Did you know that SIFT-MS has truly soft chemical ionization? Here, we provide evidence based on a sensitive measure of ion energy.
One of the ion-molecule reaction mechanisms utilized in SIFT-MS – association – is extremely sensitive to reagent ion energy. The association mechanism requires low reagent ion energies to occur because – as the name implies – the reagent ion and analyte molecule are “associated” in the detected product ion. To enable association to occur, the carrier gas (or sample matrix) itself is also implicated in the reaction, providing a “third body” particle that carries excess energy from the reaction collision, enabling the reagent ion and product ion to bind together. If there is too much energy in the reagent ion at the time of the collision with the analyte, then it is very likely that the third-body molecule cannot carry sufficient energy away and the reactants will not associate. In this case, the product ion corresponding to association will not be detected.
The table below relates experimentally observed reaction mechanisms for ketones with the SIFT-MS NO+ reagent ion to their ionization energies. If the ionization energy of the ketone is higher than that of NO (9.2642 electron volts, eV; https://doi.org/10.18434/T4D303), then electron transfer ionization becomes less and less probable as the energy difference increases. Under these conditions, association occurs as long as the NO+ reagent ion is close to ambient (or thermal) energy. The SIFT-MS NO+ ions are near thermal energy and hence the predicted ionization behavior is observed: association occurs for all ketones listed as indicated by the product ion m/z corresponding to the molecular weight + 30 (for NO+). Only 3-hexanone undergoes a small amount of electron transfer due to its similar ionization energy compared to NO. Hence SIFT-MS provides truly soft ionization.
|
Compound [molecular weight in g mol‑1] |
Ionization Energy (electron volts; eV) |
Predicted Electron Transfer Product with Thermal NO+ Ions |
Observed Product Ions with Thermal SIFT-MS NO+ Reagent Ions |
|
Acetone [58] |
9.70 |
No |
88 (100%) |
|
Methyl ethyl ketone (MEK) [72] |
9.52 |
No |
102 (100%) |
|
2-Pentanone [86] |
9.38 |
No |
116 (100%) |
|
3-Pentanone [86] |
9.31 |
No |
116 (100%) |
|
2-Hexanone [100] |
9.35 |
No |
130 (100%) |
|
3-Hexanone [100] |
9.12 – 9.30 |
Maybe |
130 (85%); 100 (15%) |
|
2-Heptanone [114] |
9.27 |
No |
144 (100%) |
Notes related to the table:
- The SIFT-MS data in this table are from P. Španěl, Y. Ji and D. Smith, Int. J. Mass Spectrom. Ion Process., 1997, 165, 25 (https://doi.org/10.1016/S0168-1176(97)00166-3) and D. Smith, T. Wang and P. Španěl, Rapid Commun. Mass Spectrom. 2003, 17, 2655. (https://doi.org/10.1002/rcm.1244).
- Two values for the 3-hexanone ionization energy (9.12 and 9.30 eV) are given in the NIST database (https://doi.org/10.18434/T4D303) and the ionization energy of NO lies between them. The occurrence of some electron transfer product for 3-hexanone suggests that its ionization energy is similar to NO.