## 4 Discussion  
An HS-SPME method was tested and applied in the study of the background interferences of three commonly used carpets, namely, Stain Master Pet Protect Dyed BCF Nylon 6,6, and Stain Master Essential carpet, for the identification of gasoline and kerosene in fire debris. The main purpose of the study was to demonstrate that SPME can be reliably used for the identification of ILs, if combined with GC-MS. Mass spectrometry provides the option of extracted ion profiles, which helps interpret the data when substrate background interferences make it difficult to make a solid decision based solely on the TICs. To show the reliability of the method in the interpretation of arson data, each carpet was burned in different conditions, namely, without ILs, with gasoline, and with kerosene. The TIC and extracted ion profiles of unburned neat gasoline and kerosene were also obtained for comparison purposes. The results showed that in general all carpets showed a few target compounds of gasoline (alkylbenzenes and polynuclear aromatics) when burned without ILs; however, the extracted profiles showed that the peak abundance ratios of alkylbenzenes and indanes were different from the characteristic ratios expected for gasoline, whereas the carpet samples burned with gasoline clearly showed the alkylbenzene and indane profiles with the same peak abundance ratios as neat gasoline. Naphthalene was identified as an interference in all three carpet substrates making the polynuclear aromatics profiles not suitable for analysis. Unlike gasoline, target alkane compounds of kerosene were not identified in carpets burned without ILs. The TICs and extracted ion profiles of carpets burned with kerosene clearly showed the characteristic profiles expected for kerosene. It can be concluded that this method can reliably be used for the identification of gasoline and kerosene in charred fire debris with the carpets included in this study as substrate material. To our knowledge, such a study of carpet substrate showing extracted ion profiles for carpet burned with gasoline and kerosene using HS-SPME as the extraction method has not been reported in the literature. Considering the advantages of SPME over other extraction techniques, the present study is valuable in providing forensic labs with a reliable method. Moreover, the provided data can help forensic labs interpret their fire debris data more reliably and demonstrates that SPME, although merely accepted as a screening test, is as reliable as the ACS method, which is accepted as the confirmatory test for IL analysis.  
The following are available online at [[http://www.mdpi.com/2297-8739/7/4/63/s1](http://www.mdpi.com/2297-8739/7/4/63/s1)]([http://www.mdpi.com/2297-8739/7/4/63/s1](http://www.mdpi.com/2297-8739/7/4/63/s1)).  
Conceptualization, S.H.; methodology, S.H.; software, S.H.; validation, S.H.; formal analysis, S.H.; investigation, A.A., E.G.-L., G.O., S.H.; resources, S.H. and A.A.; data curation, S.H.; writing--original draft preparation, S.H.; writing--review and editing, S.H.; visualization, S.H. and K.W.; supervision, S.H.; project administration, S.H.; funding acquisition, S.H. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors would like to thank SUNY Oswego's Department of Chemistry for providing the instruments and the Challenge Grant program for financial support. Special thanks to [PERSON] and [PERSON] for their help and support.  
The authors declare no conflict of interest.