Curie-point pyrolysis mass spectrometry, Curie-point pyrolysis-gas chromatography-mass spectrometry and fluorescence microscopy as analytical tools for the characterization of two uncommon lignites

Journal of Analytical and AppIied Pyrolysis, 8 (1985) 221-239 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

221

CURIE-POINT PYROLYSIS MASS SPECTROMETRY, CURIE-POINT PYROLYSIS-GAS CHROMATOGRAPHY-MASS SPECTROMETRY AND FLUORESCENCE MICROSCOPY AS ANALYTICAL TOOLS FOR THE CHARACTERIZATION OF TWO UNCOMMON LIGNITE!3

MARGRIET

NIP *, J.W. DE LEEUW

and P.A. SCHENCK

Detft University of Technology, Department of Chemistry and Chemical Engineering Geochemistry Unit, de Vries van Heystplantsoen 2, 2628 RZ De@ (The Netherlands) HENK

Organic

L.C. MEUZELAAR

Biomaterials (U.S.A.)

Profiling

SCOTT A. STOUT Pennsylvania

Center, University of Utah, Research Park, Salt Lake City, UT 84108

and P.H. GIVEN

State University, University Park, PA 16802 (U.S.A.)

JAAP J. BOON FOM - Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam (The Netherlands)

SUMMARY A case study is presented of two uncommon lignite samples, PSOC-975 and PSOC-427, which were studied by microscopic and analytical pyrolysis methods. Pyrolysis-mass spectrometric @‘y-MS), pyrolysis-gas chromatographic (Py-GC) and I’-GC-MS data demonstrate that lignite PSOC-975 contains a high abundance of resinous material of some sort, whereas PSOC-427 is characterized by two major unknown pyrolysis products, prist-1-ene and alkylphenols. The chemical data contradict the microscopic observations, which point to a maceral composition of woody and cortical tissues, although a considerable part is unrecognizable. Apparent discrepancies between low-voltage Py-MS and 80-eV Py-GC-MS results were evaluated by a study of standards. It was shown that the aliphatic hydrocarbons are highly underestimated in low-voltage Py-MS. The nature of the disagreement observed between maceral composition and the pyrolysis data needs further study.

INTRODUCTION

The chemical structure of coals is mainly determined by the input of biopolymers ‘originally present and by the degree of coalification [l]. The 01652370/85/$03.30

0 1985 Elsevier Science Publishers

B.V.

222

analysis of the contributing biopolymers and their coalified analogues is an approach we have chosen over the years to unravel the complex chemical structure of coals [2-51. Flash pyrolysis methods such as Curie-point pyrolysis-mass spectrometry (Py-MS) and Curie-point pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) have been shown useful in this respect. In this paper we concentrate on some specific problems with two lignite samples (soft brown coals) that were analysed chemically by applying analytical pyrolysis techniques and microscopically by determining their maceral composition using white light (reflected) and blue-light fluorescence. One of us (H.L.C.M.) has investigated a large series of North American lignites by Curie-point Py-MS in combination with multivariate data analysis [6]. During this study, two samples showed pyrolysis-mass spectra with specific characteristics. These samples were therefore analysed further by Curie-point Py-GC and Curie-point Py-GC-MS. The results of these analyses were not in agreement with the petrographic data. Several additional investigations were undertaken in an attempt to understand the nature of this disagreement. Much attention was given to apparent discrepancies between the Py-MS results on the one hand and the Py-GC and Py-GC-MS results on the other.

