Leidenfrost phenomenon-assisted thermal desorption (LPTD) using a metal substrate coated with a fluoro compound: Mechanism of analyte evaporation

In our previous work, direct analyses of low-volatility compounds have been performed using Leidenfrost phenomenon-assisted thermal desorption (LPTD). LPTD is affected by several factors such as substrate temperature, solvent properties, liquid volume, and surface-activity value of the analyte. In this work, the effects of surface properties of the metal substrate on LPTD were investigated by using a metal substrate with and without coating the surface with a fluoro compound. A 5 μL aliquot of 0.03 ppm of cocaine or morphine in methanol was dropped on the heater at 240 °C and mass spectra were measured as a function of time. The heater surface was polished with an abrasive agent with grit number of #5000 (alumina powder with ∼3 μm in diameter). When a non-coated metal was used for cocaine and morphine, analyte evaporation was observed only after the complete evaporation of methanol. However, when the metal surface was coated with a fluoro compound, evaporation of surface-active morphine accompanied by the rapid evaporation of methanol was observed. This is due to the rapid temperature rise of the droplet at the last moment of Leidenfrost phenomenon resulting in the occurrence of flash evaporation of morphine. This phenomenon was not observed for non-surface active cocaine.