Original Paper Environ. Control Biol., 51 (1), 23
29, 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 422
8526, 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: 2.6
3.5×10－9 cm3 s－1) than in the common monoterpenes (2.2
2.4×10－9 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: ＋81
54
264
5788, e-mail : [email protected]

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 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 (0.5
5 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 (5
85 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 10
20
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 (10－9
10－6 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 96
99% 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 7
11.8 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). ＋ ＋ Because the density of H3O ions, i.e., [H3O ] is high ＋ in the drift tube, and only a small fraction of the H3O ions ＋ reacts with the neutrals, [H3O ] remains constant, maintain- ing the pseudo-first order reaction kinetics. Under such Fig. 1 A schematic diagram of a diffusion system that has been ＋ conditions, the density of product ions [RH ] is given by used for preparing the standard atmosphere. CF: Charcoal the following relation: filter

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a flow rate of 200 mL min－1 via a length of 1/4
OD PFA to that of toluene, the reaction rate constant k for each com- tubing, on the dual bed stainless steel sample tubes (Perkin pound can be calculated. For more detailed description, see Elmer) containing Tenax-TA (200 mg) and Carbotrap (100 our previous paper (Tani et al., 2003) mg). The sample tubes were pre-conditioned at 280°C for 30 min in a stream of purified helium at 50 mL min－1 and RESULTS were sealed and stored at 4°C until sampling. Impurities in the standard vapors were identified by Effect of E/N on fragment pattern of monoterpenes, GC-MS (Hewlett Packard 5890 GC - 5870 MS) and quan- monoterpene alcohols and monoterpene ketones tified by GC-FID (Perkin Elmer Autosystem). In both Several impurities in the standard solutions and their

these systems, samples underwent two stages of thermal vapors were identified by GC-MS. C10-alkylbenzenes and desorption (Perkin Elmer ATD 400). The compound sepa- other monoterpene species were the dominant impurities in ration was achieved using an Ultra-2 capillary column these standard solutions. The total GC-FID peak area of (Hewlett Packard). GC analytical procedures and parame- the impurities in the vapors was less than 5% in all the ters are described in detail elsewhere (Hayward et al., cases. 2001). The detection limit (S/N＝3) of the GC-FID system Varying the water bath temperature was an effective is 0.03
0.04 pmol on the column. technique to distinguish the fragment ions derived from the Identification of ions derived from standards and im- standards and those originating from the impurities, (see purities Tani et al., 2003). Since the vapor pressure varies as a A monoterpene concentration of 100
300 ppbv in the function of temperature and the functions differ among gas phase was produced, and the PTR-MS instrument was compounds, fragment ions derived from standards should operated in the scan mode to select all the produced ions in increase proportionally to their protonated molecular ion as themassrangeof21
200 (including ions derived from any temperature is increased. As a result, they are distinguish- impurities present). Only the ions of which the signals ex- able from the impurities. ceeding 0.1% of the signal of the dominant ion were se- Signals of the ions derived from 13C mono-substituted lected, and in the subsequent experiments, the PTR-MS molecules must also be considered when calculating the instrument was tuned only to detect them. VOC concentrations by PTR-MS. Each ion signal is ex- The ions originated from impurities in the standards pressed as a percentage of the total signal of all the ions de- were identified using three complementary methods: GC- rived from both the non-isotopic and the 13 C mono- MS/FID analysis of diluted standard vapors and solutions; substituted molecules of the target compounds (Table 1). correlation analysis between individual ions measured by All the 5 monoterpenes were measured by PTR-MS using PTR-MS at various concentrations of standard vapors (ob- the above technique (camphene, myrcene,
-phellandrene, tained by varying the water bath temperature); and PTR-
-and-terpinene) and were found to produce fragment MS measurements at increased and decreased E/N values in ions of m/z 67, 81, and 95, as non-isotopic ions (non- the drift tube (i.e., the varying compound fragmentation). isotopic ions are those that only contain 12Cand1H and not Determination of reaction rate constant 13Cor2H). The reaction rate constants of some of the target com- The ion signal of m/z 67 accounted for less than 1.2% ＋ pounds with H3O ions at normal PTR-MS operating con- of the total ion signal for all the monoterpenes at normal ditions were determined from Eqn. 1. The concentration of experimental conditions. Ion signal of m/z 95 was also less the neutral [R] was simultaneously measured by GC-FID. than 2% of the total ion signal, except in the case of Previously determined reaction time t of 101×10－6 s (Tani myrcene (8.2%). The protonated molecular ion (m/z 137) et al., 2003) was used in the present study. As a reference at normal conditions was 50
56% for all the 5 monoter- compound, toluene was used because its rate constant with penes measured in the present study. ＋ －9 3 －1 H3O (2.2×10 cm s ) has already determined and the Among the 5 monoterpenes, only myrcene produced measurement of toluene by PTR-MS has been well an additional fragment ion of m/z 69. The relative abun- characterised and documented (e.g. Warneke et al., 2001). dance of m/z 69 in myrcene was 3.1%. To measure the concentrations of the monoterpene However, as the E/N value of the drift tube was de- families and toluene by GC-FID and also to calculate the creased, there is a steady increase in the percentage of m/z proton transfer reaction rate constant of monoterpene fami- 137 (Fig. 2). For each of the monoterpene, the percentage lies, we collected 7 to 12 samples of each compound at dif- of m/z 137 reached to 80
87% at an E/N valueof80Td. ferent vapor concentrations (5
500 ppbv). This was The percentage of fragment ions steadily decreased with a achieved by changing the water bath temperature and/or the decrease in E/N value because of the softer collisional reac- ＋ flow rates of the diffusion system. Because of the “sticky” tion between H3O and the monoterpenes. nature of these vapors, they were not sampled until their It was observed that the monoterpene alcohols linalool signal measured by PTR-MS reached a steady state. The and cineole (m.w.＝154) also produced fragment ions m/z concentrations determined by GC-FID were plotted against 69, 81, 95, and 137 at 120
122 Td (Table 2). The relative the ion count rates measured by PTR-MS, and the slopes abundance of protonated molecular ion m/z 155 was 5.8% were compared with that of toluene. Since the ratio of the and 0.4% for linalool and cineole, respectively. The value slope of the individual compounds to that of toluene is pro- increased significantly for cineole as the drift tube E/N portional to the ratio of the rate constant of the compounds value was decreased, but remained low for linalool