EXPERIMENTAL

Samples The two lignites (PSOC-975 and PSOC-427) were provided in the first instance to one of us (H.L.C.M.) by the Penn State Coal Data Bank. After analysis by Py-MS, aliquots were sent to Delft for Py-GC and Py-GC-MS analyses, as the Py-MS data indicated that these lignites were different to other lignites [6]. Preliminary investigation of the samples by Py-GC and Py-GC-MS in Delft confirmed their deviant character and therefore fresh batches of the same lignites were asked for and kindly supplied directly to Delft by the Penn State Coal Data Bank. The Py-GC and Py-GC-MS data obtained for the fresh samples were completely in agreement with the earlier results. The locations and the results of proximate and ultimate analyses of the lignites are listed in Table 1. Initial and revised microscopic data concerning the maceral composition of the samples are shown in Table 2. The initial petrographic data were obtained by conventional microscopic methods; the revised data are based on fluorescence microscopy using a Standard Leitz MPV2 microscope equipped with a Ploemopak fluorescence vertical illuminator and mercury arc lamp.

223 TABLE

1

Location

and proximate

and ultimate

analysis values for the two lignite samples PSOC-427

PSOC-975

Country State Coal province Alternative seam name Apparent rank Age of seam

U.S.A. Texas Gulf Lignite Subbit. C Eocene

U.S.A. Wyoming Northern Great Plains Lignite Subbit. C Paleocene

Proximate analysis (DMMF) % Volatile % Fixed carbon

50.08 49.92

47.93 52.07

Ultimate s&C S&H SN %O

73.11 5.54 1.24 20.11

73.02 5.29 0.99 20.70

0.91 0.20

0.87 0.21

analysis (DMMF)

Atom ratios (DMMF) Atomic H/C Atomic O/C

TABLE 2 Maceral composition of the two lignites, obtained microscopically (initial data) and fluorescence light conditions (revised data) Maceral composition

PSOC-427 l

Revised data *.**

90.6 _

78.0

8.1 1.3 _

7.5 14.6 -

83.8 6.1 0.5 0.2 1.3

77.3 -

_ -

0.5 0.2 8.1 0.5 2.8 _

2.0 2.1 3.7 -

1.8 1.9 3.4 -

0.3

0.3

* DMMF vol.%. ** 40% of the total constituents

white light

PSOC-975

Initial data Huminite Vitrinite Inertinite Liptinite Textinite Ulminite Humodetrinite Gelinite Corpohuminite Sporonite Cutinite Resinite Alginite Fusinite Semifusinite Macrinite Micrinite Sclerotinite

under reflected

not recognizable

Initial data

_ 79.6 7.6 11.8

l

Revised data *,** 71.1 _ 3.5 25.4 -

-

68.6 1.7 0.8 _

10.7 0.2 0.9 _

13.7 0.5 2.9 -

2.2 3.5 0.4 1.4 0.1

1.1 1.9 0.3 -

in blue light.

0.3

224

Sample preparation Prior to Py-MS, PJ-GC and Py-GC-MS the fresh PSOC samples were ground and the resulting fine powder (approximately 300 mesh) was suspended in Spectrograde methanol (4-10 mg/ml). Two 5~1 droplets of each suspension were applied to a ferromagnetic wire and the suspension liquid was evaporated. Curie-point pyrolysis-mass

spectrometry

of the [ignites

The coated ferromagnetic wires (Curie temperature 610°C) were inserted into especially designed reaction tubes [6] and the samples were analysed using an Extranuclear Model 5000-l Curie-point pyrolysis-mass spectrometer operating under the following conditions: pyrolysis time, 10 s; electron energy, 11 eV; mass range, m/z 20-260; scan rate, approximately 2000 a.m.u./s; and total scan time, approximately 20 s. Curie-point

evaporation-mass

spectrometry

of the standard mixture

A standard mixture containing o-cresol, 2-methylnaphthalene and ndodecane (1: 1: 2, w/w) was taken up in methanol. Fixed amounts of this solution were applied to ferromagnetic wires (Curie temperature 358°C) with a coating of water-glass and activated charcoal. The wires were activated by Curie-point heating in an inert atmosphere prior to use [7]. Spectra were obtained under the following conditions: pyrolysis time, 0.8 s; electron energies, 16, 23, 30 and 70 eV; mass range, m/z 25-170; number of scans, 150; and scan velocity, 0.15 s. Curie-point pyrolysis-gas

