Matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) postsource decay (PSD) mass spectrum of oligonucleotides of identical nucleotide composition. Spectrum A is of a 6-mer of sequence CATCCT; spectrum B, a 6-mer of sequence CACCTT. Both have parent ion mass of 1727.2 Da. Observed masses are shown above the peaks. Fragments of PSD are shown at an intensity magnification of five.