Locating　underground　pipeline　leaks　can　be　challenging　due　to　their　hidden　nature　and　variable　terrain　conditions.　To　sample　soil　gas,　solid-phase　microextraction　(SPME)　was　employed,　and　a　portable　gas　chromatography/mass　spectrometry　(GC/MS)　was　used　to　detect　the　presence　and　concentrations　of　petroleum　hydrocarbon　volatile　organic　compounds　(pH-VOCs),　including　benzene,　toluene,　ethylbenzene,　and　xylene　(BTEX).　We　optimized　the　extraction　method　through　benchtop　studies　using　SPME.　The　appropriate　fibre　materials　and　exposure　time　were　selected　for　each　BTEX　compound.　Before　applying　SPME,　we　preconditioned　the　soil　vapour　samples　by　keeping　the　temperature　at　around　4　°C　and　using　ethanol　as　a　desorbing　agent　and　moisture　filters　to　minimize　the　impact　of　moisture.　To　conduct　this　optimisation,　airbags　were　applied　to　condition　the　soil　vapour　samples　and　SPME　sampling.　By　conditioning　the　samples　using　this　method,　we　were　able　to　improve　analytical　efficiency　and　accuracy　while　minimizing　environmental　impacts,　resulting　in　more　reliable　research　data　in　the　field.　The　study　employed　portable　GC/MS　data　to　assess　the　concentration　distribution　of　BTEX　in　soil　vapour　samples　obtained　from　1.5　m　below　the　ground　surface　at　10　subsurface　vapour　monitoring　locations　at　the　leak　site.　After　optimization,　the　detection　limits　of　BTEX　were　almost　100　µg/m3,　and　the　measurement　repeatabilities　were　approximately　5%　and　15%　for　BTEX　standards　in　the　laboratory　and　soil　vapour　samples　in　the　field,　respectively.　The　soil　vapour　samples　showed　a　hotspot　region　with　high　BTEX　concentrations,　reaching　30　mg/m3,　indicating　a　diesel　return　pipeline　leak　caused　by　a　gasket　failure　in　a　flange.　The　prompt　detection　of　the　leak　source　was　critical　in　minimizing　environmental　impact　and　worker　safety　hazards.　©　2023　The　Author(s)