Anal. Chem. 1099, 65, 1937-1946
1937
Laser Micropyrolysis Gas ChromatographyIMass Spectrometry of Coal Paul F. Greenwood, Etuan Zhang, Frank J. Vastola, and Patrick G. Hatcher' Fuel Science Program, The Pennsylvania State University, University Park, Pennsylvania 16802
The focused output of a pulsed ruby laser is used for the in-situ pyrolysis of individual coal macerals. This study couples laser pyrolysis with gas chromatography/mass spectrometric analysis allowing f o r t he detection of neutral, volatile organic compounds formed in coal pyrolysis. Typically, released pyrolyzates from the vitrinite of a subbituminous coal and a coalified log of lignite rank include a wide distribution of aliphatic compounds (predominantly alkanes) in addition to aromatic compounds such as alkylbenzenes, phenols, and naphthalenes. The production of such compounds highlights the efficiency of laser radiation as a source for coal pyrolysis. The distribution of the detected components in the pyrograms is typical of compounds produced from vitrinite-rich coals by other pyrolysis methods. Differing pyrolyzates populations, including subtle intensity fluctuations for common product classes, allows for chemical distinction between the two samples examined. The potential of this technique for the in-situ chemical investigation of individual coal macerals which may be present in very small amounts within the parent coal is established.
INTRODUCTION The heterogeneity, physical domain size, and involatile nature of coal's macromolecular components has made it difficult to probe the structure and compositionof individual macerals or physically recognizable entities of the coal. Few analytical techniques have the ability to adequately investigate involatile, thermally labile materials, and even fewer techniques have the capability to do this at a microscopic level. Flash pyrolysis (i.e., thermal degradation) is one analytical technique which has had some success in providing information related to the chemical composition of heterogeneous macromolecular materials. Useful data on the molecular compoaitional units of the macromolecules have been obtained 0003-2700/93/0365-1937$04.00/0
with this procedure when applied to coals and kerogens.112 The production of structurally significant products is due to the rapid heating (Le., -lo3 O C s-l) associated with the technique.3 Since, extreme heating rates (Le., >lo8 O C 8-1) also characterize laser pyrolysis, similar high molecular weight products may also be expected from this technique. Bulk amounts of coal material are generally analyzed by flash pyrolysis, however, preventing a direct investigation of individual coal macerals. Some efforts have been directed a t physically isolating selected macerals prior to analysis. This has been performed via both hand-picking techniques and density-gradient ~ e n t r i f u g a t i o n .The ~ ~ tedious sample handling associated with these maceral isolation techniques is a major disadvantage. The purity of components separated by these techniques is also sometimes questionable. It would be far more convenient to analyze the maceral components in situ so that chemical information on microscopically recognized petrographic constituents can be obtained directly. The emergence of laser micropyrolysis as an analytical technique may facilitate this goal. Laser pyrolysis has tremendous potential as an analytical tool because laser radiation, with its highly collimated and coherent energy, can deliver very large amounts of thermal energy on a localized area of small dimension. The impact area can be further reduced, usually less than 100pm, through the use of focusing lenses, allowing very small components within complex mixtures to be isolated, hence individually analyzed. When the laser energy is focused through the optical system of a microscope onto a polished coal or kerogen surface, specific macerals can be selectively identified and pyrolyzed. In this way a systematic in-situ study can be made on the various macerals of coals and kerogens of different ranks. This technique has the potential to revolutionize the field of (1)Eglinton, T.I.; Larter, S. R.; Boon, J. J. J. Anal. Appl. Pyrolysis 1991,20,20-46. (2)Horsfield, B. Geochim. Cosmochim. Acta 1989,53,891-902. (3)Hanson, R.L.; Vanderborgh, N. E. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1979; Vol. 3,Chapter 4. (4)Zhang, E.; Hatcher, P. G.; Davis, A. Org. Geochem., in press. ( 5 ) Dyrkacz, D. R.; Bloomquist, C. A. A.; Ruaic, L. Fuel 1984,63,13671373. (6)Robert, P. Organic Metamorphism and Geothermal History: Microscopic Study of Organic Matter and Thermal Evolution of Sedimentary Basins; D. Reidel Publishing Co.: Boston, 1988; p 311. (7)Nip, M.;deleeuw, J. W.; Crelling, J. C. Energy Fuels 1991,6,125136. 0 1993 Amerlcan Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
