## 1 Introduction
The sample analysis and data interpretation are perhaps the most challenging steps of fire debris analysis. The identification and classification of ILRs is performed by a pattern recognition technique, which is a visual comparison of the Total Ion Chromatograms (TICs) obtained from the fire debris sample to those of reference ignitable liquids (RILs) obtained under the same chromatographic conditions [5]. However, the chromatograms obtained from an extracted sample from the headspace of fire debris not only include the ignitable liquid residues, but they also represent volatile compounds present in the matrix--substrate background products--as well as pyrolysis and combustion products, which can complicate the data interpretation, especially when the ILRs concentrations are very low. Thus, the standard test method for the interpretation of ILRs extracted from fire debris samples, ASTM E1618-14 [10], recommends that each laboratory should have both their own reference ignitable liquid (RIL) database, as well as a database of commonly used substrate materials that do not contain ILs [6]. However, since not all laboratories can collect and keep such a large collection of reference substrate samples, the National Center for Forensic science (NCFS) maintains a database known as the Ignitable Liquid Reference Collection (ILRC) [6; 11]. Moreover, gas chromatography coupled to mass spectrometry provides extracted ion monitoring, which is a more accurate technique for the identification and classification of ILRs. In this technique, fragment ion(s) related to different class compounds known to be present in each class of ILs are extracted from the total ion chromatograms of the heated extracted fire debris and the obtained chromatogram, which is known as extracted ion profile, is visually compared to the extracted ion profile of that class of compounds in neat ignitable liquids. For example, to decide if gasoline is present in a fire debris sample, fragment ions representative of at least alkanes, aromatics and oxygenates must be reviewed besides the total ion chromatogram [12]. This technique is particularly helpful when volatile organic compounds caused by substrate background products and pyrolysis and combustion products are present at higher concentrations compared to the ILRs and can skew the total ion chromatogram [12]. Although the ILRC database is a comprehensive library of chromatograms obtained from commonly used ILs and substrates and is gathered according to the ASTM guidelines [6], the data is obtained using the activated charcoal strip method and to our knowledge, there is no database available for laboratories that use the SPME method, which can produce a different chromatographic pattern for substrate materials due to the difference in physical and chemical characteristics of the sorbent, sorbent capacity, extraction temperature and extraction time. Besides being solvent-free, fast, reusable, and non-exhaustive, another advantage of SPME over the ACS method is the smaller sample size required for the substrate material analysis. For example, while the ACS method required burning a 6 cm\({}^{2}\) sample of the substrate carpet material [11], the SPME method requires a small sample size (e.g., a single fiber) for analysis. Therefore, SPME can provide significant substrate information even in cases where the sample size limits the use of the ACS method. Moreover, the ILRC database does not provide the chromatograms obtained from substrates burned in the presence of different ignitable liquids. Since the pyrolysis products can be different at the presence of different ILs, due to the change in combustion temperature, investigating the chromatograms of carpets in the presence of a particular IL is useful in argon identification. Due to the advantages of SPME, forensic scientists have shown more interest in using SPME for the analysis of accelerants in fire debris [13; 14]. [PERSON] et al. evaluated an SPME method for the analysis of ILs in fire debris [13] and [PERSON] et al.