A refrigerated CMA microdialysis autosampler (CMA Microdial- ysis AB, Stockholm, Sweden), operating at 5 ◦ C was used for sample injection using a 5 L loop. For cleaning of the injection needle and the loop between runs a wash solution consisting of 50% (v/v) methanol in water was used. For sample loading onto the trap col- umn, mobile phase A was delivered at a flow rate of 10 L/min. Gradient separation was performed with a flow rate of 5 L/min after back flushing the analytes with 3% mobile phase B from the trap column onto the analytical column, using a LC Packings Ultimate capillary HPLC system (Amsterdam, The Netherlands). Mobile phase A consisted of 10 mM ammonium acetate and 4.5 mM acetic acid, mobile phase B consisted of 10 mM ammonium acetate,
4.5 mM acetic acid and 95% acetonitrile (v/v). Analysis was car- ried out with a gradient starting with 3% B increased to 50% B at a rate of 6.5% a minute. After all analytes reached the detector, B was increased to 80% for 3 min to wash the analytical column from hydrophobic compounds, and then decreased to 3% to equilibrate the system for the next analysis run during 10 min. Regeneration of the trap column was performed with mobile phase A for 10 min.