organic petrography, which has traditionally relied on morphologic interrelationships of the macerals to infer origin and chemical composition and ultimately to improve our understanding of coal chemistry. The term “micropyrolysis” was used to describe pyrolysis experiments utilizing a microscope-focused laser beam to selectively irradiate a coal sample.8 Initial micropyrolysis studies incorporated conventional mass spectrometric detection (e.g., time-of-flight mass spectrometry) to probe the resultant ion populationP14 The laser microprobe mass analyzer (LAMMA), which became commercially available in the early 19809, is similar in principle to the instruments used by Vastola et al.a”Jand Karn et al.11J2 in the 1960s. LAMMA has been used with much succesa in the investigation of a variety of organic compounds, including polymers.16 Not surprisingly this technique was applied to coal by several independent groups.l”l9 Early LAMMA studies of coal detected products such as polyacetylenes (Le., C,H,+), alkylbenzenes, alkylnaphthalenes, dihydroxyphenols, and benzofurans.16J’ The instrument has also been used to monitor relative concentrations of elemental components of coal such as Ba, Cr, Ga, Sr, Ti, and V.l9 It has been shown, however, that C clusters of the type Cn+,C,H+, and C,Hz+ dominate the mass spectra of coal-based LAMMA experiments. Only charged species are analyzed by mass spectrometry, and the neutral pyrolyzates are ignored in studies employing direct mass spectrometric detection. Such a procedure restricts significantly the detection sensitivity since many more neutrals than ions are produced during any laser ablation/pyrolysis process. All volatiles are probed when online gas chromatography is incorporated into the instrumentation. Vanderborgh et al. have undertaken several on-line laser pyrolysisGC/MS studies of coals.*B Powdered samples contained in glass tubes coupled to the inlet system of a GC were irradiated by a (quartz lens) focuaed laser beam. Powders were preferred to small coal chunks as they gave more reproducible results.23 More recently laser investigations of large organic polymers24-26 and coals, sometimes doped with polyaromatic hydrocarbons (PAHs),26-28have been undertaken with on(8)Vastola, F. J.; Pirone, A. J. R e p r . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1966,10,C-53. (9)Vastola, F. J.; Pirone, A. J.; Given, P. H.; Dutcher, R. R. Spectrometry of Fuels; Plenum Press: New York, 1970; Chapter 3,pp
line GC/MS methods. Successfulproduction and entrapment of high molecular weight pyrolysis products were indicated by the mass spectral detection of various monomeric units and PAH parent ions, respectively. These results were very encouraging and suggested the technique, when applied to other macroscopic materials including pure coals, may yield structural data from the production of fragments of sufficient molecular weight size to more suitably represent coal structural fragments. Appropriate modifications including a lowvolume pyrolysis chamber and an optic fiber were, a t times, incorporated26sm to successfully improve the trapping efficiency of the released volatiles. Specially designed devices allowing online GUMS analysis of pyrolyzates from coal samples have been reported from independent studies.27,U The potential for coal fingerprinting by this technique has been demonstrated. For example, a large distribution of aliphatic materials extending to very high mass detected from an immature Torbanite alginite confirmed the highly aliphatic structure of the macromolecule(s) comprising immature alginites.28 The primary objective of the present study is to assemble instrumentationfacilitating the in-situ analysis of individual macerals in coal by laser micropyrolysis GUMS and to demonstrate the applicability of the technique for two wellcharacterized coal samples. Significant experimental parameters are investigated so as to fully test the applicability of the technique. For this reason relatively homogeneous samples were selected for the initial experiments discussed in this paper. Spectral fluctuations associated with heterogeneoussamples are not encountered with uniform materials. Because successful micropyrolysis of coal macerals requires that the pyrolysis products be transferred without fractionation to the GC column, particular attention was directed at the optimal operating temperature of a pyrolysis chamber housing the sample. The use of the visible light from a ruby laser represents a novel approach to on-line laser pyrolysis GC/MS studies since all previous investigations employed IR irradiation from either COzor Nd-YAG lasers. Few differenes from the IR-produced pyrolyzate population are expected with the 694.3-nm wavelength of the ruby laser.3 The data obtained are compared to resulta from the more conventional flash pyrolysistechnique to establish the viability of the laser as a pyrolysis source.
