Abstract
It still remains a great challenge to selectively enrich and sensitively quantify the trace volatile organic compounds (VOCs) in real samples with complex matrix. In this study, an integration method combining a selective enrichment medium of reduced graphene oxide (rGO) with a specially designed micro thermal-assisted purge-and-trap sampling device was developed for efficient enrichment and sensitive quantification of trace tobacco VOCs coupling with thermal desorption (TD)-gas chromatography/mass spectrometry (GC/MS). The prepared rGO has been proved to possess excellent enrichment selectivity and capacity for tobacco polar VOCs with the multi-layer structure, good thermal stability and large specific surface area. The specially designed sampling device was efficient and suitable for enriching and sampling trace polar tobacco VOCs coupling with rGO medium. Under the optimized sampling and analytical conditions, the established analytical method could be actually applied for quantification of typical tobacco polar VOCs with the good recoveries of 72.9–128% and the satisfied RSDs of 1.8–19.9% (n = 3). The results suggested that the developed method was selective, sensitive and reliable for enrichment and quantification of trace tobacco polar VOCs.
1 Introduction
The quality control [1–3] and safety monitoring [4, 5] of food products are closely related with human health, which are generally realized based on the accurate analysis of characteristic targets in complex food samples. Food volatile organic compounds (VOCs) constitute the unique food aroma and are closely related with food quality control [6, 7] and food safety monitoring [8, 9]. Food VOCs are usually at trace level with complex compositions and strong volatility [10, 11]. It still remains a great challenge to selectively and effectively enrich the specific food VOCs by use of efficient sampling methods. Tobacco and tobacco products as a special kind of food have a wide range of consumers around the world. Tobacco aroma is a special smell, which brings consumers a unique sense of smell [12, 13]. How to remain the special tobacco aroma but remove some harmful VOCs has been a popular issue in the field of tobacco quality control and human health management. It is an essential precondition in this interdisciplinary field to efficiently enrich and accurately analyze characteristic tobacco VOCs, which are important indicators for quality control of tobacco products [14] and reducing cigarette tar and harmful materials [15]. At present, there are still few researches focusing on the development of selective enrichment medium and efficient sampling technology for complex and trace tobacco VOCs.
Ideal enrichment medium for tobacco VOCs should possess relatively large enrichment capacity with good selectivity in order to efficiently capture the trace target VOCs from complex tobacco samples. Graphene oxide (GO), a crucial derivative of graphene, has good chemical stability, thermal stability, and large specific surface area with abundant chemical groups such as hydroxyl groups (–OH), epoxy bonds (C–O–C), carbonyl groups (–C=O), carbonyl groups (–COOH), etc. [16, 17]. The surface modification and the flexibly tunable polarity of GO can be achieved via the suitable reduction process to form the reductive GO (rGO) [18–20]. Moreover, rGO has vacant lattice and high electron density which makes rGO the great potential as a highly selective enrichment medium for trace VOCs with matchable polarity. Chen et al. [21] prepared rGO/gold nanoparticles film substrates by a chemical reduction method for the determination of 14 VOCs in human respiratory gas coupling with surface-enhanced Raman scattering. The large number of folded sheet structures of rGO provided a large specific surface area, which could effectively improve the enrichment ability for respiratory VOCs. The method possessed the potential to preliminarily distinguish patients with early and advanced gastric cancer. Niang et al. [22] prepared rGO encapsulated in a polypropylene bag as a μ-SPE sampler for the detection of polar estrogen in the water sample coupling with HPLC-UV. The detection limit of the proposed method could be achieved in range of 0.24–0.52 ng L−1. Although rGO medium shows great potential for enrichment of VOCs from simple samples, it has rarely been applied for the sampling and analysis of trace VOCs from complicated samples.
Apart from the preparation of selective enrichment media, the development of efficient sampling methods is another crucial factor for accurate analysis of trace and complex VOCs. To date, the VOCs sampling methods mainly include passive and active sampling technology. The passive sampling methods like SPME [23, 24] and static headspace [25, 26] are mainly based on the static diffusion of VOCs from the sample to the enrichment media. The sampling operation procedure is relatively simple, but the sampling efficiency and representativeness is limited due to the uncontrollable static diffusion process. Active sampling methods like purge-and-trap [27, 28] are usually based on the pump-driving VOCs sampling process. The sampling process is controllable with the better sampling representativeness and efficiency, but the instrument scale is relatively large with complicated sampling operation procedures. Moreover, the flexible design of matchable sampling devices would further improve the sampling efficiency and effectiveness according to the sample property and actual sampling requirements.
