I’m currently using the Restek method for cannabinoid analysis. It’s isocratic… ACN/H2O buffered in formic acid/ammonium formate…
Can anyone recommend a method that gives nicely baseline resolved d8/d9 peaks? The method overall works well, but I often see the d9 tail touch the d8 rise at higher concentrations. Has anyone tried any gradient methods? I’m getting great cal curves, but the subtle peak overlap worries me when dealing with less than 0.3% d9.
You need a different column to properly separate D8/D9
Which column would you recommend?
Different, or longer.
Thank you. I’ll try longer first. Then maybe a gradient. Plan C would be to try another column.
You using a Restek Raptor C-18? What dimensions?
Yup. 150 mm x 4.6 mm.
could he potentially try and extend his method as well? run a gradient as well?
i run 32% A (h2o w/ 0.1% FA) and 68% B (ACN w/ 0.1% FA)
from min 0 to 1, 32% A and 68% B
1 to 6, 68% B to 100% B
isocrat hold from 6 to 7
7 to 8 return to starting conditions.
I don’t get good sep of d8/d9 at all w/ that method either tho, but neither do most labs where im based out of (i’m in house)
Raptor C18 can easily separate the two, even on a 50 mm column.
COLUMNS
For gradient methods I’ve tried the Agilent EC-18 120 Poroshell, running Agilent’s cannabinoid method, and found it capable of baseline resolution at the concentrations I was working with.
Phenomenex provides a column that, per their isocratic method, yields a sole delta-8 peak around (RT ~9 minutes). However there are other analyses that have poor separation on that method and column.
CHROMATOGRAPHIC RESOLUTION
Often when changing method parameters, what provides an optimal separation of some cannabinoids, makes separation worse for other cannabinoids. It’s a bit of give and take.
We want to separate the peaks and to do so we need to make the peaks either more narrow or further apart. Chromatographic resolution has three key parameters: efficiency, retentivity, and selectivity.
PARAMETERS: EFFICENCY, RETENTIVITY, AND SELECTIVITY
The latter two, retentivity and selectivity, are chemical parameters and more convent to change by manipulating mobile phase composition, gradient, pH, column temperature, etc. These parameters move the peaks in relation to each other and describe how strong of an interaction they have with mobile phase vs stationary phase. Increasing your retentivity will improve the resolution but at the expense of band broadening and longer RTs.
Increasing your selectivity could also improve the resolution, but at the expense of elution order.
Efficiency is harder to manipulate. It is a physical parameter and a measure of band spreading - that is when you have a peak strongly retained on a column for a long time, the analyze begins to diffuse laterally in the stationary phase, which causes the peak to appear broad. On a large chromatography column this will also happen with non-optimal mobile phase flow rate and can be visibly noticeable.
PARTICLE SIZE: WHY INCREASING COLUMN LENGTH ONLY GETS YOU SO FAR
Here the dynamic between the column length and retentivity becomes apparent as greater retentivity means more resolution but at the expense of increased band broadening. This is something to consider when looking to increase column length in attempt to separate difficult to resolve isomers.
This spreading or band broadening can be minimized by reducing the particle size of the stationary phase. By increasing the efficiency of a column, it is said to have more ‘theoretical plates’. By running a shorter column with smaller particle size, you get the benefits of increased resolution due to sharper peaks as the analyses spend less time on the column. More theoretical plates without increasing the length. And less mobile phase is used. And the idea of no free lunch remains; your back pressure will increase as particle size decreases.
FLOW RATE: EFFECTS ON EFFIENCY & RESOLUTION
Then there’s flow rate. We want our analysis fast as possible and use the least amount of solvent while having the best resolution. We can make our peaks sharper by increasing the flow rate, but they don’t have as much time to separate. So the peaks ‘bunch up’ and resolution gets worse. We may start to see some coelution.
NO SINGLE COLUMN TO RULE THEM ALL
One approach would be to have different columns and methods which are developed to resolve select cannabinoids, like d8/d9, which otherwise remain troublesome at higher concentrations.
I have some experience using HPLC to separate d8/d9. I know it might be tempting to try to customize a method to optimize the separation of d8/d9 but just call the column manufacturer, ask them if they have a method that can use your column to separate d8/d9. They will likely say no, just ask them and another column manufacturer for a column and method to separate it.
Its literally their job to make methods and they want to find the method with the best separation for your analytes. Don’t try to compete with the experts, columns aren’t very expensive and if they do what you want its worth it.