PSI-MS

All types of paper were cut into an isosceles triangle with one apex at 30° and a height of 10 mm. The triangular papers were washed three times with methanol for 5 min in an ultrasonic cleaner, and were ready to use after solvent evaporation in a fume hood at room temperature. The mass spectrometric experiments were conducted using an LTQ Velos mass spectrometer (Thermo Scientific, San Jose, CA, United States). Optimization of collision energy and selecting product ions for each compound were performed using the LTQ Tune plus software. The data from other experiments were recorded in both full scan and selected reaction monitoring modes using the Xcalibur software with the MS/MS transition information provided in Supplementary Table S1. The triangular paper was held by a copper clip on a position-adjustable platform placed in front of the mass spectrometer inlet. A high voltage was applied through the copper clip. Thereafter, 1% formic acid in ethyl acetate as a spray solvent was continuously delivered by a solvent capillary connected to a syringe pump of the LTQ Velos spectrometer. Ethyl acetate was chosen as a common solvent for both hydrophilic and hydrophobic papers in PSI based on its substrate wettability, ionization efficiency, and solubility of the target analytes. In addition, 1% of formic acid was used as an additive to enhance the protonation of analytes. A Dino-lite digital microscope was set up to record all the spray plumes of PSI from either the top view or side view (Supplementary Figure S1). For each condition, four replicates were run.