An amount of 0.5L of the sample solution was mixed on the target with 2L of a solution of 2,5-dihydroxybenzoic acid as a matrix (15 mg/mL) in 30% aqueous ethanol. Mass spectra were recorded on a Bruker Ultraflex (Bruker Daltonik, Bremen, Germany) in the positive ion mode. A nitrogen laser (337 nm, 3 ns pulse width, 3 Hz) was used. All spectra were measured in the reflector mode using external calibration.