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Table 1 Relative abundance of monoterpene fragment ions and proton transfer reaction rate constant, k, determined by PTR-MS under normal operating conditions (E/N＝120
122 Td). 67 69 81 95 137 81＋137 Other ions k (10－9 cm3s－1) Myrcene 1.0 3.1 30.7 8.2 49.6 80.3 70, 82, 96, 138, 155 2.4 Camphene 1.2 33.7 1.1 55.6 89.3 82, 138 2.2
-Phellandrene 0.4 38.8 1.1 51.3 90.1 82, 138 2.3
-Terpinene 0.4 33.9 0.8 56.4 90.3 82, 138 2.3 -Terpinene 0.5 38.6 0.8 51.8 90.4 82, 138, 155 ―

(Comparison data from Tani et al. (2003))
-Pinene 0.6 40.9 1.3 49.2 90.1 82, 138 2.2 Sabinene 0.5 37.1 2.0 52.4 89.5 82, 138, 155 ― -Pinene 0.5 39.6 1.5 50.3 89.8 82, 138 2.3 Limonene 0.5 39.6 4.7 47.3 86.9 82, 138, 155 2.3 3-Carene 0.6 31.6 1.1 58.2 89.7 82, 138, 155 2.2

Fig. 2 Fragment patterns of ions (shown as a percentage of total ion signal) derived from myrcene, camphene, cineole, linalool and thujone affected by different val- ues of E/N. Each data point is the mean of 10
20 deter- minations.

throughout the E/N range. Further, m/z 137 became domi- ion was most abundant at E/N values of 100
120 Td, but nant (～75%) at the lowest E/N value of 80 Td (Fig. 2). below 100 Td, the protonated molecular ion m/z 153 was A monoterpene ketone, thujone (m.w.＝152), pro- dominant (Fig. 2). On the other hand, the other mono- duced fragment ions m/z 91, 93, 95, and 135 and a terpene ketone, fenchone, was observed to produce no frag- ＋ monohydrated ion m/z 171 (C10H14O·H ·H2O), in addition ment ions. It mostly produced molecular ions and small to m/z 153, at 120
122 Td (Table 2). Fragment patterns of amount (＜1%) of the monohydrated molecular ion m/z thujone was also observed to be dependent on the 171. collisional energy value, i.e., E/N. We found that m/z 135

