Original Paper Environ. Control Biol., 51 (1), 2329, 2013 Fragmentation and Reaction Rate Constants of Terpenoids Determined by Proton Transfer Reaction-mass Spectrometry Akira TANI Institute for Environmental Sciences, University of Shizuoka, Shizuoka 4228526, Japan (Received November 5, 2012; Accepted December 15, 2012) Monoterpenes and oxygenated monoterpenes emitted by plants are involved in producing photochemical oxidants and secondary organic aerosols in the atmosphere. In the present study, the fragment patterns and the reaction rate constants of some of these compounds have been determined by proton transfer reaction-mass spectrometry (PTR-MS). Five monoterpenes (myrcene, camphene, -phellandrene, - and -terpinene) and two monoterpene alcohols (linalool and cineole) were observed to produce dominant ions of m/z 81 and 137 along with some minor m/z 67 and 95 ions. Myrcene, which is a straight-chained compound, produced an m/z 69 ion along with the above mentioned ions. A monoterpene ketone, thujone, has been found to produce a fragment ion, m/z 93. Since the m/z 69 and 93 ions are the protonated molecular ions of ubiquitous isoprene and toluene, respectively, these respective fragment ions may interfere with the quantification of both isoprene and toluene. We have also revealed that the proton transfer reaction rate is faster in the oxygenated monoterpenes (rate constant: 2.63.5×10－9 cm3 s－1) than in the common monoterpenes (2.22.4×10－9 cm3 s－1). This suggests that the calibration equation obtained from the relationship between the concentration and the produced ion count (m/z 81＋137) of selected monoterpenes may underestimate the total concentration of monoterpene alcohols and monoterpenes. Keywords : fragmentation, monoterpene, proton transfer reaction, PTR-MS, reaction rate constant 2003). PTR-MS has been used for measuring BVOC emis- INTRODUCTION sion from plants (Tani et al., 2008; Okumura et al., 2008; Tani et al., 2011) and vegetation (Karl et al., 2001; Miyama Monoterpenes are the major secondary metabolites et al., 2012) and as well, for measuring oxygenated VOC produced and emitted by plants. Once emitted to the at- uptake by leaves of houseplants (Tani et al., 2007; 2009) mosphere, they are involved in producing photochemical and trees (Tani et al., 2010; Karl et al., 2010). ＋ oxidants through a series of reactions with OH radical and The transfer of protons from H3O to neutral entities NO. These reactions play an important role in influencing can be regarded as “soft” ionization; under “normal” PTR- the atmospheric chemistry from the perspective of regional MS operating conditions (detailed below), many of the photochemical oxidant formation (Fall, 1999) and also in VOCs are detected with a mass that is equal to their mo- determining the lifetime of methane (Fehsenfeld et al., lecular mass plus one. Since the quadrupole of the PTR- 1992). Recent studies have revealed that monoterpene oxi- MS instrument can discriminate different masses but dation products may significantly contribute to the forma- cannot distinguish between compounds that have the same tion of secondary organic aerosols (Miyazaki et al., 2012). mass, only the total concentration of all compounds with Measurement of the atmospheric concentrations and the same mass can be measured. In a forest atmosphere, biogenic emission rates of the monoterpenes and related more than 10 species of monoterpenes are usually found. VOCs are necessary for understanding their roles in plant Their total concentration can be determined by PTR-MS. biochemistry, plant physiology and atmospheric chemistry. However, the monoterpenes have been observed to undergo Until recently, real-time analyses of these compounds were some degree of fragmentation within the instrument (Tani not very practical because of their low emission rates and et al., 2003; Tani et al., 2004). It is thus, important to de- low atmospheric concentrations. Hence, the majority of the termine the fragment pattern of major individual analyses were earlier performed off-line, on preconditioned monoterpenes as a function of the collisional energy E/N samples using gas chromatography either by flame ioniza- (where E is the electric field strength and N is the buffer tion or by mass spectrometric detection. gas number density in the drift tube). The development of the proton transfer reaction mass Beside monoterpenes, monoterpene alcohols and ＋ spectrometry (PTR-MS), in which protonated water (H3O ) ketones are also emitted in large amounts by specific herbs is used as the primary ionizing reactant, has enabled on-line and trees such as the Eucalyptus. Since molecular weight monitoring of VOCs, including the monoterpenes and other of monoterpene alcohols and ketones are 154 and 152, re- related compounds (Lindinger et al., 1998; Hewitt et al., spectively, which are different from that of the Corresponding author : Akira Tani, fax: ＋81542645788, e-mail : [email protected] Vol. 51, No. 1 (2013) ( ) A. TANI monoterpenes (m.w.