Chromatography Forum

Hi,
in my lab, i use liquid nitrogen to cool down the oven temperature to -20(which is initial temperature) to analyze voc soil sample by GC-MS. The question is i ran blank spike (200ppb), then three rinse, then blank of batch, then samples. every time, the blank is clean, the first two samples after blank always have vinyl chloride. When i reprpe and reanalyze, the vinyl chloride is gone. The reason i reanalyze is because the client is not happy with that. However, the blank is clean which my source is clean, i do not understand why there is vinyl chloride and the management forced me to reanalyze.
thank you all
rebecca

To paraphrase, the less you tell us, the less we can help.
What are your injection settings? Direct injection,P&AT, or headspace?
Why do you feel the need to cool to -20? I do 524 and get all the 6 permanent gasses at 35C.

Are you using packed column or large bore capillary column with jet separator or 0.25ID capillary column?

If you are using a normal DB5 type column then it may not be retaining the Vinyl Chloride enough, you should be able to use a 0.25ID or 0.18ID column at normal 35C beginning temperatures and have good separation of vinyl chloride.

As Bigbear asked, what are the other parameters such as injection type, type of sample, concentration range and other information you can provide, the more information we have the better answer we can give