Determination of chemical-structural changes in vitrinite

accompanying luminescence alteration using C-NEXAFS analysis

G. D. CODY1*, H. ADE2, S. WIRICK3, G. D. MITCHELL4 and A. DAVIS4

1Carnegie Institution of Washington, Geophysical Laboratory, Washington, DC 20015, U.S.A., 2TheDepartment of Physics, North Carolina State University, Raleigh, NC, U.S.A., 3The Department ofPhysics, SUNY, Stony Brook, NY, U.S.A. and 4Coal and Organic Petrology Labs, The Pennsylvania

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

(Received 28 August 1997; returned to author for revision 5 November 1997; accepted 7 January 1998)

AbstractÐThe phenomenon of luminescence alteration has been shown to correlate with the thermalmaturity of Type III kerogens (vitrinites). In order to establish a chemical structural basis for this cor-relation, carbon near edge X-ray absorption ®ne structure (C-NEXAFS) spectroscopy is used to moni-tor the gain and loss of organic functionality in ultra-thin sections of vitrinite following timeincremental exposure to blue light (390±490 nm) irradiation in air. These data are compared with lumi-nescence alteration behavior measured at 600 nm. Three samples are studied; low maturity (%R0=0.29), medium maturity (% R0=0.73), and high maturity (% R0=1.35) vitrinite. These exhibit``positive'', ``dual'', and ``negative'' luminescence alteration, respectively. It has been previously estab-lished that the luminescence alteration of vitrinites is the result of photo±oxidation. C-NEXAFS dataare used to identify the types of reactions and correlate the chemical structural changes with lumines-cence alteration behavior. The unaltered C-NEXAFS spectrum of each vitrinite is signi®cantly di�erent,re¯ecting the broad range in vitrinite maturity. The dominant reaction is the formation of COOHgroups, through the attack of singlet oxygen on, predominantly, benzylic carbon. Carbonyl substitutedaromatics are the dominant photo±oxidation product of the most mature vitrinite. The photo±chemicaloxidation pathways and kinetics vary signi®cantly between the three samples. Virtually all of the majorspectral trends (excluding the formation of COOH groups) reverse, moving from low to high maturity,i.e. gains in absorption at a given energy at one maturity level are observed to be losses at a di�erentmaturity level. The spectral changes reveal that in the lower maturity samples aromatic acids, aliphaticketones, and hydroxylated aromatic compounds are formed; aliphatic and aldehydic carbon are lost. Inthe more mature vitrinite, aryl±ketones and aromatic acids are formed, whereas polycyclic aromaticcompounds are lost. Strong correlations exist between the development of ``positive'' alteration and theformation of COOH functionality. No obvious correlation could be made between the C-NEXAFSdata and the ``negative'' luminescence alteration, suggesting that the lumophor participating in thisreaction is below the detection limit of C-NEXAFS spectroscopy. The maturity of a given vitrinitesample, hence its molecular structure, strongly controls the speci®c reaction pathways as well as thetotal extent of reaction. # 1998 Elsevier Science Ltd. All rights reserved

Key wordsÐvitrinite, luminescence, photochemical oxidation, C-NEXAFS, thermal maturity

INTRODUCTION

Assessment of the molecular evolution of type III

kerogens (vitrinites) with diagenesis provides a

measure of the thermal history of associated rock.

Vitrinite, a macromolecular material recognized as

being derived from the biomacromolecular constitu-

ents of vascular plants, undergoes substantial

chemical structural modi®cation with the extent of

diagenetic alteration (maturation); including sub-

stantial losses in oxygen containing functionality

(carboxylic acids, ketones, and hydroxylated aro-

matics), increases in the fraction of aromatic car-

bon, as well as increases in the concentrations of

polycyclic aromatic hydrocarbons. The ability to

accurately assess the degree of maturation of vitri-

nite; hence the thermal history of the associated

lithology is crucial in oil exploration (Hunt, 1979;

Tissot and Welte, 1978) as well as providing im-

portant constraints on paleoheat ¯ow and burial/

uplift histories in geodynamic studies of sedimen-

tary basins (e.g. Beaumont et al., 1987; Furlong,

1989; Zhang and Davis, 1993).

The most commonly measured maturity par-

ameter is vitrinite re¯ectance, % R0. There are oc-

casions where the use of % R0 underestimates the

true maturity of vitrinite; for example, in the case

of so-called perhydrous vitrinite (e.g. Price and

Barker, 1985). It is worthwhile, therefore, to have a

second parameter, independent of % R0, to sup-

plement maturity assessment. Ideally, such a par-

Org. Geochem. Vol. 28, No. 7-8, pp. 441±455, 1998# 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0146-6380/98 $19.00+0.00PII: S0146-6380(98)00010-2

*To whom correspondence should be addressed.

441

ameter would be more closely tied to the chemical

structural state of the kerogen. Recently, Wilkins et

al. (1995) have proposed that the phenomenon

of luminescence alteration be used in addition to

%R0 for the determination of thermal maturity in

oil exploration.

Blue and ultraviolet luminescence analysis of

vitrinites and other kerogens is a well established

tool in organic petrography. The analytical method

involves irradiation of vitrinite with blue (390±

490 nm) or ultraviolet (340±380 nm) light using a

petrographic microscope. The luminescence is

measured using a calibrated photometric system in

a narrow band or across a spectral range using a

grating monochromator. Absorption of light energy

induces photoexcitation of valance electrons within

certain organic functional groups to various excited

states; this energy is lost through the thermal exci-

tation of vibrational modes and the emittance of

light at longer wavelengths. The emitted light may

be broadly de®ned as luminescence, avoiding the

need to di�erentiate between ¯uorescence and phos-

phorescence de-excitation pathways. It is likely that

both pathways contribute to the luminescence of

vitrinites.

