Thread: UV Spectroscopy questions

Hey all

Currently I am doing a prac writeup on UV spectroscopy, and have a few questions that I have to answer:

Why would you use standard curves in assays as opposed to the Extinction ccoefficient? (assume you have access to it)?

I'm thinking that the Extinction coefficient is probably very dependant on experimental conditions etc.

Why might the slope of the standard curve disagree with the extinction coefficient?

Any help would be greatly appreciated.
Thanks

The analytical chemist always uses a freshly-prepared set of standard solutions because it is necessary to calibrate his instrument each day.

Why might the slope of the standard curve disagree with the extinction coefficient?

Purity of sample, degradation due to age, different instruments not properly calibrated, etc.

Are you trying to figure out how much of something you have, or what you have? The extinction coefficient doesn't depend on concentration, so knowing it won't tell you anything about how much you have. That's why you have to make a standard curve with known concentrations.

Originally Posted by SteveF

Purity of sample, degradation due to age, different instruments not properly calibrated, etc.

Thanks Steve! That's exactly what I was looking for

Are you trying to figure out how much of something you have, or what you have

Thanks for your response. We are determining how much (paracetamol in a panadol tablet).

The extinction coefficient doesn't depend on concentration, so knowing it won't tell you anything about how much you have.

We dissolved the panadol tablet in water, so I was thinking you could just use Beer's law (having the Extinction coefficient) to determine concentration of this solution from its absorbance.

Thanks again guys