Chromatography Forum

Hi Terry

Damn it, you have triggered my couriosity

I just might have to spend some time on this at my lab to try this out. Most likely I will use a 6890+ GC with a 7694 HS connected to a volatile inlet and silcosteel parts in HS ie pretty much like yours. Our new HS ie same as yours are not all installed yet so acess is limited to those.

viewtopic.php?t=9658 this thread is not the same thing excatly but is related to that you sometimes have to struggle a bit to get that low level sensitivity with HS- GC.

Would not say it is just the FID. More the headspace technique itself and the method in use in combination with the low levels your looking at.
A total of 0,1ug in the vial is hard to analyse but a rough approximation could be: 50% of 0,1ug acrylonitrile ends up in 10ml gas phase, 1ml is injected with a split ratio of 1:5=> 1 ng (1000pg) on the column. As a comparision you typically can detect about 20pg toluen (that ends up on capillary column) with a FID. Sure there is a difference in the responsfaktor between the two compunds but roughly.

So purely teorectical you should be able to detect benzene at those levels.

Unsure when I can squeez in this testing but I will let you know when/if it works.

One thing to recall is also that with increased % of water in DMF you "generally" increase the sensitivity, have no good reference on that apart from graphs provided by "old" HP.

Hi Krickos

Thanks for sharing.

Is it acceptable to increase the % of water in DMF from a regulatory perspective? What other parameters I may change?

The management is really alert to any change I make to the offical method. So I have to be very cautious.

Terry

Hi Krickos

Thanks for sharing.

Is it acceptable to increase the % of water in DMF from a regulatory perspective? What other parameters I may change?

The management is really alert to any change I make to the offical method. So I have to be very cautious.

Terry

If you read 2.4.24 closely you find this:

In some cases none of the above sample preparation procedures are appropriate, in which case the diluent to be used for the preparation of the sample solution and the static head-space conditions to be employed must be demonstrated to be suitable.

So actually you may adjust the sample and standard concentrations so you get an appropiate sensitivity if the suggested ones fails, jsut make sure to document it. This should be enough for your manager.

As for headspace parameters. It is quite obvious that oven temperature may not be changed, but valve/loop/needle/transferlines temperatures may be changed dependent on what HS instrument you use, I mentioned before some instrument obviously can not prressurize the sample vial.

Regarding GC oven program you can obviuosly speeed up the program AFTER the last residual solvent of intrest has eluted, no point and very unefficient to run longer.

Finally also recall that this is a limit test method (for class 2/3 solvents std addition may be used above 1000ppm quantatively) so amount of validation if prefered is somewhat less.

One more question though, you have indicated sample preparation procedure 2 (DMF) that should use 105°C as HS oven temp but if recalling you use 80°C?
This likely would contribute to your sensitivity issue as DMF typically retains solvents more than water (typically increasing with less polar solvents).

Hi Terry,
You had listed the diluent as DMF, which I assume means you are following sample procedure 2. The headspace temperatures you listed were for sample procedure 3. According to the EP, for sample procedure 2, the headspace temperatures should be:
Equilibration temp - 105
Equilibration time - 45
Transfer line - 110

Also, even though this is a split injection, I have found that sometimes with the pharmacopeia methods (with low split flows), a 2mm splitless straight-through liner helps to increase sensitivity.

Bill

When all else fails - read the instructions

Peter

I may have misinterpreted the EP method.

On the next working day, I would use headspace parameters as Bill mentioned to see the result.

Krickos
The split ratio in the official method is 5:1, may I change this to 3:1 or 2:1? if the total flow is still more than 10 ml/min.

Bill

Also, even though this is a split injection, I have found that sometimes with the pharmacopeia methods (with low split flows), a 2mm splitless straight-through liner helps to increase sensitivity.

I am using Agilent 5181-8818, 2 mm ID, 250 uL, Splitless liner. Agilent people told me it is the right one for HS analysis.

Terry

Hi Terry

Start with the condition as indicated by myself and Bill.

With the mentioned conditions you will get alot of DMF on your column including its impurities so do not reduce the split ratio unless really needed.

Your liner choice is correct, we mostly have the volatile inlet installed with our headspace instruments.

Hello Krickos

Elevated HS oven temperature (105°C) did not make a difference to the peak response.

I guess it is the composition of the diluent, 5 ml of DMF plus 1 ml of water, to blame. I have tried 5 ml of water and 1 ml of DMF, with the same amount of benzene in the vial, and the peak is way much higher. The composition is pretty much like the USP <467> method for water-insoluble substance.

I would have to do the validation/verification thing.

P.S. Spring Festival (Chinese New Year) is coming, I am going to reunite my extended family, and I would be absent from this board during these days.

Best wishes to all of you

Terry