chromatography

Py-GC was carried out using a pyrolysis reactor suitable for Curie-point pyrolysis according to Van de Meent et al. [8]. The Curie temperatures of the ferromagnetic wires used were 358 and 610°C. The wires were kept at the end temperature for 10 s. The gas chromatograph (Packard Becker, Model 419), equipped with a cryogenic unit, was programmed from 0°C (5 min) to 300°C (25 min) at a rate of 3”C/min. Separation was achieved using a fused-silica capillary column (25 m x 0.32 mm I.D.), coated with CP-Sil-5 (film thickness 0.38 pm). Helium was used as the carrier gas. Curie-point pyrolysis-gas

chromatography-mass

spectrometty

The same pyrolysis unit and capillary column as mentioned above were also used in the Py-GC-MS mode. GC-MS was performed in a Varian 3700 gas chromatograph connected to a Varian-MAT 44 quadrupole mass

225

spectrometer. Electron impact mass spectra were obtained at 80 eV under the following conditions: cycle time, 1 s; mass range, m/z 20-450 up to scan 250 and m/z 50-450 after scan 250; m/z 28, 32, 40 and 44 were omitted from the reconstructed total ion currents, because an open atmospheric split was used as the interface.

RESULTS

Py-MS

AND DISCUSSION

of the lignites

Fig. 1 shows the pyrolysis mass spectra of the two lignites. Both spectra show series of predominant peaks with m/z 94, 108, 122, 136 and 110, 124, 138. These peaks were tentatively ascribed to phenolic and catecholic and/or methoxyphenolic compounds, respectively, characteristic pyrolysis products of lignin-derived material [9]. These tentative identifications supported to some extent the original petrographic data, which indicate that both samples mainly consist of wood-derived materials (Table 2). However, the distribution pattern of the homologous series mentioned above is not commonly encountered in lignin-derived samples. Therefore, a firmer identification of the pyrolysis products was required. Also, the mass peaks with m/z 234 in PSOC-975 and m/z 194 in PSOC-427 were shown to

WYOMING

?

PSOC

I I

(anderson

seam)

-975

3

0

So. TEXAS PSOC-L27

Fig. 1. Pyrolysis-mass

spectra

of the two lignites.

(wildcat

seam 1

I 100

1 150

16 18

I

20

22

I

24

-

-

-

_

(b) PSOC-427.

I 50

11

Fig. 2. Pyrolysis-gas chromatograms of the two lignite samples: (a) PSOC-975; temperature programmed from 0°C (5 min) to 3OO’C (25 min) at 3’C min-‘.

(‘Cl

12

250

O°C

1

-temp

10

200

I

I

(b)

(a)

28

I

PSOC427 Cu-temp610°

triterps

300 Fused-silica CP-Sil-5 column,

26

I-

1

25 mX0.32

PSOC - 975 Cu-temp 610'

mm I.D.;

22-l

be highly characteristic of these lignites [6] and also needed further identification by Py-GC and Py-GC-MS. Py-GC

and Py-GC-MS

Fig. 2 shows the Py-GC traces of both lignites. Identification was based on both relative retention times and mass spectra of the pyrolysis products compared with those of standard compounds. Major peaks in the Py-GC trace of PSOC-975 (Fig. 2a) represent compounds with molecular weights of 206 and 208. They reveal mass spectra which are highly comparable to standard spectra of sesquiterpenoid hydrocarbons [lo-121. The mass spectrum of the most abundant compound of this group is shown in Fig. 3. Similar compounds were also observed in Py-GC-MS data of a piece of pure resin, isolated from the Australian brown coal deposits in Victoria [4]. The abundant occurrence of the diterpenoid hydrocarbons norpimarane, pimarane [lo], dehydroabietane and retene [13] (Fig. 2a, peaks A, B, C and D, respectively) in combination with the above-mentioned sesquiterpenoid and triterpenoid hydrocarbons observed, strongly indicates that these compounds originate from plant resins [4,14,15]. Compounds present in much smaller amounts are an extended series of n-alkenes and n-alkanes. Their chain lengths are indicated along the upper horizontal axis in Fig. 2. Moreover, alkylbenzenes and naphthalenes were also observed. The relatively small amounts of alkylphenolic compounds together with trace amounts of dihydroxybenzenes and methoxyphenols indicate a slight, if any, contribution of lignin-type material. From these data, it is clear that the peak with

