Splitless Purge

Discussions about GC and other "gas phase" separation techniques.

4 posts Page 1 of 1
Hi,

I'm injecting (splitless) a 6 uL sample that contains 0.12 pg native and 0.6 pg -D9 in the 6 uL. The solvent is MSTFA. I'm using a pressure surge of 40 psi to compensate for the high injection volume. My inlet temp is 190. My split flow is set to 50 mL/min after 1.00 min. The septum purge is set to constant. The surge is set to 40 PSI for 0.9 minutes.
The oven ramp is 80, hold for 1 min, ramp 35/min to 295, hold for 0.5 min. The analyte elutes around 7.1 minutes (near 295 degrees).

I'm getting this little pre-peak shoulder when I am only using the pure standard that has been dried and brought up in 25 uL MSTFA, heated at 70 degrees for 20 minutes, transfer to the AS vial. Any suggestions on what these little peaks may be and how to get rid of them?

Anyone know what the rules are for setting the timing of the purge, split flow start time, how long to keep the column cooled at the start of the run?
Image
What is the column flow at 40psi?

That will determine a lot of the parameters. You want to sweep the liner volume about 3-4 times before opening the purge valve, so if you have a liner volume of 1000ul and a column flow of 4ml/min then you need a one minute valve opening time, adjust as necessary for your flow rate.

Another thing with setting purge time, is to decrease it until you see decreasing peak heights, that will be the optimal time you need to transfer the analyte to the column so just experiment with different purge valve timing until you find the optimum setting. Too long on time and you can see peak tailing from the small traces left entering the column, too short and you dump a lot of analyte to the split vent.

You want the pressure surge to be different from the valve opening about 0.1minute, if they are the same time you can get split peaks as the flow controller tries to control both the split flow and pressure that are changing rapidly, by having a delay you let one become stable before changing the other.

I normally keep the oven on hold until just after the purge valve opens, so all the analyte is transferred and focused before the ramp begins.
The past is there to guide us into the future, not to dwell in.
tracelevels100,

I am going to argue your pure is not pure.... Stick my neck out a bit and live life!

To prove it, try higher concentration, 1 uL injection. You are running a triple quad so what do the spectrum look like?

Best regards,

AICMM
I would have said impurities, but they move around which makes me think ghost peaks from earlier injections - which are also impurities.

Peter
Peter Apps
4 posts Page 1 of 1

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