Distribution measurements of 3,4-methylenedioxymethamphetamine and its metabolites in organs by matrix-assisted laser desorptionionization imaging mass spectrometry using an automatic matrix spraying system with an air brush and a turntable.
Matrix-assisted laser desorptionionization imaging mass spectrometry (MALDIIMS) is a useful tool for measuring drug distributions. To obtain reproducible analytical results with MALDIIMS, it is essential to apply a homogeneous matrix coating onto sample surfaces. A simple and inexpensive automatic matrix spraying system (AMSS) with good reproducibility was developed in this study. In addition, drug distributions in organs were measured by MALDIIMS using the AMSS for forensic toxicology applications. The AMSS was constructed from simple components, including an air brush, a turntable, and a microscope. Organ slices placed onto conductive sheets were attached to the turntable. The trigger of the air brush was held with a clamp to ensure that it sprayed continuously onto a defined area of the table. Periodic spraying of the matrix solution and evaporation of solvent were performed by rotating the turntable. The droplets and crystals on the sample surfaces were observed under a microscope attached to the turntable. The droplet size, rotation rate of the turntable, and the formulation of the matrix solution were optimized. The homogeneity of the matrix coating was evaluated using the coefficients of variation (CV) obtained by quantifying the color density of the sheet surface. The AMSS enabled more homogeneous matrix coating (intersheet CV 5.4 %) than manual spraying (intersheet CV 16.7 %) when 10 mL of 0.5 % aqueous trifluoroacetic acidacetonitrile (13, vv) containing 10 mgmL α-cyano-4-hydroxycinnamic acid were sprayed as droplets less than 50 μm in diameter onto a turntable rotating at 30 rpm. The distributions of 3,4-methylenedioxymethamphetamine (MDMA) and its main metabolites in the brain, liver, and kidney of a mouse that died from an MDMA overdose (58 mgkg i.p.) were visualized by MALDIIMS using the AMSS. The ion intensities of MDMA obtained from the same regions on three sequential kidney slices showed acceptable variations (CV 2.9-8.8 % for five different regions), implying repeatable measurements with MALDIIMS using the AMSS. It was revealed that MDMA was particularly concentrated around the brain stem and the major calix of the kidney. The AMSS would be suitable for preparing samples for measuring the distributions of drugs in organs at toxic dose levels in forensic toxicological applications.