In this paper, an efficient small-scale purge-and-trap sampling device was specifically designed with the rGO enrichment medium prepared by chemical reduction for the efficient enrichment and sensitive analysis of polar VOCs of tobacco by thermal desorption (TD)-gas chromatography/mass spectrometry (GC/MS). The preparation conditions of rGO medium and the analytical conditions for polar tobacco VOCs were optimized. The enrichment selectivity, enrichment capacity and adsorption kinetic mechanism of rGO were studied in detail. Under the optimal conditions, a selective and sensitive method for quantification of tobacco polar VOCs was established and applied in practice based on the integration of rGO medium and specially designed purge-and-trap sampling device coupling with TD-GC/MS.
2 Materials and methods
2.1 Chemicals and reagents
Propionic acid (analytical standard), 2-methylbutyric acid (98%), valeric acid (analytical standard), caproic acid (99.5%), pentanol (99.5%), benzyl alcohol (99.5%), hexaldehyde (99%), nonaldehyde (96%), 5-methylfurfural (98%), anisaldehyde (99%), acetophenone (99.5%), methyl heptenone (98%), 2,3,5-trimethylpyrazine (99%), cinnamaldehyde (99%), phosphorus pentoxide (98%) and dichloromethane (HPLC grade) were purchased from Shanghai Aladdin Reagent Co., Ltd. Butyric acid (99%) was purchased from Acros Organics. Phenylethanol (99.5%), benzaldehyde (98%) and furfural (98%) were purchased from Beijing J&W Technology Co., Ltd. Graphene oxide powder (>99%) was purchased from Shanghai Macklin Biochemical Co., Ltd. Hydrochloric acid (36%) and ethyl alcohol (analytical grade) were purchased from Guangzhou Chemical Reagent Factory. Iron powder (sphere, <10 μm) was purchased from Alfa Aesar (China) Chemical Co., Ltd. Tenax TA (60–80 mesh) was purchased from Shanghai Shuao Technology and Trade Co., Ltd. PTFE sample bags (20 L) were purchased from Dalian Hede Technologies Ltd.
Ultrapure water was obtained from Milli-Q® Ultra-pure water purification system (Millipore, USA). The mixed gas standards of target VOCs were as follows. Dichloromethane was used as the solvent to prepare mixed standard solutions of different concentrations. Then, the mixed standard solutions and internal standard (IS) standard solution of cinnamaldehyde with different concentrations were pipetted into the sample bag by microsyringe, and after volatilization at room temperature for 15 min, the mixed standard gases with the concentrations of 0.10–3.0 × 102 ng L−1 were obtained for consequent internal standard calibration. Other commonly used reagents in this work were analytical pure grade.
2.2 Instruments
The Quanta 400F thermal field scanning microscope (FEI, Netherlands) was used to characterize the surface morphology of rGO medium with the magnification of 2,500–25,000 times. The X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher Scientific Nexsa XPS instrument (Waltham, USA). Fourier transform-infrared (FTIR) was carried out using a Thermo Nicolet Nexus 670 FTIR spectrometer (Hillsboro, USA). The thermal stability was evaluated by a NETZSCH TG 209 F3 Tarsus thermogravimetric analyzer (Selb, German) from 35 to 800 °C at a rate of 10 °C min−1 under nitrogen atmosphere. X-ray diffractometry analysis (XRD) was characterized using a Bruker D8 ADVANCE X-ray diffractometer (Karlsruhe, German). The specific surface area and pore size was performed on an ASAP2020M gas adsorption instrument (Atlanta, USA) based on a Brunauer-Emmet-Teller (BET) method.
