Chromatography Forum

I'm currently using electrospray + or - and have a group of compounds that I'm trying to get a mass on that are notoriously difficult. We've gotten them in the past so we'd like to see them this time, but they were always very weak ionizers. The compounds are trichlorophenyls with various groups on them (mostly alcohols or ketones, but no nitrogens). I can't get even a tiny mass signal for any of these compounds this time around and I was curious what settings I can play around with to try to tweak something out. Below are the current settings:
Fragmentor 45V (I've tried playing around with this from 5V to 400V with no results)
Gain 1.0
Drying gas flow 350C (I tried turning this down to decrease possible compound decomposition in the oven, but it didn't have any affect)
Nebulizer pressure 30psi
Capillary voltage 3000

Any ideas? I've mostly been trying ESI+ because that is what worked in the past, but have tried ESI- as well. As a result, I've mostly been using 0.1% TFA/water and ACN, but have tried no modifier, HFBA, and have tried NH4OAc with negative mode as well. Are there any modifiers/additives that you think might work better? Sorry I'm not very knowledgeable about mass spectrometry. I just know the basics.

Anything that we should be adjusting on a machine that has been gradually ionizing worse over time (these compounds ionized worse last year than 2 years ago, etc.

You might pick up an APCI source.

For positive ionisation - I tend to go with 0.1% formic acid Then I will try alternatives. good luck

One of the early things I learned. TFA is an ion suppressor. I inherited a method using TFA and got very little response. I switched from TFA to Formic and my response increased by a factor of approximately seven.

Thanks, I'll give formic acid a try.

Unfortunately, no difference with formic acid.

Try with methanol instead of ACN? Use 0.1% formic as the additive, or try NH4OAc again but keep the level low, e.g. 5 mM. Both positive and negative mode.

I have a couple of suggestions,

1) See

Marwah, A., Marwah, P. and Lardy, H. J. Chromatog. (2002) A 964: 137-151
for ESI LC/MS using VERY low (50-200 ppb) levels of formic or TFA for steroid type compounds,

2) Probably less attractive because of extra plumbing reqd, but can work very well:-post-column addition of AgBF4
to give [M+Ag]+ signal as a characteristic 1:1 doublet.
Ballard, J.M., Zhu, L., Nelson, E.D., and Seburg, R.A. Degradation of Vitamin D3 in a stressed formulation: The identification of esters of vitamin D3 formed by a transesterification with triglycerides, J. Pharm. Biomed. Anal. 43 (2007) 142-150

Please let us know if either or both are successful.

Good Luck.

I'm not working on these compounds any more, but I did try lowering the TFA to ppm amounts and it still didn't make any difference. I think the AgBF4 might have worked since last year, many of these compounds did give a slight (M+Na)+ peak, but I really wouldn't know how to set something like that up. Like I stated before, I'm a rookie with the mass spec.

When we bought our ABSciex instrument we were advised to never use TFA because it tends to cause problems and it will hang around in the system for a long time. It is very difficult to wash all if it out, and it can cause side reactions as well.

For multichlorinated compounds like those I found it was difficult to get sensitivity mostly because you will end up with about 6 base masses because of the 35/37 m/z combinations you can have with the three Chlorine atoms on the molecule. A hexa- chlorinated compound looks really bad

I agree - if it were up to me, we'd do away with TFA altogether in the LCMS. The way I determine when TFA is gone is by taking a blank LCMS in negative mode. You realize just how long it sticks around when you do that. Actually, I prefer methanol over ACN as well, except that it creates more pressure. But I don't have the pull to get our standard methods switched away from TFA.