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Topic: Accurate Transfer of Volatile Liquid Standards  (Read 372 times)

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Offline dcormieb

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Accurate Transfer of Volatile Liquid Standards
« on: May 31, 2019, 09:12:51 AM »
Hi, first post on this forum! Haven't been on one of these before so I'm just gonna wing it and hope for the best.

Okay, long story short I work in a lab which has been working on cannabinoid analysis (in the licensed Canadian market) and have had a lot of issues surrounding the standards/response factors varying over time.

They are only available in pure methanol or ACN, are very expensive, and only come in 1mL quantities. I currently micropipette 150uL of these into a vial and then dilute, though the vapour pressure and of the solvent causes it to immediately push droplets back out of the pipette tip, meaning the volume changes slightly based on the speed at which you transfer, and I think this (among other things, like slight, slow evaporation of retained stocks in storage) is causing variation. Relative response factors (assuming concentrations consistent with the CoA, assuming no change) between cannabinoids vary up to 30% between different preparations on different days, which is unacceptable.

So I need to both conserve the liquid I use and find a way of transferring that can reduce the effect of volatility. I proposed weighing the ampule, breaking/pouring the entire contents into a volumetric, rinsing/transferring rinses to same flask, drying and weighing again, then correcting for the density of methanol, though my manager seems to believe back calculating volume via density would be unreliable. A reason was given but I don't remember it because honestly I don't think it made sense.

My question is, would my proposal be reliable? Is there a better way to quantitatively transfer small volumes of volatile liquids? Even just a reference to further reading would be great, Google isn't so great for these sorts of narrow technical things.

Offline pcm81

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Re: Accurate Transfer of Volatile Liquid Standards
« Reply #1 on: May 31, 2019, 10:22:24 AM »
Are the standard solutions certified for volume or just purity? If for both, you can dump the contents into the flask, wash the ampule like you suggested, but then you no longer need to weigh it, because you added a "certified/calibrated" volume of the standard.

If the contents are so volatile, depending on evaporation rate and accuracy you need, you may need to look into doing the transfers in a chilled and/or pressurized environment.

Offline wildfyr

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Re: Accurate Transfer of Volatile Liquid Standards
« Reply #2 on: May 31, 2019, 09:10:43 PM »
If you are doing say 1 mL of methanol into 3 mL water, then density is indeed unreliable, since density doesn't scale all nicely at such ratios. However if it's 1 mL of methanol in 99 mL of water, then the divergence in density from water is very negligible.

Even water can be kind volatile, so maybe use something like diglyme?

In general I like your procedure.

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