negative peak in fid before methane

[mallikka]

i am using an agilent gas chromatograph with tcd and fid.i am using helium as an carrier gas for the fid.i inject my gas directly in the fid and i am getting a negative peak before methane.this peak is not observed in the standard gas.even when i use a nucon gc, with nitrogen as the carrier i am facing the same problem.please help.thanks

[chromatographer1]

what is different from your sample gas from your std gas?

Rod

[mallikka]

hi! my sample gas is the gas desorbed from subsurface soil samples using orthophosphoric acid.my standard gas does not have co2 in it.i injected another standard gas having co2 and i got the same signature as my sample.there was a dip before methane.i am using agilent 7890a with helium as the carrier in fid