Column and carrier gas choice for GC-FID

[Gon]

Hi,

This is my first post over here. I'm quite new on chromatography and found this very helpful site.
I'm working with Agilent GC 7820A with FID and autosampler. We are using N2 as carrier gas and HP-5 column (15m).
Our work is based on cannabinoids.

I have some problem separating peaks of delta8 and delta9 THC due to its very close retention time.
Recently, I found a document where they suggest a Rxi®-5ms column and hidrogen as carrier gas.
http://blog.restek.com/?p=7704

So, my questions are:
- Will I improve my analysis changing the column for Rxi®-5ms (same lenght, thickness and diameter)? On description, it says tht is virtually equal to HP-5. According to despcription the column is specific for MS!
- Will H2 as carrier improve my analysis aswell?
- I'm using H2 generator for FID. Can use it for both FID and carrier gas?

Thanks in advanced.

[Peter Apps]

Both HP5 and Rtx 5 columns are 5% phenyl methyl silicone, so what works on one will work on the other, and vice versa. If you current column has deteriorated you might see an improvement with the Restek column just because you are using a new column. You would see the same improvement with a new HP5. The MS designation means that the bleed is low, there is no reason not to use it with an FID.

The advantage of hydrogen over helium is that hydrogen can be used at higher flow rates, which provides faster separations. The resolution between peaks is affected very little by carrier gas.

For carrier gas you need 99.999% hydrogen with very low levels of water. If that is what your generator produces then you can use it as carrier gas.

Peter

[ntruong]

Hi Gon,
You will get a better separation of cannabinoids using either RTX-50 or HP-50, 30m column. RTX used to have low bleed as compared to HP, but I have seen their quality is getting worse.
The isothermal program at 200°C should give you a good resolution between Delta 9-THC and Delta 8-THC, if resolution is of your concerns
Just adjust the temperature program to your liking...

[MSCHemist]

H2 often does provide slightly better separation than He or N2. The main advantage of hydrogen is that it is much faster and cheap. The column flow rate is governed by the Van Deempter equation. If you look at the Van Deemter charts for N2 He and H2 You will see that for optimal chromatography the Nitrogen flow has to be much lower than He which needs to be lower than H2. You may also need to use a narrower column because H2 is so small that it may be difficult to maintain a consistent inlet pressure on a wider column though this is more a problem with GC/MS as the column end is under vacuum.

[Gon]

Hi,
Thanks for your answers.
I have been working with HP-5 column, 30m, 0.250mm dia, 0.25micro film.
Bought some Cerilliant CRM (delta9 THC, Cannabidiol and Cannabinol) and runned with validated method. When injecting the mix of standards, got a small peak before delta9THC (see chromatogram). According to the literature that I found, this small peak should be delta8THC.
Does anyone have experience with cannabinoids Cerilliant CRM? I mean, looks like purity is not accurate making our quantitative analysis uncorrect.
Then, I decided to switch to shorter column (same charact. as previous) to shorten each analysis. When running the same standard mix, with Restek suggested method (beside carrier gas) got delta9THC and supposedly delta8THC fused (see chromatogram).
So, should I trust Cerilliant CRM? If yes, running 30m column, should I add both peaks on integration?

[James_Ball]

H2 often does provide slightly better separation than He or N2. The main advantage of hydrogen is that it is much faster and cheap. The column flow rate is governed by the Van Deempter equation. If you look at the Van Deemter charts for N2 He and H2 You will see that for optimal chromatography the Nitrogen flow has to be much lower than He which needs to be lower than H2. You may also need to use a narrower column because H2 is so small that it may be difficult to maintain a consistent inlet pressure on a wider column though this is more a problem with GC/MS as the column end is under vacuum.

\

Here is an interesting link I found when looking for alternatives to He carrier gas. It shows some research that kinda kills the theory against using Nitrogen for carrier in capillary columns.

http://www.slideshare.net/jvercamm/the- ... 49#btnLast

The author is implying that the Van Deemter equations are more suited for LC than GC at least when changing to capillary columns since the terms for the eddy currents and such that are used for packed columns do not come into play.