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I want to analyse the drug content of a sample of gelatin capsules filled with active drug (declared dose is known) and excipients such as lactose monohydrate, microcrystalline cellulose etc. with UV spectroscopy. However I have a question about the sample preparation of my capsules. So first what I want to do is dissolve the capsule content in an organic solvent (such as ethanol, chloroform etc.) because the solubility of the drug is bad in water, and then dilute the samples to a concentration which is suitable for UV analysis. The problem is that when I dissolve the content of the capsule, I'll likely get a suspension where the excipients won't dissolve in the organic solvent. This would interfere with my UV analysis. So I was wondering if it would be a good idea to centrifuge the samples, or just wait for it to precipitate on its own? And if centrifuge, would it be better to first centrifuge the stock solution and then dilute or first dilute and then centrifuge the acquired diluted solution? Thanks in advance.

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    $\begingroup$ This is a non trivial problem because some quantity of solution will be lost with the precipitate. A possible better way to do this is to apply your procedure to a sample where the exact composition is known. If the concentration is low enough, Beer's law will allow you to determine the amount relative to the reference. $\endgroup$ – Zhe Feb 25 '17 at 15:23
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I'd agree with Zhe that "this is a non trivial problem." Method development is tricky if you don't have a standard analysis procedure.

When you dissolve the capsule, if you do have suspended particles then yes they will interfere with the UV analysis. So do centrifuge.

But think of the removal of the liquid as an extraction. So add more solvent to the centrifuge tube, shake up the particles and centrifuge again. Assuming that you won't over dilute, I'd do the extraction three times, then dilute to the final volume.

The rub here in my mind is that UV is not very selective. I'd doubt that you're going to get a nice flat baseline with one nice peak. So you'll need a "pure" drug sample to compare to your extract samples to see if there is any other gunk causing interference.

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  • $\begingroup$ Thank you. And yeah I do have a reference solution of the pure drug form which I am going to measure aswell, forgot to mention. $\endgroup$ – user21398 Feb 25 '17 at 17:30

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