Determination of gas phase nicotine in mainstream smoke with denuder technology

Transcription

1 Vol. 50 Suppl. No. 1 Tobacco Science & Technology December 2017 Determination of gas phase nicotine in mainstream smoke with denuder technology GAO Linyu, WANG Jing, WANG Yalin, DENG Nan *, ZHANG Ke, WANG Le, LI Bin Zhengzhou Tobacco Research Institute of CNTC, No.2 Fengyang Street, Zhengzhou , China Abstract: The objective of the study was to establish a method to determine the gas/particle nicotine in mainstream cigarette smoke with denuder technology. The mainstream smoke from a lit cigarette was passed through a coated denuder in laminar flow by a pump and the remaining smoke was collected by a Cambridge filter pad attached to the exit of the denuder. The interior of the denuder and the filter were extracted by dichloromethane before internal standards were added. The extracts were analyzed by gas chromatography-mass spectrometry. The results show that the gas phase nicotine proportion ( f gas) increased with the exit smoke temperature (ranging from 293 K to 323 K). The determined fraction ( f gas) was ca % at 308 K. The method can be used to understand potential effects of gas/particle nicotine partitioning from cigarette designs, next generation nicotine and tobacco products such as electronic cigarettes and tobacco heating products. Keywords: Gas phase nicotine; Denuder; GC-MS; Mainstream smoke; Cigarette Introduction The mainstream cigarette smoke is predominantly generated by combustion and pyrolysis of tobacco. The nicotine in cigarette smoke consists of both gas phase and particulate phase. Nicotine is one of the most important constituents of mainstream smoke. It affects greatly on sensory qualities of cigarette smoking. The nicotine deposition in the respiratory tract have been proposed to occur by four main mechanisms [1] : 1) direct initial gas deposition; 2) evaporative gas deposition from nicotine in particulate phase; 3) particle deposition with evaporation to gas phase; 4) particle deposition with diffusion. Researchers find that the sensory quality of a cigarette is not corresponding to the total nicotine mass in its mainstream smoke. A good nicotine sensory quality may be achieved with a small mass of nicotine in its mainstream smoke. What matters is the nicotine in neutral free-base form, which is mainly in the gas phase. That is why the determination of gas phase nicotine in mainstream smoke is an important research topic. However, most nicotine in the smoke is in particulate phase and is measured by what is collected on a Cambridge filter pad [2-3] and it can be influenced by numerous factors, such as temperature and relative humidity ph value. It leads to the Received: 13 October 2017; revised: 10 November 2017; accepted: 20 November Corresponding author. address: spendid2005@126.com (N. Deng) DOI: /j.issn difficulties in determining directly in the proportion of the initial gas phase nicotine in total nicotine mass of mainstream smoke. The results consulted from different studies in the literature, ranging from % [4] to 1% [5], differ significantly. The first step to analyze the gas phase nicotine is to separate it from the mainstream smoke. Presently, the most common method to separate volatile and condense phase aerosol components is to use a filter pad to collect the particulate phase nicotine directly after the mainstream smoke generated and to allow the gas phase nicotine to pass through. This method however is unsuitable for gas phase nicotine analysis, due to possible errors by the adsorption of gas phase nicotine by the filter material. Denuder technology is hence developed to separate semi-volatile organic compounds, such as polyaromatic hydrocarbon, carbonyl compound, cyanate, etc. in aerosol stream [6-15]. This technology uses a coated denuder to selectively collect the gas phase nicotine while the particulate phase nicotine is collected by a filter attached to the exit of the denuder. It works based on the difference of radial diffusion coefficients between gas and particulate phase nicotine in a laminar flow of circular pipe. In 1994, Lewis et al. [5] made the first gas phase nicotine proportion (f gas) estimation using the denuder technique, and concluded to be about 1% of the total nicotine. In 2004, Lopowize et al. [16] built up a new mathematical model and solved it based on their experimental results, yielding a gas phase nicotine to be 0.1% of the total nicotine. John et al. presented a denuder experimental setup where the humidity and temperature on the gas phase nicotine could be studied

