Watching paint dry: Proton transfer mass spectrometry for indoor studies

Indoor air quality

Human health is affected by the quality of indoor air and there are countless sources of airborne contaminants. Emissions from adhesives in carpets, paint, wood fires, cooking, building materials and electronic devices like cell phones, TVs and computers are just some of the contributors. Being indoors, their effect is magnified because concentrations can build up if there is poor ventilation.

The quality of indoor atmospheres can be assessed by a technique called proton transfer reaction mass spectrometry (PTR-MS) due to the growing commercial availability of relatively small, mobile instruments that can be moved from location to location. PTR is finding favour in a range of applications apart from air analysis, such as breath analysis for disease diagnosis and food analysis for sensory analysis and quality control.

In the simplest set up, H₃O⁺ ions are produced by a hollow cathode discharge and are reacted with the pollutant molecules to give protonated molecules that are detected in a mass spectrometer. The proton affinity of the target molecules must be greater than that of water for the reaction to proceed but this is the case for many common indoor pollutants. If not, other reagent ions like nitric oxide or krypton can be injected to produce the protonated reagents.

The ionisation technique is gentle, generally producing no other ions apart from the protonated molecules, but this is sufficient for monitoring purposes if the analytes are known. Apart from detecting certain airborne pollutants, PTR-MS can also be used in a dynamic way to follow their levels over time. The viability of this approach has been demonstrated by European scientists who carried out a range of different experiments on different materials.

Tobias Schripp and colleagues from Fraunhofer WKI, Braunschweig, Germany, and IONICON Analytik GmbH, Innsbruck, Austria, a company which manufactures a range of PTR-MS instruments, described five applications of PTR-MS in Indoor Air.

Watching paint dry

In the first illustration, four water-based white paints known to contain triethylamine were applied to glass plates and placed in a chamber attached to a PTR quadrupole mass spectrometer. The instrument was set to monitor m/z 102, corresponding to the protonated triethylamine molecule, over the next 25 hours. The concentrations were calculated using additional thermal desorption GC/MS measurements after the amine was collected in a trap.

All paints released triethylamine at a fast rate to begin with, before flattening out, but two were classified as strong emitters and the others as weak emitters of the unpleasant smelling compound. This type of experiments is useful for manufacturers trying to change the paint formulation as well as for users wishing to optimise the drying process.

Printing volatiles and building boards

In a second example, the volatiles emitted by a laser printer during operation were monitored, following m/z 105 and 107 for styrene and benzaldehyde, respectively. The high emission rates indicated that the compounds were reacting with ozone in the testing chamber, the ozone being produced by the printing process, although the species involved could not be determined at this stage.

Diffusion of toluene through a gypsum board used as a building product was studied in a double climate chamber, measuring m/z 93. The method allowed the diffusion coefficient to be calculated, giving a value in agreement with published values. PTR-MS was also used to measure the emission of toluene from special NIST reference foils as part of a round robin test and the release rates agreed with those from other participants.

These four examples used PTR quadrupole mass spectrometers but another application used a PTR-time-of-flight instrument in which the high-resolution capabilities allowed compounds to be identified as well as measured. This ability was demonstrated by studying the steady-state emission of volatile compounds from an oriented strand board, a type of building board.

The research team recommend the use of PTR-MS for analysing processes and materials in test chambers, like those they used for the current experiments. Having said that, they point out that the technique does have its drawbacks. Target compounds with low proton affinities could be influenced by the humidity of the surrounding air. In addition, calibration to determine the analyte concentrations can be difficult.

The positive points are the good time resolution, high sensitivity and robustness of the technique. The low mass resolution of PTR-MS on quadrupole instruments can be countered by the new generation of PTR-TOF mass spectrometers to give a broadly applicable technique for studying indoor volatile compounds.