Divert valve

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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I am having issues regarding my divert valve on my Thermo Velos Pro with Agilent 1200. Every method I run using my divert valve seems to cause a spike or sharp peak right at the beginning of the sample that is around 1/4 size bigger than my initial baseline. It doesn't matter the sample, even blanks have it. If I extract the ions out of the peak, it will contain all the compounds that are in my sample. It also causes my baseline to slowly drop down after that spike making it take extra time to stabilize. My compound will still elute later in the run. The valve seems to want to hold on to a little of the sample and send it to mass spec right away. I can hear the valve switch when I tell it and I have tested the flow switch from waste to detector so I know it works.

It is a 6 port diverter which is attached to the velos pro mass spec. I have tried every port with new fittings and tubing with the same result. I changed the initial waste to detector time and also took it off but it is still there. I changed the sample delay setting also, even taking it off entirely. I have taken the port apart and cleaned it with methanol. I'm not sure if I'm setting up something wrong in my method parameters (XCalibur software) or if it is a hardware/cleaning problem. Also completely bypassing the diverter (column to mass spec) will get rid of it but I need the diverter for the method.

MPH A is 0.1% formic in water starts at 95%
MPH B is 0.1% formic in ACN starts at 5%
To some extent, this a normal effect of the spray/signal stabilizing. You can check this effect by running mobile phase, monitoring the MS signal (without injection) and manually turning the divert valve on/off. For a second or so, there will be a forest of peaks before having a normal MS background. While it's certainly possible, it does not imply that your analytes are stuck in the divert valve. If there's anything (like surfactants, phthalates, .. the usual stuff) stuck, it will be added on top of the stabilizing effect.

The solution is rather easy: you don't measure your compounds for a couple of seconds after diverting the flow to the MS. On our Waters system, there is a "solvent delay" (rather misleading term coming from GC) setting that makes sure nothing is measured during this period. An example method events setting for a compound eluting at 1.20min with a 5min injection:

- Initial: flow state = waste
- 0.00min: solvent delay = begin
- 1.00min: flow state = LC
- 1.05min: solvent delay = end
- 1.50min: flow state = waste

With these settings, the compound can be measured for 5min without having baseline spikes.
I checked the valve by manually switching it on and off and it replicated the peak each time so it narrows it down to a communication between the source and the divert valve. There is some sort of delay between the two that I can't figure out. Too much of the sample is getting pushed out of the valve for some reason.

Unfortunately I don't have that solvent delay setting on XCalibur so I can't use that. I could only use a method like you described on a targeted scan, but I need the method for broad screens as well.
Aglab wrote:
I checked the valve by manually switching it on and off and it replicated the peak each time so it narrows it down to a communication between the source and the divert valve. There is some sort of delay between the two that I can't figure out. Too much of the sample is getting pushed out of the valve for some reason


You switched it manually while running mobile phase through the system? A delay between the switch and possible effects on the MS signal is normal, the flow still has to travel from the valve to the source. I don't understand what you mean with the last sentence.

In your earlier post, you said that bypassing the divert valve gives a normal baseline. If you don't bypass it, but don't make it switch, do you get the same baseline?

Either way, targeted or not, you should be able to start measuring a couple of seconds after the valve switch.
Yes, I used direct control and I was able to manually control when I can switch the divert valve from waste to detector in real-time. The system was set up like a regular run... mobile phase, column, divert valve, etc. Every time I would switch from waste to detector I would get that giant peak at the beginning (almost looks like the tail-end of an actual peak) containing multiple ions.

The last sentence I was just thinking that there might be excess sample or solvent collecting in the divert valve and not going to waste, then it might flow to the detector once it is switched... just a thought.

If I do not bypass the valve and do not make a switch, it works perfectly fine.

I have a couple pictures but I'm not sure how to upload them since it's from my networked computer.
To post images, see the sticky.

Images would be helpful!

Aglab wrote:
Every time I would switch from waste to detector I would get that giant peak at the beginning (almost looks like the tail-end of an actual peak) containing multiple ions.


A bump in the MS signal over a large m/z range for a second or 2 after the switch is in my opinion/experience completely normal (see my first post in this topic).
Here are the pictures. The second one is zoomed in. As you can see I diverted to waste from 0.0 to 0.3 min and again at 6.5 min.

Image

Image
This looks very normal to me for a blank injection in full scan starting at m/z 50. What you see should not be interpreted as a chromatographic peak.

In my opinion this is neither a hardware or contamination issue.

If you keep the divert program timepoints the same, just change the scan so it starts at ~0.50min.
I will give it a shot and let you know how it works. Thank you.
I got rid of that spike by adding a 0.7 second start delay to the run and only diverting at the end of the sample. Thanks for your help.
That spike looks normal. Whenever a valve is switched in a high pressure system there will be a pressure disturbance. This disturbance will take a period of time to end and conditions to re-equilibrate. Most detectors will pick up this pressure change. If the method is the same, the spike should also be the same each time.

Don't forget that your diverter valve is a key component part of your HPLC system and just like all of the other modules, requires regular inspection and maintenance. The valve rotor inside the valve wear out and requires replacement. This should be part of your PM. The valve should be opened up, inspected, cleaned, worn parts replaced, re-assembled, then pressure and leak tested before use.
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