Residual Solvent (Headspace Analysis)

[GabeS]

Hi everyone,
I am trying to calibrate a SRI310C Gas Chromatograph with a Restek MXT-502.2 column for residual solvent analysis.
The analysis is for residual heptane or methanol on cannabinoid isolates.
My current procedure for calibration is:
In headspace vials,
1uL Hept. in 0.1351g sodium bicarbonate = 5000ppm
1uL Hept. in 0.1692g "" = 4000ppm
etc. until
0.2uL Hept. in 0.2717g "" = 500ppm
with a blank thats just filled with 0.3039g bicarb.

GC Program:
Heated Injector 260C
Start 45C ramp up to 200C
25uL injection

The issue I am having is that the peak area for 5000ppm ranges from 15,060 to 17,520, and the peak area for 4000ppm ranges from 16,660 to 17,190 and so on, it holds pretty constant.
The blank doesn't have any peak which is good.
The peak area should be decreasing as the ppm decreases though.

Am I right in thinking that because cannabinoids are soluble in heptane/methanol, but the sodium bicarb I am using in place of isolate is not soluble in heptane/methanol this is messing up my calibration?
If not, then what should I be trying?

[rb6banjo]

Personally, I don't like making my standards in solvents where the analytes are known to be substantially insoluble in the solvent. You can never be completely sure of homogeneity.

It seems that you're performing direct injection of a liquid. 25 µL is a lot to inject on a capillary column of any type - even a 0.53 mm diameter column. Water has a huge expansion volume. It's quite likely that you're overfilling your inlet.

If you go to restek.com, under "Resources & Education" you'll see the "Solvent Expansion Calculator". You can change your solvent, the dimensions of your inlet liner and the amount you inject and it will tell you if you're going to experience excessive backflash (over filling the inlet liner) or not.

I would start there and see how things change for you. At 5,000 ppm, you can likely run at a pretty high split flow as well and still have good sensitivity.

[GabeS]

Thanks for the reply!

Just to clarify, I am not injecting any liquid. I have a headspace vial filled with a specific weight of solid sodium bicarbonate, then I add 1uL of heptane to the vial, shake it for a bit to homogenize, then when I draw with the syringe I am just drawing the air above the sodium bicarb. So its 25uL of air containing heptane vapors that the GC is able to detect.

In theory the less ppm of heptane is in the vial, the less heptane vapor there would be in the air above the sample, which would give less peak area at the detector, but that's not what I'm seeing.

I think all I have to do is switch to a solid that is soluble in nonpolar solvents but it would be nice to know if I'm on the right track by someone with more experience than myself.

[rb6banjo]

Ok. That makes a little more sense.

Why are you adding solid sodium bicarbonate to the vial? The heptane should not interact with it much. In fact, you may be creating some unwanted adsorption phenomena in the vial with the solid there.

What is the nature of your sample? Are they really trapped on solid sodium bicarbonate?

[GabeS]

So my actual samples would be stuff like CBD, CBN, CBC, etc. in a powder form at >99% purity. And that powder when it comes from production will still contain some unknown ppm level of heptane.
To be able to calibrate the GC for ppm levels of residual solvent on the isolates, I need to be able to replicate this with something I can use as a control (ie. something that has 0 ppm heptane) I thought of using sodium bicarbonate, and sure enough when I run a blank there is no heptane peak.

[rb6banjo]

The trouble is that solid materials (like your powder) are not likely to present themselves as partition systems for HS GC. Partition systems are when Henry's Law is obeyed - the concentration of analyte in the vapor above the solution is proportional to the concentration of the analyte in the liquid. The condensed phase in the vial can be made to be in equilibrium with the vapor phase above the condensed phase.

For solids like yours, you would likely need to dissolve them in a solvent or perform some type of extraction technique like multiple headspace extraction (MHE) to get accurate and reproducible results.

The trouble with dissolving the solid in a solvent is that the sensitivity decreases, most of the time fairly dramatically. At 5000 ppm, you'd probably still be ok. If you're looking for a microsniff of stuff it may not work so well.

