Recently, I try to analyse 200 vegetable oil sample containing Methamidophos residue with the PFPD attach to our Agilent 6890 GC. I notice that after the analysis of 32 samples, the peak area for the standard methamidophos solution (0.01ppm) is decrease from about 12000 unit area to 4500 unit area. Injection of standard solution at in between several runs is our usual method to ensure the detector respond remain constant throughout the analysis. Inlet septum is checked for leakage and has been replaced, the gold seal also been replace and portion of the DB-1 column has been cut to eliminate the possible "active site" that might form and cause the decrease in respond.
By Anonymous on Thursday, June 17, 2004 - 07:09 am:
Methamidophus is a known difficult pesticide that after a few analyses develops peak tailing and is lost on active sites in both the column and injector liner. Most likely your PFPD detector is "innocent". You may test this by the injection of an inert OP pesticide and see that its response is not affected. You may learn more on this Methamidophus problem and a new remedy for it in Anastassiades M, Mastovska K, Lehotay SJ "Evaluation of analyte protectants to improve gas chromatographic analysis of pesticides" J CHROMATOGR A 1015, 163-184 (2003).
By Anonymous on Monday, June 21, 2004 - 02:09 pm:
If memory serve me correctly methamidophos is one of the low responders on the PFPD. There is a quartz liner in the PFPD, and it is possible for some compounds to break down or to be adsorbed onto the liner. You might want to try different temperatures for the detector base, as changing the temperature can have a significant effect on the response for some pesticides. Finally, if OI has worked out the issues in their 3 mm combustion tube it may have less effect on the pesticides than the standard 2 mm tube. I didn't see anything in the posting to indicate that the combustion tube had been changed. Even though it is advertised as being "self-cleaning" residue can build up on the walls of the tube and affect response.