Diminished areas on 7000 Triple Quadrupole

Hi, over a while we have noticed one of our pesticides show diminished response with time.  The areas can drop close to 50% over a run.  The pesticide is acually an internal std called phenocarbothion.

Is this a sample or a standard?

This is standard.  However, since we are following AOAC Offical Method 2007.01 we add acetic acid to each of the stanards.  The solvent we are using is methanol.  

Method details are well published here http://www.weber.hu/PDFs/QuEChERS/AOAC_2007_01.pdf

Jim wrote:

It might be a good idea to run a reference compound that you know is inert.  Most manufacturers use a compound called OFN (Octafluoronapthlene) to benchmark sensitivity and reproducibility.  Other compounds like naphthalene or chlorobiphenyl would also work

OK, we ran 100 fg of OFN using the checkout method Agilent puts in a folder called "checkout".  The method was called OFN_IDL.m.  We discovered our system performed will < 4% RSD (relative std dev = std dev / ave *100) with this compound.  We would like some guidance on what to do next.

OK, looks like this a compound specific issue.   Have you tried without adding  acetic acid to std?  Another thing, if you are using MRM in your method (I assume you are) I would try adding some of the following to the standard.

Ethylglycerol
gulonolactone
sorbitol

Here is some data collected with/without these "protectants".  

How to I make these protectants up and how do I add them?

Analyte Protectant Preparation:
L-Gulonic acid γ-lactone (L-gulonolactone), CAS # 1128-23-0: > 95% purity.

(g) D-Sorbitol, CAS # 50-70-4: > 95% purity.

L-Gulonolactone stock solution: Weigh approximately 500 mg of L-gulonolactone in a 10-mL volumetric flask. Add 4 mL of water and then bring to volume with acetonitrile. Sonicate to dissolve if needed.

(k) D-Sorbitol stock solution: Weigh approximately 500 mg of D-sorbitol in a 10-mL volumetric flask. Add 5 mL of water and then bring to volume with acetonitrile. Sonicate to dissolve if needed.

(l) Analyte protectant (AP) solution (20 mg/mL L-gulonolactone and 10 mg/mL D-sorbitol composite solution):

Add

Add 4 mL of the L-gulonolactone stock solution and 2 mL of the D-sorbitol stock solution into a 10-mL volumetric flask and bring to volume with acetonitrile.

To add protectant using Agilent 7683 autosampler

Awesome, that really is a detailed description I will try and give you feedback.

Just to let you know we were able to get <5% rsd and areas remain stable over time.  However couople things we did as well 1) used a teflon tipped syringe 2) used a DB-5 column instead of HP-MS column. THANKS FOR YOUR HELP!