SinS 2025: PTR-MS evolves to tackle complex real-time analysis across air, food, environment

At the Solutions in Science 2025 conference in Brighton, UK, Dr Philipp Sulzer presented a comprehensive overview of recent technical advances in proton transfer reaction mass spectrometry (PTR-MS) and their expanding applications across analytical science.

Representing Ionicon Analytik, Sulzer traced the development of the technique from its early use of hydronium ions – H₃O⁺ – for chemical ionisation through to its modern versatility, incorporating multiple reagent ions and enhanced resolution. He highlighted case studies in flavour chemistry, environmental monitoring and microplastic detection, underscoring PTR-MS as a maturing platform that continues to evolve in response to emerging analytical demands.

“PTR-MS enables fast-response, high-sensitivity measurement of volatile organic compounds in air and gaseous samples.

“What sets it apart is the ability to perform real-time, non-invasive analysis across a wide range of environments and matrices,” explained Sulzer.

Commercialised by Ionicon more than 25 years ago, PTR-MS operates by transferring a proton from a reagent ion – commonly H₃O⁺ – to the target analyte within a reaction chamber. The resulting product ions are then identified by time-of-flight mass spectrometry. By extending the reagent ion options to include species such as O₂⁺, NO⁺, and CO₃⁻, Ionicon has enabled selective ionisation of a broader range of compounds, increasing the technique’s flexibility.

To demonstrate the immediacy and responsiveness of PTR-MS, Sulzer described a live experiment in which two individuals exhaled into the instrument. The system successfully captured isoprene, a metabolic by-product, and acetonitrile, a known biomarker for tobacco smoke. When a cup of coffee was placed near the inlet, the instrument detected a complex and immediate array of volatile compounds from the beverage’s headspace. These demonstrations illustrated the sensitivity and real-time capabilities of PTR-MS for sampling diverse sources.

Despite its strengths, PTR-MS has faced challenges. Sulzer described how the earlier inability to resolve isobaric compounds – those with identical nominal masses – posed a limitation. In one case, a food manufacturer sought to measure retronasal retention of a garlic flavour compound during soup consumption.

Initial PTR-MS analysis failed to isolate the relevant sulphur compound, as its signal was obscured by overlapping ions. A decade later, using a system with mass resolution up to 15,000 Ionicon revisited the study. The garlic-derived molecule at m/z 119 was successfully differentiated from at least eight other overlapping species. With a revised interface and disposable nosepieces, the company captured real-time concentration data as the compound passed through the nasal cavity during and after eating, offering the client valuable insights into flavour release kinetics.

Another technical limitation addressed was the influence of ambient humidity on PTR-MS signal stability. Although PTR-MS is generally robust to humidity variation, Sulzer showed that signals for formaldehyde, hydrogen sulphide and hydrogen cyanide exhibited notable fluctuations with changes in water vapour content, despite constant analyte concentration. To address this, Ionicon implemented a ‘generic humidity control’ strategy by injecting a controlled stream of water vapour into the reagent ion source. A feedback loop stabilised internal humidity, thereby improving measurement reliability for humidity-sensitive compounds, particularly in field conditions.

Sulzer also described the integration of software-based automation through Ionicon’s Automated Measurement and Evaluation (AME) platform. AME adjusts operating conditions dynamically and applies machine learning-based pattern recognition to identify and quantify analytes with minimal user intervention.

This automation has enabled deployment of PTR-MS in high-throughput environments such as fence-line monitoring at an industrial site. In Saudi Arabia, four PTR-MS instruments have been installed around a large petrochemical facility to monitor emissions through kilometres of air sampling lines. A similar system is in use in Austria to assess urban air quality, offering continuous, high-resolution tracking of volatile organic compound pollution.

Long-term reliability remains a priority. In one study, the PTR-MS platform was operated for 18 days using gas standards of acetone and benzene and signal fluctuations remained within a 10% margin – an indication of the instrument’s stability for prolonged unattended use.

Sulzer concluded with a discussion of recent efforts to adapt PTR-MS to detect airborne and waterborne microplastics and nanoplastics – substances that do not naturally enter the gas phase and therefore fall outside conventional PTR-MS analysis. Drawing on external research, Ionicon has developed a proof-of-concept approach in which water samples containing plastic particles are dried and thermally decomposed.

The resultant pyrolysis vapours are then introduced into the PTR-MS instrument. Initial results with polystyrene showed a clear, linear response to increasing mass concentrations between 5mg and 15mg. While acknowledging the need for further refinement and validation, Sulzer noted that this work represented a promising step towards adapting PTR-MS for the growing challenge of plastic pollution.

He emphasised that despite PTR-MS’s now long-standing maturity, recent advances have enabled it to address increasingly complex analytical tasks. Improvements in mass resolution, environmental robustness and automation have broadened its applications across disciplines, including food analysis, industrial monitoring, air quality assessment and environmental science.