Proton Transfer Reactions Using H3O+ Ion Beams
In this form of chemical ionisation (CI) a primary beam, H3O+ (or 'Hydronium'), is created from an ion source, in Kore's case a hollow cathode glow discharge source. The beam is passed into the PTR section where analyte is introduced. The pressure is of the order of 1 millibar in this section. This PTR drift section has a set of parallel plate electrodes defining a field gradient, and the ions move toward the end of the PTR section under the influence of that field. The value of the electric field divided by the number of molecules gives the Townsend Number (E/n). When the E/n value is high, the analyte molecules can collide too energetically and fragmentation results. However, if the E/n ratio is too low, water molecules in the primary beam begin to form clusters and these are less effective at ionising the analyte molecules.
Like other researchers, we also have a short section between the exit of the ion source and the entrance to the PTR section called the "source drift". This section can have water vapour fed into it (to help clean up the primary beam further), or other gases can be used to exploit alternative reactions to produce other ions.
For a linear response, the technique requires that the primary beam species are always in excess compared to the analyte, which is dilute. Up to 50ppm of an analyte (approximately) can be analysed without the hydronium population being reduced significantly. In this dilute regime, simple kinetics can be used to relate the ion signals recorded by the mass spectrometer to absolute concentrations of analyte molecules
In PTRMS, a primary species such as H3O+ is the ionising species. It is an ion because it has an extra proton, not because it has lost an electron. H2O has a 'proton affinity' of 691 kJ/mol. If H3O+ collides with another molecule, that molecule can be ionised if it has a proton affinity that is greater than 691 kJ/mol.
General components in air are not ionised by the hydronium beam, but most volatile organic compounds (VOCs) are ionised by H3O+ with little or no fragmentation. Other molecules such as hydrogen sulphide (H2S), hydrogen cyanide (HCN) and ammonia (NH3) can be detectable by this H3O+ PTRMS method.
Below is a table of proton affinities for various gaseous components. Any molecule with a proton affinity less than that of water will not be ionised by H3O+, and other ionisation methods such as electron impact ionisation or chemical ionisation by ion-molecule reactions (Ar, Kr, Xe) need to be used to ionise those components.
| Species | Name | PA (kJ/mol) |
|---|---|---|
| He | Helium | 178 |
| Ar | Argon | 369 |
| O2 | Oxygen | 421 |
| H2 | Hydrogen | 422 |
| N2 | Nitrogen | 493 |
| NO | Nitric Oxide | 532 |
| CO2 | Carbon Dioxide | 541 |
| CH4 | Methane | 543 |
| N2O | Nitrous Oxide | 550 |
| HCl | Hydrochloric Acid | 557 |
| NO2 | Nitrogen Dioxide | 591 |
| CO | Carbon Monoxide | 594 |
| O3 | Ozone | 625 |
| C3H8 | Propane | 626 |
| COS | Carbonyl Sulphide | 628 |
| SO2 | Sulphur Dioxide | 672 |
| C4H10 | Butane | 677 |
| H2O | Water | 691 |
| C2H5Cl | Ethylene Chloride | 693 |
| H2S | Hydrogen Sulphide | 709 |
| HCN | Hydrogen Cyanide | 712 |
| C6H6 | Benzene | 750 |
| CH3OH | Methanol | 754 |
| C2H5OH | Ethanol | 776 |
| C4H6 | 1, 3 Butadiene | 783 |
| C7H8 | Toluene | 784 |
| PH3 | Phosphine | 785 |
| C8H10 | Ethyl Benzene | 788 |
| C8H10 | p-Xylene | 794 |
| NH3 | Ammonia | 854 |
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Last updated: 29 September 2005 22:44
© Kore Technology Limited 2005