Chromatography Forum

i have measured the flows of the two GC instruments. the first one (the one i didnt have any problems with) was giving me a flow of around 6.5ml/min with the electronic flow meter even though the front of the GC panel was saying 5.8ml/min. the second system (the one im having problems with) is giving a flow from the detector outlet of 24ml/min! so im assuming this is a major miscalculation of the GC.

i do not know how to interpret this issue.its clearly very wrong on the second system but why? will i measure flows from anywhere else or where else is best to proceed with this?

just to be clear, the retention times are earlier on the system giving the 24ml/min flow using the flow meter.

again my apologies if the info given is limited. im still very much a novice in this area.

As a novice it is easy to overlook certain things, finding them is how we learn.

On the Agilent instruments they calculate flow rate from the head pressure taking into account the column dimensions, oven temperature and pressure at the outlet.

There should be a setting for Outlet Pressure in the Column portion of the Chemstation, check to make sure it is set to 14psi or Atmosphere, if it somehow got set to something else it might be sending too much flow through the column. Also you can set the flow to 0 and see if the head pressure also reads near 0, if not then the pressure sensor is out of calibration.

Also make sure you are getting flow from the split vent and septum purge vent on the top of the instrument. If something is blocking flow there it will put extra flow into the column.

Are you sure it's He and not Hydrogen? looks like a flow rate typical of hydrogen as a carrier gas and not He. Or your head pressure is off, get a manometer and measure it.