The Single-probe SCMS technique has been used for quantification of anticancer drugs (both amounts and concentrations) in live individual cells under ambient conditions . Due to its unique design, the internal standard can be added into the sampling solvent (e.g., acetonitrile) at a known concentration. The internal standard can be an isotopically labeled compound or species with the structure highly similar to the target compound. When performing quantitative SCMS measurements of drug-treated cells, the Single-probe tip is inserted into a single living cell to extract intracellular chemicals (including drug molecules). Both the internal standard and drug molecules are simultaneously delivered to the nanoESI emitter for ionization and detected by MS. Multiple factors (e.g., the ion intensities of the drug and internal standard, internal standard's concentration and flow rate, and data acquisition time) needed to be considered for the quantification. If the isotopically labeled analog is not available, the internal standard can be selected from the species with a structure similar to the target compound, whereas a calibration curve needs to be established. The quantitative SCMS technique makes it possible to accurately estimate the amounts of drugs in individual cells, offering insights into how individual cells metabolize and retain therapeutic agents.