Chromatography Forum

Hi,

Let me start by saying I'm new at method development.

I am having problems obtaining reproducibile results on a GC 6890 with a CTC headspace sampler. I am injecting a standard solution containing TEA, methanol, ethanol, acrylonitrile, acetonitrile, toluene, pyridine, and 3-picoline diluted in buffer using a stabilwax column (30 x 0.32 x 0.5). When I analyzed my standard on the GC 6890 with a 7694 HS sampler my RSD values are <5%, but with the CTC they range from 2-30%.

Thanks.

GC parameters:
Oven: 55C hold for 4 min, 30C/min to 200C
Split ratio 5:1
Column Flow: 2.5mL/mim

HS parameters for 7694:
Vial pressure: 10psi
Oven/Loop/Transfer line temp: 85C/ 95C/ 110C
Vial Equil time: 10 min
Pressurization time: 0.2 min
Loop fill time: 0.15 min
Loop equil time: 0.05 min

HS parameters for the CTC:
Incubation temp: 85C
Syringe temp: 110C
Inj Speed: 1000 uL/s
Pre/Post inject delay: 0ms
all other at default parameters are set

What is your sample volume on the CTC?

You might want to consider using a lower syringe temperature (90 or 95°C) as the higher temperature may effect the partitioning that has already taken place during your incubation.

Depending on what your sample volume is, that injection rate may be too high for your 6890 to compensate for, and you could be having effects from the inlet EPC and inlet overloading taking place.

What kind of liner are you using?

I am injecting 1000uL using a split liner (870uL, low pressure drop, glass wool, taper, deactivated).

I will try adjusting the syringe temperature and injection volume.

Thanks.

The injection rate is far too high; it needs to be similar to the the total inlet flow to avoid a pressure spike that upsets the split ratio. Reduce it to about 150 ul/s.

Also you do not need the glass wool in the inlet, the sample is already in the vapour phase, and does not contain any non-volatile crud.

The sample temperature control on the CTC (or at least on its Gerstel clone) is poor - what is your equilibration time, and do you have more than one vial in the sample oven at a time ?

Peter

The vial equilibration time is 10 min and there is only one vial in the oven at a time.

I'll try lowering the injection speed.

Thanks.

10 min equilibrium is on the short side, and you definitely nned to be using the shaker as well.

Peter

is there any fill strokes in the default parameters on the combipal?

Are you flushing the syringe with inert gas after the injection? If the flush time is too short that can cause some carryover issues with the higher boiling compounds in a mixture.