EXPERIMENTAL SECTION Instrumentation. The laser micropyrolysis arrangement 2 ~--.4 ~ . -used in this work consisted of three principal modules: (1)the (10)Vaatola, F. J.; McGahan, L. J. Fuel 1987,66,886-889. laser and associated optical system used for pyrolysis and sample (11)Karn,F. S.;Friedel, R. A.;Sharkey, A. G. Carbon 1967,5,25-32. (12)Karn, F. S.;Sharkey, A.G.; Logar A. F.; Friedel, R.A.Bur. Mines viewing, respectively;(2) the sample chamber and cold trap; and Rep. Invest. 1970,No. 7328. (3) a GC/MS for the separation of volatile products and detailed (13)Joy, W.H.; Ladner, W. R.; Pritchard, E. Fuel 1968,19, 26. molecular-level characterization. The configuration of this (14)Biecar, J. P. J. Chromatogr. 1971,56,348-352. instrumentation is schematically shown in Figure 1. (15)Hercules, D. M.; Day, R. J.; Balneanmugam, K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982,54,280A-290A. The laseroptical system consisted of a home-built pulsed ruby (16)Dutta, P. K.;Talmi, Y.Fuel 1982,61,1241-1244. laser (wavelength 694.3 nm) and microscope system described (17)Gaines, A. F.; Page, F. M. Fuel 1983,62,1041-1045. The laser beam, with a maximum output energy of (18)Lyons,P.C.;Hercules,D.M.;Morelli,J. J.;Sellers,G.A.;Mattarn, 0.1 J, is focused by the 43X objective of the microscope onto the D.; Thompson-Rizer, C. L.;Brown, F. W.; Millny, M. A.Znt. J . Coal Geol. surface of a polished coal sample mounted in the pyrolysis 1987,7,185-194. (19)Morelli,J.J.;Hercules,D.M.;Lyons,P.C.;Palmer,C.A.;Fletcher,chamber. The microscope is equipped for reflected light illuJ. D. Mikrochim. Acta [ W e n ] 1988,111,106-118. mination and long working distance objectives to enable the (20)Hanson, R. L.; Vanderborgh, N. E.; Brookins, D. G. Anal. Chem. sample chamber to be mounted directly on the microscope 1975,47,335-338. substage. (21)Hanson, R.L.;Brookins, D. G.; Vanderborgh, N. E. Anal. Chem. The heat-controlled chamber is a separate unit which was 1976.48. ~. . .. --.-2210-2214. --- - - - ~ (22)Hamon, R.L.;Vanderborgh, N. E.; Brookins, D. G. Anal. Chem. developed in this laboratory to be compatible with both laser 1977.49.390-396. - - . ., .., - - - - - -. pyrolysis and GC/MS analysis. The sample port was kept to a (23)Vanderborgh, N.E.;Venino, W. J.; Fletcher, M. A,;Nichols, B. small size (i.d. 7 mm; depth 10 mm) to reduce the amount of dead A. J. Anal. Appl. Pyrolysis 1982,4,21-31. (24)Stout, S.A.;Hall, K. J. Anal. Appl. Pyrolysis 1991,21,195-206. (25)Pozzan, A.;Cecchinato, F.; Seraglin, R.; Traldi, P.; Cecchetti W.; (27)Maswndeh, W. M.; Arnold, N. S.; Meuzelaar, H. L. C. Prepr. Polloni, R. Org. Mass Spectrom. 1990,26,392-394. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990,35,713-720. (28)Stout, S. A. 9th Meet. SOC.Org.Petrog., R e p r . 1992, 59-62. (26)Maawadeh, W.M.;Roberta, K. A.; McClennen, W. H.;Meuzelaa~, H. L. C.; Arnold N. S. R o c . 37th ASMS Conf. Mass Spectrom. Allied (29)Cecchetti, W.;Polloni, R.;Bergamasco,G.; Seraglia, R.; Catinella, TOP.1989; pp 304-305.
S.; Cecchinnto, F.; Traldi, P. J. Anal. Appl. Pyrolysis 1992,23,165-174.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
Pyrolysis Chamber , HC4ltC.d
I
,
*en
He
Mass Spectrometer
Flgure 1. Schematicof the laser micropyrolysis gas chromatograph/ mass spectrometric instrumentation.
Table I. Maceral Composition of the Subbituminous PSOC-1632Coal maceral type maceral name d " P (~01%)
vitrinite vitrinite vitrinite vitrinite inertinite inertinite inertinite inertinite liptinite liptinite liptinite
ulminite gelinite humodetrinite corpohuminite fusinite semifusinite macrinite sclerotinite suberinite resinite cutinite
66.0 7.2
4.1 1.2
4.8 4.1 3.6 0.6 6.2
1.4 0.8
admmf, dry, mineral matter free measured on a volume basis. volume. A silica glass window on the top allowed microscopic observation and laser penetration. A thermally resistant (to>300 "C) and low contaminating Calrez o-ring was used to seal the window. The chamber was constructed of stainless steel and included a bore into which a thermocouple and heating element could be placed to heat the chamber. Inlet and outlet carrier gas ports were also included. A l/lgin. metal tubing loop connectedto the inlet port provided a flow of He carrier gas near the point of laser impact. A type J thermocouple-controlled wrap heater heated the carrier gas line loop to -300 "C to preheat the He prior to entering the chamber. From the outlet port a l/lrin. metal line tubing (0.5mm i.d.) was connected to the GC column through a modified inlet system. This transfer tubing is wrapped in heating tape and kept at elevated temperatures (i.e., >250 "C). To minimize the degree of condensation, it is necessary to heat the hardware components in contact with the pyrolyzates. A cold trap for condensing of volatiles, consistingsimply of a loop of the fusedsilica GC column submerged in a liquid nitrogen bath, is located in the GC oven. The gas chromatography/mass spectrometry was performed on a Carlo Erba GC (Model 5163)IKratos MS-80 (Kratos Analytical, Ramsey, NJ) double-focusing mass spectrometer system, The GC is fitted with a fused-silicacapillary column (30 m X 0.25 mm i.d.) with a stationary phase consisting of 50% phenylmethyl polysiloxane (RTx-50, Restek Corp., Bellefonte, PA). Sample Description. The two coals investigated were a subbituminoushumic coal (PSOC-1532)selected from the Penn State Coal SampleBank and a sample of coalified wood of lignite rank from the Potomac Groupin Maryland (i.e., Patapsco lignite). The sample description of the lignite is given elsewhere by Hatcher.30 The subbituminous coal (i.e., No. 1532) came from the No. 4 Usibelli seam of Coal Bearing Group (Suntrana formation, M. Miocene) in Koyukuk, Middle Yukon County of Alaska. The maceral composition of this coal is outlined in Table I. The Patapsco lignite is 100% ulminite. The primary reason for the selection of these essentially vitrinitic coals is their relative optical homogeneity. In the absence of spectral fluctuations associated with the random (30)Hatcher, P. G. Energy bels 1988,2,48-58.
