I am trying to prepare a standard calibration curve for 5-aminolevulinic acid (5-ALA) for a drug-release study. I made a 10 mg/mL stock and prepared working concentrations (5–100 µg/mL) in PBS and acetate buffer at different acidic pH values, but the absorbance vs. concentration curve from UV-Vis is not linear.

• Has anyone successfully obtained a linear standard curve for 5-ALA?
• Which solvent/buffer and detection wavelength worked best?
• Also, are there any alternative analytical methods that give more reliable quantification of 5-ALA?
• Are there known stability issues I should consider during sample preparation?