508
Anal. Chem. 1987. 59. 508-513
without any interferring substances. When the sample contains several mutagens, the determination of the individual constituents is more questionable. A preseparation is then recommended.
ACKNOWLEDGMENT We are grateful to Maurice Hofnung,Institut Pasteur, Paris, for providing the bacterial strain and to JBn Garaj, Slovak Technical University, Bratislava, for stimulating support of this work. Registry No. 1, 6281-23-8; 2, 1874-24-4; 3, 1874-12-0; 4, 90147-18-5;5,90147-21-0;6,90147-19-6;7,710-25-8 8,14308-65-7; 9, 804-36-4; 10, 67-20-9.
LITERATURE CITED Fishbein, L. Potenfkl Indusfrlal Carcinogens and Mutagens ; Amsterdam, Oxford, New York, 1979; Vol. 4. Qtoenen. P. J.; de Cook-Bethbedef. M. W.; Bouwman, J.; Dhont, J. H. "N-Nitroso Compounds: Analysis, Fwmatkn and Occurrence; Walker, E. A., *Me, L., Castegnaro, M., M r a h y i , M., Eds.; Lyon, 1980. Maron. D. M.; Ames, B. N. Mutat. Res. 1983, 173, 173-215. Quhrdet, Ph.; Huisman, 0.; D'Ari, R.; Hofnung, M. R o c . Natl. Acad. SCi. U . S . A . W 8 2 , 5971-5975.
Quillardet, ph.; Hofnung, M. Mutaf. Res. 1985, 147, 65-78. Qulllardet, Ph.; de Beicombe, Ch.; Hofnung, M. Mutat. Res. 1985, 147, 79-95. Hayes, S.; Gordon, A.; Sadowski, I.; Hayes, C. Mutat. Res. 1984, 130, 97-106. Karube, I.; Nakahara, T.; Matsunaga, T.; Shuichi, S. Anal. Chem. 1982, 54, 1725-1727. Hofnung, M., special publication, Institute Pasteur, Paris, 1982. McCaila, D. R. Environ. Mutagen. 1983, 5 , 745-765. Atkinson, R . L. Nutr. Rep. Inf. 1971, 3. 363-367. FarkaZ, J. C/ym.-Ab~fr.~1978, 84, 119900. KellovB, G.; Sturdlk, E.; Stibrinyi, L.; Drobnica, i.;Augustin, J. Folia Mlcrqbbl. (Pfague) 1984, 29, 23-34. Sturdik, E.; Rosenberg, M.; Stibrinyi, L.; BaPi, 5.; Chreiio, 0.; Ebringer, L.; Iiavskg, D.; VBgh, D. Chem.-Biol. Interact. 1985, 53, 145- 153. Miller, J. H. Experiments in Molecular Genetics; Cold Spring Harbor Laboratory: Cold Spring Harbor, NY, 1972. Eckschlager, K.; StQinek, V. Information Theory as Applied to Chemical Analysis ; Wley-Interscience: New York, 1979. Kaiser, H.; Menzies, A. C. The Llmif of Defection of a Complete Analytical Procedure; Adam Hilger: London, 1968. Borgatzi, A. R.; Tarozzi. F.; Grisetig, G.; Brusco, A. Nuova Vet. 1970, 46. 6-21.
RECEIVED for review April 14,1986.
Accepted September 23,
1986.
Characterization of Beech Milled Wood Lignin by Pyrolysis-Gas Chromatography-Photoionization Mass Spectrometry Wim Genuit' and Jaap J. Boon*
Mass Spectrometry of Macromolecular Systems Unit, FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, T h e Netherlands
Oskar Faix Bundesforschungsanstalt fur Forst- und Holzwirtschaft, Institut fur Holzchemie und chemische Technologie des Hokes, Leuschnerstrasse 91, 2050 Hamburg 80, West Germany
A Cwle-pobrt pyrdysls-gas chromatography-photolonlzath maw spectrometry Instrument has been used for characterIzaHon of beech mllled wood Ilgnln. The llght source for photolonlretlon Is a rare gas resonance lamp. The main pyrolyds products have been Identffled and are Wed Into a propoeed thennal degradation scheme for the beech Hgnln. I t Is shown that the responses for these products depend strongly on lonizlng energy. The cumulatlve photoionization mass spectra at two photon energie0, 11.8 eV (Ar I)and 10.6 are compared to a 15-eV electron impact pyrolysis eV (Kr I)y mass spectrun. The Ar I photdonlratlon and 1 5 4 electron Impact IOnizeUon mass spectra both show a hbgh abundance of fragment Ions, whereas In the Kr I mass spectrum predominantly molecular Ion peaks are observed.
Curie-point pyrolysis is a useful general method for thermal depolymerization of macromolecular materials, producing characteristic chemical units amenable to gas chromatographic or mass spectrometric analysis (I, 2). A large variety of materials has been characterized by automated Curie-point pyrolysis-mass spectrometry (Py-MS) (2),and many groups of biomaterial8 have been differentiated by subjecting their 'Present address: Shell Research Laboratory, Badhuisweg 3,1031 CM Amsterdam. T h e Netherlands.
