Residual Solvents by Headspace - Repeatability issue

[ChemStation1]

Has anyone experienced high %RSD when performing residual solvents analyes by headspace using headspace autosampler?
I'm using water as diluent for tert-Butanol and using the parameters specified in USP 467.
Any suggestions for critical parameters that may affect Repeatability?

[AaronAIT]

Might be an obvious one, but - how do you tighten your HS caps?

[ChemStation1]

Using a standard manual crimper.
Is there a significance for using 10ml or 20ml HS vials?

[ChemStation1]

I should also mention that the area od tert-Butanol is decreasing after each injection

[AaronAIT]

If all HS caps are crimped tight, not dented, and you can't turn them by hand, that should be tight enough.

Perhaps the problem is related to your HS sampler? How does repeatability look for other analytes?

[Peter Apps]

What septa are you using ?

Peter

[ChemStation1]

As for the repeatability of other analytes, Acetonitrile gives the same RSD as tert-butanol.
The septa is thermo scientific- Silicone/PTFE

[AaronAIT]

Have you performed any diagnostics on your HS sampler, such as a vent valve test? Your service engineer should be able to guide you through this procedure, it's fairly easy and quick.

[ChemStation1]

Thank you.

Do you know what are the recommended settings for the headspace sampler:
Fill speed
pullup delay
injection speed
pore inject delay
post inject delay

Thank you.

[Bigbear]

To make informed suggestions we need your GC and headspace parameters. Also what do you define as high % RSD?

[ChemStation1]

Headspace CTC Conditions
Incubation Temp.: 80 C
Incubation Time: 40 min
Syringe Temp: 90 C
Fill Speed-500ul/s
pullup delay-1000ms
pre inj delay-500ms
post inj delay-500ms
Flush time-200sec

GC Conditions:
Instrument: Agilent 6890B
Inlet Temp: 140 C
Inlet Liner: 1.0mm ID straight liner
Split Ratio: 1:5
Linear Velocity: 35 cm/s= constant flow-2.2 ml/min
Column: Rxi-624 Sil MS 30m, 0.32 mm ID, 1.8 um
Oven Ramp: 40 C hold 20 min, 10 C/min to 240 C, hold 240 C 20 min
Detector: 250C FID
Make up flow: 30 mL/min
Hydrogen flow: 40 mL/min
Air flow: 400 mL/min

[Peter Apps]

Two other things that we need to know about the setup:

Volume of headspace in the syringe, injection speed. ?

And you have not answered Bigbear's question about the rsd.

"The more you don't tell us, the more we can't help you."

Peter

[ChemStation1]

Injection volume-1ml
injection speed-300ul/s
RSD- about 10%
In every sequence, in 1 or 2 injections (from an overall of 6) the area of tert-butanol decreases by about 25%

[Peter Apps]

Your injection speed is 18 ml/min. This might be enough to disrupt inlet flow and split ratio. If you LOQ allows it you can decrease injection volume, keeping injection time the same, or increase split ratio. A wider inlet liner might help.

If you are consistently getting 1 or two results out of 6 that are 25% low, and the other four or five are in close agreement then I would suspect leaky vials. Check your sealing technique by putting a drop of pentane (or other low boiler) into a vial, crimp as usual, then immerse the vial in hot water, a stream of bubbles from around the cap shows a leak. Since you have an intermittent problem you need to do this for at least six vials.

Check whether it is consistently the same vials in the sequence that give low results. If it is you have a cool well in your oven (the thin aluminium shell on the CTC and clone headspacers does not spread the heat very well).

Peter