[kamiyu]

When I inject my sample (pure) for the first time in the GC, several peaks always come out. But when I inject the same sample for the second time or third time I got single peak which is my product according to MS.

I wonder why there are always several peaks in the first GC run of my sample???

[Furanone]

Did you condition the GC column first before injecting (for example heating oven to 200-250 C with carrier gas flowing through for about 15 minutes)? This cleans the column out from any residual contaminants from previous days analyses.

If so, then maybe the GC injection needle was slightly contaminated and the contaminants were injected with your sample in the first run, but later injections the contaminants had been cleaned out so you got good chromatograms. You can clean injection needle by taking a small amount of the solvent you use and pumping the syringe up and down like 10 times or so while in solvent (then discard solvent). Remember only usually 1 ul is being injected so the smallest amount of contaminants at the tip of your needle can make a noticeable mark on your chromatogram.