Py-MS fingerprints to advanced numerical data analysis methods (2-6). However, mass spectrometric results on the composition of pyrolysates depend strongly on the way in which the pyrolysate is transferred from the pyrolysis zone to the ion source and on the ionization conditions (6-9). The transfer problem can be avoided by applying in-source pyrolysis, but this in turn leads to rapid source contamination and sets and undesirable restriction to the heating rate, to prevent high-pressure pulses in the vacuum system. In the instrument described here a chromatographic inlet is used, which provides well-defined transfer conditions. Furthermore, this method enables the identification of individual pyrolysis products (Py-GC-MS mode), as well as the production, by spectrum accumulation, of a useful mass spectral fingerprint (Py-MS mode). Py-MS studies (2) generally apply low-energy (10-16 eV) electron impact ionization (EI) to enhance the characteristics of the mass spectral fingerprints by reducing fragmentation compared to 70-eV EI. At these low energies, however, small variations in electron energy lead to large differences in mass spectra, which is one of the main factors involved in poor interlaboratory reproducibility in Py-MS (8). Photoionization (PI) by means of a vacuum-UV light source is an attractive alternative to low-energy EI, because of the well-defined, highly constant, and reproducible ionizing energy. Although the ion yield of PI is much lower than that of 70-eV EI, it can be expected to be equal to or larger than that of E1 at energies near threshold, because photoionization cross sections have
0003-2700/87/0359-0508$01.50/00 1987 American Chemical Society
a much steeper onset at threshold than E1 crms sections (10). Our vacuum-UV light source allows a choice of gas for emission. In this paper the differences in response between AI I and Kr I photon as ionization media for the characterization of phenolic compounds evolved in the pyrolysis of lignin are explored. The technique of gas chromatography-photoionization mas8 spectrometry (GC-PIMS) as such has been demonstrated before by Washida et al., who investigated the fragmentation behavior of some organic compounds (11,lZ). Their instrument had a lower sensitivity than the one described here, presumably owing to a weaker lamp intensity and a lower ion source pressure. Genuit and Boon (9) have reported earlier on the results of the characterization of the glucose polymer amylose by pyrolysis GC-PIMS. In this paper, the utility of our pyrolysis-gas chromatography-photoionization mass spectrometry (e-GC-PIMS) instrument is demonstrated by the characterization of a hard wood "milled wood lignin", which is a mainly ether-linked polyphenolic polymer in plant cell walls. Lignin is a n essential component of the woody stems of arborescent gymnosperms and angiosperms which makes up 19-28% of the total wood. Lignin also forms an integral part of all vascular plants, including the herhaceous varieties. The formation of lignin proceeds via an enzyme-initiated dehydrogenative polymerization of three primary precursors: trans-coniferyl, trans-sinapyl, and trans-p-coumaryl alcohols (13). During biosynthesis, these precursors undergo structural changes due to coupling processes, displacements, and addition and subtraction reactions. The polymerization gives rise to a t least 10 types of intermonomeric linkages. In the study of the structure of lignin, the isolation and purification of lignin from the plant tissue without altering its structure are very difficult. Further, chemical degradation methods have to be applied to depolymerize the lignin, and the structure of degradation products must be elucidated. Besides chemical degradation studies, information on lignin structure can be obtained by spectroscopic methods: ultraviolet and infrared spectroscopy and 'H and '% NMR. Some attempts have been made to unify all available information into a structural scheme of lignin (14-16). For lignin classification, frequently degradation methods are applied to determine the abundance of H @-hydroxyphenyl), G (guaiacyl or 3-methoxy-4-hydroxyphenyl), and S (syringyl or 3,5-dimethoxy-4-hydroxyphenol) monomers in the degradation mixture. The lignins are classified according to their H/G and G/S ratios. Classical degradation methods are very time-consuming: the performance of these methods typically takes several days. In this respect, pyrolysis as a high-energy degradation method is a promising, rapid method for characterization of lignins. Earlier e G C (17)and Py-MS (18, 19) have been used for classification of different lignin types. PyGC-MS has been applied to identify the pyrolysis products of several lignins and lignin-containing materials (2P.22). For the PyGC-PIMS study described here, a beech milled wood sample was taken for analysis, because the structure of beech lignin is well investigated (16) and consists mainly of G (guaiacyl) and S (syringyl) units. EXPERIMENTAL S E C T I O N A scheme of the instrument is shown in Figure 1. The main parts will he described briefly helaw. Pyrolysis Unit and Gas Chromatograph. A Varian Aerograph Series 1400 gas chromatograph was modified for our purposes. The septum inlet was exchanged for a newly designed internally heatahle Curie-point pyrolysis unit FOM-SLX), suitahle for pyrolysis on-line with fused-silica capillary columns (23). In this pyrolysis unit, a glass liner amund a ferromagnetic wire coated with the sample is placed on a Kalrez interface directly above the entrance of the capillary column. The glass liner is surrounded
Figure 1. Schematic OVeNkW of the pyroiysis-gas chromatographphotoionization mass spectrometry enstrument. which consists of a Curie-point pyrolysis unit (FOM-3-X). a gas cnromatograph equipped with a fusebsilica capillary column. a direct coJpiing interface probe to tne (on sorrce. a photoionization mass spectrometer. and a data
system. hy a ceramic tuhe kept at 160 "C, which in t u r n is surrounded by a high frequency coil. The unit is flushed with oxygen-free nitrogen whirh was also used as ihe carrier gas. The GC oven wns equipped with a 30-mfused-silica capillary column (320 urn i.d.) with a 1.0 rm [hick chemically bonded Durahond DB-1coating (J&W Srientifir, Ranrhn Cordova, CAI. Nitrogen was used as the carrier gas. The rnlumn inlet pressure is 0.6 bar. The average gas velocity is 65 cm, s when the nven is at room temperature. The column inlet is heated to 180 "C. During pyrolysis injection, the nven was at roam temperature M trap the pyrolysate in the first few centimetres of the column. After injection the own was first heated rapidly LO 80 "C and then programmed from 80 10 100 "C at 2 T, min and from 100 "C' at 6 O C , min. Interface and Mass Spectrometer. The capillary column is fed from the oven M the mass spectrometer through a resistively heated stainless steel capillary. The column nulet is coupled directly to the ion source \