Take a look at this post I made a while back in regard to determination of residual limonene in a red-yeast rice sample (link below). Even if your solid is not completely soluble in the solvent, you can still get accurate results. My headspace method is solid-phase microextraction but you could try to apply it to your situation with static-headspace GC as you're doing it. The water acts as a sort of leveling solvent. In my case, it appears that limonene is soluble enough in the water to force the solid to be in equilibrium with the vapor above the slurry for the concentration of interest.

viewtopic.php?f=19&t=118884&p=444914&hilit=red+yeast+rice#p444914

I did go back and calibrate in just water (no red yeast rice) and the slopes of the calibration lines are dramatically different. To do this analysis as I've described, I'd always have to calibrate in the matrix.

[GabeS]

Okay I think I understand what you are getting at there...

Cannabinoids aren't soluble in water but would this work?:
- Dissolve 0.050g CBD (for example) in 1 mL 1-Decene then take headspace
GC same as I'm doing now? (may have to adjust injection volume)
- And for calibration just add 1 mL 1-Decene to the vials and add the appropriate volume of heptane for 5000ppm and work my way up and down in ppm to make a calibration curve?

[rb6banjo]

Decene would not be a good choice. Too volatile for sure.

When people do this sort of thing, they usually choose a heavy solvent (low vapor pressure). Something like triacetin, N,N-dimethylacetamide or N,N-dimethylformamide might work for you. Your solid might even dissolve in one of those solvents, then you can just treat it as you would any other liquid.

To get the best sensitivity, you want to use the minimum amount of solvent that shifts your system to where Henry's Law is valid for your analytes. In an effort to minimize the amount of the solvent that gets into your system, you want to operate at the lowest possible sample-incubation temperature that gives you the desired information about the analytes of interest.

Methanol is going to be tough by headspace GC with any kind of solvent added - even water. Maybe if you add salt to the water, you might be able to get it. I have had to do that and use solid-phase microextraction to determine MeOH in distilled spirits.

If you are new to this headspace analysis game and you want to learn more, there's a great book out there called "Static Headspace Gas Chromatography - Theory and Practice" by Bruno Kolb and Leslie Ettre. You can pick up an old copy on e-Bay for about $75 (US). Even an old version would be good because the fundamentals really don't change from version to version.

[GabeS]

Thank you for all your help!

After reading more research papers and guides this is making a lot more sense.
Ordering some DMAc now as it seems that is often used for cannabinoid HS-GC.

The plan so far for samples is to dissolve 0.0500g solid analyte into 1mL DMAc then transfer 200uL into a 2mL headspace vial.
Calibrations wont use any solid analyte and just put volumes heptane directly into the DMAc.

For calibrations I've done the math for 500ppm -> 10,000ppm. However, the volumes of heptane required isn't possible with the micropipettes I have, so at 500ppm I have to multiply the mass (0.0500g) by 13.7x to get 0.5uL heptane (a volume I can actually inject). I'm assuming this means I also need 13.7x more DMAc (13.7mL) to make the concentrations equal then transfer the same volume 200uL into the headspace vial.

[rb6banjo]

To make it easier to add your analytes, you can dilute them in your solvent to make a concentrated standard. Then add larger volumes of that concentrate to the solvent/sample mixture.

0.1 g of heptane in 1 g of DMA = 100,000 ppm

Add 1 µL of that to your 0.050 g/1 mL mix and you get something close to 2,000 ppm (in the solid). 2 µL gets you close to 4,000 ppm and you really don't change the volume of DMA in the vial.

[Peter Apps]

In addition to all the replies above - if you want to do accurate and repeatable headspace analysis you have to precisely control the temperature of the sample, and the syringe.

[James_Ball]

What I was seeing initially was putting the same amount of heptane in the vial but different amounts of solid (diluent) to gain the concentration, but for headspace that would not work.

What you need to calibrate is the mass of heptane per vial, that is what the headspace method will actually analyze. The matrix can contribute to the amount of analyte that goes into the headspace, but the major thing is to control the mass of analyte added to the vial.

As mentioned, make dilutions of the heptane so that if you add 1ml of solution you have varying mass of heptane to create the calibration curve. Or you can use a solid matrix such as the bicarbonate but add different masses of heptane to each constant weight of matrix, then add that same weight of sample to the vial when determining the unknown sample amount so that you can calculate mass of heptane vs mass of sample to gain the concentration that is residual in the sample.

The math works much better when you keep units as mg/kg or ug/g instead of trying to use ppm, because if you analyze 1g of sample at 10ppm or 0.1g of sample at 100ppm, the instrument will most likely give the same response, but you can still make an accurate assessment when you know the sample mass and mass of analyte detected.