5890 Flow Rates - what am I missing?

[cyork]

Hi,
I've worked as an operator on HP 5890s and 6890s but this is the first time I've had to set one up from scratch and I'm missing something in setting the flow rates. I'm working with 5890 II with EPC and an NPD. The column is a Qt Bond PLOT, 30m x 0.32 um. I'm trying to match the following parameters, using Helium as the carrier and makeup gas:

FLOW RATES
Carrier (He) 20 +/• 1 ml/min
Hydrogen 3.5 +/• 0.1 ml/min
Air 100•120 +/• 10 ml/min
Makeup (He) 10+/•1 ml/min
Split Vent N/A ml/min
Septum Purge 1•2 ml/min
(This is the NPD test method in the back of the 5890 II reference manual.)
Oven isothermal at 180C

I don't understand how to get the the carrier gas to that rate. I can set up the column head pressure to the recommended starting values in the reference manual for my column size (anywhere from 7-13 PSI), but when all other gas flows are off, I only get 1-2 mL/min out of the detector. How am I supposed to hit 20 ml/min other than crank up the PSI to an ungodly high number? When I use the HP flowcalc software it says I should only be getting 1.33 ml/min out the detector.
Also, am I supposed to get nothing out of the split flow for this?
Thanks!

[Peter Apps]

The 20 ml/min is the total flow i.e. split plus column flow. Your column flow of about 1 ml/min is correct, and you should be getting about 20 - column flow - septum purge out of the split vent. NB that the flow from the split vent will continue even if the inlet is set to do splitless injections.

Peter

[cyork]

Thank you, Peter.
Do I set the column flow and split flow rates with my flowmeter at ambient temperature or at the method initial oven temperature?

[Peter Apps]

Thank you, Peter.
Do I set the column flow and split flow rates with my flowmeter at ambient temperature or at the method initial oven temperature?

You have EPC, so you set everything on the software or the front panel (no knobs to turn). The EPC takes oven temperature into account. Checking the split flow with a flow meter is good practise.

Peter

[AICMM]

cyork,

I suspect your 5890 (being the wonderful old dinosaur that it is) still has knobs on the front. Furthermore, being that this was the first entry into EPC I would suggest that Agilent did not have it all sorted out just then. So, in this case I will respectfully disagree with Peter and provide my suggestions. Set your column flow from the EPC with the appropriate dimensions entered for the column diameter and length. Then I would use a flow meter and set the split flow where you want it with the total flow knob. Then, just for fun you can measure the septum purge. Make sure you have enough split flow because, if I am not mistaken, the GC will rob Peter to pay Paul (that is split flow to make sure you have column flow at set point.)

Best regards,

AICMM

[cyork]

cyork,

I suspect your 5890 (being the wonderful old dinosaur that it is) still has knobs on the front. Furthermore, being that this was the first entry into EPC I would suggest that Agilent did not have it all sorted out just then. So, in this case I will respectfully disagree with Peter and provide my suggestions. Set your column flow from the EPC with the appropriate dimensions entered for the column diameter and length. Then I would use a flow meter and set the split flow where you want it with the total flow knob. Then, just for fun you can measure the septum purge. Make sure you have enough split flow because, if I am not mistaken, the GC will rob Peter to pay Paul (that is split flow to make sure you have column flow at set point.)

Best regards,

AICMM

Thanks to both of you for responses.
I found this document (http://www.chem.agilent.com/Library/Sup ... a16027.pdf) that details how to set the flows for my instrument. Also, I made a mistake, the values I posted above were for a method with a purge packed inlet, not a split/splitless. I think I have it figured out at this point (at least the column flow), now I need to figure out the detector flows, bead voltage, detector temp, etc for the NPD. Lots of conflicting information out there, especially for the makeup gas... This is fun