A fast and reliable method is described for estimation of ascorbate in a small tear volume (5 to 10 mul) without pretreatment of the sample.
Howard, R. R., Peterson, T., & Kastl, P. R. (1987). High-performance liquid chromatographic determination of ascorbic acid in human tears. Journal of Chromatography B: Biomedical Sciences and Applications, 414(C), 434–439. https://doi.org/10.1016/0378-4347(87)80070-1