The main components of the sampling device developed in this study included QC-2B atmospheric sampler from Beijing Institute of Conservation Science (Beijing, China), DF-101S constant temperature heating magnetic stirrer from Gongyi Yuhua Instrument Co., Ltd (Gongyi, China) and micro diaphragm pump from Dalian Hede Technology Ltd. (Dalian, China). Chromatographic analysis was performed by a GC-MS QP 2010plus system (Shimadzu, Japan) equipped with a DB-WAXetr (30 m × 0.25 mm × 0.25 μm) (AGILENT Technologies, Palo Alto, CA, USA. Helium (>99.999%) was used as carrier gas. Temperature programming was as follows: 65 °C for 0.5 min, 10 °C min−1 to 100 °C, 2 °C min−1 to 110 °C, 5 °C min−1 to 130 °C, 10 °C min−1 to 176 °C, 1 °C min−1 to 177 °C. Transmission line temperature was set at 250 °C with the ion source temperature of 250 °C and the solvent delay time of 3 min. The characteristic ions (m/z) of target VOCs and internal standard for quantification analysis by SIM mode were shown in Table S1. Thermal desorption was performed by a Gerstel TDU-CIS4 instrument (DeMilheim, German) coupling with GC/MS. TD conditions were as follows: initial temperature, 30 °C; heating rate, 60 °C min; thermal desorption temperature, 325 °C; thermal desorption time, 16 min; desorption flow rate, 30 mL min−1. Cooled injection system (CIS) conditions were as follows: the initial temperature of CIS, −40 °C; heating rate of secondary desorption, 12 °C min; secondary desorption temperature of CIS, 250 °C; the secondary desorption time, 10 min.
2.3 Preparation of rGO medium
rGO medium was prepared according to a synthesis method in a previous report with slight modifications [29]. Firstly, 50.0 mg GO were added to 100.0 mL of deionized water followed by being sonicated for 1 h to form a brown suspension solution. Then, 3.0 g Fe powder was added to the suspension solution followed by the addition of 10.0 mL of 36% HCl and stirred at 200 rpm for 30 min, and the solution was heated at 50 °C and reacted for 6.5 h. After the reaction, 60.0 mL of 36% hydrochloric acid was added and stirred at 200 rpm for 20 min followed by removing the excess Fe powder via suction. The obtained rGO powder was rinsed with deionized water and absolute ethanol three times respectively and then dried under vacuum at 60 °C for 12 h.
2.4 Development of the micro thermal-assisted purge-and-trap sampling device
A micro thermal-assisted purge-and-trap sampling device for tobacco VOCs was designed and developed in this work. The sampling device was composed of static purging gas source, sampling chamber, adsorption tube, annular water bath heater and sampling pump, as shown in Fig. 1. The sampling system was manufactured by polytetrafluoroethylene (PTFE) without any other metal and polypropylene material, which would prevent the adsorption of VOCs by the sampler. Specific details of the sampling system can be seen in Supplementary Materials.
Schematic of sampling process of the micro-thermal assisted purge-and-trap device for tobacco VOCs
Citation: Acta Chromatographica 36, 3; 10.1556/1326.2023.01161
A computational fluid dynamics (CFD) simulation method by a commercial ANSYS 19.1/fluent CFD software was used to evaluate the effect of dead volumes of four different chamber designs on the sampling efficiency and further optimize the chamber shape. The simulation model was established to calculate the distribution of various gas flow rate in the proposed sampling chambers based on a Navier-Stokes equation with three dimensional steady and incompressible flow. Model fluid was set as air with the density of 1.29 kg m−3, and the realizable
2.5 Analysis of polar tobacco VOCs
Firstly, the tobacco samples were ground with a pulverizer followed by being passed through a 20-mesh sieve. Then, the tobacco powder was freeze-dried for 12 h to remove the water. Then, 5.0 g of dried tobacco powder were weighed into a 250 mL sampling chamber vial, and the chamber was quickly sealed for purging process. After that, the tobacco powder sample was purged by nitrogen driven by a precise gas pump for 6 min under the constant stirring conditions at the sampling temperature of 30 °C. The sample of tobacco VOCs were then collected in a storage gas bag which was then connected to the adsorption tube containing rGO medium for the consequent enrichment process at a flow rate of 0.5 L min−1 for 8 min. Finally, the rGO medium enriching target VOCs was transferred, thermally desorbed and analyzed by TD-GC/MS. The sampling system was ultrasonic cleaned with ethanol for 30 min after every sampling process, and the residual gas in the diaphragm pump should be also cleaned with high-purity nitrogen (purity >99.999%) at a flow rate of 1 L min−1 for 5 min after every sampling process.
