## 1 Introduction  
Humans have relied on canines for over 12,000 years for their ability to detect odors, using them primarily as hunting companions [1]. Canines have since demonstrated high sensitivity and selectivity for detection of contraband such as drugs and explosives [2; 3; 4; 5]. Explosive-detecting canines (EDCs) are routinely used for real-time detection of explosives as they can reliably identify chemical odors emitting from explosive mixtures. Their ability to be trained to identify new target odors, portability, and ease of handling make them an ideal detector of explosives [6; 7]. EDCs alert based on the odor of volatile organic compounds (VOCs) present in an explosive mixture [8]. VOCs such as taggants are added to the explosives at the time of manufacture to facilitate their detection and identification. Taggants are energetic volatile organic compounds such as 2,3-dimethyl-2,3-dinitrobutane (DMNB), 2-nitrotoluene, 4-nitrotoluene, and ethylene glycol dinitrate (EGDN) that serve as means of identification of explosives for both canines and analytical instruments [9]. Due to the high volatility of taggants and other VOCs in explosives and high sensitivity shown by EDCs, invisible residues of explosives can also be detected which can lead to \"nuisance alerts\". Additionally, training aids and other substrates that may be exposed to VOCs emitting from explosives during various stages such as manufacturing, transport, and storage, run the risk of becoming cross-contaminated [6]. This cross-contamination can lead to an increase in the instances of nuisance alerts which can be time and resource consuming. Therefore, it is important to study the extent of contamination that various substrates may exhibit on being exposed to explosives. Most explosives have a complex odor due to the presence of numerous VOCs present in an explosive mixture. Composition C-4 (C4) is a plastic explosive commonly used for training canines and contains VOCs such as DMNB, 2-ethyl-1-hexanol (2E1H), and cyclohexanone [10; 11]. The volatiles emitted by C4 can adsorb on various surfaces and hence there is a need to devise optimal sampling techniques and extraction procedures for determination of contamination on these surfaces. For that reason, this study will focus on a range of substrates exposed to C4. A substrate can be described as a surface where chemical processes such as adsorption and absorption occur. In parallel, 1-bromooctane (1-BO) will also be studied as a single-odor compound due to its growing application as a universal detector calibrant (UDC) for EDCs in recent times [12; 13]. Previous studies have focused on development and optimization of various highly sensitive analytical techniques for the analysis of explosive mixtures [14; 11; 15; 16; 17; 18]. Among these analytical techniques, gas chromatography coupled with mass spectroscopy (GC-MS) is a recognized gold standard for analysis of VOCs with high sensitivity and selectivity [19]. Ref. [20] compared different modes of ionization, namely, electron ionization (EI), negative ion chemical ionization (NICI), and positive ion chemical ionization (PICI) detection methods for detection of organic explosives such as dinitrotoluene (DNT), trinitrotoluene (TNT), and 1,3,5-trinitro-1,3,5-triazine (RDX) and concluded that detection limits are lower by NICI for all explosives tested, with the exception of RDX [10]. [PERSON] et al. in 2010 employed SPME-GC/MS and detection by ion mobility spectrometry (IMS) for detection of VOCs in plastic explosives. The limit of detection (LOD) for target compounds such as n-butyl acetate and DMNB ranged from 1.5-2.5 ng [20]. [PERSON] and [PERSON] in 2021 used portable GC-MS coupled with capillary microextraction of volatiles (CMV) for continuous vapor sampling of VOCs such as 3-nitrotoluene (3-NT) and 2,4-dinitrotoluene (2,4-DNT) associated with explosives. They were able to achieve rapid sampling and preconcentration of sub-ng levels of volatiles in field scenarios using a 10 min GC-MS method comparable with benchtop instruments [21].