1939
sampling of different components within heterogeneous coals, run to run differences from a uniform sample can be directly related to the variation of experimental parameters. Differences in chemicalcompositionare expected between the low-ranklignite and higher rank subbituminous coals. Spectral comparisons between the two coals should establish the viability of the technique for identifying such differences. Both the samples were prepared without use of epoxy resin impregnation to reduce contamination. The dried samples were first cut into blocks with a band saw; one surface of each block (perpendicular to the bedding or a longitudinal section)was then ground to less than 3 mm in thickness and polished by hand with grindingand polishingwheels according to conventionalmethods, taking care to minimize contamination. The polished samples were then heated in an inert vacuum oven at a constant temperature of 250 "C for at least 12 h to reduce in content any high-volatilitycomponents or contaminants. Pieces (about 3-5 m m in diameter and 2-3 mm in thickness) of polished coal were then trimmed with a knife and mounted in the sample chamber. Procedure. The area to be pyrolyzed was selected when the surface was viewedthrough the microscope. The pyrolyzed zones or ablation craters can be varied in size from 250 pm to less than 50 pm in diameter by altering the degree of laser beam focusing. These craters can be 1-5 pm in depth as determined by the size of the irradiated area (i.e., the degree of focusing)as well as the energy setting of the laser. For a typical experiment the volatiles from 15 tightly focused pulses of maximum laser energy were collectively entrapped. The coal samples were successfully pyrolyzed by tightly focused high-energy pulses, and a sufficient number of volatiles were produced from 15laser shotaat different surface sites. It was necessary to investigate fresh surfaces with each pulse so that char produced from a previous pulse is not subsequently analyzed. Throughout pyrolysis the volatile products are swept by the He carrier gas and collected in the Nz coal trap. Approximately 15min was required to collect 15shots of maximum laser energy since 1 min after each pulse was allowed for the laser power supply to again reach maximum energy. Once the pyrolyzed products from all 15 pulses were trapped, the column was temperature programmed from 40 to 280 "C at 4 OC/min. The liquid nitrogen evaporates with heat releasing the collected pyrolyzate population. Mass spectra were obtained under electron impact conditions of 70 eV and a scan rate of 1500 m/z s-l and over a mass range of m/z 40-600. Products were identified by comparison of the measured mass spectra to mass spectral libraries and other published spectra. Few peaks other than the parent ion are detected for compounds of low concentration, makingit impossible to obtain library matches forthese products. Assignment of such compounds was based on given peak(@,but only if the measured retention time for that species was also consistent with the expected elution time relative to other components as predicted by the flash pyrolysis measurements described below. The spectrum to spectrum reproducibility of each run was generally good for the same samples under the same working conditions. To investigate the temperature dependence of condensation of volatiles on chamber walls, experiments were performed over the chamber temperature range of 160-240 "C. At these temperatures, thermal desorption and/or thermal pyrolysis of the coalsamplesmay occur. Experiments at similartemperatures and trap times (i.e., 15min) were repeated in the absence of any laser pulses to differentiate which products arise from laser pyrolysis and which from thermal pyrolysis/desorption. Standard flash pyrolysis GUMS data were obtained for the two samples by a procedure described previ~usly.~~ The instrumentation used was a CDS 10oO(ChemicalData Systems,Oxford, PA) pyroprobe interfaced to a Varian 2700 (Varian Associates, Palo Alto, CA) gas chromatograph and DuPont 21-490B mass spectrometer (DuPont Instruments, Inc., Monrovia, CA) fitted with a Teknivent Vector/One data system (Teknivent, Maryland Heights, MO). The GC column was similar to the one used in the laser pyrolysis experiments. Briefly, 1mg of sample was loaded into a quartz capillary tube of the heated coil probe. This
-
h
(31)Hatcher, P. G.; Lerch, H.E., l 1988,67,1069-1075.
m,Kotra, R. K.;Verheyen, T. V.
