SPME, organochlorine pesticide, ISTD

[pipie]

Dear all,
Hello,
I am a newbie in SPME and GC/MS and have no strong background in chemical.
I would like to analyze organochlorine pesticide in water samples. I am using OCPs mix standard and pentachloronitrobenzene (PNCB) as internal standard.
As starting to check the methods according to papers and analyze spike samples which have concentration
• 80 µL of OCP mix standard with 50 ng/ml + 40 µL pentachloronitrobenzene 100ng/ml in miliQ water until the total volume 4ml. all standard I made it in acetone. volume of acetone was ≤3% of Volume total, please correct me.
• I used 30µm PDMS fiber ,immersion time 15min, agitation, no NaCl in room temperature
• I use GC 17C and QP 5050A Shimadzu with coloumn Rtx 30 m x 0.25mm I.D 0.25µ
• Operational condition, Interface 270°C, Injection 250 °C, Gas flow 1 /min, Splitless ratio 50:1, SIM mode, Open valve 2 min

PCNB peak was sharp but there were no peak of OCPs standard at all. Did my procedure was wrong, or the equipment was not enough? Did I choose wrong internal standard? Please give me suggestion because I am running out the time.

Thank you in advance and I am sorry for my poor English.


Regards,
pipie

[AICMM]

pipie,

The first place to start is to put a lot more pesticide in the water sample. Right now you are only putting about 4 ng on column which is not really all that much for an MS. If you were to spike 40 ng (either 800 ul or make a 500 ng/mL spike solution) and still not see pesticides I would start to worry more.

You also may want to increase your injector temp and make sure you are using a SPME liner.

Best regards,

AICMM

[Don_Hilton]

SPME involves partitioning between the liquid and the fiber - so you may have a partitioning problem or a method setup problem. 4 ng in sim mode should work, as long as you are not trying to acquire for too many ions at a time.

So, on your SIM set up, you should be turnign acquisition of ions on and off as groups through the chromatographic run, only acquiring masses for the compounds eluting during a portion of the run, and changing to the next set only between peaks. Have you set up the SIM descriptors this way and have you checked this bur running a sample made with a liquid injection known to contain the compunds of interest?

Confirm that you can measure when you know that you have material on the column because you put it there, then add the next level of complexity by partitioning the analyltes into the SPME fiber and getting that into the GC. (And, as suggested below, a warmer inlet would be a good idea. My default inllet temperature is 275, untill I discover that I am not getting high boilers on the column or I discover that I am degradign things in the inlet.)

[pipie]

Dear ACIMM and Don,

Thank you very much for your suggestion. I've already increased the concentration, finally I can see the peaks even though I should study further how to find the similarity because my standard mix consist 17 compounds.I will tried Don suggestion, this equipment is new for me and the manual and program is not in English so I need longer time to understand.

Anyway, thank you very much, I will do my best, but if there is anything that make me confuse, I still hope your suggestion.

Regards,
Pipie