Cell washing by cold ammonium formate (AF) solution. It has been reported AF solution (0.1427 M or 0.9%) is compatible with live cells and has a minimum influence on cell metabolism . This washing step can replace nonvolatile salts (e.g., Na + , K + , and Mg 2+ in culture medium) by volatile AF, significantly reducing matrix effect in SCMS experiment while minimizing alterations of metabolites in live cells. To perform cell washing, 0.9% of AF (w/w, 0.45 g in 50 mL LCMS-graded water (Fisher Chemical, USA)) were prepared and stored at 4°C. Next, 2 mL of cold AF solution was added into each empty well in a 12-well plate. Last, each coverslip containing cells was rapidly rinsed in a well containing AF solutions, and this washing step was repeated for the second time.