Chromatography Forum

I am looking at terpene concentrations in cannabis samples by head space analysis and I've found myself second guessing my calculation method. I've made the standard calibration curve from known concentrations in 40 ml vials to determine my concentration of injected sample as normal. How would I go about reporting the terpene concentration as a percentage?

At the moment I am using the concentration of the injected sample (as reported from the analysis method) divided by the mass of the plant material in the 40 ml vial. Is this correct for calculating the percent terpene concentration or do I need to consider the injection volume as well?

Ex. (56 ng/uL) / (200mg/40ml)
(Note: units are converted prior to final calculation)

Thanks!

Welcome

Like you I am second guessing your method.

In what matrix do you make up the standards in the 40 ml vials ? How do you calculate or measure the concentration in the headspace of the standards ?.

A concentration is a mass (of analyte) divided by a mass (of sample) , if you are dividing a concentraton by sample mass you are dividing by mass twice. Concentration per unit mass is meaningless.

What calculation does the analysis method perform to report the concentration of the injected sample ? How is this calibrated - is this calibrated from the standards in the 40 ml vials ?.

What method ate you using ? - I guess purge and trap (but maybe SPME) from the volume of the vials.

Peter

Can you describe your procedure to us in more detail? I have tried to come up with methodology to determine volatile materials in plant matter (mostly dry material, hops in particular) over the years without success because for my analytes of interest, they don't behave as partition systems. I've always had to add a fair amount of water to get the material to behave well so that I was sure I could reach equilibrium during calibration by the method of standard addition.

Hi

For this analysis I would prefer to use standard addition or multiple standard addition rather than an external standard.

What is your solvent?

Regards

Ralph

I will work on the assumption that you are using something similar to Restek's FET/HS method for this analysis.

Since your injection volume using a headspace sampler is fixed, you do not need to include it in your calculations. You would calculate the mass of the terpene that you find from the analysis (calibrate using mass, not concentration - it'll be easier) and then you would divide that mass by the total mass of the plant material that you weighed into the HS vial.

You do not mention internal standards. For FET-type analyses you absolutely, positively HAVE to use an internal standard spiked into each vial. If you do not you will get large variations in your quantification.