## 1 Introduction  
Arson is defined as a willful and malicious burning of property [1] and is considered a crime. According to the National Fire Protection Association (NFPA), approximately 1.3 million fires were responded to in the year 2019 in the United States [2], of those, 33,395 were reported as arson by the FBI [3]. Fire investigators follow standard scientific approaches to determine the origin and cause of fire [1]. The determination of the cause of fire is defined as the identification of a fuel, ignition source and the oxidizer [4]. If the use of ignitable liquids is suspected, fire debris is collected and sent to the laboratory to be analyzed for the presence of ignitable liquid residues (ILRs) [4]. Ignitable liquids are classified into two major categories: petroleum-based and non-petroleum based [5]. Petroleum-based ignitable liquids are derived from crude oil and are more often encountered in structure and vehicle fires [4, 5]. The American Society of Testing and Materials (ASTM) has further classified petroleum-based ignitable liquids according to their boiling point range (light, medium, and heavy) and general chemical composition [4, 5]. The petroleum-based ignitable liquids commonly encountered in ILR analysis are gasoline and kerosene [5]. Gasoline is mainly composed of aromatic compounds (from 4-carbon long chain, C4, to 12-carbon long chain, C12), and contains small amounts of alkanes [4]. Kerosene is classified as a heavy petroleum distillate in ASTM E1618 and contains both aliphatic and aromatic compounds in the C8-C20+ range [4]. Since ILRs mainly contain a complex mixture of volatile compounds, extraction followed by gas chromatography coupled to mass spectrometry (GC-MS) is the most predominant and effective technique for their analysis in fire debris [6]. The current standard extraction methods used for the pre-treatment of fire debris samples are: headspace concentration [7; 8], and solvent extraction [5; 9]. Among these methods, passive headspace concentration with activated charcoal (ASTM E1412-12) [7] is the most widely used confirmatory test in the United States [5]. In this method, an activated charcoal strip (ACS) is suspended in the headspace of the fire debris in a sealed container and the container is heated at the recommended temperature for relatively long durations to allow the desorption of ILRs from the fire debris into the headspace so that they can be adsorbed on the activated charcoal strip adsorbent. The activated charcoal strip is then removed from the headspace and the adsorbed compounds are desorbed into an organic solvent which is directly analyzed by GC-MS using liquid injection [5; 7]. Although simple and inexpensive, the activated charcoal strip method is time-consuming and requires the use of toxic organic solvents such as carbon disulfide [6]. Another passive headspace extraction method that has become popular in recent years due to being fast and solvent-free is solid phase microextraction (SPME) [8]. In this technique, the sorbent is a polymer, coated on a fiber, which is mounted on a syringe-like device. The sorbent is exposed to the headspace of the heated fire debris in a sealed container for a relatively short extraction time, e.g., 30 min. The sorbent is then removed from the headspace and directly injected into the injection port of the GC-MS for thermal desorption and analysis [4; 5]. Although SPME is considered a screening test, it can provide fast and diagnostic information to forensic scientists [5]. Compared with the activated charcoal strip method, solid phase microextraction is not an exhaustive method and the fire debris sample analyzed using SPME can be tested again if required [8]. Furthermore, small, commonly used glass vials (e.g., 10 mL screw cap vials) can be used for SPME, which can be conveniently heated in the autosampler heater at a controlled temperature. Unlike activated charcoal strips, SPME fibers can be baked prior to extraction to avoid any contamination and are re-used for several extractions.