This work analyses some issues that arise when using the electron-impact ionization quadrupole mass spectrometry technique for the quantitative analysis of methanol in gas streams. The mass spectrum of methanol has been found to strongly depend on the type of carrier gas (argon or helium) and the concentration/partial pressure of methanol within the low methanol concentration range. Rigorous mathematical tools have been used to determine reliable values of relative intensity of cations produced by electron-ionization of methanol. The relative intensity of cations such as m/z 2, 15, 18 or 28 grows exponentially with decreasing methanol concentration values for below 2 vol%. This could lead to errors when measuring the concentration of gases like hydrogen, methane, water or carbon monoxide alongside methanol. This effect is increased by the use of helium as the carrier gas instead of argon, although the relative intensity values in both gases tend to converge for increasing values of methanol concentration. A tentative explanation for this phenomenon is proposed, based on the different ionization energies of argon and helium, and how this difference affects the ionization of neutral fragments and the ionization equilibria in which these fragments are involved. • The mass spectrum of methanol is highly dependent on the carrier gas (He or Ar). • The mass spectrum of methanol depends largely on its concentration in the carrier gas. • Changes in the mass spectrum are observed below 2% methanol in Ar and below 8% in He. • Carrier gas-dependent ionization of neutral fragments can explain the observed changes. • If these facts are neglected, the simultaneous determination of other gases ( e.g., H 2 ) may be very inaccurate.
Marbán et al. (Fri,) studied this question.