An Absolute Quantitative Approach to Study the Desorption Step in Plasma-Based Ambient MS Methods

The desorption step in ambient mass spectrometry, concerted or decoupled with ionization, triggers the transfer of a sample (analytes) from the condensed phase or surface to the gas phase. Depending on the type of method, the desorption is caused by momentum transfer, ultrasound, thermal energy, or laser pulses, among other means. In the case of plasma-based methods, thermally assisted desorption is the most commonly discussed route for analyte desorption, and although often postulated, there is no clear evidence of other mechanisms related to high-energy species created in the discharge. This study addresses the assessment of a protocol to allow absolute quantification of the desorption step during plasma-based ambient MS experiments. As a proof of principle, we measured the desorption efficiency of low-temperature plasma (LTP), which is the more widespread DBD-based ambient MS method. Model analytes such as arginine, cocaine, rhodamine G, or imazalil have been selected to quantify the desorption efficiency using 20 ng in each experiment. Two microliters of analyte solution were deposited on a glass substrate (18 mm × 18 mm) and allowed to dry. Then, they were exposed to different LTP plasma conditions (discharge gas, probe position, and desorption time). After the sample substrate was redissolved with an appropriate solvent, quantitative data were obtained using liquid chromatography/tandem mass spectrometry. Selected experiments have been completed, demonstrating the ability to quantitatively measure the amount of analyte desorbed with high precision (RSD ≤ 7%), finding subtle changes (in the absolute picomole range) when different variables such as discharge gas nature or exposure time were evaluated. Through the use of spatially resolved fluorescence microscopy measurements, we also noticed that analyte deposition is not evenly distributed on the substrate. This evidence, together with the experimental quantitative data, confirms that the conditions used to quantify the desorption are also rugged to experimental aspects such as sample deposition, since the analyte spot size interrogated (1 mm diameter) is distinctly smaller than the LTP probe diameter (4 mm i.d.) and the plasmajet area that impinges the entire sample surface. Analyte-analyte interactions and sample thickness may also be relevant in explaining the desorption in plasma-based ambient MS methods.