Identification of lignin oligomers in Kraft lignin using ultra-high-performance liquid chromatography/high-resolution multiple-stage tandem mass spectrometry (UHPLC/HRMSn)

Abstract

Kraft lignin is the main source of technically produced lignin. For the development of valuable products based on Kraft lignin, its molecular structure is important. However, the chemical composition of Kraft lignin is still not well known. So far, the analysis of Kraft lignin by mass spectrometry (MS) has been mainly focused on monomeric compounds. Previous MS studies on lignin oligomers (LOs) considered only synthesised LO standards and/or lignins produced by processes other than the Kraft process. Furthermore, published MS methods suffer from using high resolution only in the MS1 stage in multiple-stage tandem MS methods. A high resolution in all MSn stages would provide more detailed information about LO fragmentation pathways. Since lignin samples are complex mixtures of a large number of similar phenolic compounds, the selection of tentative LOs in the MS data is challenging. In this study, we present a method for non-targeted analysis of LOs in Kraft lignin using ultra-high-performance liquid chromatography/high-resolution multiple-stage tandem mass spectrometry (UHPLC/HRMSn). A pre-selection strategy for LOs has been established based on a data-dependent neutral loss MS3 method in combination with a principal component analysis-quadratic discriminant analysis classification model (PCA-QDA). The method was optimised using a design of experiments (DOE) approach. The developed approach improved the pre-selection of tentative LOs in complex mixtures. From 587 detected peaks, 36 peaks were identified as LOs. Graphical abstractᅟ Electronic supplementary materialThe online version of this article (10.1007/s00216-018-1400-4) contains supplementary material, which is available to authorized users.

Nontargeted Analysis Strategy for the Identification of Phenolic Compounds in Complex Technical Lignin Samples

“…At the MS 2 stage, only less informative losses of methylr adicals and water molecules were detected.F or compounds of particular interest, targeted MS n experiments could be performed for more detailed structurale lucidation. [26]…”

“…The settings for the UHPSFC-HRMS method were based on two slightly modified methods previously developed by our group. [19,26] AD IOL column was used for sample separation, with ac olumn temperature of 50 8C, af low rate of 2mLmin À1 ,aback pressure of 130 bar,a nd an injection volume of 1.5 mL. After each injection, the injection syringe and injection needle were washed with acetonitrile/water (1:1 v/v,4mL) followed by methanol (4 mL).…”

“…[25] In our laboratory,w eh ave developed am ethod based on UHPLC-ESI-HRMS n for the identification of LOs in aK raft lignin sample and tentatives tructures for lignin dimers, trimers,a nd tetramers were proposed. [26] To our knowledge,U HPSFCh as to date only been used in our laboratory for the analysiso fL Ms in lignin samples. [19,27,28] We have shown that for LMs, UHPSFC provides good selectivity and chromatographic resolution for4 0l ignin-related phenolic model compounds in less than 6min.…”

“…We recently introduced a nontargeted analysism ethod for LOs using UHPLC-ESI-HRMS n in combination with ap re-selection strategy using data-dependent neutral loss MS 3 experimentsc ombined with ap rincipal component analysis-quadratic discriminant analysis (PCA-QDA) classification model. [26] With this method, tentative chemical structures for lignin dimers, trimers and tetramers were proposed.H owever,t oo ur knowledge,anontargeted analysis methodu sing chromatographic separation in combination with HRMS n covering LMs and LOs highert han dimers are currently lacking. Owen et al showed that HPLC-ESI-HRMS n can be used fori dentification of unknown lignin dimers and proposed at entatives tructure for one unknown dimer.…”

Top‐down lignomics analysis of the French pine lignin by atmospheric pressure photoionization quadrupole time‐of‐flight tandem mass spectrometry: Identification of a novel series of lignin–carbohydrate complexes

“…Lastly, it should be noted that the DBE values and C/O ratios for the identified formulae were consistent with the values reported previously in the literature. 29,31 Typically, lignin monomers possess the following values: DBE ≥4 and C/O ratio ≥2.5, while lignin dimers possess: DBE ≥8 and C/O ratio ≥2.5. 29,31 For a lignin carbohydrate dimer ion, typical values are DBE ≤6 and C/O ratio ≤ 2, which indicates that replacing a lignin monomer with a carbohydrate ring leads to a lower DBE and a lower C/O ratio.…”