Abstract
| I. | Introduction | 88 |
| II. | Lignin Paradigm | 88 |
| | A. Occurrence and Formation of Lignin | 88 |
| | B. Structural Models of Lignin | 89 |
| | C. Isolation and Chemical–Physical Properties of Lignin | 90 |
| | D. In Vitro Biosynthetic Studies | 92 |
| | E. Structural Elucidation of Lignins | 94 |
| III. | MS in the structure elucidation of degraded lignins | 94 |
| | A. Thermal Degradation of Lignins Combined with MS | 94 |
| | 1. Analytical py‐EI‐MS in the Study of Lignins | 95 |
| | 2. Py of Lignin in Conjunction with Soft‐Ionization Mass Spectrometric Techniques | 97 |
| | a. Chemical Ionization (CI)‐MS | 97 |
| | b. Photoionization (PI)‐MS | 97 |
| | c. Single‐Photon Ionization (SPI)‐Molecular‐Beam (MB)‐MS | 97 |
| | d. Fast‐Atom Bombardment (FAB)‐MS | 99 |
| | e. Resonance‐Enhanced Multiphoton Ionization (REMPI)‐MS | 99 |
| | f. FI‐MS | 100 |
| | 3. Thermal Degradation of Lignins with Slow Heating Rates Combined with MS | 101 |
| | 4. Natural Burning of Wood and GC‐MS Analysis | 102 |
| | B. Chemical Degradation of Lignins Combined with MS | 102 |
| | 1. Nitrobenzene Oxidation and CuO Oxidation | 102 |
| | 2. Permanganate Oxidation | 104 |
| | 3. Hydrogenolysis | 104 |
| | 4. Acidolysis | 105 |
| | 5. Thioacidolysis | 105 |
| | 6. Derivatization Followed by Reductive Cleavage (DFRC) | 107 |
| | 7. Ozonolysis | 108 |
| | 8. Photochemically Induced Degradation | 110 |
| | 9. Singlet Oxygen Degradation | 110 |
| | C. Thermochemolysis with Tetramethylammonium Hydroxide (TMAH) of Lignins Combined with MS | 110 |
| | D. Mass Spectrometric Investigation of Lignin Degradation Products Deriving from Fungal and Microbial Activity | 111 |
| IV. | Studies of DHPs by py‐MS | 114 |
| V. | Soft‐ionization MS as a Powerful Tool in the Study of Undegraded Lignin Macromolecules | 115 |
| | A. MALDI‐MS | 115 |
| | B. Thermospray MS | 117 |
| | C. ESI‐MS | 117 |
| Acknowledgments | 119 |
| Abbreviations | 119 |
| References | 120 |
Lignin, a resistant cell‐wall constituent of all vascular plants that consists of ether and carbon‐linked methoxyphenols, is still far from being structurally described in detail. The main problem in its structural elucidation is the difficulty of isolating lignin from other wood components without damaging lignin itself. Furthermore, the high number and variegated forms of linkages that occur between the monomeric units and the chemical resistance of certain ether bonds limit the extent to which analytical and degradation procedures can be used to elucidate the lignin structure. Most of our present knowledge about the molecular structure of lignin is based on the analysis of monomers, dimers or, at the most, tetramers of degraded isolated lignins. Mass spectrometry (MS), which offers advantages in terms of speed, specificity, and sensitivity, has revealed to be a very powerful technique in the structural elucidation of lignins, in combination with the great number of chemical and thermal degradation methods available in the study of lignin. Moreover, the recent development of new ionization techniques in MS—electrospray ionization (ESI)‐MS and matrix‐assisted laser desorption/ionization (MALDI)‐MS—has provided new possibilities to also analyze the undegraded lignin macromolecule. © 2003 Wiley Periodicals, Inc., Mass Spec Rev 23:87–126, 2004.