3 Results and discussion
3.1 Preparation and characterization of the rGO
rGO medium was prepared based on a chemical reduction method [29] by using graphene oxide, iron powder and hydrochloric (36%) as raw materials. In order to obtain the rGO with satisfied thermal stability, the ratio of GO and iron powder, the volume of hydrochloric and reaction temperature were optimized in detail, which was based on an orthogonal design method, as shown in Table S2. TGA was used to investigate the thermal stability of rGO medium obtained under the corresponding synthesis conditions in Table S2. It could be seen from Fig. S1 that rGO medium prepared under the conditions which were 3.0 g of Fe with 10.0 mL of 36% HCl at the reaction temperature of 50 °C demonstrated the relatively lower weight loss rate of 9.4% at 300 °C, which suggested the good thermal stability, so this set of synthesis conditions was selected as the optimized preparation conditions of rGO medium in the following study.
The surface morphology of rGO medium was characterized by SEM, as shown in Fig. 2. It could be seen that rGO had a layered loose structure with homogeneous surface morphology. Thin two-dimensional rGO sheets were bent and folded to form thermodynamically stable porous three-dimensional particles, which was conducive to increasing the specific surface area of rGO and further the extraction capacity. XPS was used to investigate the elemental composition and chemical bonds of rGO medium, as shown in Fig. 3A and B. The XPS full-scan spectra in Fig. 3A had two peaks at 296.03 and 540.67 eV attributing to C1s and O1s respectively, indicating the main elements of the rGO medium were carbon and oxygen and the ration of C:O was 7.7 around. Then, the C1s spectrum of rGO medium was further interpreted as Fig. 3B. It could be seen that there were 4 peaks at 284.6, 285.5, 286.9 and 288.2 eV corresponding to C=C, C–C, C–O and C=O, respectively.
Scanning electron micrographs of rGO adsorbent with the magnification of 5,000 × (A) and 25,000 × (B)
Citation: Acta Chromatographica 36, 3; 10.1556/1326.2023.01161
Characterizations of rGO adsorbent including the corresponding (A) General XPS spectrum, (B) C1s spectra, (C) FTIR spectrum, (D) TGA curve, (E) XRD curve and (F) Adsorption desorption isotherm of nitrogen curves
Citation: Acta Chromatographica 36, 3; 10.1556/1326.2023.01161
FTIR was used to study the characteristic functional groups of rGO medium, as shown in Fig. 3C. It could be seen that the rGO medium had characteristic absorption peaks at 3,400, 1,500 and 1,200 cm−1 attributing to the contraction vibration of C=C, the vibration of C=C and the C–OH vibration respectively [30]. The results showed that rGO was synthesized successfully and contained hydroxyl groups. TGA was used to characterize the thermal stability of rGO medium, and the thermal gravimetric curve was shown in Fig. 3D. The results showed that the weight loss rate of rGO medium was less than 9.8 and 15% within 350 and 800 °C respectively, indicating that the thermal stability of rGO medium could satisfy the consequent TD-GC/MS analysis.
XRD was used to characterize the crystal change of GO and rGO before and after the reaction, as shown in Fig. 3E. It could be seen that the diffraction peak of GO disappeared at 2θ = 10° after reduction, indicating that the (001) crystal plane in the crystal disappeared. At the same time, the diffraction peak of 2θ = 25° appeared, indicating that the synthesized rGO medium had a (002) crystal plane, which was consistent with the previous literature [31]. The nitrogen adsorption-desorption isotherm of rGO medium was measured under 77 K condition, and the specific surface area and pore size distribution of rGO medium were characterized, as shown in Fig. 3F. The results showed that the specific surface area of rGO medium was 87.39 m2 g−1, the surface area of Langmuir was 128.29 m2 g−1, the total pore volume of adsorption was 0.214 cm3 g−1, and the average pore size of adsorption was 8.998 nm. The BET results suggested that the rGO dielectric micropores were slit holes with parallel plate structure. And capillary agglomeration occurred when p/p0>0.50, which caused the rapidly increasing nitrogen adsorption.
