Measurement of Anions in Tear Fluid Using Ion Chromatography

Abstract

Purpose: Tear fluid (TF) contains a variety of electrolytes that exhibit a strong correlation with its osmotic pressure. These electrolytes are also related to the etiology of diseases on ocular surfaces such as dry eye syndromes and keratopathy. Although positive ions (cations) in TF have been investigated to understand their roles, negative ions (anions) have hardly been studied because applicable analytical methods are restricted to a few kinds. In this study, we established a method to analyze the anions involved in a sufficiently small amount of TF for in situ diagnosis of a single subject. Methods: Twenty healthy volunteers (10 men and 10 women) were recruited. Anions in their TF were measured on a commercial ion chromatograph (IC-2010, Tosoh, Japan). Tear fluid (5 μL or more) was collected from each subject with a glass capillary, diluted with 300 μL of pure water, and conveyed to the chromatograph. We successfully monitored the concentrations of bromide, nitrate, phosphate, and sulfate anions (Br−, NO3 −, HPO4 2−, and SO4 2−, respectively) in TF. Results: Br− and SO4 2− were universally detected in all samples, whereas NO3 − was found in 35.0% and HPO4 2− in 30.0% of them. The mean concentrations (mg/L) of each anion were Br−, 4.69 ± 0.96; NO3 −, 0.80 ± 0.68; HPO4 2−, 17.48 ± 7.60; and SO4 2−, 3.34 ± 2.54. As for SO4 2−, no sex differences or diurnal variations were observed. Conclusions: We established an efficient protocol to quantitate various inorganic anions involved in a small amount of TF using a commercially available instrument. This is the first step to elucidate the role of anions in TF.