Chromatography Forum

Could someone help me?
I am developing a method to analyze glyphosate and AMPA (solutions in water 0.8 ppm) without derivatization. Mobile phase: formic acid in water 0.5%. The problem is that at first I had a good signal but day after day, with the same solution and method the signal was lowing until almost 20 times. I've tried with new solutions at the same concentration, new mobile phase, varying the IS, the CEM, the TEM, capilar position but never can return to the original signal.
Any suggestions?
Thanks

It sounds like your system may be charging try cleaning the instrument front plate then q zero and ensure you have a decent curtain gas setting. I see lots of method which aim for the maximum signal from the system which isnt necesary the optimum for the system.

As stated, cleaning the front end sounds like a good idea.

0.5% formic acid might be a bit high I think. Do you really need to go this high?
0.1% is a very common concentration.

What else is in the water samples?

Are you using old solutions or have you tried preparing fresh solutions (made by weighing out fresh standard and preparing fresh stocks) to compare to old solutions? I am not familiar with glyphosate or AMPA. Some compounds stick or adsorb into to the containers, degrade, dimerize, or bind to things in the matrix.

You mention the mobile phase is formic acid in water. No organic?
If you are using ionspray then you'll likely get a better signal with some methanol or ACN, if the chromatography can handle it.

Thanks everyone for your suggestions. I will answer some of the comments:
I have tried formic in water 0.1 y 0.5%; I've cleaned the interface and Q0; I used old and new solutions of the std's in water; I am using a ionic exchange column and an organic solvent (ACN y MeOH) did't make good results.
Yesterday I tried new optimization but the parameters don't change; new injections (8 in total, one after another with the same method) gave very different signals. The MS performance is OK (calibration with reserpine). I really don't know what's happening.

Do you still have this problem? Have you tried to perform a charging test? Infuse your reserpine solution and monitor the resperine mass (609) in positive mode until you have a stable signal, then switch to negative mode for 5 minutes. After these 5 minutes, switch back to positive and your signal should be the same height as before the negative switch. Is the signal decreasing rapidly in the first minute, than you have charging. If cleaing Q0 doesn't help, then it is in Q1.

almondeyes...did you ever find the solution to your problem?

I am also trying to analyse AMPA and glyphosate and I am having a lot of trouble by LC-MS/MS. Can you share your experiences?