3.2 Design and development of the sampling device
The purge-and-trap method, which has the characteristics of simple operation, high enrichment capacity and no use of organic solvents, has been widely used in the separation and analysis of VOCs [32]. However, the commercial purge-and-trap instrument usually adopts infrared heating module and is more suitable for analyzing liquid samples with uniform heat transfer. When being used for analyzing solid samples such as tobacco, the sample would be heated unevenly without stirring. Moreover, the types of commercial adsorbents are limited with non-specific selectivity to the polar tobacco VOCs. In most cases it is used for qualitative or semi-quantitative analysis of tobacco VOCs [33, 34]. Therefore, the development of a rotary heating purge-and-trap sampling device for efficiently sampling tobacco VOCs, combined with selective enrichment media would be an effective way to improve the selectivity, sensitivity and accuracy for separation and analysis of trace and complex tobacco VOCs.
The ideal tobacco VOCs sampling device should be able to heat tobacco samples uniformly with a small dead volume, and more samples can be placed in the sampling chamber to improve the sampling flux and obtain the representative tobacco VOCs samples. The material of the sampling system will not adsorb tobacco VOCs with low sampling blank. At the same time, the purging flow rate should be accurate and controllable to improve the sampling stability and reproducibility. In this study a micro thermal assisted purge-and-trap sampling device designed and developed for tobacco VOCs based on the reasonable design and optimization of the sampling chamber shape and pipeline material. This sampling device was mainly composed of static purging gas source, sampling chamber, adsorption tube, annular water bath heater and sampling pump, as shown in Fig. 1.
Firstly, the shape of the sampling chamber would be directly related to the dead volume of the chamber and the airflow distribution in the chamber, which would greatly influence the retention of tobacco VOCs in the sampling process and further the sampling efficiency. CFD can directly reflect the dead volume of the cavity, and is often used to evaluate the distribution of gas flow velocity in the sampling chamber with different shapes [35–38]. Generally, the distribution area of low velocity is positively correlated with the dead volume of the sampling chamber. In this study, CFD was used to evaluate the distribution of gas flow velocity of four different sampling chambers including sphere, cuboid, tube and cone shape (Fig. S2) so as to study the dead volume ratio of the chamber during the sampling process. The results are shown in Fig. S3. It can be seen from Fig. S3 that the area of low gas velocity (<0.2 m s−1) in the sphere, cuboid, tube and cone sampling chamber accounts for 47.58, 58.86, 96.38 and 78.67% respectively. The results showed that the dead volume of the sphere sampling chamber was relatively low, so the sphere sampling chamber was selected as the optimized sampling chamber.
Secondly, the appropriate pipeline material would effectively reduce the adsorption residue and sampling blank during the sampling process of tobacco VOCs and improve the sensitivity of the proposed analytical method. In this study, common silicone tube and polytetrafluoroethylene tube were used as the pipeline materials of the sampling device respectively for the evaluation. The corresponding sampling chromatograms were shown in Fig. S4. The results showed that the adsorption residue and sampling blank of tobacco VOCs were significantly reduced, and the analytical sensitivity was effectively improved when PTFE was used as the pipeline material. Therefore, PTFE was selected as the optimized pipeline material for the sampling device.
3.3 Enrichment selectivity, capacity and mechanism of rGO medium
At present, there has been no specific medium for the enrichment and analysis of tobacco VOCs. Tenax TA as a broad adsorption medium is widely used for the enrichment of VOCs in environmental samples. Therefore, Tenax TA is selected as a comparison medium for the study of enrichment selectivity of the prepared rGO medium to the target tobacco VOCs including propionic acid, butyric acid, 2-methylbutyric acid, valeric acid, caproic acid, pentanol, benzyl alcohol, phenylethanol, furfural, 5-methylfurfural, benzaldehyde, anisaldehyde, acetophenone and methyl heptenone. It could be seen from Fig. S5A that with the increase of the polarity of the target VOCs, the enrichment efficiency of rGO medium increased accordingly. The enrichment efficiency of propanoic acid, butyric acid, 2-methylbutyric acid, valeric acid, 5-methylfurfural, benzaldehyde, acetophenone by rGO was better than that by Tenax TA. The maximum improvement of corresponding enrichment efficiency by rGO could reach 2.8 times in comparison with Tenax TA, indicating that rGO had good enrichment selectivity for polar tobacco VOCs.
