NOVELTY - Gas chromatographic method to detect first and second marker compounds in petroleum hydrocarbon or liquid biologically derived fuel, comprises: e.g. providing two analysis channels (H1) and (H2), where channel (H1) comprises first and second capillary columns and channel (H2) comprises third and fourth capillary columns; determining retention times of first marker in first and second capillary columns and second marker in third and fourth capillary columns; and introducing first sample of e.g. petroleum hydrocarbon into first channel, and allowing sample to flow through first capillary column.    USE - The gas chromatographic method is useful for detecting a first marker compound and a second marker compound in a petroleum hydrocarbon or a liquid biologically derived fuel (claimed) including petroleum hydrocarbon, biodiesel fuel or ethanol fuel.    ADVANTAGE - The method avoids the removal of a marker by distillation.    DETAILED DESCRIPTION - Gas chromatographic method to detect a first marker compound and a second marker compound in a petroleum hydrocarbon or a liquid biologically derived fuel, comprises: providing two analysis channels (H1) and (H2), where channel (H1) comprises a first capillary column i.e. an open tubular column coated with a polysiloxane stationary phase and a second capillary column i.e. an open tubular column coated with a polyethylene glycol, and the channel (H2) comprises a third capillary column i.e. an open tubular column coated with a polymethylphenylsiloxane having at least 30 mole% phenyl substitution and a fourth capillary column i.e. a deactivated open tubular column; determining retention times of the first marker compound in the first and second capillary columns, and the second marker compound in the third and fourth capillary columns; introducing a first sample of the petroleum hydrocarbon or liquid biologically derived fuel into the first channel, and allowing the first sample to flow through the first capillary column to produce a first effluent stream; introducing only a portion of the first effluent stream having a retention time range that includes the retention time of the first marker compound into the second capillary column, and allowing the portion of the first effluent stream to flow through the second capillary column to produce a second effluent stream; allowing the second effluent stream to pass through a mass spectrometer; introducing a second sample of the petroleum hydrocarbon or liquid biologically derived fuel into the second channel, and allowing the sample to flow through the third capillary column to produce a third effluent stream; introducing only a portion of the third effluent stream having a retention time range that includes the retention time of the second marker compound into the fourth capillary column, and allowing the portion of the third effluent stream to flow through the fourth capillary column to produce a fourth effluent stream; and allowing the fourth effluent stream to pass through a mass spectrometer. The first marker compound is substituted aromatic compound of formula (I). The second marker compound has 3 or 4 un-fused benzene rings, a total of 20-60C and at least one alkyl or alkoxy substituent having 1-18C, where each marker compound is present in the petroleum hydrocarbon or liquid biologically derived fuel at a level of 0.01-100 ppm.    A1 = 6-20C aromatic ring system;    R1 = 1-12C alkyl or 2-12C alkenyl;    R2 = 1-12C alkyl or 3-12C alkenyl;    m = 0-5; and    n = 1-3.