A gas chromatographic method for detecting a marker compound in a fuel by (a) introducing a sample of fuel into a first capillary column coated with a stationary phase based on polydimethylsiloxane and allowing the sample to flow through the first column to produce a first effluent; (b) allowing the first effluent to pass through a detector and identifying a retention time range in it which includes a retention time of the marker compound; (c) introducing only a portion of the first effluent stream which is within the retention time range into a second capillary column coated with either (i) an ionic sorbent or (ii) a polyethylene glycol, and allowing said portion to flow through the second capillary column to produce a second effluent stream; and (d) allowing the second effluent to pass through a detector; wherein the marker compound has formula Ar(R2)m(OR1)n and is present in the fuel at a level from 0.01 ppm to 100 ppm.