Apparatus and reagents
Gas chromatography (GC) coupled with a triple quadrupole mass spectrometer (GCMS-TQ8040, SHIMADZU) and HP-5MS (30 m × 0.25 mm i.d., 0.25 μm) (Agilent Technologies, CA, USA) were used to quantify furans, pyridines, and other additives in the mainstream smoke in this study. GC coupled with a thermal conductivity detector (TCD) and a packed column (Porapack Q2 m×3 m i.d., 80–100 mesh deactivated stainless, GL Sciences, Tokyo, Japan) was used to quantify water in the mainstream smoke in this study. Nicotine 97%, n-hexane > 96%, 2-propanol > 99.7%, methanol (high-performance liquid chromatography [HPLC] grade) > 99.9%, acetic acid > 99.7%, and 2,6-dimethylpyridine were purchased from FUJIFILM Wako Chemicals, Ltd. (Osaka, Japan). 2,5-Dimethylpyrazine, 2(5H)-furanone, 5-methylfurfural, nicotine, acetonitrile, and ammonium acetate (≥ 99.99%) were purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA). Pyridine, furfural, 2-furanmethanol, 2-ethenylpyridine (2-EP), 3-ethenylpyridine (3-EP), 4-ethenylpyridine (4-EP), 3-ethyl pyridine, 2,3,5-trimethylpyrazine, benzyl alcohol, linalool, menthol, 5-hydroxy-2-methylpyridine, isoquinoline, eugenol, and 4-ethyl guaiacol were purchased from Tokyo Kasei Co., Ltd., (Tokyo, Japan). The water used for sample preparation and analysis was deionized, and it was further purified using a Milli-Q water system (Millipore Co., Bedford, MA, USA).
In this study, we used conventional combustion cigarettes (3R4F) from the University of Kentucky (Lexington, KY, USA), IQOS (regular and menthol) from Philip Morris International Inc. (NY, USA), glo (berry boost and dark fresh) from British American Tobacco (NC, USA), and ploom S from Japan Tobacco Inc. (Tokyo, Japan). According to the International Organization for Standardization (ISO) 3402, these cigarettes were used for measurement after being placed at 22°C and 60% humidity for 2 days [
7].
Method of sampling main stream smoke of HTPs
For sampling of the gas phase and particle phase of mainstream smoke of HTPs, a sorbent cartridge and CFP were used. To prepare the sorbent cartridge, 150 mg of Tenax GR (GL Sciences, Tokyo, Japan) was packed in a Rezorian tube (1 mL), and the cartridge was washed with 10 mL of 2-propanol and dried with nitrogen gas at a flow rate of 1.0 L/min for 5 min. Mainstream smoke of conventional combustion cigarette was collected with this sorbent cartridge connected between tobacco product and CFP according to the intense regime described in the standard operating procedure (SOP) 01 [
8] and Health Canada, official method T-115 [
9]. Briefly, mainstream smoke was collected under the following conditions: 55 mL puff volume, 2 s puff duration, 30 s puff interval, and 100% blocking of the filter ventilation holes with Mylar adhesive tape. The puff number of 3R4F as a standard conventional combustion cigarette was nine times. In case of HTPs, we produced a puffing profile for the collection of mainstream smoke of HTPs from battery time conditions of HTPs as described below. The maximum number of smoking absorption was twelve times for one dedicated stick from the calculation of smoking interval based on the HCI method (30 s) and battery time of IQOS (6 min). When the number of puffs was calculated for glo and ploom S by the same method as IQOS, the number of puffs for IQOS was the largest. Therefore, in order to compare the concentrations of three types of HTPs in the mainstream smoke, the total number of puffs was set as twelve puffs. So far, no official law has been established regarding the smoking interval and amount of smoke absorbed for HTPs. Actually, it is possible that research on smoking behavior of smokers who use HTPs has not been sufficiently conducted. Therefore, it is necessary to consider it as a future issue. Furthermore, for volatile compounds, many of them pass through the CFP at the time of collection and are collected in the sorbent cartridge, but those collected in the CFP may be slightly volatile during the collection of mainstream smoke of HTPs. Therefore, we need to do quickly pretreatment after the sample collection.
Analytical method
Both the sorbent cartridge and CFP were extracted using 5 mL and 10 mL of 2-propanol, respectively, for 30 min using a rotary shaker at 120 cycles per min. After extraction, isoquinoline was added as an internal standard and analyzed via GC-MS/MS. The GC oven temperature program was 50 °C (held for 2 min) and increased to 200 °C at 6 °C/min and 310 °C (held for 3 min) at 20 °C/min. Splitless injections were carried out at an injector temperature of 280 °C and splitless time of 1 min. Helium (99.99995%) was used as the carrier gas at a flow rate of 1.3 mL/min (pressure 75 kPa). Mass spectrometric ionization was performed in the electron impact (EI) mode at a voltage of 70 eV. The transfer line and ion source temperatures were set to 280 °C and 230 °C, respectively. To identify suitable precursor ions, full scan monitoring was performed for all compounds at the first MS with a mass range of m/z 50–500. Argon (99.999%) was used as the collision-induced dissociation (CID) gas. To determine suitable product ions, full product ion spectra of selected precursor ions were obtained, and the optimum collision energies were determined for each identified m/z transition. The most abundant and apparent ions were chosen as the targets for quantification. The peaks of each compound that showed a signal-to-noise ratio (S/N) of 3 in the sample were identified by comparing the retention times (within 0.1 min) and the ratios (within 20%) of the two selected reaction monitoring transitions in the sample with those of the standards.
For the analysis of tar, we referred to the previous report by Bekki et al. [
1]. The amount of tar exhausted in the mainstream smoke was calculated by subtracting the amount of nicotine and water from the total particulate matter (TPM) collected by CFP.