GC/MS Doubt

[oscarBAL]

Hello, I am here again with the same question:
I injected DDT (1ppm) on my new GC/MS, PE Clarus 500 I didn't see anything on Full Scan and SIR Mode either, the people from technical service asked me to run the senciblility test with Octaflouronaphtalene, and I didn't see anything again, so I injected one mix of plaguicides: Clorotalonil, Chlorpyriphos, Diazinon, Endosulfan, PCB using Both scan mode; at arround of 1.2ppm on spiltless, but they say is normal to don't see anything at that concentration, because the precense of electro....

If that is true, What is the real Adventage of work with GC/MS for Pesctiside residues analysis?

I just Use this technic to confirm what I see on ECD or NPD, do not for Screening?
I think I should be able to see these compounds on Full scan Mode with electroninc impact ionition. at 1 ppm, now I dont know what to believe.

[jimmyhu]

There are some limitations on analysis of certain pesticides using GCMS. Many pesticides have a higher boiling point, such as Diazinon [333-41-5] with 306 C. Normally one would not use higher than that temperature in you GC system such as injector and oven.
On the other hand, concentration does matter using GCMS for pesticides analysis.

Hope this would help.

[WK]

oscarbal,
Surely there is something wrong with your instrument if you cannot detect the test compound in your test mix? This needs sorting out first. Does the MS or injector need a clean?
What type of liner do you use (wool?)?
Why bother testing other samples when you cannot find compounds in the test mix?
WK

[adam]

GC/MS with electron ionization does give a somewhat lower response for compounds with a lot of electronegative atoms - like Cl and F. I think that is what they were trying to say.

You can think of it this way, the sensitivity with GC/MS is exactly opposite of ECD. The reason is that the exact opposite process is happening. With an ECD you are "putting" an electron onto the molecule; whereas the electron ionization process in GC/MS "rips" an electron off the molecule. There is no question that ECD gives better sensitivity for these types of molecules. But, GC/MS (with selected ion monitoring) will give you a much cleaner baseline - hence less noise. So it may still have an advantage.

[CE Instruments]

You have something wrong with your GC/MS. Are you sure it is injecting anything ? Does your MS tune OK and give good s/n ?

Specifications on a Thermo Trace MS from 2002 OFN 1pg/ul in EI 50:1 s/n, CI- 100:1 Scanning 200-300AMU in 0.2 seconds. Splitless.

From my calculation 1ppm is 100pg/ul so you should be able to easily see it in full scan. Assuming you inject 1ul splitless.

If my calculations are correct you should see 100pg of pesticides on column as a standard. Once you have the retention times and spectra you can then look at lower levels.

People run MS because it "sees" everything not just Halogenated species and because it provides conformation of idenification. Your MS has limited sensitivity capability you will need to improve either your extraction method or get a large volume injection capabilityto match the ECD sensitivity
ECD is more sensitive for Chlorinated pesticides than MS but you do not get spectral conformation for identification. I note that the quoted sensitivity test for an ECD is for 0.1pg Lindane on-column. From this and memory it suggests ECD can be 10 times more sensitive than MS in Full scan EI. Comparable with SIR