Chromatography Forum

Hello all,

I am operating a Thermo Ultra Trace GC-MS. I recently installed a new column (Rtx-5MS 30mx0.25mmx0.25um). At the same time, I also changed the septum, seal, and ferrules. I performed a column evaluation and the K value was a little low, but when I ran multiple leak checks it always came back as 'passed'.

However, I had the split flow set at 25 ml/min with the carrier gas (helium) set to constant flow of 1 ml/min. I noticed on the GC control panel and on computer screen that the setpoint was set to 25 ml/min while the actual said it was at 180 ml/min (with total flow rate of 185.6 ml/min) with a pressure of about 10 psi.

I have tried turning the GC off and then restarting. I have tried changing the split to different flow rates (50, 100, 150, 200, 250 ml/min) and nothing would change it from 180 ml/min. When I did turn this carrier flow to 'off', the split flow did go to 0 ml/min, so this is a good sign. I tried changing from 'constant flow' to 'constant pressure'. I tried turning the Helium pressure at the gas tank down from 90 psi to 30 psi. None of these worked.

Any suggestions to troubleshoot this would be most greatly appreciated.

Thank you.

I preume that the leak checks that you ran are an on-board diagnostics test ? If so, you need to check for leaks using a leak seeker, or failing that with some 1:1 isopropanol and water as a bubble fluid.

Peter

It sounds like you have a very large leak in the injection port, and the fact that appeared after you had the injection port apart supports this idea. A mass spec leak check is good for finding leaks at the MS end of the column, not so good for inlet leaks.

Did you change the column dimensions so the EPC module wouldn't be confused?

Remove the plastic cover at the back of the GC and locate the in-line filter between the split vent and the DPFC unit. I suspect you have a leak there. It will pass the leak check as this is not part of the flow circuit but the gas is not making it back to the DPFC module.

Note Thermo GCs do not require to enter any values to the EPC as they run a column evaluation to measure the actual back pressure of the column rather than a rough calculation based on the printed column dimensions. If it passed the leak check you cannot have an injector leak, do not try tightening the septa tighter.

When you talk about the K factor what do you mean by, "a little low"? If it's still a 30m column (not trimmed) and the K value is below 1.6 something is probably leaking no matter what the leak check says.

It's also worth noting that the leak check is not infallible. You can have a leak in the injector system, which will become evident when looking at the air/water spectrum, but the diagnostics will still pass the leak test.

When you talk about the K factor what do you mean by, "a little low"? If it's still a 30m column (not trimmed) and the K value is below 1.6 something is probably leaking no matter what the leak check says.

It's also worth noting that the leak check is not infallible. You can have a leak in the injector system, which will become evident when looking at the air/water spectrum, but the diagnostics will still pass the leak test.

By the K Factor being a little low I meant it was 1.40 when for the 30m column with 25mm ID the acceptable range the Thermo Technician told me (based on a K Factor chart) was between 1.42 and 1.99.

When you say that the leak will become evident from the air/water spectrum, how much greater will the water <18>, N2 <28> and CO2 <44> peaks become? My typical TIC baseline is around 1.2^6

I thank everyone for their help and insight. The problem is now solved. It did turn out to be a leak at the injector at the terminal fitting nut, after I tested using a solvent to check for air bubbles. I guess the excess split was to compensate for the constant flow setting of the column since helium pressure was being lost out the leak. I replaced the silver seal and re-tightened the terminal fitting nut and there was no more leak.

Now, I remain puzzled why it passed the leak check.

Thanks to all~!

Now, I remain puzzled why it passed the leak check.

Glad you fixed it, and thanks for the feedback. Moral of the story ? - never trust onboard diagnostics.

Peter

By the K Factor being a little low I meant it was 1.40 when for the 30m column with 25mm ID the acceptable range the Thermo Technician told me (based on a K Factor chart) was between 1.42 and 1.99.

When you say that the leak will become evident from the air/water spectrum, how much greater will the water <18>, N2 <28> and CO2 <44> peaks become? My typical TIC baseline is around 1.2^6

The column evaluation diagnostic is far more use (and more reliable) than the leak check. If my K factor was coming in at 1.42 on a 30m x 0.25um new column I'd know there was a problem. There may be differences between machines but I run four of them and they're all pretty much the same.

The air peak at 18 needs to be larger than the 28, ideally at least twice the size and small leaks are easily detected using this rule of thumb.....tracking them down is the tricky part!

Glad you sorted your problem.