Introduction - Paper Spray (PS) mass spectrometry (MS) has become an important tool in the analysis of complex and unpurified mixtures - PS-MS is ideal for use with portable mass spectrometers because nebulizer gases are not required, but there are still some aspects of the PS experiment that need improvement - For example, the use of high DC voltage (>3kV) to in PS-MS to generate stable ion signal poses safety concerns - High wicking rates generated at high voltage conditions causes the time for analysis to be narrow and requires large volume of the spray solvent (>10 μL) - A new hydrophobic paper spray ionization method is developed that allows the use of low spray voltages (0.3 kV). No spray solvent is required Approach Significance - Reduction in spray voltage and solvent during PS-MS is important in the creation of miniature, handheld mass spectrometers for on-the-spot chemical - Decrease in power and other resources needed for mass analysis means a reduction in battery size, cost and weight; all allowing for easy, widespread use of miniature mass spectrometers - Longer window of analysis is essential for accurate chemical characterization, especially in complex mixtures where extra time is needed for tandem MS (MS/MS) Experimental Procedure Mechanistic Consideration - The mechanism of hydrophobic paper spray ionization is not clear, but we rule out the possibility of inlet ionization - The application of high DC voltage is essential in the ionization process (Figure 5) - Ions are formed possibly via a typical electrospray mechanism - Dry paper conducts electricity through an electron tunneling process Summary - The use of hydrophobic paper decreased the adhesion forces between the sample being analyzed and the paper by forming a droplet - New capabilities include controllable wicking rate, and lower limit of detection - Graphite pencil line provides a simple means to increase conductivity of a non-wetting hydrophobic paper Acknowledgement This research was supported by The Ohio State University start-up funds References 1. Badu-Tawiah, A.K., Thou M.M., Whitesides, M.G. US Patent 2013 2. Wang, H., Liu, J., Cooks, R.G., Ouyang, Z. Angew. Chem. Int. Ed. 2010, 49(5), 877-80 Hydrophobic Paper Spray Ionization Mass Spectrometry Kathryn M. Davis and Abraham Badu-Tawiah* Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210 Results and Discussion We hypothesized that by using hydrophobic paper: - Adhesive forces between paper and analyte, due to entrapment of analyte in paper pores, can be minimized to allow the use of low voltages for sample desorption and ionization - Wicking rates can be minimalized to allow smaller volume of liquid to be analyzed over a longer period of time. Lower Spray Voltage: Conductivity of hydrophobic paper was increased by applying a thin line of graphite pencil on its surface The graphite line creates less electrical resistance, and enables the use of 0.3 kV DC spray voltage ( Figure 4 ) Untreated Paper Treated Paper - Filter paper was cut into triangles using a laser cutter, and was then treated with trichloro(3,3,3-trifluropropyl) silane vapor under vacuum (~20 torr) to create a thin hydrophobic layer only on the surface of the paper (Scheme 1) - Slow salinization (with no heating) over a period of four hours ensures that only the exposed surface hydroxyl (OH) groups were derivatized, while most of the internal OH groups involved in intermolecular hydrogen bonding within the fiber core were left unaltered RSiCl 3 -H 2 O - HCl Figures of Merit: Hydrophobic surface allows the formation of a droplet that sits on top of the paper surface This enables (1) lower limit of detection ( Figure 3 ), lower wicking rate (2), and (3) longer time frame for analysis ( Figure 1 ) for a smaller volume of the target mixture Scheme 1 50 100 (A) 100% = 1.53E6 0 0.3 0.6 0.9 1.2 1.5 1.8 Time (min) 0 50 100 (B) 100% = 1.54E8 0 Relative Abundance Figure 1. Extracted ion chromatogram for methamphetamine (m/z 150) obtained from paper spray ionization at 3 kV using (A) untreated hydrophilic paper (B) hydrophobic paper (A) Time = 0 – 0.8 minutes; 100% = 2.17E5 70 120 170 220 270 0 50 100 Relative Abundance 245 150 157 87 (B) Time = 0 – 0.8 minutes; 100% = 3.70E7 70 120 170 220 270 m/z 0 50 100 Relative Abundance 150 91 Figure 2. Mass spectra recorded at 3 kV using (A) conventional paper spray and (B) hydrophobic paper spray. Figures 2A and B were obtained from 0 - 0.8 minute time range of Figures 1A and B, respectively Comparison: Hydrophilic versus hydrophobic paper spray ionization - in terms of analysis time (Figure 1) and signal intensity (Figure 2) when using 2 μL of 5 ppm methamphetamine and 3 kV 80 100 120 140 160 180 m/z 0 50 100 Relative Abundance 119 132 150 91 MW 149 Methamphetamine m/z 150 Figure 3. Tandem MS spectrum for methamphetamine at the detection of 1 ppb in methanol/water (1:1, vol/vol) solution Figure 4. Analysis of 5 ppm methamphetamine at 0.3 kV using hydrophobic paper having a thin graphite line at its surface 100% = 1.05E7 80 120 160 200 240 280 m/z 0 50 100 Relative Abundance 150 91 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Time (min) 0 50 100 Relative Abundance 3 kV 0 kV Figure 5. Total ion chromatogram showing the effect of DC high voltage on hydrophobic paper spray. Signal is observed only after the application of 3 kV