Proton transfer reaction mass spectrometry

Another technique called selected (onflow tube (SIFT)-MS [188-190] is closely related to PTR-MS. It involves reactions of neutral analytes with selected ions such as H30, NO, or 02 . The reagent ions are generated (e.g., using microwaves), and selected by a quadrupole mass filter. They enter a flow tube where they encounter analytes. Following the reaction, the newly formed ions are separated in the second quadrupole downstream from the fiow tube. SIFT-MS has successfully been used in the monitoring of gaseous samples, including air pollutants as well as breath (e.g., [191-195]). 

PTR-MS combines a soft, sensitive and efficient mode of chemical ionisation, adapted to the analysis of trace VOCs. Briefly, headspace gas is continuously introduced into the chemical ionisation cell, which contains besides buffer-gas a controlled ion density of H3O. VOCs that have proton affinities larger than water (proton affinity of H2O is 166.5 kcal/mol) are ionised by proton transfer from H3O+, and the protonated VOCs are mass-analysed. The chemical ionisation source was specifically designed to achieve versatility, high sensitivity and little fragmentation, and to allow for absolute quantification of VOCs. To 

The four key features of PTR-MS can be summarised as follows. First, it is fast. Time dependent variations of headspace profiles can be monitored with a time resolution of better than 1 s. Second, the volatile compounds do not experience any work-up or thermal stress, and very little fragmentation is induced by the ionisation step hence, measured mass spectral profiles closely reflect genuine headspace distributions. Third, measured mass spectral intensities can be directly related to absolute headspace concentrations, without calibration or use of standards. Finally, it is not invasive and the process under investigation is not affected by the measurements. All these features make PTR-MS a particularly suitable method to investigate fast dynamic process. 

PTR-MS was introduced in 1993 by Lindinger and co-workers at the university of Innsbruck. A schematic drawing of the apparatus is given in Fig. 15.15. Here, only a brief description will be given. A more detailed discussion of the technical aspects of PTR-MS has been published in a series of review papers [193-195]. 

Primary (reactant) ions AT generated in a hollow cathode ion source, travel through a buffer gas within the drift tube, to which the reactant gas (VOC) is added in small amounts, so that the density of the buffer gas is much larger than the density of the VOC. On their way through the reaction region, ions perform many non-reactive collisions with buffer gas atoms or molecules however, once they collide with a reactant gas particle, they may undergo a reaction  

When H3O+ is used as the proton donor, most of the organic trace components R in air are ionised by proton-transfer processes  

Reagent gas Reactant ions Neutral from reactant ions PA of neutral product Analyte ions 

By combining the principles of PICI with an apparatus derived from selected-ion flow tube (SIFT) experiments, the technique of proton transfer reaction mass spectrometry (PTR-MS) has been developed as a dedicated tool for the analysis of volatile organic compounds (VOCs) at parts-per-trillion by volume (pptv) level in air [51,52]. PTR-MS is highly useful as a fast and quantitative method for the determination of VOCs as proven by numerous applications in food control, envi- 

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Karl, T., Yeretzian, C., Jordan, A., and Lindinger, W. Dynamic measurements of partition coefficients using proton-transfer-reaction mass spectrometry (PTR-MS), Int. J. Mass Spectrom., 223-224 383-395, 2003. 

Steeghs M, Bais HP, de Gouw J, Goldan P, Kuster W, Northway M, Fall R, Vivanco JM (2004) Proton-transfer-reaction mass spectrometry as a new tool for real time analysis of root-secreted volatile organic compounds in arabidopsis. Plant Physiol 135 47-58... 

Kushch 1, Arendacka B, Stoic S et al (2008) Breath isoprene - aspects of normal physiology related to age, gender and cholesterol profile as determined in a proton transfer reaction mass spectrometry study. Clin Chem Lab Med 46(7) 1011-1018... 

Coupling of Proton-Transfer-Reaction Mass Spectrometry with Gas Chromatography-Mass Spectrometry... 

Lindinger, W., Fall, R., Karl, T.G. (2001) Environmental, food and medical applications of proton-transfer-reaction mass spectrometry (PTR-MS). Adv. Gas Phase Ion Chem. 4 1-48. 

