Repairing gradient mixer in Waters 600 system

[akots]

First, I wanted to say that I'm reading this forum for a couple of years already but did not register until really stuck and need some help.

It is a typical 4-channel Waters 600 mixer with solenoid valves. When I run the tests, channels A and D are more or less OK (still gradient shape is not perfect), channel B is somewhat malfunctioning, and channel C is completely failing to work in combination with A but somewhat works in combination with B. A+D and B+D work rather well. I called the tech support and they recommended to replace the whole mixer which is very expensive. We are on a rather tight budget at the moment and cannot really afford that. The 600 machine is about 10 years old and is not in the best condition.

We however have an older 600 system and the solenoids, mixer, and connectors there look the same and are also IEEE-compatible even though they are linked with green wires instead of red on the newer system. That older machine is not used at all and I can salvage whatever parts I need from it (already replaced the plungers on the newer pump heads). I was thinking whether it is reasonable to try and replace one-by-one the defective solenoids. It is a lot of work, so it would be very nice if somebody had some experience with mixer rebuilding or repairs or may be there is something else wrong like defective central block or some tubing. While tubings do not visibly leak and the system had been washed through with about 3 liters of methanol and isopropanol for the past month, I'm not so sure about actual valves.

I searched the fora but could not find anything at all.

Any advice is highly appreciated, please help!

[AA]

Swap out the entire GPV (gradient proportioning valve) assembly. You should be able to salvage the part from the unused 600 system. Its about a 20-30 minute job to swap it out parts in hand, so a little longer in you case. The sealing surfaces on those parts do wear out, they are moving parts after all.

[akots]

Unfortunately, the old mixer block is visibly leaking.

You are probably right, there might be a problem with wear and tear of the teflon interfaces but we first thought it is not the case since the whole assembly did not leak visibly. Also, there was a problem exchanging the solenoids because those with red wires are for 9V and green wired ones are for 12V. When we switched one of the platforms with springs and attachment screws to both solenoid and the main block, the mixer actually started leaking. So, we put back the one which were there in the beginning and decided to play it safe and work with one normal (A+D) and one somewhat crooked gradient valve combination (B+D).

So, I guess, the tech support people at Waters were right about the mixer being not serviceable.

Now, another question, more on a general subject. How bad this mixer defect can be if we are trying to develop a method potentially admissible for USP? Do we need to get a new mixer asap or for current purpose, we can live with what we got?

[AA]

The question really is "Can you afford to be wrong?". A replacement GPV cost abot $1200, the amount of effort to develop, validate, and submit that method (converted into dollars) is WAY higher. To me, it makes little sense to move forward with a broken system. You could think of it this way, would you drive your car across country with one flat tire? Or this, would you feel confident taking a drug substance that was analysed with a method developed on a broken system? I, personally, would not (although there is no way I could ever know that).

[akots]

Wise words, I hear you. For now, however, we are still working on the draft of the protocol and not the final version and the valve combination A+D passes the test. I just hope that we can get enough funding for the project soon to buy some decent instrument because, metaphorically speaking, trying to fix this old car would most likely just burn the money and something will get broken again. If it is too old to run normally, you've got to dump it or continue risking it on a highway.

On the other hand, being in academia, you never have a decent budget, money is always an issue.