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Synchrotron Vacuum-Ultraviolet Postionization Mass Spectrometry with Laser and Ion Probes for Intact Molecular Spatial Mapping of Lignin

  • Author(s): Takahashi, Lynelle Kazue
  • Advisor(s): Leone, Stephen R
  • et al.
Abstract

A secondary ion mass spectrometer is coupled to a vacuum-ultraviolet (VUV) synchrotron beamline to perform neutral postionization experiments. The goal of the project is to develop a new molecule-specific imaging technique that can provide maximum chemical information of chemically heterogeneous samples while also maintaining information on their spatial distributions. Modifications are made to the beamline branchline to accommodate the new imaging instrument.

Initial experiments focus on the application of ion sputtering with VUV postionization to simple metal and semiconductor systems. From these experiments, VUV-secondary neutral mass spectrometry (VUV-SNMS) is established as a complementary imaging technique to SIMS. Signal intensities for select species are determined to be comparable to SIMS mass spectra, however VUV-SNMS signal intensities are more generally found to be limited by the low ionization cross sections of molecular analytes and low synchrotron flux. Despite this, it is observed that postionization significantly reduces the "matrix effects" that makes SIMS analysis difficult to quantitate. No significant differences were observed in the internal energies of atoms/clusters desorbed with different primary ions (Bi+, Bi3+, and Bi32+) from the photoionization efficiency curves of the sputtered atomic and cluster species. In atomic Au and As, no electronic excitation was observed.

Organic molecules are also investigated; however the non-electrically conductive nature of most organic surfaces leads to sample charging during VUV-SNMS analysis. To deal with this problem, a double pulse extraction scheme is developed. Organic test molecules thymine and tryptophan are examined and used as molecular thermometers to determine the approximate internal energies and temperatures of the ion-sputtered organics. By approximating the energies by two different methods, these molecules are found to have on the order of 2.5 eV to 3 eV of internal energy.

The feasibility of using VUV-SNMS for the study of lignin within plant systems is evaluated by studying the energetics and fragmentation mechanisms of coniferyl and sinapyl alcohol, two monomer units of the lignin polymer. The mass spectra are compared to those obtained with positive mode secondary ion mass spectrometry (SIMS) and thermal desorption molecular beams (TDMB) mass spectrometry. While SIMS shows extensive fragmentation of the monolignols and a non-distinctive mass peak at the parent m/z, both TDMB and VUV-SNMS show prominent parent signal. Furthermore, it is found that many of the major VUV-SNMS peaks of the monolignols (m/z = 124 and 137) are observed in the TDMB mass spectra, suggesting that the fragments may arise due to dissociative photoionization of ion-sputtered molecular parent molecules. With the tunability of the synchrotron light, ionization energies of coniferyl alcohol are measured to be 7.60 eV ± 0.05 eV, and dihydrosinapyl and sinapyl alcohol were found to have ionization energies of <7.4 eV.

Alkali lignin from Sigma Aldrich is analyzed with VUV-SNMS. Extensive fragmentation is observed, and signatures from the monomer mass region provided the only characteristic ion signals in the mass spectra. Thus, while VUV-SNMS may be useful for probing monomer composition of lignin, it cannot provide much more chemical insight on lignin structure and how it changes upon different perturbations. Laser desorption is therefore developed in order to provide a desorption method that imparts less internal energy into the analyte molecules.

Lower internal energies can be achieved by using a nanosecond-UV desorption laser, which is applied to study coniferyl alcohol and lignin. As expected from previous VUV-laser desorption postionization (VUV-LDPI) work, coniferyl alcohol molecules are largely desorbed intact into the gas phase by minimizing the laser dosage and laser peak power. Similarly, when the laser is used to desorb molecules from the sample of alkali lignin, signatures in the dimer mass region appear. Several monomer masses are identified by their molecular weights and photoionization efficiency curves. Lignin dimers could not be assigned and are an area for much future work. Different mass spectral features are observed from different lignin extracts, illustrating that chemical differences between samples are reflected in the mass spectra. Preliminary results also suggest that it may be possible to follow the chemical changes that occur during laser pyrolysis of lignin and other important plant biopolymers.

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