2 58 Tobacco Science & Technology 2017 systematically [17]. This paper describes the combination of denuder and GC-MS to determine the initial gas phase nicotine at different aerosol temperatures and compares two calculation methods on processing the denuder experimental results to obtain f gas values. 1 Materials and methods 1.1 Materials and reagents All solvents were chromatographic grade. Dichloromethane was purchased from Sigma (USA). Nicotine, greater than 97% pure, was purchased from Toronto Research Chemicals (Canada). Water was purified through Milli-Q system (Millipore, USA). Citric acid was greater than 98% pure and quinolone greater than 98% pure (Tokyo Chemical Industry, Japan). Sodium hydroxide was greater than 97% pure (Aladdin, China). In this study, 3R4F research reference cigarettes were conditioned for at least 48 h at (60±2)% relative humidity and (22± 1) before using Denuder experiments For each denuder experiment, four 3R4F cigarettes were separately connected to the denuder apparatus and smoked (Fig.1). Each cigarette was used only for a 14 mm length of the tobacco rod, namely, a section from 20 to 34 mm of the lit cigarette end. This was found useful to reduce the scatter of the experiments and to ensure laminar flow of the smoke into the denuder. The smoke passed though the denuder in a constant velocity of Q=4.17 ml s -1, another measure to ensure the laminar flow in the denuder. The denuder was set vertically to reduce the influence of gravity and this also simplified the mathematical treatment. A Cambridge filter was attached to the exit of the denuder, into which the remaining particulate phase nicotine was trapped and analyzed. 1.2 Methods Denuder preparation The denuders used in this study consisted of a cylindrical glass tube with an internal diameter of 8 mm and length of mm. In preparation, they were first washed by deionized water and then dried. Citric acid was selected as the coating agent. Uniform coating was applied by putting 10 g of citric acid into the tube and slowly melted in a heating tube furnace set at 473 K for about 15 min. The glass tube was rotated simultaneously on a rotator with a constant velocity until a layer of liquid citric acid coated all the tube surface and then allowed to cool down. After the operation, the inlet and outlet of the coated denuder were sealed and ready for experiments. Each coated denuder was conditioned for at least 1 h under a selected experimental temperature Standard solution preparation A series of standard solution were prepared by stepwise diluting. 1 ml nicotine solution was diluted by dichloromethane to 10 ml and then thoroughly mixed. This step was repeated again by taking 1 ml solution and diluted by dichloromethane to 10 ml and so on, until the final solution of 1.01 μ g ml -1 was prepared. Other standard solutions in different concentration (5.05 μg ml -1, μg ml -1, μg ml -1, μ g ml -1, μ g ml -1, μ g ml -1 and μg ml -1 ) were prepared in the same way. 1. 3R4F Cigarette 2. Cigarette holder 3. Denuder 4. Thermostatic pipe 5. Cambridge filter 6. Constant temperature water-bathing 7. Valve 8. Mass flowmeter 9. Pump 10. Glass tube 11. Coating Fig. 1 Apparatus of denuder setup. After each smoking of consecutive 4 cigarettes, the whole denuder was cut into 5 cm long segment and a total of 20 segments were collected and extracted by the extraction solution (2 ml dichloromethane and 2 ml of 5 mol L -1 sodium hydroxide solution) in small glass bottles. Then they were put under ultrasonic shaking for 15 min. The extracted solutions were poured into 20 customized separation funnels and lower portion of the solution was collected before 100 μl internal solution (containing 5 μg quinoline as internal standards) was added separately into each bottle. Finally, the liquid samples were analyzed by GC-MS Particulate phase nicotine collection After each smoking, the Cambridge filter pad was extracted with 10 ml dichloromethane and 10 ml of 5 mol L -1 sodium hydroxide solution, followed by ultrasonic shaking for 15 min. The solution was poured

