Detecting drug in ethanol solvent using UV-Vis

[AndrewUV]

Hey Everyone,

I'm having trouble detecting my drug dissolved in ethanol using UV-Vis spectroscopy. The peak absorbance of the drug is at 240 nm while the peak absorbance of the ethanol seems to be around 190 nm. Are these peaks too close to each other to get an accurate measurement? In the wavelength scan of the drug dissolved in solvent, the graph has a higher peak up to 3 AU for ethanol at 190 nm and a lower peak of 1.5 AU at 240 nm for the drug. The peaks, however, are not separated... from the higher ethanol peak at 190 nm, it drops to only around 1.25 AU at 217 nm before going up to the drug peak of 1.5 AU at 240 nm. I've done a standard and that seems to work out fine. However, when I encapsulate the drug inside polymer and then dissolve that polymer to extract the drug again and detect the amount encapsulated inside, I get a number greater than 100%. So I'm detecting more drug than what I put in... would this have to do with how close the peaks are in absorbance and the ethanol peak possibly contributing to the absorbance? Thanks for your help.

Andrew

 

[Ygggdrasil]

Are you measuring your solutions in quartz cuvettes? Glass and plastic cuvettes absorb UV and will interfere with your measurements. I don't think ethanol should have any strong absorbance around 240 nm. Are you sure it's not the polymer that absorbs at 190 nm and is interfering with your measurements? You say your standard looks fine. Is this standard also dissolved in ethanol (but with no polymer)?

 

[AndrewUV]

Thanks for the help! Yes the solutions are measured with a quartz 96 well plate. The standard is dissolved in ethanol with no polymer. Hmm yeah that's a good point... it may be the polymer contributing to the measurements. The polymer, however, is not soluble in ethanol. We are dissolving the polymer with DCM and then adding ethanol so the polymer stays in the DCM and the drug is extracted into the ethanol. The two layers are then centrifuged and supernatant removed to be analyzed. Do you think there is a better way to extract the drug from the polymer?

To see if the polymer absorbs near 240 nm I'm thinking of dissolving the polymer in DCM and comparing that to 100% DCM. Does this make sense?

 

[Ygggdrasil]

We are dissolving the polymer with DCM and then adding ethanol so the polymer stays in the DCM and the drug is extracted into the ethanol.

There's your problem. DCM will absorb light and interfere with your readings below 230 nm (http://macro.lsu.edu/HowTo/solvents/UV%20Cutoff.htm). It would also be good to check to see whether your polymer also absorbs in the UV/vis (but check in a solvent other than DCM, if possible).

 

[AndrewUV]

So even though we are removing the supernatant (ethanol + drug) there might still be DCM present in that supernatant? And if we are looking at the peak at 240 nm, DCM may interfere with that peak? Also, for using a different solvent, would I want to use something with the UV cutoff lower than 230 nm then? eg. trifluoroacetic acid?

 

[Ygggdrasil]

DCM is fairly soluble in ethanol, so you are probably getting quite a bit of carry over of DCM into the ethanolic phase after extraction. And, yes, using an extraction solvent with UV cutoff lower than 230 nm would be helpful.

 

[AndrewUV]

Great. Thanks so much for the advice! I'll see if using a different solvent works.