clasperj wrote:
Dear all,
My lab manager changes the GC columns (on our GC-MS) almost every other day because of the samples analysis. He explains it's ok to do so (at least in theory) but I don't really believe him.
Can anyone shed some light here and explain whether it's really ok to switch between columns this often? Thank you so much!
First, since you normally break vacuum it takes about a day for it to become completely stable again so changing every other day will limit how many samples you can run when the instrument is stabilized.
Also the more the Electron Multiplier is at atmospheric pressure the shorter the life it will have. It may not change it drastically, but it will go bad faster than if it remained under vacuum for months at a time.
If you have a turbo molecular pump for the high vacuum, the most stress on it is when it is accelerating up to full speed. The more often you vent then pump down the more stress you put on the turbo pump motor and bearings.
Opening the vacuum chamber also introduces moisture and oxygen into the system and if it is not properly cooled before venting and not completely evacuated before sending power to the filaments the quicker the source will oxidize and the shorter the lifetime of the filaments.
If you have the oil diffusion pump, that does not suffer the stress on restart that the turbo pump can, but it takes longer for those to reach a good stable vacuum which again reduced the number of samples you can run between changes.
What are the different columns being used for the analysis?
If you use different columns that have similar maximum temperatures but different phases then you can do a setup where you have two injectors, two columns and they connect with a Y connector prior to the MS. You create a method that has a low flow of about 0.3ml/min to the column not being used while you have 1-1.3ml/min to the column being used for analysis. With two methods setup like this for the front and rear injector you can switch between two columns without needing to vent the instrument. I use this currently and it works well, but it does work best if you use 0.18ID columns since with 0.25ID the head pressure for the low flow can be two low for the pressure controller to be steady at low temps, but it can work.
