[nobodytobe123]

Hi chem wizards!

I'm not a chemist, only had a few basic chem classes in college. But I happen to have some H. pluvialis powder here that I need to analyze for cheap and I can get a cheap spectrophotometer on ebay, so I've been looking at articles to try to figure out what wavelength to look for.

Strangely, different sources disagree on the wavelength! So I'm confused.

One source says, "The data show that 530 nm is the most suitable wave length for spectrophotometric determination to the astaxanthin in Haematococcus crude extract."

https://link.springer.com/article/10...343-012-1217-5

But another source, https://www.ncbi.nlm.nih.gov/pubmed/22320081 seems to indicate that 432nm is the magic wavelength.

And other sources say other wavelengths!

Please end this confusion, someone! Thank you!

[Corribus]

Astaxanthin is a hydroxy functionalized carotenoid. I didn't look at the articles you linked to,  but the absorption band is quite broad,  so there's not necessarily a single magical wavelength you'd need to use.  Moreover the absorption band appears to be quite sensitive to the solvent,  meaning the best wavelength probably changes depending on what solvent the substance is extracted into (eg water vs alcohol).

[Babcock_Hall]

There is quite a large amount of data and procedural help on the spectrophotometry of carotenoids available.  One such source is a lipids database in Japan, and another is a highly useful two-volume carotenoid handbook from the late 1990s, the name of which escapes me at the moment.  I will keep my comments general at first.  One, carotenoids may have considerable shifts in their wavelength maxima in different solvents, on the order of 20 nanometers or so IIRC (obviously depending on which solvents are involved).  Two, a typical carotenoid spectrum has a three-finger appearance, with each finger being a local maximum.  It is possible that some people make their measurements at one maximum and other people make theirs at a different maximum.  Three, a measurement in a crude extract may require that one take into account the presence of other chromophores.  In my case chlorophyll was one such pigment.  Point three is merely my conjecture, and even if it is critical for one situation might easily be different in another.