Ionization involves adding or removing an electron or proton from a molecule, giving it a recognizable charge. This requires a type of energy jolt where molecules break apart, and free electrons or protons attach to others, putting them in an ionized state. Only then, the mass spectrometer can detect the analytes. When it comes to pharmaceutical testing, Liquid-Chromatography Mass Spectrometry (LC-MS) with Electrospray Ionization (ESI) is most commonly the method of choice. This method is especially suitable for ionizing solvable, polar substances (e.g. active ingredients in medications for hypertension). However, many chemical compounds, such as the volatile nitrosamine NDMA, cannot be ionized by ESI due to their volatile nature. The Atmospheric Pressure Chemical Ionization (APCI) is typically used to ionize these substances, as it is well-suited for the task.