Anal. Chem. 1994,66, 543-550
Resonance-Enhanced Multiphoton Ionization Mass Spectrometric Analysis of Lignin Using Laser Pyrolysis with Entrainment into a Supersonic Jet Erik R. E. van der Hage,' Jaap J. Boon, Ruud J. J. M. Steenvoorden, and Tina L. Weeding Unit for Mass Spectrometry of Macromolecular Systems, FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
Resonance-enhanced multiphoton ionization (REMPI) as a means of specifically ionizing aromatic compounds is extended to the analysis of lignin pyrolyzates. First, mass-resolved excitation spectra of phenolic model compounds of increasing complexity were measured to determine optimal resonant wavelengths for jet-cooled phenol, guaiacol, 4-methylguaiacol, homovanillic acid, and syringol. The obtained set of resonant wavelengths is unique for each benzene derivative, depending on the number and types of substituents on the aromatic ring, different isomeric conformations, and stable rotational conformers which are frozen out in the supersonic jet. In this work, the one-color REMPI results are used to identify and quantify the main pyrolysis products of a cottonwood milled wood lignin. The insoluble lignin preparation is introduced directly inside the ion source of a time-of-flight (TOF) mass spectrometer and analyzed without any sample pretreatment. Flash pyrolysis is achieved by means of a pulsed C02 laser. Analytical calibration curves are linear over a dynamic range of 4 orders of magnitude. Limits of detection obtained at a resonant wavelength of homovanillic acid are homovanillic acid, 400 pg, coniferylalcohol, 3 ng, and sinapyl alcohol, 5 ng. Results are compared with high-resolution in-source pyrolysis mass spectrometry data obtained under E1 conditions. Lignin is a relatively resistant cell wall constituent of all vascular plants and consists of ether- and carbon-linked methoxyphenols. This cross-linked polyphenolic macromolecule, with a molecular mass of over ten thousand, is insoluble in virtually all simple solvents and seems to contain no chains of repeating units, nor are there bonds that are easily hydrolyzed. These physical and chemical properties complicate the characterization of lignin on a molecular level. Over the past five years, pyrolysis mass spectrometry (PyMS) has become a successful microanalytical tool for the structural study of synthetic polymers and complex macromolecular materials such as lignine2 Resonance-enhanced multiphoton ionization (REMPI) under supersonic jet conditions has great potential as a soft and highly selective ionization method in the analysis of complex mixtures such as lignin pyrolyzates. The 1 + 1 REMPI process proceeds via the absorption of two photons in which the first photon excites the molecule to a resonant intermediate excited state and the second photon ionizes the molecule. The ability to obtain energy-resolved ionization is (1) Adler, E. J . Wood Sci. Technol. 1977, 11, 169. (2) Boon, J. J. Inr. J . Mass Specrrom. Ion Processes 1992, 118/119, 7 5 5 .
0003-2700/94/0366-0543$04.50/0 0 1994 American Chemical Society
a unique feature of REMPI as compared to other means of ionization. Mass-resolved excitation spectra are obtained by monitoring the intensity of a molecular ion ( m / z ) as a function of wavelength by scanning the wavelength of a frequencydoubled dye laser. The molecules are rotationally "frozenout" in a supersonicjet to increase the optical spectral resolution and sensitivity by minimizing thermal population of internal modes. In addition to cooling for spectroscopic purposes, the supersonic jet also prevents thermal decomposition of neutral molecules formed upon laser desorption and pyrolysis. REMPI has been applied successfully for the spectroscopic and selective chemical analysis of a great variety of jet-cooled benzene derivatives. It has been shown that REMPI can distinguish between cis and trans isomers of hydroxylated aromatics such as 0-, m-,and p-dihydroxyben~enes,~.~ metasubstituted phenols and j3-napth01,~and various other isomers of mono- and polysubstituted benzenes.6-9 Lubman et al. demonstrated the applicationof REMPI as a soft and selective ionization technique in the analysis of laser-desorbed thermally labile molecules such as indole and catechol derivatives, peptides, and At present, REMPI mass spectrometry has been mostly limited to the analysis of volatiles and laser-desorbed model compounds. In this study, the application of REMPI is extended to the characterization of laser pyrolysis products of a milled wood lignin (MWL) isolated from cottonwood. The structure of this relatively homogeneoushardwood-derived lignin was previously found to be a copolymer of mainly alkyl-aryl ether linked 2-methoxyphenol (guaiacyl) and 2,6dimethoxyphenol (syringyl) derivatives.13J4 PyMS results depend strongly on the transmission of thermal decomposition products from the pyrolysis zone to the ionization zone. The use of interfaces in on-line PyMS (3) Dunn, T. M.; Tembreull, R.; Lubman, D. M. Chem. Phys. Lerr. 1985, 121, 453. (4) Oikawa, A.; A&, H., Mikami, N; Ito, M. Chem. Phys. Lerr. 1985,116, 50. (5) Oikawa, A.; Haruo, A.; Mikami, N.; Ito, M. J . Phys. Chem. 1984,88, 5180. (6) Breen. P. J.; Bernstein, E. R.; Secor, H. V.;Seeman, J. I. J. Am. Chem. Soc. 1989, 111, 1958. (7) Aoto, T.; Ebata, T.; Ito, M. J . Phys. Chem. 1989, 93, 3519. (8) Oikawa, A,; A&, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1983, 87, 5083. (9) Yamamoto, S.; Okuyama, K.; Mikami, N.; Ito, M. Chem. Phys. teff.1986, 125, 1. (10) Li, L.; Lubman, D. M. AMI. Chem. 1988, 60, 2591. (11) Li, L.; Lubman, D. M. Anal. Chem. 1989, 61, 1911. (12) Tembreull, R.; Sin, C. H.; Li, P.; Pang, H. M.; Lubman, D. M. Anal. Chem. 1985, 57, 1106. (13) van der Hage, E. R. E Mulder, M. M.; Boon. J. J. J . Anal. Appl. Pyrolysis 1993, 25, 149. (14) Milne, T. A.: Chum, H. L.; Agblevor, F.; Johnson, D. K. Biomass Bioenergy 1992, 2, 341.
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