A Quadrupole Mass Spectrometer is an analytical instrument that uses an electric field and a magnetic field to measure the mass-to-charge ratio of ions in a sample. It consists of four parallel rods, or "poles", that form a quadrupole electric field. When an ion passes through the field, it is subjected to forces that cause its trajectory to be deflected and focused in the direction of the detector. The detector then measures the ion's mass-to-charge ratio and displays the result. This technique is used for a variety of applications, such as the analysis of proteins, proteins, and small molecules.