＝136), they can be easily discrimi- ＋ ＋ RH H3O Rkt (1) nated by quadrupole, simply if they produce significant ＋ ＋ proportion of protonated molecular ions (m/z 155 and 153 where [H3O ] is the density of H3O ; [R], the molecular ion for monoterpene alcohols and ketones). However, only a density of the trace component R; k, the reaction rate con- ＋ few studies in the scientific literature have addressed the re- stant for the proton transfer reaction between R and H3O ; ＋ ＋ action of these compounds with H3O (Tani et al., 2003; and t, the time taken for H3O ions to traverse the drift Maleknia et al., 2007). tube. A potential source of inaccuracy in a PTR-MS meas- Standard preparation using diffusion system urement of the common and oxygenated monoterpenes is In order to identify the individual monoterpene frag- the uncertainty in the proton transfer reaction rate constant ment patterns, to investigate the effects of the collisional values of the individual species. These values can be esti- energy on these patterns, and to experimentally determine mated using the parameterized trajectory formulation the rate constants, a diffusion system was constructed. A method developed by Su and Chesnavich (1982) that uses range of nominal gaseous concentrations could be achieved the dipole moment and the polarizability of the compounds. using this system (Fig. 1). The system comprised of two There is, however, very limited information available for air streams, which were dried by CaSO4 and purified by the monoterpenes (Nelson et al., 1967; Zhao and Zhang, charcoal filtration and regulated by mass flow controllers 2004). As a result, a nominal value of the proton transfer (0.55 L min－1, MKS Instruments, USA). One of the air reaction rate constant (2.0×10－9 cm3 s－1) has widely been streams was passed through a temperature-controlled (585 adopted for the PTR-MS quantification of monoterpenes. °C, with an accuracy of ±0.1°C) monoterpene diffusion Owing to the different physico-chemical properties of indi- system, while the other acted as a bypass. The diffusion vidual monoterpenes, they may have different rate con- system had a glass chamber (～100 mL) housing a sealed stants; it is thus highly desirable to determine the rate vial (1.5 mL) containing 1020 L of a pure monoterpene coefficient for all such individual monoterpene species so standard. The septum of the vial was pierced by a syringe, that their concentrations can be correctly measured. enabling monoterpene vapor to diffuse out at a constant In the present study, we have used PTR-MS to inves- nominal rate into the air stream. The resultant air stream tigate the fragmentation patterns of many of the was subsequently combined with the bypass air stream, and monoterpene species and oxygenates. We also show how mixed over a length of ～1 m; thereafter, it was sampled by these fragmentation patterns are affected by different val- PTR-MS, and/or adsorbed onto solid adsorbent sample ues of E/N in the drift tube. Using the relative rate constant tubes for GC-FID analysis. Monoterpene concentrations in determination method and simultaneous gas chromatogra- the range of parts per billion by volume to parts per million phy quantification of gaseous standards under varying con- by volume (10－910－6 volume mixing ratio) were achieved centrations, we obtain proton transfer reaction rate by manipulating the two air flow rates and by controlling ＋ constants for the compounds with H3O ions. Finally, we the water bath temperature. The common and oxygenated discuss the error in the determination of total monoterpene monoterpene standards used in the study including concentration caused by the fragmentation patterns of myrcene, camphene, -phellandrene, -and-terpinene, monoterpene families. linalool, cineole, thujone, and fenchone of purity 9699% were obtained commercially. EXPERIMENTAL METHODS PTR-MS operation The sample air containing variable amounts of PTR-MS instrumentation monoterpenes was introduced to the PTR-MS drift tube via PTR-MS has been described in detail elsewhere a ～1-m-long PFA tubing (outer diameter: 1/8) at a flow (Lindinger et al., 1998; Hewitt et al., 2003); therefore, only rate of 711.8 mL min－1. To assess fragmentation patterns, the points relevant to this paper have been described in this the E/N value of the drift tube was varied from 80 to 170 section. The PTR-MS instrument comprises three parts: (1) Td (normal value＝120 Td) by altering both the drift tube an ion source, (2) a drift tube (reaction chamber), and (3) voltage and the pressure. ＋ an ion separation/detection system. In general, H3O ions Gas sampling and GC analysis formed in the hollow cathode ion source react with the neu- To identify and quantify the impurities included in the trals (R) in the drift tube, resulting in the proton transfer re- standard vapors, the samples were periodically collected at actions. This results in the production of RH＋ ions that are separated by a quadrupole mass spectrometer (Balzers QMG421) and detected and quantified in terms of ion counts per second (cps) by a secondary electron multiplier (Balzers QC422).