3 Vol. 50 Suppl. No. 1 GAO Linyu,et al:determination of gas phase nicotine in mainstream smoke with denuder technology 59 into a separation funnel and the lower portion of the liquid was collected before 1 ml internal solution (containing 50 μg quinoline as internal standards) was added. Finally, the sample was analyzed by GC-MS GC-MS analysis All samples were analyzed by using an Agilent 7890A/5975C MSD equipped with a DB-5MS (30 m mm i.d., 0.25 μm) column. The chromatographic conditions were set as follows: carrier gas, He; flow rate, 6 ml min -1 ; simple size, 1 μ L; inlet temperature, 250 ; splitless mode. The GC oven was programmed with the initial temperature of 70 for 1 min, followed by a ramp of 10 min -1 and up to 250. The mass spectrometer was operated in the selected-ion monitoring (SIM) mode. Characteristic ions were monitored for nicotine (m/z 84, m/z 133, m/z 162) and quinoline (m/z 102, m/z 129) Mathematical model of denuder gas/particulate equilibrium The total nicotine adsorbed in the denuder consists of two parts: 1) an initial gas phase nicotine; 2) vapor phase nicotine escaped from the aerosol droplets as the smoke passes through the denuder. To identify the portion of the gas phase nicotine, two groups (Lewis et al., Lipowicz et al.) have separately developed two mathematical treatments, namely, the LCM model [5] and LP model [16], respectively. The LCM model is derived from fitting the denuder experimental data curves, which were treated as the naturally double-exponential to account for both the initial and gradual nicotine adsorption. Part 1 represents the initial gas phase adsorption while part 2 expresses the vapor phase nicotine adsorption. The nicotine mass of the initial gas collected by j th denuder section can be written as [18] : m Dx j=kexp( ) (1) Q The mass of the volatile nicotine in the particulate phase is expressed as: D m Ax j=k Aexp( ) (2) Q Wherein D and D A mean respectively the diffusion coefficient of the initial gas and vapor nicotine from the particulate phase in cm 2 s -l, Q means the volumetric flow rate through the denuder in cm 3 s -1, x means the axial distance from the end of j th section to the denuder inlet, and k and k A mean the two proportionality coefficients of the two equations. The LP model is derived from coupled partial differential equations and the Henry Law to describe the gas/particle equilibrium [16]. 2 P= u 3+1.2u u3 u<0.02 P= e ( u) e ( u) e ( u) e ( u) u 0.02 (4) u= fgasπdl Q (5) Wherein P is a ratio of the amount of nicotine adsorbed in the denuder to the total amount, u is the deposition parameter and f gas means the initial gas phase nicotine proportion. 2 Results and discussion 2.1 Establishment of analytical method The prepared nicotine-dichloromethane solutions were analyzed by GC-MS with internal standards (quinoline dissolved in dichloromethane in 50 μg ml -1 ). The mass spectrometer was operated in the selected-ion monitoring (SIM) mode. The retention time of nicotine is 7.75 min and that of quinoline is 6.63 min. The result showed that the concentration of nicotine had good relationships to the intensity of the chromatographic peaks, while the equation of the standard curve is: y= x , R 2 = The distribution of nicotine absorbed in denuder The distribution of the nicotine deposited in the denuder at different temperatures ranging from 293 K to 333 K is shown in Fig 2. Although the total amount of nicotine absorbed in the denuder was slightly different, the general tendency remained the same. The most rapid absorption of nicotine occurred in the initial segments of the denuder. The higher the aerosol temperature, the larger the amount of the initial absorption. The amount of nicotine in the denuder sections decreased as the distance to the denuder inlet increased. Fig. 2 Distribution of deposition nicotine per denuder segment at different temperatures. The denuder separation efficiency to the gas phase nicotine increased from 4.87% to 40.71% with the 4 (3)