10a
98 I
,g
80 70
-
produced but excluded from the TIC in order to highlight the organic compounds volatilized. The high abundance of this product is consistent with the general observation from earlier high-powered laser pyrolysis studies of low molecular weight species including COz t o be t h e major pyrolysis
benzene naphthalene
-
C
p~~~~~~~~8,9,11-14,16,17,20,21-23
60-
64:
50-
I
vinyl benzene
1
8.29
A large number of peaks identified as n-alkanes (Figure 21, (alkyl)benzenes,(alky1)phenols(including xylenol, cresol,and guaiacol),(alkyl)naphthalene, and PAHs are readily observed. Many of these compounds, with naphthalene a notable exception, are also identified to be major flash pyrolysis products and are believed to be fragmented components from the coal macromolecules. Evidence of their presence together with relative concentrations provides direct chemical information regarding the macromolecular components of Patapsco lignite. The high concentration of naphthalene may arise from contamination. A blank run (i.e., without a coal sample) at 200 "C revealed both naphthalene and benzene to be background products. The Calrez O-ring, despite property specification claims of thermal stability below 300 OC, is a likely source of contamination. Aromatic contaminants such as these have been observed in high concentration from standard rubber and Teflon O-rings. While it may be reasonable to assume that a majority of the high molecular weight products observed from the summed chromatogram are indeed products from laser pyrolysis, it still remains possible that some products arise from thermal desorption (Le., evaporation and/or pyrolysis) initiated by the high temperatures of the pyrolysis chamber. The question of thermal desorption is discussed in detail in a following section, where it is shown unequivocally that laser pyrolysis is the major contributor to the formation of all assigned pyrolysis products. Many of the aliphatic and aromatic pyrolyzates detected with these experiments have also been observed with other pyrolysis techniques. To allow a comparison of the laser pyrolysis results with those from flash pyrolysis,the TIC from the flash pyrolysis of Patapsco lignite is shown in Figure 3.
indene
16.59
33.59
25.29
42.29
Retention Time
Flgure 2. Total Ion chromatogram of ions over the range mlz 55-260 from laser pyrolysis of Patapsco lignite at a chamber temperature of 195 O C . The symbol A represents alkanes.
tube was placed inside the coils of the pyroprobe. The probe and sample were then inserted into the injection port (temperature maintained at 280 O C ) of the gas chromatograph, and the sample was pyrolyzed. Flash pyrolysis conditions were the following: temperature, 610 "C for 10 s with a heating rate of 5 OC/ms. The pyrolyzate was cryotrapped with liquid nitrogen prior to being chromatographed on a 25 m X 0.25 mm i.d. J&W Scientific DB17capillarycolumn. The GC was temperature programmed from 40 to 280 O C at 4 OC/min. The effluent was swept into the source of the mass spectrometerfitted with the data systemfor detection and compound identification. Compoundswere identified by a combination of methods which include comparison of mass spectra to the NBS/Wiley library, to published mass spectra, and to authentic standards whenever possible.
RESULTS AND DISCUSSION Patapeco Lignite. The total ion chromatogram (TIC) trace, representing a sum of m / z 55-260, for the laser pyrolysis of Patapsco lignite coal, analyzed under the conditions described above and with the cell chamber at a temperature of 195 "C, is shown in Figure 2. A large amount of COz is
21. Ct-guaiacol 22. C1-guaiacol 23.4-methvl rmaiacol 24. C p g u d a G l 25. catechol 26. ethvl methvluhenol 27. CZIguaiacol' 28.4-ethylguaiacol 29.3-methyl catechol 30.4-methyl catechol 3 1.4-propyl guaiacol 32. CZ-catechol 33. ethyl catechol 34. C2-naphthalene 35. trans-isoeugenol 36. vanillin 37. C4-phenol 38. acetoguaiacone 39. pmpan-2-one guaiacol 40. C5-naphthalene
1. benzene 2. toluene 3. m+p-xylene 4. o-xylene 5. n-propyl benzene 6. methoxy benzene 7. C3 -benzene 8. Q -benzene 9. methyl pmpylbenzene 10. phenol 11. methoxytoluene 12. indene 13. o-cresol 14. m+p-cresol 15. methyl indene 16. guaiacol 17. 2-ethyl phenol 18. 2,4 xylenol 19. ethyl phenol 20. dimethyl indan
18
19
3&-
1
I
10
15
25
30
35
40
Retention Time (min)
Flgure 3. Total ion chromatogram (m/z 47-500) from CDS pyroprobe flash pyrolysis of Patapsco lignite at 610 O C .
