THE EFFECTS OF A QUADRUPOLE MASS ANALYZER ON MEASUREMENT OF UHV RESIDUAL-GAS COMPOSITION

The effect of both the quadrupole mass analyser (OMA) and the ion-gett er-pump (IGP) on the residual gas composition and on the composition d uring gas dosing were investigated. At the background pressure the mea sured height of mass 16 was more than 10-14 times the sum of contribut ions of methane and oxygen ions from other molecules and a significant amount of inert gases were found at this pressure level. Pure oxygen, carbon monoxide, hydrogen and a mixture of carbon monoxide and hydrog en were introduced into the vacuum system while the IGP was running. T he final concentration of the dosed gas depended on the ratio of its p umping speed to that of the inert gases. The pumping speeds followed t he sequence: oxygen > carbon monoxide > hydrogen. Exceptional behaviou r of mass 16 was observed during introduction of carbon monoxide and h ydrogen.