Current Crash in MEKC

Discussions about CE, MEKC and related topics

4 posts Page 1 of 1
I'll supply as many details about my problem as I can, but I've been working on this for weeks now, so I'm sure I'll forget something that I have already tried. I'm looking for some advice on how to proceed with analysis, because my current keeps crashing unexpectedly within the first couple of injections.

First, I am using a Beckman P/ACE MDQ, my capillary is effective length 50cm, 50um i.d.x375um o.d. fused silica capillary. My running buffer is 40mM SDS with 20mM Phophate buffer ph 7.0 and the separation Voltage is 25kV.

I have spoken with Beckman's MEKC expert, and they have given us a new rinse procedure to try, but I still see my current crash. I can provide that if people find it is necessary to correcting my problem, but it's fairly standard 0.1M NaOH, Water, buffer, etc..... I have also varied my rinse times, and nothing fixes this. This includes the new column rinses as well as the daily and in between injection rinses.

I am injecting small organic molecules (anisole, cyanophenol, etc.) and they are all diluted in pure methanol, so I am doing a 0.4psi injection for 5 seconds. I can track my current with no injection and see that it is fairly stable, although for the first 6 minutes or so, it will increase from 20-25 uA before it stabilizes. Once we inject even pure methanol, the current drops within a few minutes (time varies) from ~20 to ~3-4 uA. This type of work is performed throughout the literature, so I'm confused as to why the methanol seems to be contributing to this issue. We've injected Phosphate buffer, with 1% methanol and it will run ok for an injection or two, then just crash inexplicably.

I agree this sounds like a current leakage issue, but I have checked my electrodes, installed new columns, had other people install the columns to make sure it isn't specific user error, etc. One of my colleagues suggested increasing my buffer concentration, so I ran 100mM Phosphate alone (no SDS, so effectively running CZE), and I achieved a decent current profile for a couple of injections, but on the third injection of 1% methanol it crashed again.

Every "expert" I have spoke to about this says my methods are sound. I'm completely dumbfounded. I almost forgot, I also tried using other stocks of buffer to make sure I didn't have an isolated contamination of my phosphate bottle or SDS bottle, and that made no difference either.

Anyone have any other insight into what I could be overlooking? I know this is a long post. If you want more info, please ask! Thanks for any help you can provide!
I have the same problem with the current, but i am now quite sure that is from methanol as when i am using a solvent rather than the methanol the problem resolved but i do not have any explanation for that
Did your degas you buffer and your sample and filtered it? Usually when you mix solvents of different polarities air bubbles could form, maybe this is why your current drops. Also it may depend on the length of the sample plug, having to much methanol could work as an insulator, but i believe the air bubbles are the problem
I agree with DeusEx. Degassing is first and foremost needed but not for the SDS solution. For this you need to leave it once made to de-gas naturally for about a week which means making fresh stocks ahead of time. Also for the current leakage problem if a bubble is not causing it and you have kept up the rinse steps: the cleaning of the interface block is important. I can send a presentation on how to do this by contacting me at AB SciEX ( use their main number and use the Separations option) which helps to uncover whether the interface block on the Beckman CE needs replacement.

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