Tandem mass spectrometry: A procedure used in medical laboratories consisting of two mass spectrometers in series connected by a chamber known as a collision cell. The sample to be examined is essentially sorted and weighed in the first mass spectrometer, then broken into pieces in the collision cell, and a piece or pieces sorted and weighed in the second mass spectrometer. Tandem mass spectrometry is used in newborn screening to detect molecules such as amino acids (the building blocks of proteins) and fatty acids. Tandem mass spectrometry may be abbreviated as Tandem MS or MS/MS. See also: Mass spectrometry. See: Mass spectrometry.