Restrictions with 105m column on 6890?

[cawarhur]

We're currently running a 30m x .25mm x .25uM HP5-MS on our Agilent 6890 GC with a MS attached and considering using a 105m x .25mm x .25uM HP5-MS. Are there any foreseeable issues regarding flow, pressures, etc that we'll run into should we switch to the longer column?

[GasMan]

You should do some calculations to check if you will have enough pressure in your capillary inlet. Agilent supplied two versions of the capillary inlet, one with a max pressure of 100 psi and the second had a max pressure of 150 psi. You should be able to check what you have installed by setting your 6890 to run in constant pressure mode, and then check what the max pressure that you enter is.

Gasman

[cawarhur]

It is the 100psi max inlet.

Where do I go from here?

[Consumer Products Guy]

Use Agilent's downloadable free flow calculator software to see what column flow 90 psi head pressure (or whatever you choose) can deliver with your column dimensions.

[cawarhur]

Inlet Pressure: 90psi
Inlet flow: 1.1mL/min
Split vent flow: 98.4 mL/min (20:1)
Outlet flow: 4.92mL/min
Average Velocity: 43.9cm/sec

[Don_Hilton]

Note Temperature. Gas viscocity increases with temperature. Thus higher pressuire is needed to drive a column in a warmer oven.

[chromatographer1]

time to consider hydrogen carrier.

Rod

[GasMan]

chromatographer1

They may not have a choice, as our helium supplier has notified us that helium is going to be rationed in the US. From April until the end of the year we will only be getting 70% of our normal helium supply.

Gasman

[Bigbear]

One question why do you need to go to a 105M when you use a 30M? You may do better to go smaller ID ( 0.18). You will use less He as column flow will be less and your resolution should improve.

[cawarhur]

@Bigbear: Sort of an arbitrary order...we're using a 105m Rtx-5 on our GC and we want our GCMS column to match for conformity. I know retention times won't be exactly the same, but they'll be close.

[chromatographer1]

Not as close as you might find out.

The difference of 15 psi over a 105 meter column might be a bit more than you anticipate. Of course you can always lower the flow rate on the MS column, or increase the temperature for the other GC.

I guess H2 carrier is out with the MS.

Rod