Chromatography Forum

Hi, recently acquired a HP 7694 headspace sampler with a hp 6890 GC. Trying to determine terpene concentration in food, hops, and herbs samples. More information can be provided if necessary. Not entirely sure how to go about this. Used terpene standards from Restek and have determined the retention times for different terpenes, but I'm not sure how to find the concentration in a sample.

In theory, I should be able to calculate concentration using the partition coefficient, but I don't know how to experimentally determine that. I'm not sure of any other way to determine concentration using a headspace sampler.

Any thoughts?

Are you analyzing the plant matter directly? If that's the case, are you sure that a partition mechanism is operable? A partition coefficient doesn't do you any good if the mechanism of release of the volatile material doesn't follow a partition mechanism.

Are you analyzing the plant matter directly? If that's the case, are you sure that a partition mechanism is operable? A partition coefficient doesn't do you any good if the mechanism of release of the volatile material doesn't follow a partition mechanism.

Yes I have been analyzing the plant matter directly. I was under the impression that there is a partition coefficient between solid/gas phases. I could be mistaken.

For the sake of making the partition coefficient easier to determine, I have thought about extracting terpenes into a solvent. However, I feel as though this makes for a less accurate analysis. Also defeats the purpose of easier sample prep.

Not all polymers behave as partition systems. Sometimes, even if they are partition systems it takes a prohibitively long time to reach equilibrium. For quantitative analysis, you can try to calibrate in the matrix (method of standard addition).

Another thing you can try is to slurry your sample with a solvent (like water in this case) and analyze the mixture. I will refer you to a book like:

"Static-Headspace Gas Chromatography" by Bruno Kolb and Leslie Ettre, Wiley-VCH (1997)

for the information on how to figure out if your system is a partition system or not.

Something I do is to extract the plant matter with a strong solvent (like methanol), filter out the plant matter and then dilute it with water for the headspace analysis (I use SPME). You have to be careful with the results because you're pulling lots more "stuff" out with the strong solvent that a beer (e.g., hops used for dry hopping) won't touch.

There was similar thread not so long ago - something to do with acids in fermentations if I recall correctly.

Do not worry about the partition coefficients, ignore them completely, pretend that they do not exist. Run calibration series with known concentrations of terpenes spiked into blank matrix, or added to samples. Then run the samples and calculate analyte concentrations from the calibration exactly as you would for any other analysis.

Peter

Thanks Peter, I had a feeling I was overthinking it.

And thanks banjo for the suggestions as well. I ordered that book, it looks like it will be useful.

Cheers,
D

In the case of terpenes you need to spike the samples and use the method of known addition. They behave very differently depending on your matrix. Since there is no adequate way other than known addition in headspace to compensate for matrix effects you are pretty much limited to using known addition when you get these kind of matrices.

Also try matrix swamping; it helps a lot. Add 1-2 mL of saturated salt solution for every 10 grams of plant matter. Make sure you add enough water to cover the plant material; it helps when dealing with non-polars.