Ambient level hydrocarbons lighter than C 6 were analyzed by the Deans switch-modulated comprehensive two-dimensional gas chromatography (GC×GC) method with flame ionization detection (FID). A thermal desorption (TD) device built in-house connects the GC×GC system to pre-concentrate the target compounds at ambient levels prior to GC analysis. Because the conventional orthogonality based on polarity difference for normal GC×GC separation does not provide sufficient retention for the target compounds of extremely high volatility, the orthogonality of non-polar vs. adsorptive force was adopted instead. The system employed a 100% polydimethyl siloxane column serving as the first-dimension column to provide separation based on dispersive interaction, with a short PLOT column serving as the second-dimension column to provide the needed retention based on gas–solid adsorption interactions.The shortest possible length of the PLOT column was tested to minimize the modulation period (P M ) and wraparound and, at the same time, to maintain the desired resolution. The tests led to the final optimal parameters of 1.1m for the PLOT column length, 9s for the P M , 0.013 for the modulation duty cycle (DC) and a modulation ratio (M R ) of 3.7 with minimal wraparound. Important criteria for quality assurance of precision and linearity are reported. The low cost and ease of construction and operation make the in-house Deans switch TD-GC×GC-FID system practical and useful for the analysis of light hydrocarbons in urban or industrial environments.