Improving liquid chromatography-mass spectrometry sensitivity for characterization of lignin oligomers and phenolic compounds using acetic acid as a mobile phase additive

Characterization of complex lignin degradation products by liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) requires a high sensitivity, which can be improved by use of optimized mobile phase additive. In this study, chromatographic separation and ionization efficiency for LC-MS analysis of lignin oligomers and lignin-derived phenolic compounds were compared among six different mobile phase additives. We found that acetic acid exhibits analytical performance superior to the other mobile phase additives for characterization of lignin degradation products by LC-(-)-ESI-MS/MS. The retention of phenolic compounds (PCs) containing carboxylic group was improved by using acidic additives instead of their ammonium salts. The use of acetic acid as an LC mobile phase additive enhanced the ionization efficiency of PCs in (-)-ESI-MS about 2 to 4 times higher than the other additives. The improved MS sensitivity with acetic acid yielded a higher coverage of fragment ions in LC-MS/MS analysis, leading to identification of four additional synthetic G-type lignin oligomers (LOs) than ammonium acetate. A similar degree of sensitivity enhancement was also demonstrated for degradation products from rice straw lignin using acetic acid additive over the others. The increased chromatographic retention and enhanced sensitivity of PCs and LOs provided by the use of acetic acid as an LC-(-)-ESI-MS mobile phase additive will facilitate structural analysis of lignin degradation products without relying on additional instrumentation.