Negative inlet pressure GC-MS

[rubyfan]

Has anyone here done any negative inlet pressure GC-MS? I work for an R&D company and we're trying to do some testing of a 1.5 meter, 0.25mm ID analytical column. I have the column connected to a 1079 PTV and plumbed directly into the Saturn 2000 mass spectrometer. For my initial start temperature of 85C, the inlet pressure is at -9.3psi. I found initially that when the split comes on at 0.75 minutes into the run, at 100:1, the inlet pressure would jump way up to over 1psi. So I changed the method to keep the split closed through the analysis. That still hasn't worked. Now with the oven at 190C, the inlet should be at -7.9psi, but it's reading 0psi. So, I'm having difficulties maintaining the negative pressure. Seems fine for the first several minutes, then it can't maintain.

Also, I notice a lot of air/water with the negative pressure.

I just need to do a couple of quick tests with the instrument configured like this. Any comments or suggestions? I could try installing the column into the front injector, which is an 1177.

-Aaron

[Peter Apps]

Hi Aaron

What is the actual problem - tha pressure change or the air and water ? The pressure change when you open the split is the MS sucking air into the inlet, which is to be expected. The pressure change as you temperature programme is due to slower flow through the column as the gas viscosity increases with temperature. The air and water is due to air being sucked in through any tiny leak, and possibly back through the split and septum purge.

The pressure controls on GC inlets are designed (for obvious reasons) to control positive pressures. To run this experiment you need a controller designed to bleed helium into a vacuum. These do exist, but interfacing one to your GC is not likely to qualify as a couple of quick tests.

Peter

[AICMM]

Rubyfan,

Dr. Richard Sacks did work on vacuum outlet chromatography if I am not mistaken. I believe it was published in Analytical Chemistry but I do not have a reference. Very similar to what you are looking to do if I understand things correctly.

Best regards.