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Table 2 Relative abundance of oxygenated monoterpene fragment ions and proton transfer reaction rate constant, k, determined by PTR-MS under normal operating conditions (E/N＝120
122 Td). 69 81 95 137 155 81＋137 Other ions k (10－9 cm3 s－1) (Monoterpene alcohol) Linalool 3.9 36.7 0.7 44.2 5.8 80.9 82, 138, 156, 173 2.6 Cineole 0.5 35.7 0.9 54.7 0.4 90.4 82, 138, 173 3.5
-Terpineol 0.4 39.3 2.2 48.7 1.5 88.0 82, 138, 173 ―

91 93 95 135 153 171 Other ions k (10－9 cm3 s－1) (Monoterpene ketone) Thujone 0.5 17.6 4.6 69.5 0.0 0.1 136, 154 ― Fenchone 90.1 154, 171 3.5 (Comparison data from Tani et al. (2003)) Camphor 89.9 154, 171 4.4

DISCUSSION

In our previous papers (Tani et al., 2003; 2004), we in- vestigated the fragment patterns of the major mono- terpenes,
-and-pinene, sabinene, 3-carene, and limonene in detail and showed that they produced fragment ions of m/z 67, 81, and 95. We found that relative abundances of the fragment ions of these 5 monoterpene species were not the same and that they also varied with the Fig. 3 Relationship between the concentrations of mono- E/N values. Following these reports, the fragment patterns terpenes and oxygenated monotrpenes measured using of several monoterpenes (Steeghs et al., 2007; Misztal et GC-FID and the total signal of all ions derived from al., 2012), oxygenated monoterpenes (Maleknia et al., them. The count rate was standardized to 1×106 cps of ＋ 6 ＋ ＋ 2007), and sesquiterpenes (Demarcke et al., 2009) have H3O for toluene and 1×10 cps for H3O plus H3O subsequently been reported. The fragment ions of the 5 H2O for the monoterpenes, at 2.0 mbar of the drift tube pressure. The linear regression for each data set were: monoterpenes (myrcene, camphene,
-phellandrene,
-and Camphene, y＝0.0943x, r2 ＝0.9992, n＝8; Myrcene, -terpinene) and 2 monoterpene alcohols (lionalool and 2 y＝0.0792x, r ＝0.9968, n＝8; Cineole, y＝0.0574x, cineole) used in the present study were also addressed r2 0.9968, n 8; Linalool, y 0.0811x, r2 0.9962, ＝ ＝ ＝ ＝ (Steeghs et al., 2007), and the major fragment ions (m/z 81 n＝5; Fenchone, y＝0.0549x, r2 ＝0.9963, n＝5; Toluene, y＝0.0912×x, r2＝1.000, n＝11. and 95) found in our study were also reported by the authors. However, we found that myrcene exclusively pro- duces a fragment ion m/z 69. Myrcene is the only straight- Reaction rate constant calculation chained compound among all the monoterpenes, and it 2 ＋ Figure 3 shows the good linear relationship (r ＝0.98
seems to produce a fragment ion C5H9 when the 4 and 5 1.00 for n＝5
12) between the concentrations of positions of the carbon-carbon bond are attacked. The camphene, myrcene, cineole, linalool, and fenchone (meas- other major monoterpenes are the cyclic monoterpenes and ＋ ured with GC-FID) and the sum of the signals of all the are believed to produce a fragmentary cyclic ion C5H7 (m/z ions derived from these individual compounds (including 67) (Misztal et al., 2012). The m/z 69 is also a protonated 13C monosubstituted molecules) that were simultaneously molecular ion of isoprene. Both isoprene and measured by PTR-MS. The signals were shown as stan- monoterpenes are present in the forest atmosphere and in dardized count per second (SCPS), i.e., standardized to 1 many cases isoprene concentration is much higher than ×106 cps of the proton donors and 2.0 mbar of the drift myrcene. However, when measuring BVOC emitted from tube pressure (Tani et al., 2004), because small differences monoterpene emitting coniferous trees, of which emission in these values occurred between individual experiments. includes a large amount of myrcene, it should be noted that The rate constant k for monoterpene species (myrcene, myrcene fragment ion m/z 69 may interfere with isoprene camphene,
-phellandrene, and -terpinene) was experi- quantification. mentally determined to be in the range 2.2
2.4×10－9 cm3 Fragment patterns of the monoterpene ketones, s－1 (Table 1). The monoterpene alcohols, linalool and thujone and fenchone, have not been investigated so far. cineole, have a higher rate constant of 2.6×10－9 and 3.5 We have found an obvious contrast between thujone and ×10－9 cm3 s－1 , respectively (Table 2). A monoterpene fenchone: thujone produces several fragment ions, includ- ketone, fenchone, also has been observed to have a high ing m/z 93 and 135, but fenchone yields no detectable frag- rate constant (3.5×10－9 cm3 s－1). ment ions. We previously reported that no fragment ions were produced from the monoterpene ketone, camphor (Tani et al., 2003). Among the 3 monoterpene ketones, only thujone has a three-membered ring that is likely to get