Then, butyric acid was selected as a typical tobacco polar VOCs to study the enrichment capacity of rGO medium. The results was shown in Fig. S5B. When using rGO and Tenax TA to enrich butyric acid with different concentrations, the enrichment equilibrium was reached at 4.5 × 103 and 2.0 × 103 ng L−1 respectively, and the enrichment capacity was calculated as 100 μmol mg−1 for rGO and 9.4 μmol mg−1 for Tenax TA. The enrichment capacity of rGO to butyric acid was about 10 times as that of Tenax TA. The results suggested that rGO medium had higher enrichment capacity for polar tobacco VOCs. This might be due to the intermolecular hydrogen bond formed between –OH and –COOH in the rGO medium skeleton, which increased the enrichment capacity of polar VOCs such as butyric acid.
Moreover, the enrichment mechanism of rGO to butyric acid as a typical polar VOC was further investigated based on a dynamic adsorption experiment which demonstrated the relation between the enrichment amount and the sampling time. It could be seen from Fig. S6A that the enrichment equilibrium was reached at 8 min. The slope of the kinetic curve was large at the beginning sampling phase with a relatively fast adsorption rate. Then, the slope became smaller with a decreasing adsorption rate, which was attributed to the saturation of the surface adsorption site, leading to the decrease of mass transfer efficiency and adsorption efficiency. The enrichment capacity of rGO gradually slowed down with the increase of time, and finally reached the adsorption equilibrium. After that, the enrichment mechanism of rGO medium to butyric acid was studied by the fitted pseudo-first-order (Fig. S6B) and pseudo-second-order kinetic (Fig. S6C) model respectively. The fitted pseudo-second-order model showed a higher linear correlation coefficient (R) than the fitted pseudo-first-order model. The result suggested that enrichment kinetic process of rGO medium to butyric acid was more in line with the pseudo-second-order model, indicating that the enrichment mechanism of rGO to butyric acid was mainly based on chemical adsorption [39].
3.4 Method validation and application
Before the establishment of the analytical method, the crucial sampling conditions including sampling flow rate and sampling time were optimized in detail (Fig. S7). The optimized sampling flow rate and sampling time was 0.5 L min−1 and 8 min, respectively.
Under the optimized sampling and analytical conditions, an efficient and sensitive method for the enrichment and analysis of 12 trace polar tobacco VOCs was established by use of rGO media coupling with the micro thermal-assisted purge-and-trap sampling device and TD-GC/MS. The linear ranges for these 12 polar tobacco VOCs by the proposed method were listed in Table S3 with the LODs of 0.02–0.4 ng L−1. The one-batch and batch-to-batch relative standard deviations (RSDs) of rGO media for sampling and analysis of target VOCs standards were in range of 1.4–18.5% and 0.3–20.1% (n = 3), respectively.
Then, the established method was applied for the efficient enrichment and quantification of trace polar tobacco VOCs, and the corresponding chromatograms were shown in Fig. 4. The quantification results in Table 1 showed that 12 polar tobacco VOCs could be actually detected and quantified to be 24.0–504.0 ng g−1. To further validate the method reliability, the spiked recovery experiment was conducted. The recovery in this work was achieved in range of 72.9–128% with the satisfied RSDs of 1.8–19.9% (n = 3). The results showed that this method was accurate and reliable, and was suitable for the efficient separation and sensitive quantification of trace polar tobacco VOCs.
