(81c) Integrating Pulsed Pyrolysis/Laser Ablation with High Sensitivity Laser Ionization Mass Spectrometry to Measure Plant Cell Wall Composition of Single Cells
Biomass composition can have a significant impact on pretreatment and bioconversion processes. Understanding the chemical changes that occur at the cellular level in heterogeneous biomass materials can aid in identifying chemical reactions important for improving pretreatment processes and help engineer cell walls with traits more amenable to biomass conversion. This will require highly sensitive and selective experiments in which chemical measurements can be made on small groups of cells or even single cells. For example, laser ionization techniques combined with time-of-flight mass spectrometry (TOF-MS) may be useful. We have designed and developed a laser ablation/pulsed sample introduction/mass spectrometry platform that integrates pulsed pyrolysis with single photon ionization (SPI) and resonance-enhanced multiphoton ionization (REMPI) reflectron TOF-MS. We have used this experiment to measure cell wall chemical traits of biomass materials. In this study, the 3rd harmonic of a Nd:YAG laser (355 nm 3.50 eV) is used to pyrolyze the sample. The products are entrained and cooled in a He supersonic jet exiting the orifice of a solenoid pulse valve placed directly above the sample. The resulting molecular beam is then skimmed and introduced into an ionization region. The one color REMPI was used to improve the ionization efficiency of aromatic compounds, leading to highly selective detection of lignin and aromatic metabolites. Lignin pyrolysis products based on both guaiacyl and syringyl monomers have been assigned. The SPI (118.2 nm, 10.49 eV) is suitable for detecting all other compounds with ionization potentials below 10.49-eV. The recorded photoionization mass spectra are compared with that obtained from 22.5-eV electron-impact ionization in a molecular beam mass spectrometer. The mass spectra obtained using photoionization are characterized by high sensitivity, selectivity, high resolution and they display predominantly molecular ion peaks (soft ionization) compared with that of electron impact. The implication of using REMPI under supersonic jet conditions for measuring cell wall composition at sub-tissue level will also be discussed. Samples consisting of single types of cells are collected for pulsed introduction using laser microdissection.