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Mass spectrometry of thermally unstable molecules. Evaluation of ionization techniques using glutamine as a reference compound

1985; Wiley; Volume: 12; Issue: 12 Linguagem: Inglês

10.1002/bms.1200121205

2376-3876

William M. Lagna, Patrick S. Callery,

Electrohydrodynamics and Fluid Dynamics

A limiting factor in the application of mass spectrometry to biochemically important molecules has been the inability to analyze compounds without inducing thermal decomposition. Glutamine, which readily cyclizes to 2-pyrrolidone-5-carboxylic acid with loss of ammonia, is one of the more difficult of these non-volatile, thermolabile biomolecules to determine. Using a heated, direct probe, no molecular ion for glutamine was observed with electron impact ionization. A protonated molecular ion was detected with direct probe chemical ionization and thermospray. In both cases, the major ion formed was derived from the thermolysis product, 2-pyrrolidone-5-carboxylic acid. Analysis by fast atom bombardment and 252Cf plasma desorption mass spectrometry yielded a molecular ion as base peak with a small ion from the 2-pyrrolidone-5-carboxylic acid. Ion evaporation produced only the molecular cations, and collisionally activated dissociation demonstrated that glutamine does not cyclize appreciably once protonated. These results suggest that glutamine can be used as a reference compound when analysis requires the optimization of conditions to produce a molecular ion. Furthermore, the relative intensity of the pyrolytic products, 2-pyrrolidone-5-carboxylic acid derived molecules, divided by the relative intensity of the molecular ions of glutamine provides a numerical evaluation of ionization conditions.