THE DIFFICULTY OF DETECTING LOW VOLATILITY SPECIES FOR VOC ANALYSIS TECHNOLOGIES

Time evolution of the signals detected by the SUPER SESI-MS.

As you probably already know, current VOC MS analyzers can detect high volatile species, even when they are present in very low concentrations. For instance, in 2009 P. Spanel and D. Smith measured the concentration of ammonia, acetone, methanol, ethanol, and isoprene in human breath using Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) [1] . In another study by B. J. Prince, M.J. McEwan et. al , 1,3-Butadiene was detected at a concentration of 9 ppt [2] in ambient air. This study also measured Toluene, Benzene, and Ethanol. Proton Transfer Reaction Mass Spectrometry (PTR-MS) has been used in several applications to detect VOCs at very low concentrations, providing limits of detection in the ppt range.

THINGS GET MORE COMPLICATED WITH LOW VOLATILITY SPECIES

With a ppt sensitivity, and if sensitivity was the only limiting factor, one would expect to be able to detect very low volatility species, with vapor pressures in the range of 10 -12 Bar. However, the less volatile species mentioned in the recent review on PTR-MS are Phenol and Aniline [3] (see Table 1 of the mentioned review). The vapor pressures of Aniline and Phenol are 5·10 -4 Bar and 8·10 -4 Bar, which are much higher than the theoretically expected limit of 10 -12 Bar for a sensitivity in the ppt level. This mismatch of over seven orders of magnitude shows that VOC analysis technology has a great potential for improvement.

OPTIMIZATION FOR LOW VOLATILITY SPECIES

Over the last years, we have dramatically improved the ionization efficiency of SUPER SESI, but other important effects hold the Limits of Detection (LoD). SUPER SESI has greatly improved background levels, which are dominated by condensation effects. Currently, these are the limiting factors defining the LoD for low volatility species.

We have worked in SESI-MS for 10 years now, if you would like to know more about how our core solution SUPER SESI works, contact us to ask any questions or keep on reading here.

References:

[1] Patrik Spanel and David Smith; Progress in SIFT-MS: breath analysis and other applications; Mass Spectrometry Reviews, 2011, 30, 236– 267

[2] B. J. Prince, D. B. Milligan and M. J. McEwan; Application of selected ion flow tube mass spectrometry to real-time atmospheric monitoring; Rapid Commun. Mass Spectrom. 2010; 24: 1763–1769

[3] X. Zhan, J. Duan, and Y. Duan; Recent developments of proton-transfer reaction mass spectrometry (PTR-MS) and its applications in medical research; Mass Spectrometry Reviews, 2013, 32, 143–165