Ion-Selective Optodes in a Sampling Capillary for Tear Fluid Analysis


Plasticized PVC membrane-based potassium or sodium ion-selective optodes were incorporated into sampling capillaries in combination with sol-gel-based hydrogen ion-selective optodes for the sampling and analysis of minute tear fluid samples. The performance characteristic of the K +/pH and Na +/pH sensor arrays were optimized for an adequate dynamic response range, sensitivity, stability, and response time. The precision and accuracy of the combined sampling/measurement system was evaluated both in stopped flow and continuous flow modes using aqueous standard solutions. By using a two point calibration protocol, with pH standards bracketing the sample, the pH of 20μL pooled human tear samples could be determined with a precision less than ±0.03pH units. Errors in the pH measurements have a significant influence on the attainable precision and accuracy of the ion-exchange-based K + and Na + optodes. Nevertheless, the results of the quantitative assessment of the K + and Na + concentrations in "check" standards and pooled human tear fluid samples were within ±5% of the nominal values or of the results of the determinations with atomic absorption spectrophotometry. Cationic surfactants in artificial tear drops were found to interfere with the K + and Na + optode responses. No interference was detected in the presence of anionic and nonionic surfactants. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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