SIL-10AXL injecting air

[schielj]

I have a SIL-10AXL autosampler that has been injecting air. I noticed this when the outlet of my detector was submerged in the excess mobile phase eluting from the detector. Injections of air coincide with very large, random peaks. I have degassed everything properly, my sample vials are filled and of the correct type, and I still see the bubbles when I use a union (no column). I believe that the problem resides in the connection between the injection port and cleaning port. These ports connect while the needle goes to the sample vial. If these are slightly unlevel I would expect that solution may leak one way or another as it would do if you hold two ends of a hose unlevel. The design of the instrument does not seem to allow the levels of these ports to be adjusted. Has anyone else exerienced this problem with this model? Any other ideas as to what could be the problem? I have done a number of tests to try to figure out the problem and can eleborate further if necessary. Thanks for any help you can offer!

[DR]

And you have a solvent reservior that is full and has the second line from your 3-way valve in it?

[schielj]

Yes, the solvent reservoir and rinse reservoir are all full and have been degassed. I have replaced the needle, rotor and stator of the six port valve, and needle seal. The drain port also seems to be draining fine. If I just switch the valve, no bubbles are produced. However, I I program the instrument to inject, or lift the needle, then go back to the injection port with no sample injected the bubbles are still introduced. Thanks for any help you can offer.

John

[JJG]

Are you making partial loop injections, or full loop injections? If you are making partial loop injections, I would suggest trying full loop injections (3X the loop volume).

[Alfred88]

Dear schielj:
You may want to purge the bubbles trapped in the autosampler.

Press the purge button on the instrument. This will "rush" the new washing solution through the tubing into the autosampler.
Also, use a syringe with adaptor/needle to "Prime" the autosampler (do you still remember how to prime the flow line?). You may need to open the valve, pull the syringe, then close the valve.

Good luck.
Alfred.

[schielj]

This instrument is one where the needle is in the flow line so it only does partial loop injections.

Also, I have purged the autosampler with the purge function each day. This Puts quite a bit of rinse solution through (I can't remember the volume), but it should be more than enough. I am not sure what you were reverring to with the manual purging of the line with an adapter, however. Each day I prime the pump and autosampler with thier built in functions, but nothing manually. Can you clarify?

[Mark Tracy]

When the needle is up, the needle seat is connected to the waste line via the 6-port valve. Is that waste line simply a long tube leading to the waste container? If so, eliminate it. Use a short bit of tubing and let it spill into the drip-catcher. Adjust it's outlet to the same level as the needle seat.

This is a common issue with autosampler designs, and sometimes someone in the field will defeat the design. Then you inherit that system, and...

[schielj]

Mark,

Thanks for your reply. I have been thinking this is the problem for a while now (The rinse port is slightly lower than the injection port therebye pulling air in when the needle is at the vial). I have tried multiple lengths of tubing on the drain all the way to no tubing and still see the problem. You are right in that the rinse port and injection port are connected when the needle goes to the vial. The two ports sit on the same plate and in theory should be at the same level. The rinse port design is such that the inlet to the rinse port comes up the middle as a tube, hits a septum at the top and drains over the side where the waste tubing connects below it. I was thinking of moving this rinse port up or down until the air was no longer pulled into the injection port when the two ports were connected, however, there is a C-clip that prevents the rinse port from being adjusted up or down. The injection port also cannote be adjusted in height. The company technitians say this is the first time they have heard of this problem, but it seems to me it would be common by design. I am thinking of taking off the rinse port and placing it much higher than the injection port, then putting a "home made" assembly that is essentially a check valve sandwiched between two column end caps in between the six port valve and the rinse port. This would allow solution to the rinse port, but not flow back due to gravity to overflow the injection port. Do you think this is a descent idea or do you have a different suggestion? Thanks for any help you can offer.

John

[Mark Tracy]

Hmmm. I would avoid modifying the alignment of the rinse port. You might want to use it to rinse the needle some day. (Dionex has something similar to what you describe.) When you inject, is the rinse port filled? If it is empty, it could siphon from the inject port. The other thing you can try is to use a rather restrictive bit of capillary between the 6-port valve and the rinse port. The tube between the inject port and 6-port valve ought to be rather small already.

Best of luck

[schielj]

Thanks, I will try a smaller i.d. capillary ( I am waiting on a new sample loop right now, but I will try it when it comes in). Right now there is just normal HPLC tubing between the valve and the rinse port. Is small i.d. tubing going to be enough to restrict the flow? I am thinking gravity will take over unless there is an absolute stop mechanism in place.

Looking at the design, I think If I just raise the rinse port a couple of mm with a washer, and add the check valve apparatus it would be able to be used for rinsing the needle if needed later on.

I appreciate all your help!

[Mark Tracy]

Upchurch has spring-loaded checkvalves that operate in any position. Putting one of those between the 6-port valve and the rinse port will stop gravity flow because they need 1 psi in order to open. A gravity operated checkvalve would not help anyway because the unwanted flow is in the same direction as normal flow when you do a needle wash. Definitely don't put a checkvalve on the inject port; that will hurt the chromatography. You should not need to move the rinse port if you use the checkvalve.

[DR]

Check the union of the syringe with the 3-port valve assembly. Watch the syringe draw during injections. If you are seeing air in the syringe barrel, there's your problem. If fittings are all snug, replace the syringe (or at least the plunger portion) - Hamilton 1750.

[schielj]

Thanks again for the input. I think the syringe is fine (I don't see any air in it) and I have massed the vial after multiple large volume injections and it is injecting (drawing up) the correct volume. I suppose it could be such a small quantity of air that I don't see it? This may be the culprit since when I put the three port valve and rinse solution higher than the autosampler the air is injected less frequently. I will see if I have another syringe and try it.

When the needle lifts up from the injection port, then comes back down I can see a slight bit of solution around the needle. It is not leaking when flowing, but a little solution is backing up into the injection port which tells me the rinse port is not lower than the injection port. However, I still get air injected 4 times out of twenty. Is it ok that I can see a tiny bit of solution here?

[schielj]

I plugged the inlet of the six port valve where the external syringe unit attaches, then lifted the needle and put it back down (a pretreatment file with all valve changes, just no injection). No air was introduced in 30 injections. This likely implies the syringe or the rotor and stator of the threee port valve. The faces of the rotor and stator did not look compromised, but the pins that hold the rotor in place seemed worn and the insert for the pins in the rotor were worn and larger than they should be. Do you think a replacement of these parts will fix the issue for the long term? Have you heard of the pin inserts going bad before the contact surface of the rotor and stator? Thanks for any help you can offer. Problem almost solved!

john

[DR]

Sounds like you need to replace the ceramics (3-port rotor & stator & pins). I htink there's a kit for that. Usually, 3 ports fail by generating a "no home" message which means that the mechanism couldn't rotate the rotor. This happens with age - as the thing gets used, it polishes itself to the point that there is a lot more surface area contact between the parts than when new. The result is too much friction for the little motor to overcome.

When you get the kit, do NOT lose the little SS ball behind the stator (or is it behind the rotor?).