Negative peak with Pulse Flame Photometric Detector

[along]

Hi all,
I am analyzing Organophosphorus pesticide residue using OI PFPD 5380 version. Recently, I have a problem where there are few negative peaks appear in my chromatogram after I change to a new Column.
There is no leakage any where and I dun know how to solve this problem...any one of you facing this problem before....????

[BG]

HI along,

with PFPD in general when you have negative peaks it's due to a high concentration of your sample but you sayed that appeared after a change of column............maybe in that case , you have a high bleeding from this column or eventually a high concentration of hydrocarbure in your gas.
With my little experience in PFPD that are the only explanations i can give you for negative peaks........perhaps it could help you!!!!

[along]

Thanks BG, what you said are correct, I solve the problem by flushing the column with one end opened and let the detector heated at 350oc for a day.
Thanks