Chromatography Forum

Hello!
My lab has just installed a two-hole ferrule at the aux of our GCMS so me and my other department don't have to keep venting and switching our columns.
In chemstation, I now have both the inlets set to the same parameters as my previous method.
Upon running my standard I'm seeing a 10x drop in sensitivity, and poor peak shape for my early eluting compounds (running EPA 8270, so things like 1,4 dioxane)
I'm sure something probably needs to be modified on the GC end but I would appreciate if anyone has tips of where to start troubleshooting - I've still got plenty of other issues to sort out with the method itself, but can't start that until I have decent peak shapes
Thanks!

ajc

So you have one column going to each inlet? Then they meet at the inlet and both extend into the MS analyzer chamber? Or do you have some kind of Y splitter at the end that joins them, and a third piece of column actually connecting to the interface?

And you keep gas flowing through both columns? Constant pressure or constant flow?

Are your air & water checks good?

What if you plug one column with a septum and try to run the other one?

You have doubled the gas flow into the MS, which may be struggling to maintain the vacuum in the source as a result. What are the column dimensions? What is the "aux" you refer to? - the transfer line to the MS?

Peter

You have doubled the gas flow into the MS, which may be struggling to maintain the vacuum in the source as a result. What are the column dimensions? What is the "aux" you refer to? - the transfer line to the MS?

Peter

Hi Peter! This is something along the lines of what I was concerned about. Both columns are 30mx.25umx.25mm. And yes, the transfer line.

So you have one column going to each inlet? Then they meet at the inlet and both extend into the MS analyzer chamber? Or do you have some kind of Y splitter at the end that joins them, and a third piece of column actually connecting to the interface?

And you keep gas flowing through both columns? Constant pressure or constant flow?

Are your air & water checks good?

What if you plug one column with a septum and try to run the other one?

Hi Michael!
One column into each inlet, they meet at the inlet to the transfer line using a 2-hole ferrule, so no Y splitter involved.
I've been keeping the gas flow through the columns at the same rate in each in my previous tests.
Air and water have been good. Last week was below 1% for both, they're up to around 4 this morning after I did inlet maintenance.

I haven't tried the plug idea - after my tests this morning I might try that out.

Could you try halving the pressure in each column for a run?

I'm trying to picture what it looks like where the two columns extend (1 mm I think?) into the analyzer chamber. A cross-section would look like two small circles (the columns) inside a larger circle (the transfer line). So nether column would be in the middle of the transfer line opening. Could it be that they're not quite centered on the source opening? I don't know if that would make a difference, but it comes to mind as a possibility.

Could you try halving the pressure in each column for a run?

I'm trying to picture what it looks like where the two columns extend (1 mm I think?) into the analyzer chamber. A cross-section would look like two small circles (the columns) inside a larger circle (the transfer line). So nether column would be in the middle of the transfer line opening. Could it be that they're not quite centered on the source opening? I don't know if that would make a difference, but it comes to mind as a possibility.

Hi Michael - Firstly, I did try plugging the column but saw no change except an elevated baseline.
Dropping the pressure in both made no change except RT shifts. I'd like more sensitivity but peak shape is my biggest issue. I'm not sure if there's anything I can do with the GC parameters to get higher responses.

So far my biggest success at improving peak shape has been to set my unused inlet to splitless and lower that column pressure as much as possible. I at least can recognize my 1,4 dioxane d8 IS with that.

And also, you're probably right about how the columns are sitting. I don't think the two hole ferrule was the best idea..

Have you looked at the columns from the analyzer chamber to see if they are doing anything weird, like one sticking out further than the other?

Even if the two-hole ferrule doesn't work out, it was worth a try! I wonder if doing a Y connection right before the inlet and having a third small piece of column going into the transfer line would make a difference.

I've tried press-tight and MTX connectors from Restek. The press-tight ones are simple but really fragile - the first time I bought a pack of 3, I broke 2 of them almost immediately. But it's the simpler option. The MTX one seems sturdy but need special ferrules, and I found it a bit awkward to find a good way to hang it in the oven so it's not bending the column in weird ways.

Do both of the coumns run through the transfer line to the source ? Or do they extend just a little way past the two-hole ferrule?

Hi all, just wanted to update this thread.
We came to an agreement to all use the same column and ditched the two-hole ferrule. Sensitivity and solvent fronting/peak shape are all back to where it was previously. Thanks for everyone's help!