I would like to see a spectral analysis of grapefruit rind to see if there really is any quinine there and, if so, how much. I am rather unconvinced that there is. Cinchona and grapefruit are not closely related taxonomically. Also, the quinine bark you buy in the health food store probably has had most of the quinine removed.
https://nutritionj.biomedcentral.com/articles/10.1186/1475-2891-6-33
https://www.mdpi.com/1420-3049/21/2/247/pdf
Feb 22, 2016 - peel, leaves, juice and roots of C. sinensis, which include the following ... quinine (3D7 strain IC50 0.02 µg/mL) [60]. ... for determining carbosulfan and its metabolites in citrus by liquid chromatography/mass spectrometry.