A.S. Borodkov - Ph. D. (Chim.), Research Scientist, «Energomashtekhnika» Ltd; Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences, Moscow, Russia E-mail: abors@mail.ru I.I. Kuzmin - Junior Research Scientist, «Energomashtekhnika» Ltd; Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences, Moscow, Russia E-mail: mclaren.ilya@gmail.com A.A. Grechnikov - Ph. D. (Chim.), Head of laboratory, «Energomashtekhnika» Ltd; Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences, Moscow, Russia Email: agrech@bk.ru Ya.O. Simanovsky - Ph.D. (Phys.-Math.), Senior Research Scientist, A.M. Prokhorov General Physics Institute of Russian Academy of Sciences, Moscow, Russia E-mail: yasimanovskiy@mail.ru

The detection of biologically active metal coordination compounds by laser-induced electron transfer desorption/ionization (LETDI) in negative ionization mode was investigated. In LETDI, the analyte is deposited on the surface of a specially-designed solid substrate, followed by the exposure of the surface to pulsed laser radiation. The effect of irradiation produces molecular ions of the analytes, which are then desorbed from the surface and detected in a mass spectrometer. Previously reported results were obtained with the chemical compounds, which are detected in positive ionization mode of LETDI. In this paper, LETDI technique was tested for the production of negatively charged ions. Cisplatin (diamminedichloroplatinum, a widely used anticancer drug), lutetium complex with crown ether- substituted phthalocyanine and copper complex with dithizone (a well-known organic reagent for the extraction and concentration of metal ions) were used as test analytes. The forth harmonic of the radiation of the Nd:YAG laser (the wavelength 263 nm, the pulse duration 4.5 ns) was used for laser desorption/ionization. In order to find the optimal parameters of laser system, the analyte ion yields were measured as a function of laser fluence for various solid substrates, including stainless steel, graphite, silicon and titanium dioxide. The comparison of these substrates at optimal laser fluencies showed that silicon substrates possessed the best ionization properties. For all studied metal complexes, mass spectra were recorded and identified in negative ionization mode. Mass spectra are dominated by the molecular ions; however, the degree of fragmentation is higher as compared with positive ionization mode. The observed isotope distributions for the molecular ions coincide with the calculated distributions with a good accuracy. Calibration curves were constructed as a function of the total ion signal (including fragment ions and adduct ions) on the actual amount of analyte deposited onto the silicon substrate. Calibration curves were found to be linear in the range of at least two orders of magnitude with limits of detection of studied metal complexes in the range of 1 - 25 pg. It is shown that negative ionization mode of LETDI allows for an expansion of the range of compounds determined by laser desorption/ionization.

 

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