81

19'

2 Fig. 3. Mass spectrum of one of the compounds identified as being a sesquiterpenoid hydrocarbon.

PSOC -975

0

present in the pyrolysate

of PSOC-975

228

m/z 234 in the Py-MS data obviously is the molecular ion of retene (peak D, Fig. 2a). Peaks with high intensities observed in Fig. 2b (PSOC-427) are alkyl phenols, prist-1-ene and two other compounds with molecular weights of 168 and 194. Although alkylphenols are present in significantly larger amounts than in the other sample, the dihydroxybenzenes and methoxyphenols are represented by peaks of minor importance. Here, too, an extended series of n-alkenes and n-alkanes is encountered, together with alkylbenzenes and naphthalenes. The relatively high abundance of phenolic compounds in the pyrolysate of this sample might reflect a considerable contribution of lignin-derived material. However, the ratio of dihydroxybenzenes and methoxyphenols to alkylphenols indicates that the original lignin must have been highly altered by means of biodegradation

PSOC -427 (a)

139 168 I

52

6g

78

/L\JLJJ I" 20

70

109 iI, 95 .II; .,L&&&_++ 125 158 120

v

1‘!l

179

I

(b)

I

M/Z

270

PSOC -427

151

136

, 165

1, , , ,

( , , ,

220tq? Fig. 4. Mass spectra of the compounds indicated as (a) m/t pyrolysis-gas chromatogram of PSOC-427 (Fig. 2b).

270

168 and (b) m/r

194 in the

229

[16] and/or chemical diagenesis [17]. A purely chemical transformation is hardly expected because of the low degree of maturity of the samples. Another as yet unknown source of the phenolic compounds might be considered. A possible contribution of tannin-like material for the. explanation of the phenols is discussed later. Prist-l-ene might indicate a contribution of chloroplast material [18]. The mass spectra that correspond to the peaks indicated as m/z 168 and m/z 194 (Fig. 2b) are shown in Fig. 4. So far, we have not been able to propose structures that explain the mass spectral fragmentation patterns satisfactorily. These compounds are certainly not pyrolysis products of the syringyl-type lignin [16]. The Py-GC and Py-GC-MS data obtained for both samples contradict the original petrographic data, which indicate a major contribution of lignin-derived material. It is also noted that discrepancies between the Py-MS data on the one hand and Py-GC and Py-GC-MS data on the other are present. Fluorescence microscopy

Because of these contradictory results, a more refined petrographic investigation was carried out on both lignites by fluorescence microscopy [19]. Fig. 5 shows two typical photomicrographs of the samples under blue-light irradiation. The upper photomicrograph shows a representative ulminite-rich particle recognized in the PSOC-975 sample. Ulminite is a maceral that belongs to the huminite/vitrinite maceral group [l]. This group of macerals is thought to be derived from woody and cortical plant tissues. The lack of vessels in this tangential section of ulminite shown in this photomicrograph suggests it was derived from coniferous woody material. There is a clear distinction between the primary cell wall (dark brown) and the secondary cell wall (orange-brown). The herringbone-type structures in the cell wall may be due to microbial action, which was oriented parallel to the cellulose micelles in the original cell wall (perhaps due to soft-rot fungi). Some tracheid lumina are filled with small orange bodies, resembling tannin-like material in wood. It cannot be ruled out though that these structures are resinous. The lower photomicrograph (Fig. 5b) shows a liptinite-rich coal particle found in PSOC-427. Liptinite is thought to represent remains of plant constituents other than woody and cortical tissues [l]. This liptinite particle contains primarily sporinite (yellow), some small resinites (solid oblong, yellow to yellow-orange) and bituminite [20,21] (small brown to green wisps in the groundmass). Much of the groundmass is non-fluorescent and was therefore not characterized. The results of this refined petrographic study are summarized in Table 2 (revised data). Although in second instance both samples appeared to contain significantly larger amounts of liptinite material (14.6% in PSOC-427

