Peak appear after switch from Load to Injection Position

[Anthony_Ng]

Hello all,

The instrument use Rheodyne 7125 manual injector. And no column is attached.

When I switch from Load position to Injection Position, there is a large peak appeared (~1500mAU, 254nm) without any sample load actually.
Sometimes it is large and sometimes it is small (~600mAU).

As I know sometimes there is system peak occur, but the size should be very small (around 1-5mAU).

If, excluding dirty loop, what else can I troubleshoot? (What is the symptom of Air peak/air leak?)

[tom jupille]

With no column attached, two possible causes are entrapped air (as you suggested) or a pressure pulse (which causes a slight change in RI, which the detector "sees").

Does the size of the "peak" vary with how quickly you turn the valve? If it does, that would argue for a pressure pulse. The confirmation would be if you get the same sort of "peak" when you go from the inject to the load position.

You can easily get air entrained in those valves, especially if you have a long drain tube attached to the waste outlet line. In effect, the solvent in that line "siphons" down, entraining air into the loop.

[Anthony_Ng]

>>Does the size of the "peak" vary with how quickly you turn the valve?

No, it doesn't vary with the speed of turing the valve. And there is no "peak" when I turn it from Injection to Load position.

>>the solvent in that line "siphons" down, entraining air into the loop
I replace a 0.17mm id capillary to the drain tubes. And the "peak" appeared also.

One strange thing I see is that, when I turn in Load position for longer time, the "peak" is bigger. And if I keep it in Injection all the time, just turn to Load position for 1-2 seconds and back to Injection, the "peak" is small.

Will the air "suck" into the loop via Load position? If so, how can I solve it?

Thanks!!!

[tom jupille]

What you're seeing is consistent with air being drawn into the loop. The fact that you have a small capillary on the outlet may slow it down, but wouldn't prevent it.

I have a dim recollection that Rheodyne sells a "blank" needle (same outside gauge as the standard injection needle, but no hole in the middle) that you can stick in the injection port to close it off. If they don't (or no longer make it), you should be able to simply get a piece of stainless steel wire of the appropriate outside diameter (same as the needle) and stick it in.

Or, you could simply leave the syringe in the injection port when you turn the valve.

[Bruce Hamilton]

The obvious solution is to have the drain lines go to a small reservoir that is the same level as the valve, and I think that's what Rheodyne recommends for their large volume loops on those valves.

The manuals for the valves should also be available from the Rheodyne WWW site. I also leave the syringe in while injecting. Too narrow drain lines can encourage leaks around the syringe needle when quickly filling large loops.

Please keep having fun.

Bruce Hamilton

[Anthony_Ng]

Thanks experts,

What I have done so far is changing the drain tubes (ports 5 & 6) to capillary tubes (0.17mm id), keeping it on the same level as the Rheodyne suggested.

And I use samll volume 20ul loop only. I also leave the syringe in the injection port and Injection position all the time.

But when I load the sample, there still have 5-10 seconds turn to Load position and air goes in.

Is this an inevitalbe symptom that some air will go into the loop from injection port? (If siphon from drain tube is excluded)

Will there any help if I replace the parts (e.g. rotar seals) to minimize the problem?

[Bruce Hamilton]

20uL loop, so presumably you are pushing through at least 30-40 ul from your syringe to completely flush the loop?.

The next obvious action is to deliberately inject some air and ensure that the peak you see is caused by air. Just fill your syringe with air and inject that.

If the peak is air, then check to ensure that lines are correctly fitted, and that your mobile phase is premixed and fully degassed. an unlikely, but possible, explanation is that the switching is reducing pressure and liberating dissolved gases from the mobile phase. You should be injecting the mobile phase whilst testing the valve.

If you still have an air peak, then your valve may be leaking, so check leak performance according to the manual. Before disassembling it, try to ensure that the valve is the cause, perhaps by also putting avery short column in place, and observing the effect on the peak.

Please keep having fun,

Bruce Hamilton.

[HW Mueller]

We just fitted about 3cm tubes to the waste exits, straight out along the axis of the valve. No more syphoning. One must be aware, though, that a small amount of air is pushed in due to inserting the syringe, another reason for overfilling the loop (to push out this air, . . . . the other reason for overfilling: laminar flow and resulting loss of sample).

[Anthony_Ng]

Got some result today:

(no column connected) For the first 4 injection, I fully fill the 20ul loop with mobile phase. Switch to injection position and leave the needle there until the next injection.

For the middle 4 injection (2.5-4min). I just turn the valve from injection to load position without filling any new mobile phase in it.

For the last 4 injection, I fully inject air into the loop and leave the needle there until the next injection.




The following chromatogram is run with column installed:
RT left to right: Phenol, 4-chloronitrobenzene, Tolune.



And you will note that the "Air peak" disappeared when the column installed. This means there is not any leak, siphon, etc. in the injection valve.

How can I explan why "air peak" disappeared after column installed?

[HW Mueller]

One can not see what the retention of injected air is (the last four peaks, first chrom.), also I wonder what you mean with "fully fill". The first four peaks are much larger than the pressure disturbance (2nd series of four), and have a negative peak. Tentatively, if the positive peaks are air then the negative ones indicate that the injected mobile phase absorbs less than the stuff flowing through the system. These negative peaks should show up with the column in-line, though, the positive air peaks sometimes dissipate in a column (spread so that one doesn´t see them). If I remember correctly, someone suggested to inject different amounts, a good idea. Not bad either: inject the different components of the mobile phase (aqu. and org).
One other thought: Maybe with the first set of four peaks (1st chrom.) you switched the valve more slowly or just differently, so that all you see there is a flow fluctuation.

[Bruce Hamilton]

The first 4 runs look like the Refractive Index/ pressure pulse effect in the detector cell that I saw when injecting solvent into such valves.

The second set would appear to verify that, as a small pulse is present.

The last set of air peaks are positive, and probably not the cause of your concern.

If the peak goes away when the column is added, that suggests it's probably mainly a pressure effect, perhaps with a little air ( but I doubt that much air is present ).

I assume you want to use the system with a column, so you should be "good to go".

Please keep having fun,

Bruce Hamilton

[HW Mueller]

Since Anthony claims to have injected mobile phase (1st four) one needs to account for the difference to the 2nd series of peaks, here the loop also contained mobile phase.

[Bruce Hamilton]

On a cursory examination, the second set look similar ( going above and below baseline - but smaller magnitude ) to the first set. This is what I would expect from reinjection of the recently-obtained sample of the mobile phase. Still looks like the same effect to me, perhaps minus any temperature difference that the first set might generate.

Please keep having fun,

Bruce Hamilton