A new method for the simultaneous determination of two tumour biomarkers, homovanillic (HVA) and vanillylmandelic acid (VMA), using flow injection analysis (FIA) with amperometric detection (AD) at a commercially available boron doped diamond electrode (BDDE) was developed. It was found that this method is suitable for the determination of HVA (in the presence of VMA) and VMA (in the presence of HVA) in optimum medium of Britton-Robinson buffer (0.04 mol L-1, pH 3.0). Calibration dependences consist of two linear parts for both biomarkers, the first one being in the concentration range from 1 to 10 μmol L-1 and the second one from 10 to 100 μmol L-1 (with obtained LODs 0.44 μmol L-1 for HVA and 0.34 μmol L-1 for VMA, respectively). To minimize any negative effects related to the passivation of the working electrode, suitable cleaning pulses (+2.4 V for 30 s) were imposed on the working electrode after each measurement. An attempt to use FIA with multiple pulse amperometric detection to determine both analytes in one run was not successful. Changing potentials in short intervals in multiple pulse detection probably results in mutual interaction of analytes and/or products of their electrochemical oxidation, thus preventing the application of this approach.

Charles University Faculty of Science Department of Analytical Chemistry UNESCO Laboratory of Environmental Electrochemistry Hlavova 2030 8 128 43 Prague 2 Czech Republic

Charles University Faculty of Science Department of Analytical Chemistry UNESCO Laboratory of Environmental Electrochemistry Hlavova 2030 8 128 43 Prague 2 Czech Republic; J Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences Dolejškova 2155 3 182 23 Prague 8 Czech Republic

Institute of Physics of the Czech Academy of Sciences Cukrovarnická 10 162 00 Prague 6 Czech Republic

J Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences Dolejškova 2155 3 182 23 Prague 8 Czech Republic

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