230

Fig. 5. Photomicrographs of the two lignites under fluorescence light conditions. (a) Ulminite-rich particle of PSOC-975 (enlargement 600 x ); (b) liptinite-rich particle of PSOC427 (enlargement 600 X ).

231

and 25.4% in PSOC-975), the major part of the recognizable material, about 60% of the sample, is still ascribed to woody and cortical tissues. It is unclear whether or how these materials are reflected in the pyrolysis data. The question now arises of the extent to which the pyrolysis data reflect the chemical constitution of the samples. It may be that the 40% microscopically unrecognizable material relates to a substantial part of the P y - G C and P y - G C - M S data.

Evaporation-GC and extraction of PSOC-975 It would be important to estimate how much of a sample is actually pyrolysed and the extent to which this pyrolysate is revealed by the analytical method used. As a first approximation, an attempt was made to estimate the weight percentage of the GC-amenable part of the pyrolysate of the PSOC-975 sample. The total sample was therefore subjected to evaporation using a wire with a Curie temperature of 358°C. Fig. 6a shows the GC trace. Obviously, the compounds released in this way ("thermal extraction") are the sesqui-, di- and triterpenoids that are either adsorbed to the lignite matrix or are entrapped within the pore system. Therefore, they were thought to be extractable with organic solvents. For this reason, the sample (PSOC-975) was extracted ultrasonically with dichloromethane [22]. Approximately 20% (w/w) of the total sample was solvent extractable and, indeed, was identical with the thermal extract. The residue after extraction, representing approximately 70% of the total sample (the ash content is ca. 10%), was pyrolysed at 610°C and the results are shown in Fig. 6b. Now the compounds that represent the resin-like material (sesqui-, di- and triterpenoids) have been completely removed by this extraction. The polymeric framework (the residue after extraction) is characterized by an extended series of n-alkenes and n-alkanes, prist-l-ene and some alkylphenols. These pyrolysis products probably reflect highly aliphatic polymeric structures in certain plant cuticula [23]. Assuming that the solvent extract is totally GC-amenable, it is estimated from the P y - G C trace of the whole sample (Fig. 2a) that the P y - G C and P y - G C - M S data represent approximately 40% of the total sample at the most. About half of the P y - G C trace (ca. 20%) can then be ascribed to thermally extractable terpenoids, whereas the other half (ca. 20%) represents the non-extractable part of the sample. This second half can represent either the total non-extractable material or a selective (easily pyrolysable) part of it. The H / C ratio of this sample (Table 1) is estimated to be 0.87. This value is not in agreement with the H / C ratio of the compounds present in the pyrolysate, which is thought to be more than 1.00. This could indicate that a selective part of the non-extractable material of this sample is reflected by the P y - G C trace. As the microscopic and pyrolysis techniques do not describe the total

232

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233

sample completely, it is likely that the discrepancy in their results is due to some extent to the incompleteness of both applications. Further investigations are necessary in order to understand the discrepancy between the petrographic and pyrolysis data for the lignites.

Comparison of the Py-MS and Py-GC and Py-GC-MS results The earlier mentioned apparent discrepancy between the P y - M S data on the one hand and the P y - G C and P y - G C - M S data on the other has a two-fold nature. One part might arise from the fact that the identification of individual components present in very complex mixtures such as these pyrolysates on the basis of presence or absence of nominal masses is highly tentative. The other part is due to the fact that different detection systems are used to monitor the pyrolysate: a mass spectrometer operating at low eV (Py-MS), a mass spectrometer operating at high eV ( P y - G C - M S ) and a flame ionization detector as a detector for the P y - G C . Most of the confusion may be due to the different responses of the detectors to the various compounds in the pyrolysates investigated. In order to solve these problems, a number of major peaks in the pyrolysis-mass spectra were evaluated in the G C - M S data. Furthermore, responses in phenolic, aromatic and aliphatic hydrocarbons at low voltages were studied.

