CID and CIU

CID stands for collision-induced-dissociation a common fragmentation technique to create tandem MS spectra (MS2). In CID precursor ions are accelerated and subsequently injected into a ion guide filled with inert neutral gas molecules, typically N2, where the analyte of interest undergoes single or multiple collisions. The charge of the analyte, the accelerating (collisional) voltage and the gas pressure can determine the extent of collisional Impact.

Ions are vibrational excited by the collision(s) with a time frame of a few femtoseconds. Depending on the chemical bonds present the ions can break apart into charged, radical or neutral fragments. In proteomics weaker bonds, such a post-translational modification, tend to get lost during CID.

In beam type instruments a special version of CID, In Source CID, can facilitate MS3 fragmentation for deeper structural elucidation. Herein, ion guides on the front end of the MS serve as a 2^nd collision device for fragmentation of all ion species injected. Out of this very complex MS2 spectrum precursors can be selected for an MS3 in the actual collision cell downstream.  

When it comes to analysis of intact proteins and protein complexes having high molecular weights, In-Source CID at elevated pressure, can be applied but in most cases it directly leads to dissociation. Secondary or high order protein structures cannot be detected easily.

Besides the dissociative nature of CID for large biomolecules there is a much elegant appoarch that utilizes collisional activation to study structures intactly. It is called CIU, which stands for collisional induced unfolding, and can be conducted with an ion mobility detector coupled to an MS.

During a CIU experiment intact biomolecules, such as antibodies, are ionzied and undergo a stepwise increase of collision heating which induces a gradual change in conformation (unfolding).

For every stepped potential an ion mobility scan, plus the nested MS scans, are recorded so that one can follow the structural changes. These multidimensional datasets (see image) help to distinguish isoforms, determine number for disulfide bonds and degree of modifications or monitor ligand binding.

Source: https://www.sciencedirect.com/science/article/pii/S1367593117301266?via%3Dihub#fig0010