Chromatography Forum

I was asked to quantify a drug residue in a a plastic bottle. My concern is how can I make sure to extract the entire amount (supposedly sub mcg level). I am planning to do that with methanol (in this case the drug of interest is phenethylamine HCL). What is the best way to ensure full extraction? Does HCL or sodium attached to the main drug have any effect on extraction?

I'd use a solvent that the sought-for material is very soluble in. I think that's the best you can do.

You can take a known clean plastic bottle, spike it at low level, evaporate the solvent and extract. This would give you and idea of recovery. This is a bit iffy because the history of the bottle may have some effect on the recoverability of small quantites of the analyte. (For example if the bottle has been filled wiht pills and emptied, coating from the pills can affect the bottle or the pills can mechanically affect the surface of the bottle. But it gives you a starting place.)

Do the extraction twice and compare the quantities in the extracts - the ratio gives you the fraction of the residue that was extracted at the first step.

Peter

You can take a known clean plastic bottle, spike it at low level, evaporate the solvent and extract. This would give you and idea of recovery. This is a bit iffy because the history of the bottle may have some effect on the recoverability of small quantites of the analyte. (For example if the bottle has been filled wiht pills and emptied, coating from the pills can affect the bottle or the pills can mechanically affect the surface of the bottle. But it gives you a starting place.)

Yes, the bottle had been emptied and it appears to have some colored coating residue which I probably need to filter out. I'm going to do two identical extractions just to prove that the extraction was complete (provided that the second extraction will reveal no analyte). I wonder if evaporating the solvent can also reduce the amount of analyte... But I guess the concentration shouldn't change as I am using internal standard..

On evaporation - phenthylamine is a small enough molecule, you need to use care. May I suggest: Try the steps with the spiked, clean, empty bottle first and be sure you get good recovery and no surprises. You can always get a new clean, empty bottle to try again if needed. Your real sample, you get only once.

You can always get a new clean, empty bottle to try again if needed. Your real sample, you get only once.

Don, that is a VERY good point !

Peter

On evaporation - phenthylamine is a small enough molecule, you need to use care. May I suggest: Try the steps with the spiked, clean, empty bottle first and be sure you get good recovery and no surprises. You can always get a new clean, empty bottle to try again if needed. Your real sample, you get only once.

Would that be ok to add PFPA to about 100ul methanol solution instead of completely drying samples?

Assuming tha PFPA is pentafluoropropionic anhydride:

The amide formed is pretty volatile - so reduction in volume after derivitization would still remain an issue.

I'm not sure that methanol would be my first choice of solvent with an acid anhydride. You have the solvent competing for the reagent - this will form the methyl ester.

Try on a small portion of analyte under contions you intend to use with the real sample -- see what happens.

Assuming tha PFPA is pentafluoropropionic anhydride:

The amide formed is pretty volatile - so reduction in volume after derivitization would still remain an issue.

I'm not sure that methanol would be my first choice of solvent with an acid anhydride. You have the solvent competing for the reagent - this will form the methyl ester.

Try on a small portion of analyte under contions you intend to use with the real sample -- see what happens.

How about filtering the residue in methanol through a filter paper, any concern loosing the analyte this way? The residue has some color from tablet coating...

Will the analyte be adsorbed by the filter paper? Will the color be a problem in the GC?

At low levels, I would be careful about adding ways for the compund to get lost - that includes large glass surfaces and adsorbant materials, like paper.

Dependign on the color it will go into solution and can be come part of the stuff that collects in the inlet liner. Or it may not go into solution - and if it does not adsorb analyte is a non-issue.

If you have access to some of the type of tablets that were in the bottle, you can take small portions and try it out.

If this is a one of a kind analysis, I woud just make the extract and shoot the unfiltered extract onto the GC, perhaps with derivitization. I would avoid this only if there are other materials in the residue that would react with the analyte in the GC inlet. If the ome materials are insoluable in your extraction solvent, let it settle or centrifuge it. The rextract the residue, as Peter has described.

Model the analysis the best you can by using similar bottles and material from similar tablets. Check it out and give it a try.