Problem with Chlorpyrifos at GC/MS

[astroguille]

Hi people!

I analize pesticides in fruits, vegetables, water and soil with an Agilent GC6890/MSD5973.
I use a column DB-1701 and the Multi-Pesticide Residue Analysis method of Fillion-Thorp.

I have just a problem with the Chlorpyrifos.
Normaly, the qualifying-ion/target-ion ratio is approximatly 122%. But two months ago this ratio became approximatly 35%: the qualifying-ion abundance has decreased. Moreover, this ratio is holded at the standard and at the sample, and only for this pesticide: the other qualifying-ion/target-ion ratios are the same.

Can anybody help me?

[Don_Hilton]

The change in ion ratio, particularly of a single compund can be an indication of coelution of some other compund. Some ways to check this are 1) obtain a full spectrum through the peak - but if you are using a quadropole in SIM mode for sensitivity, you would have to find someone with a TOF instrument to run this for you. Or, 2) you can do the analysis on a different column and see if this resolves the coelution. Or 3) It may be possible to look at other ions in the chlorpyrifos spectrum and see if any of these gives a better result.

The last thing I can suggest is that you can spike the extract with chlorpyrifos and observe that the ion ratios of the chlorpyrifos peak will go back to the normal ratio as you increase the chlorpyrifos and swamp out the signal from the coeluting compund. It won't help to resolve the problem, but it will demonstrate that the problem is a coelution.

[astroguille]

Hi Don!

The ion ratio of chlorpyrifos (35%) is holded at the standard always (from 0.1 ppm to 50ppm), and the problem disappears when I run a blank, or when I analyze a sample without chlorpyrifos.

[Don_Hilton]

I may have missed something on the first look at your problem. Which masses are you monitoring? And is there any chance that someone has accidentally changed the acquisition method - and one of the masses you are monitoring is not the same as in the past?

[astroguille]

Hi Don!

The first picture shows the ions 314 and 199 with the original ion ratios and retention times. The second picture shows the same ions with after to cut 1 meter of column (notice the retention times).

[Rick]

It is absolutely normal that your retention time have shift after cutting your column.
Your problem is because Chlorpyrifos have shift and it is outside the SIM group ion create for Chlorpyrifos.
Reajust pressure to replace the RT of Chlorpyrifos