Biomass processing: HPLC with ionic liquid for polysaccharides and lignin
Plant biomass extraction
Plant biomass is an important source of polysaccharides like cellulose, as well as lignin, a substance found in the cell walls that binds the cells, fibres and vessels. For efficient extraction of lignin and polysaccharides, ionic liquids have been used with some success, although the elevated temperatures that are required can also induce decomposition.
Both of these substances consist of a mixture of components of different molecular weights, so the extracts will exhibit a molecular weight distribution (MWDs) for each, rather than a single molecular weight. The success of an extraction process and the degree of decomposition will be reflected in the MWDs and a team of Japanese scientists has proposed an HPLC method for their estimation.
Hiroyuki Ohno, Kosuke Kuroda and Yukinobu Fukaya from Tokyo University of Agriculture and Technology and Tatsuhiko Yamada from the Forestry and Forest Products Research Institute, Tsukuba, concentrated on extraction of the polysaccharides and lignins with an ionic liquid followed by HPLC using the same ionic liquid as the mobile phase.
The scheme has the added advantage that no derivatisation is needed for the extraction. As well as speeding up the process, this minimises the losses of low-molecular-weight components that derivatisation and subsequent washing causes, a factor that distorts the true MWD.
Ionic liquid HPLC with dual detection
To begin with, suspensions of cellulose or lignin were analysed by HPLC to check the chromatographic characteristics. Two columns filled with silica gel were operated in tandem and the compounds were eluted with the ionic liquid 1-ethyl-3-methylimidazolium methylphosphonate. The eluting compounds were directed to a UV detector and a refractive index detector.
Lignin absorbs light from 500 nm so can be detected on both types of detector but celluloses has no UV absorption and is only visible on the RI detector. So, both compound classes can be detected separately using both detectors.
The team turned next to wheat bran, representing a herbaceous plant, which they extracted with the same ionic liquid. The peak intensities after RI detection were far greater than those from UV detection, indicating that the former can be almost wholly attributed to cellulose and hemicellulose. One of the three peaks observed in the chromatogram was assigned to monomeric or oligomeric sugar and other low-molecular-weight compounds. The two others corresponded to hemicellulose and cellulose, respectively.
The peak in the UV spectrum at high retention volume was caused by a combination of lignin and low-molecular-weight aromatic species whereas that at low retention volume was probably due to lignin-carbohydrate complexes.
The chromatograms illustrated the effects of different extraction conditions. At 25°C with the ionic liquid, only low-molecular-weight polysaccharides were obtained but the yield of high-molecular-weight components increased with rising temperature. For lignin, the yield increased at higher temperatures too but there was some evidence of partial decomposition at 120°C.
Woody biomass
Milled wood samples from the pine tree Picea jezoensis and the oak Quercus crispula, representing softwood and hardwood, respectively, were also extracted with the ionic liquid and the extracts subjected to HPLC. The extraction yields were lower than for wheat bran, which is consistent with the knowledge that it is more difficult to remove polysaccharides from woody species than herbaceous plants.
The pine extracts contained polysaccharides and lignin of low-molecular-weight compared with oak, agreeing with published reports for cedar, another softwood. For pine, the lignin yield was unchanged as the extraction temperature was increased but the polysaccharide yield rose, indicating that preferential extraction of the latter can be achieved.
The ionic liquid extracts of the leaves, petals and twigs of the flowering cherry shrub Prunus yedoensis were also analysed by HPLC with the ionic liquid mobile phase. The average molecular weight of the polysaccharides from leaves was lower than that from twigs while lignin from twigs had the largest molecular mass of all three plant parts.
The results will help to formulate efficient extraction procedures for biorefineries and the principles could be applied to the extraction and analysis of lignin and polysaccharides from other types of plants. The ionic liquid HPLC system will be able to provide the molecular weight distributions of the products and determine if any decomposition has occurred during the procedures.
