Proton-transfer-reaction mass spectrometry

PTR-TOF mass spectrometer

Proton-transfer-reaction mass spectrometry (PTR-MS) is an analytical chemistry technique that uses gas phase hydronium ions as ion source reagents.[1] PTR-MS is used for online monitoring of volatile organic compounds (VOCs) in ambient air and was developed in 1995 by scientists at the Institut für Ionenphysik at the Leopold-Franzens University in Innsbruck, Austria.[2] A PTR-MS instrument consists of an ion source that is directly connected to a drift tube (in contrast to SIFT-MS no mass filter is interconnected) and an analyzing system (quadrupole mass analyzer or time-of-flight mass spectrometer). Commercially available PTR-MS instruments have a response time of about 100 ms and reach a detection limit in the single digit pptv region. Established fields of application are environmental research, food and flavour science, biological research, medicine, etc.[3]

Theory

With H3O+ as the primary ion the proton transfer process is (with R being the trace component)

Fig. 1: Evolution of reagent ion yields and sensitivities of PTR-MS instruments taken from peer reviewed journal articles

 

 

 

 

(1)

Reaction (1) is only possible if energetically allowed, i.e. if the proton affinity of R is higher than the proton affinity of H2O (691 kJ/mol[4]). As most components of ambient air possess a lower proton affinity than H2O (e.g. N2, O2, Ar, CO2, etc.) the H3O+ ions only reacts with VOC trace components and the air itself acts as a buffer gas. Moreover due to the low number of trace components one can assume that the total number of H3O+ ions remains nearly unchanged, which leads to the equation[5]

 

 

 

 

(2)

In equation (2) is the density of product ions, is the density of primary ions in absence of reactant molecules in the buffer gas, k is the reaction rate constant and t is the average time the ions need to pass the reaction region. With a PTR-MS instrument the number of product and of primary ions can be measured, the reaction rate constant can be found in literature for most substances[6] and the reaction time can be derived from the set instrument parameters. Therefore the absolute concentration of trace constituents can be easily calculated without the need of calibration or gas standards. Furthermore it gets obvious that the overall sensitivity of a PTR-MS instrument is mainly dependent on the primary / reagent ion yield. Fig. 1 gives an overview of several published (in peer-reviewed journals) reagent ion yields during the last decades and the corresponding sensitivities.

Technology

Hydronium ions generated from water vapor in an ionizer are reacting with analytes in a drift chamber. Newly generated ions are separated in an analyzer based on the mass to charge ratio, and are subsequently transferred to a detector where identification takes place

In commercial PTR-MS instruments water vapour is ionized in a hollow cathode discharge:

.

After the discharge a short drift tube is used to form very pure (>99.5%[5]) H3O+ via ion-molecule reactions:

.

Due to the high purity of the primary ions a mass filter between the ion source and the reaction drift tube is not necessary and the H3O+ ions can be injected directly. The absence of this mass filter in turn greatly reduces losses of primary ions and leads eventually to an outstandingly low detection limit of the whole instrument. In the reaction drift tube a vacuum pump is continuously drawing through air containing the VOCs one wants to analyze. At the end of the drift tube the protonated molecules are mass analyzed (Quadrupole mass analyzer or Time-of-flight mass spectrometer) and detected.

Advantages of PTR-MS

Disadvantages of PTR-MS and countermeasures

Applications

The most common applications for the PTR-MS technique are (including some relevant publications):[3]

Extensive reviews about PTR-MS and some of its applications were published in Mass Spectrometry Reviews by Joost de Gouw et al. (2007)[21] and in Chemical Reviews by R.S. Blake et al. (2009).[22] A special issue of the Journal of Breath Research dedicated to PTR-MS applications in medical research was published in 2009.[23]

Examples

Fig. 2: PTR-MS measurement of vanillin dissemination in human breath. Isoprene is a product of human metabolism and acts as an indicator for breath cycles. (The measurement was performed utilizing a "N.A.S.E." inlet system coupled to a "HS PTR-MS.)
Fig. 3: PTR mass spectrum of laboratory air obtained using a TOF based PTR instrument.

Food science

Fig. 2 shows a typical PTR-MS measurement performed in food and flavor research. The test person swallows a sip of a vanillin flavored drink and breathes via his nose into a heated inlet device coupled to a PTR-MS instrument. Due to the high time resolution and sensitivity of the instrument used here, the development of vanillin in the person's breath can be monitored in real-time (please note that isoprene is shown in this figure because it is a product of human metabolism and therefore acts as an indicator for the breath cycles). The data can be used for food design, i.e. for adjusting the intensity and duration of vanillin flavor tasted by the consumer.

