Lactic acid is one of most studied analytes. Its concentration is an important parameter in clinical diagnostics and sport medicine [1]. Various methods using different types of sensors based on lactate oxidase (LOD) have been proposed for the determination of lactate concentration in solutions.
Increasing demand for non-invasive methods for metabolites determination in human fluids resulted in extensive research of exhaled breath condensate (EBC) [2]. However, some components of EBC may affect the results of the lactate determination.
Lactate biosensors based on screen-printed electrodes modified with Prussian blue were developed [3]. LOD was immobilized on the surface of working electrodes into polymer membrane containing high percentage of organic solvent. The biosensor was used for the lactate determination in flow injection analysis (FIA) techniques. It was showed that lactate could be measured over a dynamic range of 2-1000 µM which was linear up to 200 µM. The highest corresponding sensitivity was 0.17 A/M•cm². The lower detection limit was 0.8 µM. The response time was 30 seconds. The obtained results of the lactate determination in real objects were comparable to the measurements performed with automated Biosen C-line (Germany).
The developed lactate biosensor was used in FIA techniques with preliminary concentration. The ion-exchange column was filled with silica phase with quaternary amine groups providing a strong anion-exchange mechanism of retention. Thus the influence of hydrogen peroxide is eliminated. Moreover, the sensitivity went up by a factor of fifty and the highest one was 8.1 A/M•cm². The lower detection limit was 0.02 µM.
The developed electrochemical system was applied to the lactate determination in EBC (120 samples) collected from healthy patients before and after physical exercise as well as from patients suffering from different pulmonological diseases. The increasing of lactate concentration in EBC during physical exercise was shown.

Acknowledgments
The authors are grateful to the Research Institute for Pulmonology and the Research Institute for Physical Culture and Sport for providing the samples of EBC. Also we acknowledge the financial support from Russian Ministry of Science and Education, project №14.740.11.1374.

References
1. Sakharov D. A., Shkurnikov M. U., Vagin M. Yu., Yashina E. I., Karyakin A. A., Tonevitsky A. G. Relationship between lactate concentrations in active muscle sweat and whole blood. // Bulletin of Experimental Biology and Medicine, 24 Nov 2010, pp. 1-3.
2. Gessner C., Wirtz H. Continuous exhaled breath condensate measurements in ventilated patients. // Resp. Care. 2006, V. 51, pp. 1118-1119.
3. Yashina E.I., Borisova A.V., Karyakina E.E., Shchegolikhina O.I., Vagin M.Yu., Sakharov D.A., Tonevitsky A.G., Karyakin A.A. Sol-gel immobilization of lactate oxidase from organic solvent: toward the advanced lactate biosensor. // Anal. Chem. 2010, 82, pp. 1601-1604.

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