strange ionization in LC-MS/MS

[MG]

I am running out of ideas. I keep coming back to your problem where your ISTD increases in response as your analytes increase in response (and concentration), even though your ISTD concentration is fixed. You have checked for crosstalk or cross-contamination and found none. Since your ISTD's are deuterated versions of your analytes (hopefully d3 or more), they should coelute, and if anything I would expect your ISTD signal to be suppressed if anything at higher analyte concentrations rather than enhanced,according to some data I saw at ASMS (Zhou et. al., poster TPF_103, ASMS 2003).

This sounds to me like it could be an adsorption problem as others have suggested. Are these the only compounds with which you have had this problem?

Just out of curiousity, what is your approximate analyte concentration range and how much ISTD is spiked into the standards?

[kazy]

Hi MG,

yeah, these are my only problem children (in the moment). The concentration range is about 1 - 1000 ng/ml and the IS concentration is 100 ng/ml. First thing tomorrow I try some pp-vials. I also think it could be some adsorption phenomena. I've also tried to change from acetonitrile to methanol, but to no avail. My results are the same.

kazy

[Ericz]

I saw you mentioned that you got same results when you injected one sample 5 or 6 times (how high is the concentration?). Carry-over might be an issue. It sounds it not a satuaration as I thought before (oops: ).

[kazy]

I’ve tried several vials today (glass, polypropylene, silanized glass) but none of it works. Still the same strange ionization. So I guess it wasn't an adsorption problem after all, at least not in the vials. I just dont understand it.

Has anyone of you guys here done tricyclic antidepressants with a mass spec? And if so, how did you do it?

kazy

[fishin' addict]

Kazy, maybe you stated this earlier but, what temperature are you running this method at, what's the run time, what are the retention times, what are the analyte curve ranges for each MRM, and where if anywhere is there linearity in the calibration?

Some quick suggestions:

1) Have you checked your water source/for adducts? That is, would you think there may be potential for adduct formation with the analytes.

2) Alternatively, but along the same lines, try making the solutions up in Mobile phase instead of water and look at responses?

3) Use a scouting gradient to find a quick and dirty gradient to run and see if that fixes the problem.

Hope this helps, I'll be looking for the post concerning the questions above.

[RA Koster]

Kazy,

I've read that you tried a different internal standard.
Did this internal standard also show enhancement to its own signal as the concentration of the curve increased? Is it really component dependant, or machine dependant?

Are you doing tricyclic antidepressants?
If so, because of their three rings, they can be difficult to ionise.
Do you have the opportunity to try a different ionisation method?
Like APCI or APPI, these ionisation methods are usually more suitable for these kind of components.

Best regards

RA Koster

[MG]

If so, because of their three rings, they can be difficult to ionise.

Has that been your experience? Looking at the structures, I would think they would work well by ESI(+). If the basic nitrogen is not contained in the ring itself, then what is the reason for the ring causing problems with ionization?

[RA Koster]

MG

You're right, my mistake sorry.
Actually i meant something more like apolair components with ring structures work well with APCI/APPI.