The pressure change reading of the capacitance manometer over time gives the flow rate which is directly correlated to the ion current of a specific mass peak in the span of 3 orders of magnitude. The extracted calibration curves of the QMS for each gas species are applied in the second stage of the calibration when known gas mixtures are prepared in the calibrated volume to verify the gas composition determination procedure based on the fractionation. Such procedure is revealed fairly accurate at high (above ∼6 × 10−5 mbar L/s) flow rates, however a significant error appeared at lower flow rates. Possible explanations for erroneous hydrogen determination at very low fluxes by the QMS in the mixture with argon are presented.