PTR Method Using H₃O⁺ (Ionisation of analyte by proton transfer from H₃O⁺).

In this mode a primary beam, H₃O⁺ (or ‘Hydronium’), is created within the hollow cathode glow discharge source. The beam is passed into the PTR section where analyte is introduced. General components in air are not ionised by the hydronium beam, but some inorganic gases and most organic gases are ionised in a ‘soft’ manner that preserves the molecule.

The technique requires that the primary beam species are always in excess compared to the analyte, which is dilute. Up to (approximately) 50ppm of an analyte can be analysed without the hydronium population being reduced significantly (thus ensuring a linear response).

Example Data: H₂S detection in PTR Mode

In a conventional mass spectrometer, detecting S in the presence of O₂ is not trivial. The main isotope of S is ³²S with an exact mass of 31.972. The most abundant isotope of oxygen is ¹⁶O, and the exact mass of ¹⁶O₂ is 31.989. Thus, both ions appear at mass 32. This is known as an ‘isobaric mass interference’.

The H₂S ion occurs at mass 34 (¹H₂³² S). Unfortunately, one of the (isotopic) ions of O₂ is ¹⁶ O¹⁸O⁺ , and this also has a mass of 34. The ‘exact’ masses of these two molecules are 33.9877 and 33.994 respectively, and thus a mass spectrometer would need a resolving power of 5,400 to separate the two mass peaks. This is certainly possible (though not with a quadrupole MS), but it has implications for the size and cost of such an instrument.

The PTR-TOF instrument offers an alternative way to overcome such ‘mass interference’.

In the PTR-TOF (proton transfer reaction TOF), it is possible to employ an ionisation scheme that uses the ion H₃O⁺ to ionise the compounds of interest, by a method known as “proton transfer”. H₂O has a ‘proton affinity’ of 691 kJ/mol. This is less than H₂S, which has a proton affinity of 709 kJ/mol, and so in a collision H₂S will remove the proton from H₃O⁺ and become ionised. By contrast, O₂ has a proton affinity of 421 kJ/mol, and will not be ionised by H₃O⁺. Thus it should be possible to distinguish completely between H ₂S and O₂, because only the H₂S will be ionised in this particular PTR scheme.

This is mass spectral data was acquired from a sample of H₂S (50ppm) in Nitrogen (log scale Y-axis)

600,000 c/s of were recorded at mass 35 (H₂S⁺), i.e. 12,000 c/s per ppm. This is equivalent to a detection sensitivity of 50ppb in 0.1 second.

This is data acquired immediately after the H₂S sample, from room air. The signal at mass 35 has disappeared. Example Data: Methanol Detection in PTR Mode

Methanol (CH₃OH) has a proton affinity of 754kJ/mol compared to 691kJ/mol for water. Thus, methanol can be ionised by proton transfer.

The adjacent data was acquired from a gas mixture of 10ppm methanol in nitrogen. The molecular mass of methanol is 32, but with an added proton the appearance mass is at 33. Greater than 9 million c/s second were recorded at mass 33.

Chemical Ionisation by Ion Molecule Reaction

(Ionisation of analyte by ion – molecule reactions)

In this mode, the glow discharge is run on an inert gas such as argon, xenon or krypton, instead of N₂/H₂O mixture. The primary ion produced from the glow discharge source has a specific energy in its ionised state. If it impacts a molecule in the collision cell and that molecule requires less energy to be ionised, then that molecule can become ionised and the primary ion will convert to a neutral particle. The sequence is as follows:

Kr⁺ (13.99 eV) + CO → CO⁺ (13.98 eV) + Kr
(Chemical Ionisation by Ion-Molecule Reaction)

However, if the molecule requires more energy than is available in the rare gas ion, the molecule remains neutral, e.g.

Kr⁺ (13.99 eV) + N₂ → Kr⁺ + N₂
(No Chemical Ionisation, because 15.5 eV is required to ionise N2).

Example Data in Ion-Molecule Mode (Ar+ Ion Beam)

Argon has an ionisation energy of 15.76eV. SO₂ has an ionisation energy of 12.32eV, thus should be ionised in a collision with Ar⁺.

Adjacent is data acquired from a 50ppm SO₂ in Nitrogen sample. 100,000 c/s were at mass 65 for this analyte using Ar⁺

Electron Impact Ionisation (EI Mode)

The instrument features a conventional electron impact ionisation source in the high vacuum section. When an analyte of interest cannot be ionised by the proton transfer method, the glow discharge source may be switched off and the electron impact source activated.

Electron impact ionisation is a very common method, and a large database of EI spectra exists (NIST database). All components are ionised by this method, but the excess of energy does result in fragmentation. At times this can be useful for identifying a compound.

Example Data in EI-Mode

The adjacent spectrum was acquired from a calibration gas mixture containing 2.6% Hydrogen. The signal at mass 2 was 3.6 million counts per second in EI mode, and this equates to a detection sensitivity of 0.005% (limited by background count at mass 2 due to hydrogen in the system).