GC/MS chromatograms of tobacco samples and spiked samples. a. original tobacco samples, b. spiked tobacco samples. The spiked concentrations were set at 36.0 ng g−1 for 2,3,5-trimethylpyrazine (1), 56.0 ng g−1 for furfural (2), 168 ng g−1 for benzaldehyde (3), 176 ng g−1 for propionic acid (4), 36.0 ng g−1 for 5-methylfurfural (5), 60.0 ng g−1 for butyric acid (6), 80.0 ng g−1 for 2-methylbutyric acid (7), 60.0 ng g−1 for valeric acid (8), 88.0 ng g−1 for caproic acid (9), 404 ng g−1 for benzyl alcohol (10), 152 ng g−1 for phenylethanol (11), 20.0 ng g−1 for anisaldehyde (12) and 1.0 × 103 ng L−1 for cinnamaldehyde as IS (13)
Citation: Acta Chromatographica 36, 3; 10.1556/1326.2023.01161
The determination of polar VOCs from tobacco sample and the results of spiked experiment
| VOCs | Original content (ng g−1) | Spiked content (ng g−1) | Found content (ng g−1) | Recovery (%) | RSDs (%, n = 3) |
| 2,3,5-Trimethylpyrazine | 48.0 ± 5.4 | 24.0 | 76.0 | 127 | 11.7 |
| 36.0 | 92.0 | 125 | 8.8 | ||
| Furfural | 72.0 ± 13.0 | 36.0 | 112 | 112 | 3.9 |
| 56.0 | 116 | 78.5 | 4.0 | ||
| Benzaldehyde | 208 ± 38 | 104 | 336 | 128 | 7.3 |
| 168 | 408 | 120 | 6.7 | ||
| Propanoic acid | 220 ± 25 | 112 | 328 | 103 | 19.3 |
| 176 | 392 | 97.7 | 19.0 | ||
| 5-Methylfurfural | 52.0 ± 9.4 | 24.0 | 72.0 | 93.2 | 12.7 |
| 36.0 | 96.0 | 114 | 14.2 | ||
| Butyric acid | 76.0 ± 8.8 | 40.0 | 112 | 87.5 | 11.9 |
| 60.0 | 144 | 107 | 16.1 | ||
| 2-Methylbutyric acid | 100 ± 10 | 52.0 | 148 | 99.5 | 18.4 |
| 80.0 | 188 | 108 | 18.8 | ||
| Valeric acid | 76.0 ± 6.9 | 40.0 | 108 | 79.8 | 19.9 |
| 60.0 | 144 | 111.0 | 13.8 | ||
| Caproic acid | 112 ± 8.4 | 56.0 | 156 | 72.9 | 17.3 |
| 88.0 | 180 | 75.6 | 12.4 | ||
| Benzyl alcohol | 504 ± 61 | 252 | 800 | 118 | 1.8 |
| 404 | 1,010 | 126 | 5.9 | ||
| Phenylethanol | 192 ± 26 | 96.0 | 300 | 109 | 16.0 |
| 152 | 376 | 121 | 9.3 | ||
| Anisaldehyde | 24.0 ± 4.0 | 12.0 | 32.0 | 89.4 | 18.6 |
| 20.0 | 40.0 | 87.6 | 11.7 |
4 Conclusions
In this study, a kind of selective rGO enrichment medium and an efficient micro thermal assisted purge-and-trap sampling device were specifically prepared and developed for efficiently enriching and sampling trace polar tobacco VOCs. rGO enrichment medium prepared in this work were characterized and proved to have multilayer structure, good thermal stability and large specific surface area. Moreover, rGO medium had higher enrichment selectivity and enrichment capacity for polar tobacco VOCs than commercial Tenax TA medium. The adsorption process of rGO media for target VOCs was also investigated and confirmed to fit the secondary kinetic law, so the corresponding adsorption mechanism was mainly based on the chemical adsorption. Under the optimized sampling and analytical conditions, an efficient and sensitive analytical method for trace polar tobacco VOCs was established by use of rGO medium coupling with the micro thermal-assisted purge-and-trap sampling device and TD-GC/MS detection. Twelve polar tobacco VOCs could be quantified to be 24.0–504.0 ng g−1 by the proposed method with the good recoveries in range of 72.9–128% and the satisfied RSDs of 1.8–19.9% (n = 3). It was expected that this efficient and sensitive analytical method was suitable for the efficient enrichment and analysis of trace polar tobacco VOCs.
Acknowledgments
The work was financially supported by the Science and Technology Project of China Tobacco Guangdong Industrial Limited Corporation (No. 2020009).
Supplementary material
Supplementary data to this article can be found online at https://doi.org/10.1556/1326.2023.01161.
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