Chromatography Forum

Hello,

I have a problem with the LC-MS/MS... The solution is probably silly, but I just don't see it.

Regular analysis showed no signal when mainly aqueous mobile phase was in the gradient. When more and more organic was used, situation turned normal...

I ran some tests:
* pressure is ok, the flow is reaching the MS (even when 100% aqueous is used)
* I used fresh aqeous mobile phase, even did some experiments with pure water, the mobile phase itself seems ok
* the column is not the problem, I removed it and did some tests without column, same problem was seen
* when running 100% water, no signal is seen, nothing, even when measuring in full scan!!! When organic modifier is added, signal is seen.

I don't understand it: the LC seems to work fine (the flow reaches the MS, pressure is normal), the MS seems to work fine (since organic mobile phase works as normal), but the LC-MS doesn't work?!

I would really appreciate some advice!

Thanks!

I assume you are working with electrospray ionization. In highly aqueous mobile phases, the formation of the spray is more difficult to achieve since water is quite viscous. When you increase the organic solvent %, the viscosity is reduced and the spray gets more stable. Check how your spray looks like when you use 100% water: on some old instruments, the signal is sometimes completely lost, especially at high flow rates.

Hello,

Thanks for your repley. Yes, I work with electrospray. Spray looks ok. Our system is only two years old, and always worked fine even with highly aqueous mobile phases. The method used to work fine, not anymore, so I am wondering what went wrong...

Problem solved!
There was a problem in the electronics of the ESI, probably causing a too low ionspray voltage which resulted in bad ionisation.

Great! Anyway keep in mind that if ionization occurs with organic eluent and not with water, this is logical: in your case, the capillary voltage was high enough to form a spray in an organic solvent, but too low for water.