In tandem MS, the ion selected for fragmentation is referred to as what?

In tandem MS (mass spectrometry), the ion selected for fragmentation is referred to as the precursor ion or parent ion. The precursor ion is the original ion that is chosen based on its mass-to-charge ratio (m/z) and is subjected to further fragmentation to produce smaller ions, known as fragment ions. This process allows for the analysis of the structure of the precursor ion by studying the resulting fragments.

Selecting the appropriate precursor ion is crucial in tandem mass spectrometry because it determines the relevant fragmentation pathways that will be explored. Once the precursor ion is fragmented, the resulting ions can then be analyzed to provide insight into the molecular structure or composition of the initial molecule. Understanding this process is foundational in mass spectrometry, particularly in the context of identifying and characterizing complex biological molecules.