Chromatography Forum

Hi Folks,

I have GC Varian CP-3800 and need your expert advice regarding problem with increasing RT in mole sieve column.

my first corrective action was increased pressure in rear EFC type 4 (molesieve column) after that performed calibration, the RT was returned to original and the result passed.

and the next day, I perform calibration and the result failed and I checked the RT was different, so I increase again pressure in rear EFC type 4 then rerun calibration and the result passed.

But today, the RT was increased again. Based on your expert, should we increase the pressure in rear EFC type 4 again or otherwise?

currently the actual pressure in rear EFC type 4 more higher than setting (set 4 psi actual 12 psi)

any ideas?what should I do?

I'm working with a Varian 3300 so not the same system but when my RT start fluctating it's usually a blockage. This may be to basic but did you trim your column recently?

Hi Stef,

I'm not insert new colomn recently.This problem happen suddenly.

Currently status of the GC have differential between set (5psi) and actual (50 psi) pressures in rear EFC, type 4. Leaks test (Connection of column, carrier gas connection, connection indicating filter trap) have been conducted but no luck.

what steps do I take?

Could you kindly help.

Sorry like I said I doin't know anything about the 3800.

It just happens for me sometimes that I inject something that will block my column and I have to cut the beggining of the column to get rid of it.

If you have a high pressure it really does feel like a blockage. Nothing to do with having an old or new column. Have you injected dirty stuff recently, or something with higher boiling point?

you first need to check if the flows are the same. so for this you need flow meter or inject a non-retained component on teh molsieve 5A.

Molsieve 5A is highly hygroscopic, emaning it will attract water from its surrounding. Once water is in molsieve, retention time will reduce. When molsieve is used, some water may desorb and retention will increase again.
If it;s water, condition the molsieve at 300 C for a couple of hours.

make sure that no water enters this column.(use carrier gtas filtration). If sampels conmtain water, use a pre column to remove water, or heat the 5A every analysis to 300C to remove the water,

jaap de Zeeuw, restek Corporation