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cleaved, and this seems the reason for its fragmentation. the total concentration of both monoterpenes and Ion of m/z 93 is a protonated molecular ion of toluene, monoterpene alcohols emitted by plants. This is especially which often has the highest concentration among all non- noted in cases where the herb plants and trees that emit a methane hydrocarbons in the atmosphere. Ion of m/z 93 is large amount of monoterpene alcohols are used for emis-

also produced by p-cymene (C10H14), which is emitted by sion measurements. Thus, it is essential to first quantify many plant species (Tani et al., 2003). In the present study, what compounds are emitted by a target plant using GC- we found that m/z 93 ion also originate from thujone, and MS before directly quantifying the total monoterpene con- that these fragment ions derived from two different com- centration by PTR-MS. By calibrating the standard pounds may interfere in the measurement of toluene. This mixture of major compounds emitted from target forests is particularly important when the measurements are made and trees we can achieve greater precision in measuring the in a rural atmosphere. Fragment ion of m/z 93 originated total concentration of monoterpenes and monoterpene from other compounds was also considered for toluene alcohols. quantification (Ambrose et al., 2010). The reaction rate constant k for the 4 monoterpene This research was partially supported by the Ministry of species (myrcene, camphene,
-phellandrene, -terpinene) Education, Science, Sports, and Culture, Grant-in-Aid for has been experimentally determined for the first time and Scientific Research (B), No. 21310026 and Grant-in-Aid for has been found to be in the range of 2.2
2.4×10－9 cm3 Scientific Research on Innovative Areas No. 20120005. It was s－1. The rate constant of myrcene (2.4×10－9 cm3 s－1)was also supported by the Japan Society for the Promotion of the theoretically calculated to be 2.57×10－9 cm3 s－1 (Zhao Science, A3 Foresight Program “CarboEastAsia”. and Zhang, 2004), which is very similar to that observed in our experimental results. The previously reported rate con- stants of
- and -pinene, limonene, and 3-carene (Tani et REFERENCES al., 2003) were also in this range. From the present study, we estimate that linalool, Ambrose, J. L., Haase, K., Russo, R. S., Zhou, Y., White, M. L., cineole, and fenchone have high values of proton transfer Frinak, E. K., Jordan, C., Mayne, H. R., Talbot, R., Sive, B. C. 2010. A comparison of GC-FID and PTR-MS toluene reaction rate constant compared to that of the most com- measurements in ambient air under conditions of enhanced mon monoterpenes. We had previously reported a high re- monoterpene loading. Atmos. Meas. Tech. 3:959
980. －9 3 －1 action rate (4.4×10 cm s ) of another monoterpene Demarcke, M., Amelynck, C., Schoon, N., Dhooghe, F., van ketone, camphor (Tani et al., 2003). This value was also Langenhove, H., Dewulf, J. 2009. Laboratory studies in very similar to the theoretical value that was calculated support of the detection of sesquiterpenes by proton-transfer- using the dipole moment and the polarisability of camphor. reaction-mass-spectrometry. Int. J. Mass Spectrom. 279:156
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