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, A W S T 1, 1993
Table 11. Products of Common Classes of Compounds from Both Flash Pyrolysis and Laser Pyrolysis of the Patapsco Lignite flash pyrolysisa (610 OC)
laser pyrolysisb
Benzenes benzene toluene m+p-xylene o-xylene n-propylbenzene Cs-benzene methoxybenzene Cd-benzene methylpropylbenzene methoxytoluene
benzene toluene xylene vinylbenzene Cs-benzene
Phenols phenol o-cresol m+p-cresol 2-ethylphenol 2,4xylenol 4-ethylphenol ethylmethylphenol 4-methyl-2-propylphenol C4-phenol Polycyclic Aromatics indene methylindene dimethylindan
phenol o-cresol m+p-cresol 2,4-xylenol
indene methylindene biphenyl acenaphthalene fluorene
Guaiacols guaiacol
guaiacol C1-guaiacol C1-guaiacol 4-methylguaiacol Crguaiacol ethylguaiacol 4-propylguaiacol isoeugenol vanillin acetoguaicone propan-2-one guaiacol
4-methylguaiacol
vanillin
Catechols catechol 3-methylcatechol 4-methylcatechol Cz-catechol ethylcatechol Naphthalenes Crnaphthalene methylisopropylnaphthalene 1-naphthalenol 2-naphthalenol Ca-naphthalene C1-naphthalenol 5,7-dimethyl-l-naphthol 1,2-naphthalenedione a ~~
naphthalene Cz-naphthalene
Cb-naphthalene
DuPont 21-490B. b Kratos MS80. ~
Consistencywith laser pyrolysis is evident with the detection of (alkyl)benzenes, (alkyl)phenols, and alkylnaphthalenes. Listed in Table I1 are classes of compounds from the Patapsco lignite detected by both flash pyrolysis and laser pyrolysis. It is clearly evident from the comparative data shown in Table I1 that both types of pyrolysis experiments produce many of the same aromatic products such as (alkyl)benzene, (alky1)phenol (including (alkyl)guaiacols), alkylnaphthalenes, and several polycyclic aromatics. The formation of the same types of products suggests a similar pyrolysis mechanism operates during both laser and flash heating. In view of the much higher heating rate associated with laser
1841
pyrolysis (>lo8 OC 5-11 than with flash pyrolysis (-109 OC s-9, such similarities are perhaps unexpected. However, the formation of some of the same products at the different heating rates suggests that heating rate is an insignificant parameter above a certain value. It has been previously proposed that the devolatilizationof coals remains essentially unchanged over the heating rate range of 10-"106 "C 8-l.n Also notable in Table I1is the larger range of species which are detected by flash pyrolysis of the Patapsco lignite. The absence from the laser pyrolysis experiments of many compoundsobserved by flash pyrolysis may be due to a lower concentration of these species produced by the laser. Flash pyrolysis does reveal a decreasing concentration with an increase in the alkyl moiety of these aromatic classes. The sensitivity of the present laser micropyrolysis GC/MS instrumentation may not be sufficient to detect such laser pyrolyzates of extremely low intensity. A comparison of the >2O-pg samples typically wed in flash pyrolysis to the 190 " 0 ; tridecane (i.e., CIS-alkane),the most dominant, and other alkanes (>200"C); and toluene (>200 "C)appear, in that order, as the temperature is progressively increased. Since these aromatic and alkane species are predominant flash pyrolysis products it is reasonable to suggest they arise from thermal decomposition of the coal in the chamber, which clearly may occur at hightemperatures. Other major products detected from the laser pyrolysis experiment such as cresol, xylenol, and C1-guaiacol are not observed in the temperature range of these experiments. Insufficient thermal energy may be available to effect the release of these components. In general, the concentration of all thermally desorbed species from Patapsco lignite increases with temperature. The kinetics associated with thermal desorption are enhanced at higher temperatures. The concentrations of some of the thermal and laserproduced high molecular weight products from Patapsco lignite at a chamber temperature of 220 "C are shown in Figure 6. The relative intensities of the species produced at this chamber temperature through (1)both thermal and laser production (lightbars) and (2) thermal productionalone (dark bars) are indicated. It is evident from Figure 6 that the intensities of the thermally desorbed products are less, and in most cases considerably so, than when the pyrolyzed volatiles of 15 laser pulses are included. This unequivocally indicates that the production of these products is contributed to by the laser pulse. Several species such as cresol, 2,4xylenol, and C1-guaiacol were not produced by thermal desorption at 220 "C; hence the intensities indicated by the
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
1944
53
a
... r
1:
Y
.7.-
'
Inn Jlonitorctl
(
'27
JVZ)
Figure 6. Parent ion intensity of products produced from Patapsco ligniteat a chamber temperature of 220 "C with (light bars) and without (dark bars) laser pyrolysis.
