FRONT INLET PRESSURE PROBLEM

[AICMM]

Che313,

Frankly, I am surprised the Agilent rep let you get away with installing a 0.32. I was always told that was a big no-no due to the pumping capacity of the mass spectrometer. If you are using a diff. stack then you probably should not be using a 0.32 but if turbo you might get away with it. In my opinion, better to return to the 0.25. You don't say what the ghost peak problem was about but what if you changed column vendors or phases (0.25 RTX-1 or DB-1 for example?)

One other possibility that was just presented at Gulf Coast Conference. You might try a restricted inlet (or narrow bore pre-column) in conjunction with the larger bore analytical column. Restek presented this and I am sure you could get the presentation from them if you asked. I will look to see if I can find the title.

Best regards.

[Blossom]

If you want to use a 0.32mm id column on a mass spec you need to make sure that ms pumping system can handle the flow rate you want to use. If the flow rate is too high for the MS then you must split the flow between the mass spec and the column so that the flow to the ms is reduced to an acceptable level.
The ms is at vacuum, inlet pressure controllers (electronic or otherwise) on GCs are not designed to maintain pressures below ambient pressure and are not very good at maintaining pressure near their lower limits. A 30m x 0.32mm column does not have sufficient resistance to flow to ensure that at optimum flow rates the inlet pressure is above ambient, therefore to use a 0.32mm column you need to increase its resistance by either making it longer or by adding on a length of inert column of a smaller diameter. You add this piece of column not at the beginning but at the end. Calculate the diameter and length of column required to give you about 1 atmosphere at the start of this piece of column when the other end is under vacuum. The result of this will be that, the inlet pressure required will be above ambient and in the range that the pressure controller can operate accurately. There will be no vacuum in the analytical column thus improving performance. It will improve the quality of splitless injection because the higher inlet pressure will reduce the volume of the vaporised sample. Lastly when you change columns all you need to do is lower the MS temp to about 50C, you won't need to turn of the pumps or vent it ( if you're quick!).
I've used this method (when I was young) for measuring ethanol using static headspace, an FID and a 0.53mm id column.
My advice is, throw the 0.32mm column away and get the smallest diameter column that has the capacity for your analytes, try 0.25 or 0.2. Good luck

[Don_Hilton]

One can use a 30 m by 0.32 column into a mass spec. I used to do this with an HP 5972 (back when they were the latest thing out). We did this for a specific purpose: we were looking for things at low levels in tobacco extracts. And tobacco extracts are very complex. To get enough material onto the column to see some of the smaller peaks, you needed a lot of stationary phase to handle the column loading.

Unless there is a column loading issue that prevents using a smaller column, the smaller diameter column is the better option. The column geometry can be entered directly into the Chemstation. And, the narrower column should give better separation than the 0.32 column.

And, I'd love to know how things are working for CHE313...

[Peter Apps]

I have used 30 m x 0.32 mm column in both an Agilent 5973 (with bigger pump option) and a Varian 320. The inlet pressures are low, but workable.

Peter