Page "Selected-ion flow-tube mass spectrometry" Paragraph 10

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, H < sub > 3 </ sub > O < sup >+</ sup >, NO < sup >+</ sup > and O < sub > 2 </ sub >< sup >+</ sup >, 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 H < sub > 3 </ sub > O < sup >+</ sup > as a precursor ion ( because it has a proton affinity of 543. 5kJ / mol, somewhat less than that of H < sub > 2 </ sub > O ), but can be analysed using O < sub > 2 </ sub >< sup >+</ sup >.

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 sulphide ( C < sub > 2 </ sub > H < sub > 6 </ sub > S, 62amu ) accepts a proton when it reacts with H < sub > 3 </ sub > O < sup >+</ sup > to generate C < sub > 2 </ sub > H < sub > 7 </ sub > S < sup >+</ sup > 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, H < sub > 3 </ sub > O < sup >+</ sup > CO < sub > 2 </ sub >, and the single hydrate of the protonated acetaldehyde ion, C < sub > 2 </ sub > H < sub > 5 </ sub > O < sup >+</ sup >( H < sub > 2 </ sub > O ), which also appear at m / z 63, and so it may be unidentifiable in certain samples.

However dimethyl sulfide reacts with NO < sup >+</ sup > by charge transfer, to produce the ion C < sub > 2 </ sub > H < sub > 6 </ sub > S < sup >+</ sup >, which appears at m / z 62 in resulting spectra, whereas carbon dioxide does not react with NO < sup >+</ sup >, and acetaldehyde donates a hydride ion, giving a single product ion at m / z 43, C < sub > 2 </ sub > H < sub > 3 </ sub > O < sup >+</ sup >, and so dimethyl sulfide can be easily distinguished.