Chromatography Forum

In the past we have observed unknown peaks in residual solvents tests using GC with FID detection. When trying to identify the unknowns by GC/MS, a higher signal to noise was observed by the MSD when directly inserting the column into the ion-source of the MSD.

After installing a quickswap valve, we have often observed unknown peaks by GC-FID that are not detected or have a much smaller S/N ratio by GC/MS. Has anyone else observed this and would there be a quick fix other than removing the valve?

Can the quickswap valve be easily bypassed?

It may be a pollution.

Have you try to wash the valve with acid and used it several time to move the seals ?

We have not tried cleaning the valve. However, wouldn't you inherently loose sensitivity with these valves as you are diluting the efluent with enough gas to raise the backpressure by ~4psi?

We are able to see impurities that provide enough signal on FID (S/N > 100) that are still observed by the GC/MS (S/N ~ 20) which leads me to believe that all seals are properly seated.

I have never used a commercial quickswap type valve, only a home made version, but I would have thought that extra gas is needed only while the column is disconnected, in order to have an excess of helium flowing into the MS vacuum. Don't you turn the shield gas off once everything is coupled up ??

Peter

The vendor never mentions cutting of the shield gas as an option in the manual. They always have a flow through the valve during the run. They provide software that is supposed to predict how you should change your chromatographic conditions to account for this.

What you say makes sense about turning off the shield gase, but that does not appear to be an option based on the manual.

It seems pretty clear the peaks are from the Quickswap valve, or the gas supply or tubing connected to it. Have you contacted the company that sells this valve and see what their suggestion for addressing the problem is? Adjusting the shield gas might minimize the effects, I tend to look at the values produced by software as a starting point, not necessarily the final answer.

I have never seen one of these valves that requires a constant flow of gas through when the column is not being disconnected. The only reasons I can think of for a continual flow are not good.

We are on our second quickswap (we had trouble with the first one) and I only turn ours on when we need to change a column. I have found that having it on (even 'on' and set to zero) affects retention times and introduces water. I'm not impressed with the siltite ferrules, their surface delaminates and flakes off.

cheers
Liz