First, I exploited previous observations that neutral loss of ribose occurs from UVcrosslinked RNA nucleosides . Fragpipe open search with diagnostic ion mining followed by inspection of fragment ions (b, y) for ribose loss unraveled two additional UVC-crosslinked RNA moieties (Table ). One RNA adduct mass corresponded to guanosine with ammonia loss, while the other could be either cytosine with ammonia loss or uridine with water loss.