Mass chromatography of important Py-MS peaks Based on the P y - M S data, the peaks with m/z 94, 108, 122, 136, 150, 164 and 110, 124, 138 were tentatively ascribed to lignin-derived components. The P y - G C and P y - G C - M S data, however, indicated a relatively low abundance of these compounds, especially in PSOC-975. Therefore, it was necessary to investigate whether other compounds could contribute to these mass peaks in the pyrolysis-mass spectra. Mass chromatography was performed on the peaks at m/z 94, 108, 122, 136, 150, 164 (Fig. 7) and m/z 110, 124, 138 (Fig. 8) in the P y - G C - M S data of both samples, assuming in the first instance that the fragmentation patterns at low and high eV are to some extent similar. Fig. 7a (PSOC-975) shows that the peak with m/z 94 can almost exclusively be ascribed to phenol. The mass chromatograms for m/z 108 and 122 show fragment ions originating from the sesquiterpenoid hydrocarbons and molecular ions of the alkylphenols. The peaks with m/z 136 and 150 are almost exclusively fragment ions from sesquiterpenoids, whereas with that for m/z 164 an additional contribution of a fragment ion from pimarane is observed. The same phenomenon is seen in Fig. 7b (PSOC-427), where apart from the molecular ions of the (alkyl)phenolic compounds with m/z 94, 108 and 122 fragment ions that belong to the mass spectra of the unknowns with molecular peak values of 168 and 194 are present.

234 PSOC

- -

M/z 206

975

,

OH M/Z 108

M/Z~2

U

M/Z ~36

M/Z ~50 (a)

__3_...

M/Z~64

L

PSOC-/,27

I

A~~

..., ...... •

..

.~IA,~

A

OH Mlzme

~

, .........

.,

Mlz ~22

__~

A i

M/Z 130 M/Z 150

i

(b) M/Z ~6~,

Fig. 7. Mass chromatograms for m/z 94, 108, 122, 136, 150 and 164 of (a) PSOC-975 and (b) PSOC-427. For the peaks with m/z 136, 150 and 164, contributions of fragment ions belonging to the compound with a molecular weight of 194 are observed. Fig. 8a (PSOC-975) shows that the peaks with m/z 110, 124 and 138 are mostly fragment ions originating from sesqui- and diterpenoid hydrocarbons. For the peak with m/z 110 an additional contribution of a fragment peak from prist-l-ene is observed.

235

PSOC-

MIz ~oo

"'z 206

975

L,,,L,',,k,,',L

i

\

M/Z 12/.,

M/Z 138

(a) P50C--427

OH

M/z 168

M/z 19~

L MIz ~0

MI z ~24 ,,

(b)

l

~ . . . . .

,,___

MI z ~3B

Fig. 8. Mass chromatograms for m / z 110, 124 and 138 of (a) PSOC-975 and (b) PSOC-427.

The peak with m/z 110 in PSOC-427 (Fig. 8b) originates from both the molecular ion of a dihydroxybenzene and a fragment ion originating from the unknown with the molecular weight of 168. For the peaks with m/z 124 and 138, contributions are observed from dihydroxybenzenes and methoxyphenols on the one hand and fragment ions from the compound with a molecular weight of 168 on the other. As it is known [24,25] that low eV mass spectra of sesqui- and diterpenoids show fragments more or less similar to those at 80 eV, we must conclude that with PSOC-975 at least part of the intensities of the peaks with rn/z 94, 108, 122, 136, 150 and 164 and 110, 124 and 138 can be ascribed to lignin and resin compounds. Whether fragments of the compounds with molecular weights of 168 and 194 contribute to the masses previously mentioned for PSOC-427 cannot be concluded because of their unknown identity and hence unknown behaviour at low eV.

