Proton Transfer Reactions and Real-time VOC Analysis

The proton transfer reaction (PTR) is a mild chemical ionisation technique specifically designed for the real-time detection of volatile organic compounds (VOCs). At its core is a beam of reagent ions, H₃O⁺ (hydrated hydrogen ions): when the sample gas mixes with H₃O⁺, any compound with a ‘proton affinity’ higher than that of a water molecule (which applies to the vast majority of VOCs) will readily snatch a proton from H₃O⁺, transforming into a positively charged molecular ion, MH⁺. As water molecules themselves are not easily ionised, the background remains exceptionally clean.

The appeal of PTR lies in its ‘gentle’ and ‘rapid’ nature. Gentle ionisation means that VOCs undergo virtually no dissociation; the spectrum consists predominantly of intact molecular ions, allowing for direct identification. Its rapidity is evident in the fact that no chromatographic separation is required; results are available as soon as the gas enters the instrument, with a temporal resolution of seconds or even higher, making it suitable for monitoring real-time fluctuations in concentration. When combined with high-resolution mass spectrometry, sensitivity can reach the ppt–ppb range, sufficient to detect trace emissions.

It is a ‘cousin’ of photochemical ionisation (PI): PI uses vacuum ultraviolet photons to ionise neutral molecules, whilst PTR uses proton transfer from H₃O⁺ to ionise molecules; both are forms of soft ionisation, both provide real-time results, and both eliminate the need for chromatography. The difference lies in the distinct ‘triggering agents’ for ionisation—photons are sensitive to ionisation energy, whilst proton transfer is sensitive to proton affinity—therefore, each has its own focus in terms of the range of compounds covered and fragmentation behaviour, and the two are often used complementarily.

Relevant to New Source’s products: The ProC series of online time-of-flight mass spectrometers offers real-time, chromatography-free analytical capabilities, enabling online analysis of VOCs and reaction gases in scenarios such as catalytic reaction exhaust, flue gas and ambient air. By combining the PTR-style soft ionisation approach with high-resolution TOF, researchers can observe the temporal evolution of product spectra whilst the reaction is underway, without having to wait for results from offline chromatography.

In terms of applications, PTR is widely used in environmental VOC mobile monitoring, breath gas biomarker screening, and the monitoring of flavour release in food; it is an efficient option for any volatile system requiring ‘measurement whilst it occurs’.

A clear example is breath gas analysis: VOCs such as acetone and isoprene in human exhaled breath are linked to metabolic status, and PTR-TOF can report the real-time concentrations of dozens of biomarkers simultaneously from a single breath, which can be utilised for disease screening or nutritional research. Interestingly, PTR and photochemical ionisation (PI) are often found on the same online mass spectrometer—the former utilises H₃O⁺ proton transfer, whilst the latter employs vacuum ultraviolet photons. By switching between the two depending on the sample characteristics, users can cover a broader range of compounds on a single platform. For the New Source ProC series, this ‘switchable soft ionisation’ design enables a single instrument to analyse both reaction gases and environmental VOCs.