4 60 Tobacco Science & Technology 2017 increment of the temperature, which was due to the rise of the gas partial pressure of the nicotine. 2.3 Model comparison The initial gas proportions calculated by LCM model and LP model are shown in Fig.3. It is clear that the stability and regularity of the results in LCM model are worse than that of LP model, resulting from the fact that LCM model used empirical fitting of the experimental data. The results in LCM model mostly depend on the experimental data, and a slight error of the data may influence the final result. However, LCM model is useful at describing the process on nicotine depositing in the denuder. On the other hand, the results obtained by the LP model seemed more stable and less dependent on the variability of the experimental results. That was because LP model relied partially on the experimental data and partially on its gas/liquid equilibrium theory. These two models contribute to the two completely different results although they were originated from the same experimental data. Based on the comparison, the results for the initial gas phase nicotine proportions ( f gas) are calculated by the LP model. denuder is μg, while the total mass of nicotine collected in the Cambridge filter is μ g and the separation efficiency of the denuder is 6.8%. In the distance of 5-10 cm to the denuder inlet, the nicotine deposition reached the highest and decreased subsequently. The deposition curve fits naturally by a double-exponential function. The mass of initial gas phase nicotine deposition decreased while the aerosol passing through the denuder (Fig.4) and most of all the initial gas phase nicotine was absorbed before the end of the seventh section. Hereafter, most deposited nicotine was vaporized from particle phase nicotine. a. Deposition of all nicotine b. Deposition of initial gas nicotine c. Deposition of evaporative gas nicotine Fig.4 Distribution of gas nicotine deposition in denuder(t/ 308 K, RH/60%, LCM model). D at any temperature can be calculated by D T = D 298 K (T/298 K) To obtain the value of f gas, D 298 K = cm 3 s -1 [19], L=100 cm, Q=4.167 cm 3 s -1 was used to calculate it. The f gas in the aerosol at 308 K was equal to %, obtained by the LP model which was proposed by Lipowicz et al [16]. a. LCM model b. LP model Fig.3 Results of initial gas phase nicotine proportions. 2.4 Nicotine absorption in denuder at 308 K The nicotine deposition in the denuder is shown in Fig 4. The total mass of nicotine absorbed in the whole 2.5 Nicotine evaporation and diffusion in denuder at different temperatures This section described the phenomenon of nicotine evaporation by the LCM model. Gas phase nicotine deposited in the denuder with the aerosol passing through it and the mass of deposition nicotine decreased exponentially with the distance. When the temperature increased from 293 to 333 K, the initial gas phase nicotine was absorbed near completion before 15, 50, 30, 30, 20 and 60 cm, respectively. Besides, the mass that particle phase nicotine evaporated to the gas phase nicotine showed incremental rise with increasing smoke temperature (Fig.5). 2.6 Mass distribution of initial gas phase nicotine at different temperatures The f gas in the aerosol is summarized based on the LP model (Table 1). At 293 K, f gas is equal to %,

5 Vol. 50 Suppl. No. 1 GAO Linyu,et al:determination of gas phase nicotine in mainstream smoke with denuder technology 61 Fig. 5 Distribution of gas phase nicotine deposition in denuder. (LCM model). Table 1 Fraction of initial gas phase nicotine at different temperatures (LP model). Temperature (T/K) while the temperature increases to 333 K, the f gas rises to 1.56%. 2.7 Abnormal absorption of nicotine at the inlet In order to investigate the reasons of abnormal absorption of nicotine at the inlet of the denuder, the temperature distribution and velocity distribution in the tube inlet at the 333 K were simulated by Fluent software, and the results are shown in Fig.6. a. Temperature distribution f gas/% The results show that the aerosol flow was instable in the first section, where the laminar flow was not developed fully (Fig.6). That caused the much higher nicotine deposition in the first section, than those of the second and the rest of segments. The error, generating from the first section, was estimated to be within 1.35% ± 0.28%. 3 Conclusions The work established the denuder technology to separate the gas phase and particle phase nicotine in mainstream tobacco smoke. An advantage of this method is that it minimize the adsorption of gas phase nicotine by collecting gas phase nicotine in a denuder before reaching common Cambridge filter pads. The results indicate that the mass fraction of nicotine deposited in each sections of the denuder followed a double-exponential curve as the smoke passing down the denuder. The initial gas phase nicotine proportions ( f gas) could be described by a diffusion and evaporation mechanisms of the smoke nicotine between its gas phase and particle phase. This method can be applied not only for cigarette smoke but also for aerosols produced from novel tobacco and nicotine products such as electronic cigarettes and heat-not-burn tobacco products. Acknowledgement Financial support from China National Tobacco Corporation is greatly appreciated (Grant No ) b. Velocity distribution Fig. 6 Distribution of temperatures and velocities in denuder inlet at 333 K. References [1] Pankow J F. A consideration of the role of gas/ particle partitioning in the deposition of nicotine and other

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