[Doris Dey]

I am working on a project in my lab, and we are analyzing capsaicin using HPLC. I am trying to find an appropriate wavelength for the HPLC using a UV-Vis spectrometer. When analyzing the pure capsaicin standard dissolved in methanol, I of course use methanol as a blank. However, I got two peaks, one around 230nm and one at 280nm. Since the system automatically subtracts the methanol, I can't understand why I am getting two peaks.
Anyway, 230 yielded the maximum absorbance, so I used that in my HPLC. I didn't get great results and decided to read some literature on the project. I saw that most other people had a UV-Vis spectrum like mine, but they decided to use 280nm. Why is that?

[Bidagdha_TADIR]

Our teacher says that measuring absorbance at below 250nm would require using expensive cuvette like one made of quartz and therefore it is preferable to use wavelength above 250nm for measurements. Of course it was said purely from an economic point of view.

I didn't think it mattered because I think that even if pyrex/plastic cuvette absorbs strongly below 250nm blank cuvette should cancel it out.

Still, that could be a reason to chose 280nm. But then again most laboratories use quartz, choosing 230nm shouldn't be a problem.

[Arkcon]

Some substances have multiple maxima.  This Googled reference contains a spectrum just like you described: http://www.mdpi.com/1420-3049/16/10/8919/pdf

[mjc123]

What sort of absorbance values are you getting? For good linearity vs. concentration, you ideally want A < 1 AU. If your absorbance at 230 nm is greater than this, and at 280 is less, 280 would be a better choice for your analysis. Or use more dilute solutions.

[Doris Dey]

Thank you so much everyone. @mjc123, that seems exactly the problem. I went back to the articles I was looking at and realized they were all using 280nm because the absorbance at 230nm was above 1AU. Mine is as well, so I will be using 280nm. And thank you for your article @Arkcon because it verified my understanding of the subject. The person in that reference had an absorbance under 1 AU at 230nm, which is why he decided to go with that. Thank you!