Another example for the application of PTR-MS in food science was published in 2008 by C. Lindinger et al.[24] in Analytical Chemistry. This publication found great response even in non-scientific media.[25][26] Lindinger et al. developed a method to convert "dry" data from a PTR-MS instrument that measured headspace air from different coffee samples into expressions of flavour (e.g. "woody", "winey", "flowery", etc.) and showed that the obtained flavor profiles matched nicely to the ones created by a panel of European coffee tasting experts.

Air quality analysis

In Fig. 3 a mass spectrum of air inside a laboratory (obtained with a time-of-flight (TOF) based PTR-MS instrument), is shown. The peaks on masses 19, 37 and 55 m/z (and their isotopes) represent the reagent ions (H3O+) and their clusters. On 30 and 32 m/z NO+ and O2+, which are both impurities originating from the ion source, appear. All other peaks correspond to compounds present in typical laboratory air (e.g. high intensity of protonated acetone on 59 m/z). If one takes into account that virtually all peaks visible in Fig. 3 are in fact double, triple or multiple peaks (isobaric compounds) it becomes obivious that for PTR-MS instruments selectivity is at least as important as sensitivity, especially when complex samples / compositions are analyzed. Methods to handle this issue have been suggestested in literature as:

See also

References

  1. Andrew M. Ellis; Christopher A. Mayhew (17 December 2013). Proton Transfer Reaction Mass Spectrometry: Principles and Applications. Wiley. pp. 15–. ISBN 978-1-118-68412-2.
  2. A. Hansel, A. Jordan, R. Holzinger, P. Prazeller W. Vogel, W. Lindinger, Proton transfer reaction mass spectrometry: on-line trace gas analysis at ppb level, Int. J. of Mass Spectrom. and Ion Proc., 149/150, 609-619 (1995).
  3. 1 2 Publications – University of Innsbruck
  4. R.S. Blake, P.S. Monks, A.M. Ellis, Proton-Transfer Reaction Mass Spectrometry, Chem. Rev., 109, 861-896 (2009)
  5. 1 2 Lindinger, W.; Hansel, A.; Jordan, A. (1998). "On-line monitoring of volatile organic compounds at pptv levels by means of Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS): Medical applications, food control and environmental research, Review paper". Int. J. Mass Spectrom. Ion Proc. 173: 191–241. doi:10.1016/s0168-1176(97)00281-4.
  6. Y. Ikezoe, S. Matsuoka and A. Viggiano, Gas Phase Ion-Molecule Reaction Rate Constants through 1986, Maruzen Company Ltd., Tokyo, (1987).
  7. 1 2 Jordan, A.; Haidacher, S.; Hanel, G.; Hartungen, E.; Herbig, J.; Märk, L.; Schottkowsky, R.; Seehauser, H.; Sulzer, P.; Märk, T.D. (2009). "An online ultra-high sensitivity proton-transfer-reaction mass-spectrometer combined with switchable reagent ion capability (PTR+SRI-MS)". International Journal of Mass Spectrometry. 286: 32–38. doi:10.1016/j.ijms.2009.06.006.
  8. Sulzer, P.; Edtbauer, A.; Hartungen, E.; Jürschik, S.; Jordan, A.; Hanel, G.; Feil, S.; Jaksch, S.; Märk, L.; Märk, T. D. (2012). "From conventional Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) to universal trace gas analysis". International Journal of Mass Spectrometry. 321-322: 66–70. doi:10.1016/j.ijms.2012.05.003.
  9. Krypton
  10. Müller, M.; Graus, M.; Ruuskanen, T. M.; Schnitzhofer, R.; Bamberger, I.; Kaser, L.; Titzmann, T.; Hörtnagl, L.; Wohlfahrt, G.; Karl, T.; Hansel, A. (2010). "First eddy covariance flux measurements by PTR-TOF". Atmos. Meas. Tech. 3: 387–395. doi:10.5194/amt-3-387-2010.
  11. R. Beale, P. S. Liss, J. L. Dixon, P. D. Nightingale: Quantification of oxygenated volatile organic compounds in seawater by membrane inlet-proton transfer reaction/mass spectrometry. Anal. Chim. Acta (2011).
  12. F. Biasioli, C. Yeretzian, F. Gasperi, T. D. Märk: PTR-MS monitoring of VOCs and BVOCs in food science and technology, Trends in Analytical Chemistry, 30/7, (2011).
