Microanalysis of Lung Airway Surface Fluid by Capillary Electrophoresis with Conductivity Detection

The thin layer of fluid that covers the surface of the epithelia lining the conducting airways plays an important role in primary pulmonary defense, and its composition may be a critical factor in the pathogenesis of several lung diseases including cystic fibrosis. Despite its physiological importance, the composition of airway surface fluid (ASF) is poorly understood due to considerable difficulties in sample collection from the 5-30 μm thick layer and subsequent analysis. We have used a novel technique for sample collection and microanalysis of ASF (nanoliter sample required) by capillary electrophoresis with conductivity detection. Limitations on the diameters of capillary required for the sample injection process and for the conductivity detector require the use of coupled separation capillaries with different external diameters. Two different methods were used to construct a butt-joint coupling for capillaries of different outer diameters. Reasonable efficiency is observed with the coupled capillaries (N = 100 000 plates m^-1) compared to an unbroken single capillary (N = 180 000 plates m^-1). The use of conductivity detection allows greater flexibility in method development and the possibility of determining a greater variety of ions than with a previous indirect-UV method. In the present study, we describe the analysis of cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and anions (Cl^-, NO₂^-, NO₃^-, SO₄^2-, PO₄^2-, HCO₃^-) in rat ASF. Particular attention was paid to developing washing procedures which limited fouling of the conductivity sensor. In healthy rats, ASF was found to be hypotonic compared to plasma levels, consistent with some observations made in human airways.