Column flow calc help

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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I'm currently using a DB5-MS column that is 30m x.25x.25 with hydrogen carrier gas at 1.2psi on a 5890 with a 5972 MS. I have to run the head pressure at 1.2psi to keep the flow at around 1ml/min, which seems a bit low to get reliable results from the EPC flow controller. I'd like to add a short .1um guard column to the transfer line, but I can't find any calculations for flow when using a hybrid column like this. I tried 1 meter to start, but I had to raise the pressure way to high to get the right holdup time, which surprised me. Any suggestions?
Controlling pressure with a MS and hydrogen can be tricky on a 30m column. I've had a lot of experience using guard columns, and I would be nervous connecting a 0.25mm column to a 0.10mm guard. Have you considered a 20m 0.18mm, 0.18um column? With a 1.0mL outlet flow, that would put your head pressure over 8psi at 40C starting oven temp.

As above, we use a 20mx0.18mmx0.36um film Rxi-5SilMS column with hydrogen in our MS and it works very well. At 50c oven temperature the head pressure is around 10psi for 1.2ml/min. If you want to use a guard column you can add a couple meters of a 0.25ID column and the flow will still be the same as without since the 0.18 is the more restrictive portion of the flow path. I have even used a 0.53 guard column when injecting Ethyl Acetate with good success.
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