light bars from these products are exclusively attributable to laser pyrolyzate intensities. While it is possible that the laser, a phenomenal localized heating source, contributes further heat to enhance thermal desorption, other high-energy mechanistic routes are likely at the extreme heating rates of the laser. It can be concluded that the predominant formation route for all products shown in Figure 6 is laser pyrolysis although an alternativethermally operating formation route exists for many. Rarely does a single mechanistic process operate exclusively throughout high-powered laser ablation experiments. Several different mechanistic pathways, including both laser pyrolysis and thermal desorption, are expected to contribute to the overall product population. Trends identified from relative abundances of some laser pyrolyzates compared to the relative abundances of these same products from thermal desorption experiments may provide additional information regarding formation mechanisms. Unique mechanistic relationships experienced by the products for the different pyrolytic techniques lead to abundance variations. For example, consider the ratio of benzene to toluene concentrations from Patapsco lignite a t a chamber temperature of 220 "C. With laser (+thermal) production this ratio is -51 while for thermal desorption alone it is -11:l. The relative variation in the values, consistent at all chamber temperatures, indicates either toluene to be more susceptible to laser production or alternatively benzene to be more susceptible to thermal production. The different abundances may simply reflect differing concentrations of these products as clathrated and rigid species. It is likely that only the more mobile clathrated componentswithin coal are released by the low heating rates of thermal desorption, while the higher energies associated with laser pyrolysis can also probe the "backbone" macromolecules within coal. Pyrolysis reactions become more significant with higher heating rates, and relative bond strengths within a structure will favor specific routes of pyrolysis. Consider the pyrolysis of an aromatic moiety attached to the macromolecular coal structure by C-C linkages. Energeticallythe most favorable cleavage is that of /3-scission which leads to the production of toluene. This route is likely preferred over the production of benzene, which requires the cleavage of the stronger alkyl-
aryl bonds. Thus, pyrolytic degradation of the coal macromolecules will probably lead to the enhanced relative production of toluene over benzene. This may partially explain the lower benzene to toluene ratio which is consistent throughout the laser pyrolysis experiments. Such a result is quite significant as it highlights the selective nature of laser pyrolysis toward desired products (e.g., toluene). In future work, further attention will concentrate on this phenomenon to confirm whether, and the degree to which, certain products are favored over others by laser pyrolysis. In the absence of laser irradiation a more extensive range of thermally produced volatiles arises from the No. 1532coal. In addition to large concentrations of naphthalene, benzene, and CO1 a t the relatively low temperature of 160 "C, signals are also observed from toluene, phenol, a distribution of alkanes (of which tridecane is again dominant), and a variety of biomarkers. The detection of the alkane distribution and such aromatic compounds a t this low temperature is intriguing. Although these species represent anticipated pyrolysis products, the results from Patapsco lignite suggest thermal pyrolysis will not commence until chamber temperatures in excess of 190 "C are reached. Products detected a t higher chamber temperatures includexylene (>190 "C), cresol (>210 "C), and guaiacol (>210 "C). The detection of xylene and cresol is of further interest since neither product could be thermally desorbedfrom Patapsco lignite. A consistency with the results from the Patapsco lignite is the increase in concentration with temperature of all thermally produced species from the No. 1532 coal. It is apparent that thermal desorption of products similar to those observed under pyrolytic conditions is also observed at temperatures below the expectedonset of thermal pyrolysis. A plausible suggestion is that the detected speciesare trapped within the coals and evaporated a t these relatively low temperatures. However, it is generallybelieved such produds are componentsof coal's rigid three-dimensionalnetwork and severe operational conditions which can initiate pyrolysis are required to facilitate their release. It is very possible that structural changeswithin the condensed phase of the coal are thermally driven at high temperatures (i.e., >200 "C). The transformation within the coal of "backbonencomponents to more mobile material clathrated within the network will result in these relatively isolated components becoming more accessible to release upon thermal disturbance. This proposal of thermally decreasing the stability/rigidity of a component within the coal may account for the subsequent thermal desorption of such species a t low temperatures. There is little evidence to indicate the preheat treatment initiates a significant chemical transformation within the Patapsco lignite since pyrolyzates from this coal are not observeda t low temperatures. The respective results suggest the Patapsco lignite has greater thermal stability than the No. 1532 coal. 2. Condensation. Condensation of laser-generated volatiles in the pyrolysis chamber is thought to be a significant sensitivity limiting factor of the present technique. To reduce the degreeof condensation,all hardware componentsbetween the pyrolysis chamber and cold trap were maintained a t high temperatures. The experimental relationship between condensation and temperature is investigated by monitoring the relative concentrations of the various volatile compounds produced from the two coals over the temperature range of 160-240 "C. As expected, a general increase in product concentration is observed with higher temperatures. In addition to a possible reduction of condensation, concentrations will be further increasedby the enhancement of thermal desorptionprocesses a t the higher temperatures. The increasing concentration of
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
f-*
Chamber Temperature ( O C )
Flguro 7. Intensky of the cresol product at various chamber temperatures from (A) laser pyrolysis of Patapsco lignite, (B) laser pyrolysis of the subbituminous PSOC-1532 coal, and (C) thermal desorption of the subbituminous PSOC-1532 coal (Le.. without laser pyrolysis).
