## 3 Results and Discussion
### Qualification Tests of Flow Rates and BTEX Extraction Rates for NTS  
The NTS packed with 60-80 and 80-100 mesh DVB adsorbents were respectively used to examine sampling flow rates and then sample gaseous BTEXs in glass bulbs simultaneously with 100 \(\upmu\)m PDMS-SPME fibers. All duplicate experiments were performed three times. The RSD values of sampling flow rates were below 5%, indicating a high degree of uniformity of the packed DVB phases for all NTS (Table 2).  
\begin{table}
\begin{tabular}{c c} \hline
**Test Item** & **Value** \\ \hline Sampling flow rate of NTS (mean values, mL/min) \({}^{\mathrm{a}}\) & \\ Hollow needle & 790 (RSD = 4.88\%) \\ NTS packed with 60–80 mesh DVB adsorbents & 64.2 (RSD = 4.79\%) \\ NTS packed with 100–120 mesh DVB adsorbents & 47.1 (RSD = 4.98\%) \\ VOC extraction rates (mean values, ng BTEX/min) \({}^{\mathrm{b,c,d}}\) & \\
60–80 mesh DVB NTS & \\ Benzene & 2.22 (RSD = 3.73\%) \\ Toluene & 3.29 (RSD = 4.66\%) \\ Ethylbenzene & 4.58 (RSD = 4.37\%) \\ _o_-Xylene & 6.13 (RSD = 4.90\%) \\
100–120 mesh DVB NTS & \\ Benzene & 4.58 (RSD = 4.36\%) \\ Toluene & 5.55 (RSD = 3.00\%) \\ Ethylbenzene & 7.19 (RSD = 4.63\%) \\ _o_-Xylene & 8.58 (RSD = 4.08\%) \\
100 \(\upmu\)m PDMS-SPME fiber sampler & \\ Benzene & 1.56 (RSD = 0.9\%) \\ Toluene & 3.14 (RSD = 1.0\%) \\ Ethylbenzene & 4.52 (RSD = 1.6\%) \\ _o_-Xylene & 5.33 (RSD = 2.8\%) \\ \hline \end{tabular} Notes: \({}^{\mathrm{a}}\) Nine NTS were tested. Five of NTS were packed with 60–80 mesh DVB, and four were packed with 100–120 mesh DVB; \({}^{\mathrm{b}}\) All VOC sampling periods were 1 h; \({}^{\mathrm{c}}\) The specific concentration is around 100 ppm for benzene, toluene, ethylbenzene and _o_-xylene; \({}^{\mathrm{d}}\) The exactly extracted mass of BTEX by 60–80 mesh DVB NTS were \(133\pm 5.3\), \(197\pm 9.0\), \(275\pm 10.9\) and \(368\pm 15.1\) ng; by 100–120 mesh DVB NTS were \(275\pm 10.8\), \(333\pm 13.0\), \(432\pm 23.5\) and \(515\pm 38.0\) ng; by SPME fibers were \(93\pm 0.5\), \(189\pm 0.8\), \(271\pm 3.6\) and \(320\pm 6.3\) ng.  
\end{table}
Table 2: Tests of sampling flow rates and volatile organic compound (VOC) extraction rates by solid phase microextraction devices in this study.  
Table 2 also shows the BTEX extraction rates (BTEX extraction mass/sampling time) for the NTS and SPME fibers during 1 h. The DVB-NTS extracted more BTEX mass than the 100 \(\upmu\)m PDMS-SPME fibers during the extraction duration of 1 h. The smaller DVB adsorbents (120-100 mesh DVB) packed in the other NTS extracted more BTEX than the 60-80 mesh DVB-NTS. The BTEX extraction rates typically followed trends of adsorption performance for both DVB-NTS and PDMS fiber samplers [12, 13].  
According to experimental data (Table 2), total VOC adsorption capacities for 0.347 mg DVB in each NTS were 793 ng BTEXs for the 60-80 mesh DVB-NTS and 1555 ng BTEXs for 100-120 mesh DVB-NTS. That is, the equivalent BTEX adsorption capacities were 2285 ng/mg and 4481 ng/mg for the 60-80 mesh and 100-120 mesh DVB-NTS, respectively, under total BTEX concentrations of 400 ppmv (see notes in Table 2). In Equation (1), the VOCs mass (\(n\)) obtained via NTS extraction correlated with sampling time (\(t\)) given constant gaseous VOC concentrations (\(C\)). However, all concentrations of gaseous by-products were approximately 1.124 ppmv, much lower than the 400 ppmv simulated in the bulbs (Table 1). Based on experimental results, we reasonably infer that the amount of VOCs extracted does not reach the equilibrium adsorption mass of the DVB adsorbent, and these self-manufactured NTS can be used for passive TWA sampling.