Temperature gradient for MSD transfer line (bakeout)

Question

Good afternoon,

I am investigating the possibility of measuring diketones in a matrix of glycerol, which has a BP of 290*C.

The column I'd like to use is the VF-624ms 30 x 250 x 1.4, with the idea that this column is in-house and has heat resistance up to 280*C isocratic or 300*C gradient.

The up to 300C will allow me to bake out the glycerol after each run, but in my preparation, I wondered what to do with the MS transfer line temperature.

If I would set the MS transfer line to the isocratic maximum of 280C, I'll just be left with a glycerol buildup inside the column. Or is the drop in pressure due to the MS vacuum enough to keep glycerol in the gas phase? And if I would set the MS transfer line at 300C it will bleed the column bone dry, creating active zones at the end of the column.

Is there any way to have the MS transfer line temperature increase in tandem with the GC oven temperature gradient?

I understand that a Headspace method or buying a different column with a higher heat capacity while being more polar would help but unfortunately, that is not a current option.

Thanks and have a good day!

Instrument:

7890B GC, 5977B MS, VF-624ms column (-40*C to 280*C isocratic, up to 300*C for gradient)

Masshunter GC/MS Acquisition B.07.04.226

paulsalverda · Accepted Answer

Peaks move in a column without being fully vaporized or else columns would work only by each compound's boiling point. Temperature, gas flowing across the peaks, column chemistry, the column internal pressure gradient, and temperature drive the peaks along. Oven temperature is the main driver for GC peaks. The column internal pressure is a gradient from the inlet to the MS vacuum. The linear velocity is not constant! What is talked about is Average Linear Velocity. For optimum column and MS performance, the volumetric flow should be constant, but the velocity increases from the inlet to the MS as the internal pressure decreases. At the very end of the column somewhere inside the transferline, the flow changes from laminar to molecular. The mean free path is longer than the column internal diameter and the bouncing of molecular interactions dramatically diminishes. The molecules are moving very, very fast into the source - in the hundreds to thousands of miles per hour. The MSD transferline is isothermal, not programmable. All that needs to happen is that the peaks continue moving at the end of the run, so a typical transferline temperature setting is the final oven temperature. Don't exceed the isothermal column maximum, of course, and not too cold, either! An easy guideline would be no colder than the ion source temperature. MS Ion Source temperature. The default is 230 but many methods benefit from a bit higher temperature. The MSD tune ion ratios, the spectral tilt, are affected a bit by the ion source temperature so set it to the new temperature, wait at least one to two hours for thermal stability, tune, and then use that tune file in your method. Up to a point, higher source temperatures will keep the source cleaner longer. I like to set it the same as the transferline if possible - only because that seems logical. I like 275 to 300. Leave the Quad at 150. It does not need to be heated up any hotter than that. In fact, it would be just fine at a lower temperature. I did a test with the source at 325 and the quad temperature set to OFF. The quad went down to 96 degrees. We need thermal stability for best spectral stability, so any quad temperature between 125 and 150 is fine. Too hot is not good and may reduce the life of the quad over time. Have you tried a basic first pass at a method to see where glycerol elutes? It may be much earlier than you think. 1.2 ml/min constant flow. 1uL injection, split 25:1 with a split liner installed, inlet 275, oven 35 for 1 min then 30 degrees/min to 275 for 10 minutes, transferline 275. That's a simple method to try to see what comes out and then post the results here!

ron_tackett · Answer

One option is to purchase an ultimate union from Agilent. https://www.agilent.com/cs/library/usermanuals/public/G3182-90580.pdf This way a deactivated piece of column can be used after the ultimate union to the ms and a higher temp then could be used on transfer line without damaging the analytical column. Regards Ron Tackett

Temperature gradient for MSD transfer line (bakeout)

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