## 3 Results and Discussion
### Sampling Products from Incense Coils by SPME Fiber and NTS Samplers  
Figure 3 plots the GC-FID extracted amount (integral area) of pollutants from burning incense coils when using SPME fibers and NTS for a TWA sampling time of 60 min. Figure 4 is the typical GC diagram for main by-products methanol, ethylbenzene, chloromethane, dichloromethane and 1,3-butadiene via extraction of 100-120 mesh DVB-NTS. Comparison of VOC adsorptions shows that VOC extractions by the NTS packed with DVB were higher than that of the 100 \(\upmu\)m PDMS SPME fibers; the 100-120 mesh DVB-NTS performed better than the 60-80 mesh DVB-NTS and SPME fibers. For 1,3-butadiene, as an example, the extracted mass by the 100-120 mesh DVB-NTS was 1.47-fold than that by 60-80 mesh DVB-NTS, and 2.53-fold than that by the 100 \(\upmu\)m PDMS SPME fibers. Experimental results demonstrated that commercially available SPME fibers could generally extract wide distributions of organic compounds, which is suggested by SUPELCO, the SPME manufacturer [17]. Man-made NTS packed with adsorbents with a proper diameter and affinity to adsorbates can extract a larger mass of alkane, aromatic and chlorinated compounds. These extraction distributions of combustion by-products from indoor consumer goods by NTS have roughly followed the same trends as those in earlier studies [11, 12]: the smaller diameter adsorbent 100-120 mesh DVB-NTS can extract more VOCs than the 60-80 mesh DVB-NTS.  
An earlier study [11] reported that after 3-5 samplings using the DVB-NTS for air-borne products of mosquito coils combustion, gaseous VOCs could not be collected due to clogging of the diffusion path between adsorbents packed in the samplers, and no further integral peak counts of VOCs could be analyzed via GC-FID. This implies that the main limitation of aerosol extraction by NTS may be clogging during the packing phase when sampling products from heated sources, particularly large diameter aerosols such as mosquito coil aerosols and cigarette smoke. To determine whether burning incense coil clogs the NTS, this work performed nine continuous 1-h samplings, using a single 60-80 mesh DVB-NTS and a 100-120 mesh DVB-NTS. The notable declined rates of extraction performance for the eight compounds (Figure 3) was observed ranging from 31.8 to 60% for the 60-80 mesh DVB-NTS and from 40% to 64.4% for the 100-120 mesh DVB-NTS. The ESEM-FEI images (Figure 5) show surface views of the DVB particle before and after extracting the combustion products of incense coils, indicating that residues on the surface of the 100-120 mesh DVB particle were remarkably smaller than those on 60-80 mesh DVB particles. The phenomena may be resulted from the paths between the 100-120 mesh DVB particles are narrower than those between 60 and 80 mesh DVB particles. In other words, the 100-120 mesh DVB-NTS will clog more when sampling suspended aerosols than 60-80 mesh DVB-NTS.  
Figure 4: Typical gas chromatograph diagram of incense coil combustion using needle trap sampler. The sequence of retention times (min) respectively indicates the main by-products methanol, ethylbenzene, chloromethane, dichloromethane and 1,3-butadiene.  
Figure 3: Main hydrocarbon analysis from combustion of incense coil. Columns \(\blacksquare\), \(\blacksquare\) and \(\blacksquare\) respectively indicate the integral area extracted by 60–80 mesh, 100–120 mesh NTS and SPME fibers via GC-GID analysis.