Chromatography Forum

Hai, I have some very basic questions regarding GC gas flow.

(A) How should I determine the correct flow for:

1) Inlet septum purge flow (mL/min)
2) Inlet purge time (sec)
3) Inlet purge flow (mL/min)
4) inlet total flow (mL/min)

(B) Secondly, how about the flow rate of gases for FPD detector (air, H2,)?

Do I need to do some optimizations depend on the analyte studied?

Thank you.

1) Septum purge flow shuld have a vendor reccomendation - check the user manaual. If you can not find it, you only need a low flow rate to keep junk from the seppptum out of the inlet. This will be in the neighborhood of 1 to 3 mL/min.

2) Septum purge depends on A) the flow rate through the column and b) the discrimination you can tollerate. Using with an estimation of the volume of your liner and the column flow rate, determine how long it takes for gas to move gas from one end of the linter to the other. Thiat is your first estimate. If you are trying to resolve peaks close to the solvent, you may need to go for shorter time. If you need to avoid discrimination, then go longer. And expect the solvent tail to grow.

3) purge flow should be enough to clear residucal solvent out of the inlet so that it no longer taills in the chromatogram - This should be a flowrate that will move gas from one end of the liner to the other in a few seconds. This depends on how close your analytes are to the solvent peak. I use 35 ml/min in some work I am doing - but the analytes I am looking for are well away from the solvent peak.

4) total gas flow is the sum of the gas flows you set

Optimization of the inlet conditions - for serious work, yes.

And I am sure that someone with experience with the FPD will post shortly, but until then...

For the FPD, start with the parameters in the user manaual. Check the manual to see the sensitivity of the detector to operating conditions.

You will have to provide more information about your system to make a reasonable answer to the FPD flow rate question possible. There are many FPDs on the market, and they all have different recommended flow rates, and the flow rates are different for sulfur and phosphorus.

Ron, I'm using Agilent GC 7890 with FPD detector. I'm analysing OP residues using rtx-5 column. These are my GC conditions:

Injector: Splitless
Front inlet flow: 1 mL/min
Front inlet septum purge flow: 3 mL/min
Front inlet purge time: 5 sec
Front inlet purge flow: 0.5 mL/min
Front inlet total flow: 1.5 mL/min
Front inlet gas saver flow: 20 mL/min
Front inlet gas saver time: 1 min

Is the flow conditions right? Thank you.

There seems to be some confusion:

The 'inlet flow" cannot be 1 ml per minute if the septum purge flow is 3 ml/min. I suspect that you mean that the volume flow rate through the column is 1 ml/min, which is approximately correct for a 0.25 mm column (but you do not tell us the column diameter).

A purge time of 5 s gives you the worst features of both split and splitless injections. The shortest sensible splitless time (set as the time at which the inlet purge switches on) is about 20s, more often it is 30 - 60 s to give time for the heavier analytes to get onto the column.

An inlet purge (split) flow of 0.5 ml/min is 100 times too slow - are you sure that the units are ml/min and not ml/sec ? Set it to between 30 and 50 ml/min.

Presumably the total inlet flow is the sum of the column flow (inlet flow above) and the split purge flow, but ignores the septum purge.

The gas saver flow is correct but the time at which it comes on is way too short - it needs to come on at 5 min or later.

What you are doing with these settings is a splitless injection for 5s, followed by a split injection with a split flow so low that it is in effect still splitless, and then the gas saver increases the flow to purge the inlet at 1 min.

It would be a good idea to have a detailed look at the users manual for the intrument and its control software, and the conditions set in the method if you have one.

Peter