236

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Q

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21

142

108

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30

71

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Fig. 9. Pyrolysis-mass spectra of a standard mixture containing n-dodecane (MW 170), o-cresol (MW 108) and 2-methylnaphthalene (MW 142) ( 1 : 1 : 2 , w/w). Electron energy conditions used: 16, 23, 30 and 70 eV. The corresponding histograms were constructed by the summation of the masses 43, 57, 71, 85, 99 and 170 for n-dodecane; 77, 79, 107 and 108 for o-cresol; 89, 115, 141 and 142 for 2-methylnaphthalene.

237 Low and high e V conditions in the mass spectrometer

The influence of low and high eV conditions in the mass spectrometer was investigated with the object of evaluating response differences of the compounds in the pyrolysates. Curie-point P y - M S was applied to a standard mixture containing o-cresol (MW 108), 2-methylnaphthalene (MW 142) and n-dodecane (MW 170) (1 : 1 : 2, w/w). The influence of the electron impact energy was studied by analysis of the mixture at 16, 23, 30 and 70 eV. Fig. 9 shows the four pyrolysis-mass spectra obtained. Summation of the appropriate masses of the different components resulted in the histograms indicated. These show that the relative responses of the individual compounds depend highly on the electron impact energy. At low voltages, the molecular ion of the aromatic compounds predominate over fragment ions of the aliphatic hydrocarbons. At higher voltages, the fragment ions of aliphatic hydrocarbons are much more abundant than the molecular ions of the aromatics. Extrapolating these data to the high-voltage G C - M S results, we conclude that aliphatic and probably also alicyclic hydrocarbons substantially fragment at low voltages but their absolute response declines with respect to phenolic compounds. It is clear that the aliphatic compounds in the pyrolysis-mass spectra of the lignites (generated at 11 eV) are highly underestimated. The peaks with m / z 94, 108, 122, 136 and 110, 124, 138 in these spectra (Fig. 1) are probably mostly from phenolic compounds and derived from lignin-like material, despite the fact that this material is present in the samples at low or moderate concentrations. The flame ionization detector used in P y - G C probably gives the best results as far as the relatively uniform response to the different types of compounds is concerned. The total ion current traces, shown in Figs. 7 and 8 (upper traces), are more or less similar when compared with the P y - G C traces. This was due to the fact that the total ion currents were generated using m / z 50-450, neglecting high-intensity fragment ions below m / z 50, which are commonly present in high eV spectra of aliphatic compounds.

CONCLUDING REMARKS P y - G C - M S at 80 eV is a necessary complementary analytical technique to low-voltage P y - M S whenever a firm structural identification of compounds present in a pyrolysate is needed. In this case study, both sets of results have been shown to be consistent with each other when the difference in response of the various classes of compounds is taken into account and the structural identification is based on P y - G C and P y - G C - M S data. This means that not only P y - G C and P y - G C - M S results, but also the P y - M S data, are in disagreement with the petrographic data for the two samples under study. The question remains of the extent to which pyrolysis data

238 reflect the c h e m i c a l c o n s t i t u t i o n of the samples. F u r t h e r i n v e s t i g a t i o n s are n e e d e d in o r d e r to u n d e r s t a n d the r e l a t i o n s h i p b e t w e e n m a c e r a l c o m p o s i t i o n and pyrolytic depolymerization.

ACKNOWLEDGEMENT P a r t of the p y r o l y s i s w o r k in this investigation was s u p p o r t e d b y the F o u n d a t i o n for F u n d a m e n t a l R e s e a r c h o n M a t t e r ( F O M ) , a s u b s i d i a r y of the N e t h e r l a n d s O r g a n i z a t i o n for the A d v a n c e m e n t of Pure R e s e a r c h ( Z W O ) .

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