  13. Simpraga, M.; Verbeeck, H.; Demarcke, M.; Joó, É.; Pokorska, O.; Amelynck, C.; Schoon, N.; Dewulf, J.; Langenhove, H. Van; Heinesch, B.; Aubinet, M.; Laffineur, Q.; Müller, J.-F.; Steppe, K. (2011). "Clear link between drought stress, photosynthesis and biogenic volatile organic compounds in Fagus sylvatica L.". Atmospheric Environment. 45: 5254–5259. doi:10.1016/j.atmosenv.2011.06.075.
  14. Wisthaler, A.; Strom-Tejsen, P.; Fang, L.; Arnaud, T. J.; Hansel, A.; Märk, T. D.; Wyon, D. P. (2007). "PTR-MS Assessment of Photocatalytic and Sorption-Based Purification of Recirculated Cabin Air during Simulated 7-h Flights with High Passenger Density". Environ. Sci. Technol. 1: 229–234.
  15. Kolarik, B.; Wargocki, P.; Skorek-Osikowska, A.; Wisthaler, A. (2010). "The effect of a photocatalytic air purifier on indoor air quality quantified using different measuring methods". Building and Environment. 45: 1434–1440. doi:10.1016/j.buildenv.2009.12.006.
  16. Han, K.H.; Zhang, J.S.; Knudsen, H.N.; Wargocki, P.; Chen, H.; Varshney, P.K.; Guo, B. (2011). "Development of a novel methodology for indoor emission source identification". Atmospheric Environment. 45: 3034–3045. doi:10.1016/j.atmosenv.2011.03.021.
  17. Herbig, J.; Müller, M.; Schallhart, S.; Titzmann, T.; Graus, M.; Hansel, A. (2009). "On-line breath analysis with PTR-TOF". J. Breath Res. 3: 027004. doi:10.1088/1752-7155/3/2/027004.
  18. Brunner, C.; Szymczak, W.; Höllriegl, V.; Mörtl, S.; Oelmez, H.; Bergner, A.; Huber, R. M.; Hoeschen, C.; Oeh, U. (2010). "Discrimination of cancerous and non-cancerous cell lines by headspace-analysis with PTR-MS". Anal. Bioanal. Chem. 397: 2315–2324. doi:10.1007/s00216-010-3838-x.
  19. Jürschik, S.; Sulzer, P.; Petersson, F.; Mayhew, C. A.; Jordan, A.; Agarwal, B.; Haidacher, S.; Seehauser, H.; Becker, K.; Märk, T. D. (2010). "Proton transfer reaction mass spectrometry for the sensitive and rapid real-time detection of solid high explosives in air and water". Anal Bioanal Chem. 398: 2813–2820. doi:10.1007/s00216-010-4114-9.
  20. Petersson, F.; Sulzer, P.; Mayhew, C.A.; Watts, P.; Jordan, A.; Märk, L.; Märk, T.D. (2009). "Real-time trace detection and identification of chemical warfare agent simulants using recent advances in proton transfer reaction time-of-flight mass spectrometry, Rapid Commun". Mass Spectrom. 23: 3875–3880. doi:10.1002/rcm.4334.
  21. de Gouw, J.; Warneke, C.; Karl, T.; Eerdekens, G.; van der Veen, C.; Fall, R. (2007). "Measurement of Volatile Organic Compounds in the Earth's Atmosphere using Proton-Transfer-Reaction Mass Spectrometry". Mass Spectrometry Reviews. 26: 223–257. doi:10.1002/mas.20119.
  22. Blake, R. S.; Monks, P. S.; Ellis, A. M. (2009). "Proton-Transfer Reaction Mass Spectrometry". Chem. Rev. 109 (3): 861–896. doi:10.1021/cr800364q.
  23. Jens Herbig and Anton Amann " Proton Transfer Reaction-Mass Spectrometry Applications in Medical Research" Journal of Breath Research Volume 3, Number 2, June 2009.
  24. C. Lindinger, D. Labbe, P. Pollien, A. Rytz, M. A. Juillerat, C. Yeretzian, I. Blank, 2008 When Machine Tastes Coffee: Instrumental Approach To Predict the Sensory Profile of Espresso Coffee, Anal. Chem., 80/5, 1574-1581.
  25. Electronic nose knows quality coffee - Technology & science - Innovation - Frontiers | NBC News
  26. https://www.nytimes.com/2008/02/19/science/19objava.html?ex=1361077200&en=393e1220ba7a4cf1&ei=5124&partner=permalink&exprod=permalink
  27. A. Jordan, S. Haidacher, G. Hanel, E. Hartungen, L. Märk, H. Seehauser, R. Schottkowsky, P. Sulzer, T.D. Märk: A high resolution and high sensitivity time-of-flight proton-transfer-reaction mass spectrometer (PTR-TOF-MS), International Journal of Mass Spectrometry, 286, 122–128, (2009).
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