Dear Sam Daniel Venturina ,

Unfortunately you are somewhat unclear in what you have done exactly.

I see a number of issues:

-Are you aware that Bixin is virually insoluble in water and only slightly soluble in methanol?

-Did you use the seed (powdered or so) purely or is it some kind of extract?

See for both issues:

Giridhar, P., Venugopalan, A., & Parimalan, R. (2014). A review on annatto dye extraction, analysis and processing–A Food Technology Perspective. Journal of Scientific Research & Reports, 3(2), 327-348.

http://www.journalrepository.org/media/journals/JSRR_22/2013/Nov/Giridhar322013JSRR5870_1.pdf

or this excellent review with good information about the absorbence:

https://www.researchgate.net/publication/249067011_The_chemistry_and_analysis_of_annatto_food_colouring_A_review

-Based on what I think that you have tried I feel your problem is that there is not enough colour substance in your sample. In a paper with a good description of the extraction:

https://www.researchgate.net/profile/Laxmikanta_Behera2/publication/301477412_Efficacy_of_different_oils_used_for_the_extraction_of_annatto_colour_from_the_seeds_of_Bixa_orellana_L/links/571619ba08ae730bc5ae3472.pdf

you can see that only small amounts of bixin can be extracted. So without extraction I think your sample is simply not concentrated enough.

I suggest you have a good look at the references given and hope they will help you any further.

Could you show the obtained spectrum?

If the spectrum reached a plateau, it may be because you exceeded the dynamic absorbance range of your spectrophotometer, i.e. the absorbance was too high to measure.

sorry for the late reply, here is the graph

As I suggested, you exceeded the dynamic range of the spectrophotometer at just over 3 absorbance units. You should dilute your sample at least 10-fold. There is also a discontinuity at about 535 nm, which may be due to a filter switching problem. Sometimes this can be alleviated by running through the spectrum a few times to loosen up the mechanism.