Identifying fragments using a Neutral Loss spectrum

May 21, 2008

by Arvin Moser, Team Manager, Application Scientists, ACD/Labs

A calculated neutral loss spectrum is obtained from a mass spectrum by determining the mass differences between the precursor ion m/z and each of the other peaks in the spectrum and plotting the original intensity versus neutral mass. Neutral losses with small masses have limited possibilities for their composition and thus can facilitate the identification of specific species.

Common losses (based on the J.H. Beynon table) for fragments with C, H, O, N elements are reported below:

OH, NH3                                                     nominal mass of 17

CO, N2 C2H4, CH2N                                    nominal mass of 28

CHO2, CH3NO, CH5N2, C2H7N, C2H5O        nominal mass of 45

A positive ion EI mass spectrum of benzoic acid is shown below. The molecular ion (M+’) appears at an m/z of 122. Two additional ion clusters appear at m/z of 77 and 105. The Beynon table reports 4 and 13 possible fragments for m/z 77 and 105, respectively. Examining the neutral loss spectrum, shown below, 3 basic ion clusters appear: m/z 17, 28 and 45. The Beynon Table (listed above) reports 2, 4 and 5 possible fragments for m/z 17, 28 and 45, respectively. When working with fewer fragment possibilities, one can reduce the time spent on an elucidation problem.

TIP: Knowing a partial fragment for an unknown can aid in narrowing down a molecular formula and limit the number of candidate structures.

Spectra courtesy of Joe DiMartino, M.Sc.