I want to use UV-vis spectrometer to measure the concentration of drugs released from nanoparticles in each time point. I wonder why they added equal volume of fresh PBS as they extract for each time point. Does adding fresh PBS affect concentration and not accelerate drug release?
(the drug release experimental parts are attached)
It would be very helpful if you could reply.
The reason is to keep the volume constant. Drug release rate is a first order process, so the external volume doesn't make any difference as long as the volume is large enough that the system does not approach equilibrium (because then the rate of release slows down). Replacing the withdrawn volume prevents the volume from decreasing so much after repeated samplings that it is too small to prevent the system from approaching equilibrium. Keeping a constant volume also simplifies the calculation of concentration.
Dear, Adam B Shapiro
Thank you very much for your detailed answer. I have additional questions. Wouldn't the concentration change immediately after sampling and adding PBS of the same volume? The volume extracted at the first sampling includes drugs, and the volume I put in after sampling does not contain drugs. Can the concentration of sample be lowered according to the sampling interval?
The rate of release of drug from the particles should not depend on the concentration of the nanoparticle suspension, so the dilution with PBS to keep a constant volume should not affect the rate.
Personally, I would probably not have added back PBS to keep the volume constant. I would have adjusted the calculation for the reduced volume.
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