Chromatography Forum

What are the symptoms of a failing proportioning valves.

Anyone have any advice for an LC system that's producing sporadic retention times between normal retention time and +/- .6 min. With no pattern to them. I use an agilent 1100 quat pump with 4 channel degasser? I'm stumped I replaced all the check valves, seals, and pistons. The instrument passes the canned tests but the peaks still wonder around the chromatogram?

Q

Hi,
In passing the instrumental tests, why would one suspect a hardware fault? For your GPV test, I assume (with a UV detector) an absorbing marker was used in lines B,C,D and their percentages stepped up - there's a wealth of information posted and cited in the following historical threads;

Gradient PQ
Gradient Mixing Test

If yes, then it's down to the physics or chemisty of your separation and for that it would help if you post some more details of the application. Is it an established method which is now failing system suit or a rough-and-ready screening method?..
Regards

Collect the pump pressure as another data channel. See if the changes to retention correlate to changes in pressure. Also look for sporadic pressure anomalies.

If you are using phosphate buffer (?), the Agilent Quat pump cannot handle to mix buffer and acetonitrile. Especially if you run 100% buffer in one channel and 100% acetonitrile in the other.

You have to premix trhe mobile phase (or use binary pump instead)!

If you are using phosphate buffer (?), the Agilent Quat pump cannot handle to mix buffer and acetonitrile. Especially if you run 100% buffer in one channel and 100% acetonitrile in the other.

That's news to me. I've been using Agilent 1100 quaternary pumps for nearly the last 8 years to mix 100% phosphate buffer ( Channel A )and acetonitrile ( Channel B ). Seems to work fine, and the 1100 quaternary pump appears to handle 100% acetonitrile better than some other brand pumps - which may encounter sticking valves, unless water or water blends are regularly pumped in the acetonitrile channel.

Occasionally pumping an hour or two of water in the acetonitrile channel has reduced sticking valve problems with a colleague's binary 1100 pump.

Yes, mixing may be improved if you start with solvent blends in each channel, but that's probably more to do with proportioning accuracy and less different physical properties, such as density and viscosity, rather than any design fault of the 1100 quaternary pump. Obviously, you've had problems, but I've used three quaternary pumps with 100% of aqueous phosphate and acetonitrile with no obvious mixing issues.

If you were talking about large gardient mixing of aqueous buffers with 100% methanol, I'd probably agree, as I've experienced some issues with baseline stability and degassing noise in the detector, probably due to the large pressure changes. But that's a fairly severe test of any pump.

Bruce Hamilton

First, I'd re-purge all four channels again (you need to purge all channels, even if you're only using two).

If you pre-mix a mobile phase (e.g. 30% aqueous-70% organic) and put all four inlet tubing into it, then purge each channel, then you can run and see if your retention times jump around. If your pressures, retention times, and peak sizes shapes are reproducible this way (example: 6 injections) this way, then you know the pump itself is OK. Maybe the quat mixing vlave is bad, or maybe it wasn't purged completely (occasionally we need to purge twice).

I have seen a few cases of exotic problems like kinked supply lines, solvent filters that are too restrictive/hydrophobic/dirty, outgassing in the supply lines, reservoirs that are too far below the pump inlet.

I once was asked to diagnose a system with retention instability. It turned out that the mobile phase supply lines were being routed past the air vent of the pump; the hot air would cause outgassing in the supply lines. They moved the lines and the problem was cured.

For a good article describing a failing gradient proportioning valve (GPV), follow this link:

http://www.lcgcmag.com/lcgc/data/articl ... rticle.pdf

Pay particular attention to the GPV Results section. I would strongly recommend running the GPV test described in the article if you want to determine whether it is the GPV that is causing problems. I have had to replace a couple before. Just make sure each line is adequately primed.

Hey thanks to all but it was the proportioning valve that led to the problems.

appears to handle 100% acetonitrile better than some other brand pumps - which may encounter sticking valves, unless water or water blends are regularly pumped in the acetonitrile channel.

Bruce, please don’t misunderstand me – I’m not questioning your observation but I’m just curious about the reason for this malfunctioning of the valves, when 100 % ACN is pumped through some pumps. Does something dissolve?

Best Regards

appears to handle 100% acetonitrile better than some other brand pumps - which may encounter sticking valves, unless water or water blends are regularly pumped in the acetonitrile channel.

I’m just curious about the reason for this malfunctioning of the valves, when 100 % ACN is pumped through some pumps. Does something dissolve?

Not as far as I'm aware, but I don't know the details. It's been suggested that the stickiness is mainly due to active seat surfaces binding some molecules - producing a very smooth surface that no longer has microscopic grinding marks. The smooth surface permits a thin film to hold the valve shut just by surface tension.

For some reason, acetonitrile or acetontrile/water cause the problem.
In my limited experience, occasionally flushing the pump with water seems to prevent the problem, but why is a mystery to me, although some obvious speculations appear....

The problem was initially defined as occuring with ruby ball valves and sapphire seats, as in many early HPLC pumps. There is a Spectra-Physics US patent ( 4,862,907 to Ledtje , et al. September 5, 1989 ) for a check valve claiming to solve the problem. They investigated the water/acetonitrile problem using SEM and FTIR, and finally offered up ceramic valves as the solution.

You can read the patent for free at the USPTO www site. Their abstract claimed:-
" An improved liquid flow check valve using a ball and seat wherein the seat is made of sintered ceramic. The improved valve will operate in an environment of acetonitrile and water without sticking shut, thereby providing more stable liquid flow rates in liquid chromatography systems. "

Hope this is interesting.

Bruce Hamilton

Bruce, it is interesting. Many thanks.

Best Regards

Dionex offers ceramic-ball checkvalves as an option for our HPLC pumps. Third-party vendors (Optimize for one) offer them for a variety of pumps.

I wonder if the patent on ceramic seats is to get around the fact that ceramic balls are an already established technology?