Photoionisation (PI) and Photochemical Ionisation

A photoionisation source (PI) is a device that utilises the energy of photons to cause molecules to lose electrons and form ions. The key principle is that the energy of the photons emitted by the light source must exceed the ionisation energy of the molecule under investigation; when a molecule absorbs a photon and its energy exceeds the ionisation energy, it releases an electron and becomes a positive ion. This method of ‘ionisation by photon impact’ falls under the single-photon ionisation pathway.

Photochemical ionisation is a form of ‘soft ionisation’. The energy of the photons is precisely controlled; it is usually sufficient only to cause the molecule to lose a single electron, but insufficient to cause extensive bond breaking, and therefore almost no fragments are produced during the ionisation process. This characteristic offers two direct benefits: firstly, the spectrum consists mainly of molecular ions and a small number of adduct ions, making it clean and easy to interpret; secondly, as there are few fragments, the peaks of different substances on the mass axis are clearly separated from one another, allowing the chromatographic separation step to be bypassed and enabling real-time analysis of mixed gases or complex samples.

Among New Source’s products, the photoionisation source is a core module found across multiple instruments. The ProC-1 and ProC-2 online time-of-flight mass spectrometers, as well as the ProC-1C in-situ catalytic/pyrolysis mass spectrometry analysis system and the ProC-1S flue gas multi-component dynamic analysis system, all utilise a specialised photoionisation source in their ionisation section. This is capable of performing single-stage ionisation of virtually all constituents in environmental gases—including hydrogen, oxygen, carbon monoxide, carbon dioxide, nitrogen oxides, sulphur-containing/ chlorinated compounds, hydrocarbons, polycyclic aromatic hydrocarbons and virtually all other constituents in ambient gases through a single ionisation step, thereby obtaining mass spectra for all analytes within seconds. In the field of mass spectrometry imaging, the MSI DPI incorporates a photodissociation system downstream of the DESI nebuliser to perform secondary photochemical ionisation on the desorbed neutral molecules; MSI LDPI, on the other hand, combines laser desorption with post-photochemical ionisation; both utilise photochemical ionisation to achieve ‘non-polarity-discriminating’ broad-coverage detection.

It is important to note that the photoionisation source is both a standalone component and the energy source for the ‘post-photoionisation/secondary photoionisation’ technical approach. Operating at atmospheric pressure and working in concert with the downstream ion transfer lines and mass analyser, it determines the instrument’s overall sensitivity and non-polar discrimination capability. In short, the ionisation source is responsible for ‘converting molecules into ions’, whilst the distinctive feature of photochemical ionisation lies in achieving ‘clean and comprehensive’ results; this is precisely the technological foundation that enables several of New Source’s products to perform direct, real-time analysis without the need for chromatographic separation.