Chromatography Forum

Hi everyone,

I have been running my mobile phase for months now on our ELSD system with zero issue, exceptionally stable baseline at 5 mV. All of a sudden, with 2 new batches of mobile phase prepared, it just gives a very noisy baseline at 25-30 mV that oscillates wildly. The system needs to be cleaned with water whenever i try to pump my mobile phase through, or else the noisy baseline stays. Water gives a stable baseline, even 50:50 ACN:water gives stable baseline. But when I run my mobile phase again (see below) it just goes back to this highly variable noise level. I have tried discarding and making new mobile phase twice. It makes me suspect some sort of salt precipitation issue? I dont understand why this happens now and not the countless times ive run it before. I vacuum filter the mobile phase every time using a .45 µm filter, never had issues with that before. The only thing we did was switch supplier of mobile phase solvents. However, seeing as both are reputable suppliers and its the same gradient grade solvent, I cant see how that would be the issue.. any help is greatly appreciated! I am at a loss.

System:
Sedere - Sedex 90 LT-ELSD
3.5 bar - nitrogen - 94 degrees
Mobile phase:
MeOH:ACN:Water (53:24:23) + 30 mM ammonium acetate.
0.7 mL/min
no coloumn attached.

Do a stepwise exclusion of the soruce. What happens when you run: MeOH:ACN:Water (53:24:23) without 30 mM ammonium acetate?

What happens when you run: MeOH:ACN:Water (53:24:23) without 30 mM ammonium acetate?

When I was working, with ELSD I never even tried any mobile phases with acetate.

I used with acetic acid primarily if I needed a modifier. So I'm of little help except that I go along with the above recommendation. If that looked OK, I'd halve the level of acetate and try that, if you're stuck needing acetate.

Thanks alot to you both for taking the time to respond! Greatly appreciated. I have the tried the exclusion method, and 30 mM ammonium acetate does not seem to be an issue. However, when I move on and start adding acetic acid to to bring the pH down to 5.6, my problem arises. I tried switching to an older bottle of acetic acid (glacial) and i got the same problem, albeit much less pronounced.. is it really possible that both these bottles are contaminated? This was all repeated.. and once again as acetic acid was added.. the noise started.

To exclude the contaminated botles as the source you probably will have to open a brand new and ensure contaminations caused by wathever you use for the transfer of the acid from the the bottle is avoided. Always best to never bring any pipettes etc. into the bottle, but to pour a small amount of the acid into a beaker.
However, a contamination of the system, which is dissolved at the lower pH, also might be the reason. Especially matrix heavy applications run at a different pH than yours prior to your analysis might be a problem.