Application of Chloride Adduct Ionization Tandem Mass Spectrometry for Characterizing and Sequencing Synthetic Lignin Model Compounds

The need for renewable bioenergy sources has renewed interest in lignin chemistry; however, structural elucidation and characterization of lignin degradation products remain a challenge because of lack of effective analytical methods. The analysis of lignin oligomers has been accomplished by simple deprotonation of weakly acidic phenolic moieties using NaOH and analyzed in a negative ESI mass spectrometry. Although simple deprotonation works to produce excellent results for many types of lignin compounds, others can undergo extensive in-source fragmentation for certain bond types making structural elucidation more complicated. Herein, we present an alternative method for analyzing lignin model compounds using chloride adduct chemistry. In this study, nine β-O-4 dimers, an (4-Ο-α)(β-Ο-4) trimer, and a (β-O-4)(β-O-4) trimer were synthesized and analyzed using chloride adduct mass spectrometry in the negative mode using NH₄Cl as the chloride source. Stable chloride adducted molecular ions were observed for all analyzed compounds. Tandem mass spectrometry experiments performed on each precursor ion produced "signature" fragment ions specific to each analyte. The compelling features of this method include the production of stable chloride adduct molecular ions that do not undergo in-source fragmentation, in contrast to simple deprotonation methods that can lead to extensive fragmentation for some structures, the appearance of the chlorine isotope pattern for enhanced recognition of molecular ions, and production of monolignol sequence specific fragment ions using tandem mass spectrometry.