laser pyrolyzates at temperatures below the threshold for thermal pyrolysis (e.g., -190 "C for Patapsco lignite) is unequivocal evidence for a thermally related decrease in the degree of condensation. The production of cresol from the two coals by both laser pyrolysis and thermal desorption as a function of the cell chamber temperature is shown in Figure 7. Presented are the relative intensities of the cresol product a t the different chamber temperatures for both laser pyrolysis and repeat experiments without the laser (i.e., exclusive thermal desorption). Cresol is not thermally produced from Patapsco lignite over the temperature range of interest; hence only three line shapes are illustrated. The cresol laser pyrolyzate from Patapsco lignite (i.e., curve A) first appears at a chamber temperature of 195 "C. The intensity of this product is then observed to increase rapidly with temperature to 210 "C.A steady state of evolution is established above this value with no further increase in intensity with temperature observed. This suggests the enhancement of ion production and entrapment, through minimizing condensation, is highest at temperatures above 210 "C. The value 210 "C hence represents an optimum value for this procedure. The threshold chamber temperature for detection of the cresol laser pyrolyzate (i.e., curve B)from the No. 1532 coal is less than 165 "C. An increase in concentration for this product is observed at higher temperatures. It also apparent that the rate of evolution lessens between 165 and 190 "C with a further surge above 200 "C. The reduction of condensation with temperature may be maximized at temperatures above 190 "C as indicated by the almost uniform concentration of cresol in the 180-195"C range. This proposal is consistent with the result from Patapsco lignite, where a similar behavior for the cresol pyrolyzate was observed over the temperature range 195-210 "C. This proposal is consistent with the result from Patapsco lignite, where a similar behavior for the cresol pyrolyzate was observed over the temperature range 195-210 "C.
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If, as with the No. 1532 coal, condensation is minimized above 190-200 "C, then all products formed a t these and higher temperatures will condense a t the same minimal rate. The concentration of cresol products detected from laser pyrolysis (Le., laser conditions remain uniform throughout) will therefore be expected to be uniform for temperatures above 190 "C. The rapidly increasing cresol concentration at the higher temperatures (Le., >200 "C) in the laser experiment with the No. 1532 coal can be attributed to the additive contributions from thermal desorption. Thermal production of cresol (i.e., curve C) commences at 200 "C and closely mirrors the line shape of the summed laser pyrolysis and thermal desorption concentrations of cresol at higher temperatures. From these results it is evident (for the cresol product at least) that condensation becomes a minimum in the temperature region 1 W 2 1 0 "C. This range is, therefore, established as the most suitable operating temperature for the technique since higher temperatures will only serve to further enhance thermal desorption. Comparing the relative intensity of cresol from Patapsco lignite with that from the No. 1532coal at lower temperatures (Le., 195 "C) where cresol is also formed exclusively from laser pyrolysis reveals a much higher abundance of cresol in Patapsco lignite. Similar results were obtained from the comparison of flash pyrolysis data associated with the two coals. The consistency revealed by the two techniques suggests the laser pyrolysis technique may also have much potential for quantitative analysis. It is further evident from a comparison of the temperature behavior of cresol from the two coals that the thermal characteristics of the coalmust be considered. Products (e.g., cresol from Patapsco lignite) from laser pyrolysis will be easily identified from thermally resistant coals which do not undergo thermal desorption at low temperatures. Such may not be the case for less thermally resistant coals. The detection of cresol at a chamber temperature of 165 "C from the No. 1532 coal and the absence of cresol from Patapsco lignite a t this temperature (with identical laser conditions) may be related to the relative stabilities of the two coals. Preheating the samples (i.e., -250 "C) may thermally initiate structural transformation resulting in some components within the macromolecules becoming more vulnerable to physical disturbance. For example the cresol within the No. 1532coal may rearrange to eventually become entrapped components easily accessible to evaporation. The higher thermal resistivity of Patapsco lignite may prevent a similar chemistry being experienced by the cresol components within this sample. In addition to an influence from the thermal characteristics of the coals, the thermal history of the samples will also be a significant parameter.
CONCLUSIONS The results outlined in this report from preliminary experiments performed in our laboratory have proved very encouraging. Low concentrations of aliphatic and aromatic pyrolyzates have been detected from two coals of different rank. These products included a distribution of alkanes and a variety of alkylated aromatics. A number of biomarkers characteristic of the No. 1532 coal were also observed from this sample. Mass spectral data together with information from flash pyrolysis allowed for the successful identification of most pyrolyzates. In addition to this qualitative analysis, some quantitative information was also obtained. Distinction between the two coals was observed from the intensity distributions which differed for the two coals. An increase in pyrolyzate concentrations with temperature to a uniform value at 190-210 "C highlights the successful
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optimization of the instrumentation in regard to condensation of volatiles within the chamber. Higher temperatures will not be necessary or desirable when one considers that many coals are thermally unstable at these temperatures. Severe sample preheating (1250"C) to remove loosely bound volatiles may not be appropriate for thermally unstable coals that experience chemical transformations at these temperatures. Although the sensitivity of the present basic instrumental arrangement appears to be restricted to some extent, the chemical analysis of the major componentsof individual coal sampleshas been demonstrated. Sincethe very small sample volumes examined in this study (15 etched holes of
