Another key feature of SIFT-MS is the upstream mass quadrupole, which allows the use of multiple precursor ions. The ability to use three precursor ions, H3O+, NO+ and O2+, to obtain three different spectra is extremely valuable because it allows the operator to analyse a much wider variety of compounds. An example of this is methane, which cannot be analysed using H3O+ as a precursor ion (because it has a proton affinity of 543.5kJ/mol, somewhat less than that of H2O), but can be analysed using O2+. Furthermore, the parallel use of three precursor ions may allow the operator to distinguish between two or more compounds that react to produce ions of the same mass-to-charge ratio in certain spectra. For example, dimethyl sulfide (C2H6S, 62 amu) accepts a proton when it reacts with H3O+ to generate C2H7S+ product ions which appear at "m/z" 63 in the resulting spectrum. This may conflict with other product ions, such as the association product from the reaction with carbon dioxide, H3O+CO2, and the single hydrate of the protonated acetaldehyde ion, C2H5O+(H2O), which also appear at "m/z" 63, and so it may be unidentifiable in certain samples. However dimethyl sulfide reacts with NO+ by charge transfer, to produce the ion C2H6S+, which appears at "m/z" 62 in resulting spectra, whereas carbon dioxide does not react with NO+, and acetaldehyde donates a hydride ion, giving a single product ion at "m/z" 43, C2H3O+, and so dimethyl sulfide can be easily distinguished.