Taylor, A.J., Sivasundaram, L.R., Linforth, R.S.T., Surawang, S. (2003) Time-resolved head-space analysis by proton-transfer-reaction mass-spectrometry. In Deibler, K.D., Delwiche, J. (eds) Handbook of Flavor Characterization. Sensory Analysis, Chemistry and Physiology. Dekker, New York, pp 411-422. 

However, the desire to reduce or even eliminate the required sample preparation has led to the development of newer techniques, such as proton-transfer-reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry coupled with ion mobility spectrometry (MS-SIFT IMS). These techniques allow real-time measurements within a single second. This enables analysis of gas composition during technological processes, monitoring of indoor air, and investigation of VOCs emitted by living organisms. 

Fig. 14.8 Scheme of proton-transfer-reaction mass spectrometry system [123]... 

Zhan, X., Duan, J., Duan, Y. Recent developments of proton-transfer reaction mass spectrometry (PTR-MS) and its applications in medical research. Mass Spectrom. Rev. 32, 143-165 (2013)... 

Aprea, E., Biasioli, F., Carlin, S., Versini, G., Mark, T. and Gasperi, F. (2007) Rapid white truffle headspace analysis by proton transfer reaction-mass spectrometry and comparison with solid-phase microextraction coupled with gas chromatography/mass spectrometry, Rapid Commun. Mass Sp., 21(16), 2564-2572. 

More experiments are needed to determine the yields of reactive oxygenated intermediates formed in aromatic degradation. Ideally these would be carried out with high time resolution as an aid to distinguishing primary and secondary oxidation products. Sensitive, on-line analytical techniques, such as proton-transfer-reaction mass spectrometry, should be employed to detect and quantify such intermediates in chamber experiments carried out under NOx conditions representative of atmospheric levels. 

The traditional equilibrium method of flavor release study mentioned above is extremely time consuming, and several weeks are commonly needed to obtain full release profiles of flavors from powders. Recently, thanks to the pioneering work of Dronen and Reineccius (2003), proton transfer reaction mass spectrometry (PTR-MS) has been used as a rapid analysis to measure the release time-courses of flavors from spray-dried powders. The PTR-MS method has been applied extensively to analyze the release kinetics of volatile organic compounds from roasted and ground coffee beans. The release profiles could then be mathematically analyzed by means of Equation 1.1 to obtain the release kinetic parameters, A and n (Mateus et al., 2007). 

ENVIRONMENTAL, EOOD AND MEDICAL APPLICATIONS OF PROTON-TRANSFER-REACTION MASS SPECTROMETRY (PTR-MS)... 

The development of proton-transfer-reaction mass spectrometry (PTR-MS) as a tool for the analysis of volatile organic compounds (VOCs) is described. PTR-MS is based on the rapid, non-dissociative transfer of protons from H30 to most common VOCs, but not to the principal gases in the air sample. Recent developments in the design of PTR-MS instruments allow detection of some VOCs in the parts per trillion by volume range. This sensitivity and the capability of PTR-MS instruments to be operated for extended periods in both laboratory and field settings has allowed exploration of many aspects of VOC analysis in environmental, food and medical applications. 

Warneke, C., de Gouw, J. A., Kuster, W. A., Goldan, P. D., and Fall, R., Validation of atmospheric VOC measurements by proton-transfer-reaction mass spectrometry using a gas-chromatographic preseparation method, Environ. Sci. TechnoL, 37, 2494-2501, 2003. 

Anderson, G.M., M.L. Alexander, and R.B. Westerberg Vapor phase analysis of mainstream tobacco smoke by proton transfer reaction mass spectrometry (PTR-MS) 57th Tobacco Science Research Conference, Program Booklet and Abstracts, Vol. 57, Paper No. 8, 2003,... 

We have briefly mentioned the use of data on IMR in plasma modeling and for the understanding of interstellar molecular synthesis as well as of ionospheric chemistry, and we also want to point out the applications of IMR in various methods of chemical ionization. The most recent one, developed in our laboratory, namely proton-transfer reaction mass spectrometry (PTR-MS), allows for on-line monitoring of volatile organic compounds at levels as low as a few parts per trillion and is therefore applicable for environmental, food, and medical research involving investigations of fast metabolic and enzymatic processes. 

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