IAQ : The value of proton transfer reaction mass spectrometry

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Indoor air pollution
The versatility of the oft-forgotten technique proton transfer reaction mass spectrometry for indoor air applications has been demonstrated by European scientists who carried out five contrasting experiments, including the emission of volatiles from a laser printer and building board.

Indoor chemistry is vitally important to the health and welfare of workers and residents due to the myriad of airborne pollutants that can invade the space. They can originate from outdoors, as in the case of exhaust gases which enter via open doors and windows, as well as indoors where carpets, furnishings, machinery and building materials are all contributors.

Although many people with a knowledge of science would expect GC/MS to be used to detect and measure these pollutants indoors, a less well-known technique has been highly successful in this area. Proton transfer reaction mass spectrometry (PTR MS) can be performed at ambient air pressure and is only affected marginally by humidity. It functions by producing hydroxonium ions (H3O+) from water vapour in the air which react with the target compounds to produce protonated molecules for detection. In some cases, other ionising species, such as Xe+ or NO+, are employed.

The low proton affinity of H3O+ ions means that they do not react with the other major gases present in air but they do react with trace species, with readily attainable detection limits of ppt by volume. The proton transfer reaction is gentle, so that there is little fragmentation of the protonated species that are formed, giving simple spectra that are useful for determining the molecular masses of the volatile compounds.

PTR MS is gaining popularity in other areas too, like the analysis of human breath for markers of disease and studying the sensory properties and origins of food but a team of European scientists have carried out a number of experiments to illustrate its effectiveness and versatility for analysing pollutants in indoor air. Tobias Schripp and colleagues from Fraunhofer WKI, Braunschweig, Germany, and Lukas Maerk from IONICON Analytik GmbH, Innsbruck, Austria, who manufacture PTR instruments, shared their results in Indoor Air.

Watching paint dry
All of the experiments were carried out in environmental test chambers with H3O+ as the reagent ion and commercial PTR quadrupole of time-of-flight instruments. The target volatile compounds were quantified from a knowledge of the rate constants of the reactions with H3O+ and the reaction times.

In the first case, they followed the emission of triethylamine from four different paints after they were applied to glass plates. This amine is often added to paints to neutralise acids that are released during drying, but it has an unpleasant fishy aroma. The paints could be classified into strong and weak emitters from their emission curves, levelling off after various times up to 24 hours.

PTR MS is better than thermal desorption GC/MS, which introduces large margins of error as the emission profile changes during heating. It is also more accurate than methods like photoacoustic monitoring, which cannot distinguish between different volatiles.

Building materials
The emission of volatiles from a piece of oriented strand board was studied after a 24-hour equilibration period in parallel with GC/MS to assist with product identification. About 20 compounds were detected, including acetic acid, pentanal, benzaldehyde and α-pinene.

The TOF instrument can distinguish between ions of the same nominal molecular mass due to its higher resolving power. When combined with the selection of a panel of emitted compounds rather than the whole set, PTR MS can provide a rapid system for testing the boards to ensure compliance with industrial guidelines.

Another type of building material, gypsum boards, can be fabricated to inhibit the diffusion of volatile compounds from one side to the other or to act as buffers for organic compounds. The suitability of PTR MS for testing these properties was demonstrated in a double climate chamber with the diffusion of toluene from one side to the other through the board. The diffusion coefficient calculated from the results agreed with reported values.

Printing output and reference materials
Laser printers emit a mixture of ultrafine particles, and organic compounds, some of which interact with ozone to form secondary organic aerosols, which can cause health problems indoors. Here, styrene was targeted because it reacts with ozone, along with its by-product benzaldehyde, and their levels both increased sharply after the beginning of each printing phase.

A NIST foil reference material loaded with a known amount of toluene was the final example. In this case, an unexpected short peak appeared in the depletion curve due to a nearby heating source that was switched on briefly. After the source was switched off, the curve resumed its expected path, confirming that short-term aberrations need not necessarily affect PTR MS experiments in chambers.

These examples illustrate the range of indoor air investigations that PTR MS can assist but it is not all straightforward. Sometimes, as was shown, GC/MS is needed as back up for compound identification. In addition, the rate constants for the reaction with the ionising H3O+ species may be unknown, so calibration of the instrument is difficult. Nevertheless, it remains a valuable way to monitor the effects of many materials on indoor air quality.