TeichmuÈ ller (1974) ®rst observed that when ana-

lyzed in air vitrinites exhibit luminescence intensity

variations with irradiation exposure time. Ottenjann

et al. (1982) described three distinct classes of

alteration behavior; positive, ambivalent (or dual),

and negative. Positive alteration refers to the pro-

gressive increase in luminescence intensity with time

during continuous exposure to blue or UV light,

negative alteration is the opposite trend, and dual

describes an initial decrease in luminescence inten-

sity followed by a later increase. These classes were

shown to correlate well with the thermal maturity

level of vitrinite; immature vitrinites (% R0<0.60)

exhibit exclusively positive alteration, vitrinites in

the range, % R0=0.60±1.0, exhibit dual alteration,

and mature vitrinites (% R0>1.0) exhibit negative

alteration. Wilkins et al., 1995 established that these

alteration trends persist even in kerogens exhibiting

``suppressed'' values of % R0, implying that the two

parameters are largely independent, hence the po-

tential utility for vitrinite maturation assessment in

oil exploration.

By monitoring the e�ects of atmosphere on the

time dependent luminescence intensity, Davis et al.

(1990) established that the alteration is the result of

photo±oxidation chemistry. There have since been a

number of studies aimed at determining the chem-

istry of luminescence alteration employing FTIR

(e.g. Pradier et al., 1990; Mitchell et al., 1996).

These studies have clearly de®ned losses in the ali-

phatic region and gains in the carboxyl C.Ostretching region that appear to correlate with lumi-

nescence alteration. Other subtle changes across the

FTIR spectrum were also recognized but have not

been described in detail. FTIR is a powerful tech-nique, however, spectral complexity and base line

artifacts related to scattering often complicate in-terpretation.In the present study, carbon near edge X-ray

absorption ®ne structure (C-NEXAFS) spec-troscopy was employed to systematically monitorthe change in concentration of various organic

functional groups following progressive photoche-mical alteration. Soft X-rays (l140 AÊ ) are used topromote carbon core level (1s) electrons into var-

ious bound and virtual excited states consisting, lar-gely, of anti-bonding molecular orbitals. Localvariations in electron density surrounding thephotoexcited cores impart di�erences in the energy

gaps between the core and excited states leading torelatively well resolved spectra of absorption bandscorresponding to carbon in di�erent organic func-

tional groups. C-NEXAFS spectroscopy is particu-larly appropriate for the present study in that theinner shell spectrum reveals the manifold of excited

states available for the photoexcited electrons. Inmany regards these excited states are well describedby the lowest unoccupied molecular orbitals

(LUMOs) of organic functional groups; thus, C-NEXAFS is complementary to UV-visible absorp-tion and luminescence spectroscopy.C-NEXAFS has been previously applied to probe

the electronic structure of organic carbon withinmicroheterogeneous kerogeneous materials (Cody etal., 1995a,b, 1996) and has been shown to provide

fairly well resolved functional group information.An experimental requirement for C-NEXAFS isextremely thin samples (150±300 nm), due to the

very large absorption cross section of carbon for X-rays in the 280±300 eV range (Henke, 1986). Thisrequirement is ideal for the present experiment inthat the samples are essentially ``surfaces''. In this

regard, C-NEXAFS should be extremely sensitiveto changes in chemical structure of vitrinites occur-ring at the sample surface.

EXPERIMENTAL

Samples

Vitrinite samples used in this study were selectedfrom blocks of coal collected from freshly exposed

mine working face during sampling for the PennState Sample Bank collection supported by theDepartment of Energy. These samples were sealedin foil multi-laminate bags under argon gas at the

mine and were stored under refrigeration until used.Three vitrinite samples were selected to representdi�erent luminescence alteration behaviors, i.e.,

positive, dual and negative. Analytical dataobtained from the nearby channel samples areshown in Table 1 to denote the approximate matur-

ity of the vitrinites used in this study.

G. D. Cody et al.442

Sample preparation

Ultra-thin sections of vitrinite were preparedusing an ultra-microtome with a diamond knife.Sample thickness were estimated to on the order of

100±200 nm based on their characteristic interfer-ence colors. Details on the sample preparation havebeen published previously (Cody et al., 1995a).

Blue-light irradiation and luminescence measurements

A Leitz Orthoplan MPV-II microscope photo-

meter system was used for both photo±oxidationand luminescence measurements. This system gener-ates excitation energy from a 100 watt mercury±arclamp which is then passed through a series of inter-

ference ®lters to give an excitation wavelengthbetween 390±490 nm. This light is re¯ected by a510 nm re¯ection short-pass dichromatic beam split-

ter and condensed onto the sample through a 50XNPL FLUOTAR air objective. Part of the absorbedexcitation energy is released as a longer wavelength

emission that is passed back through a 515 nm bar-rier ®lter to remove any re¯ected excitation lightand then through a 600 nm measurement ®lter. Thelight energy is transformed into an electronic signal

and ampli®ed by an EMI 9558 photomultiplier.System calibration is achieved using a ®ltered ura-nyl glass standard.

Measurement of the change in luminescenceintensity were made at 6 s intervals during thephoto±oxidation sequence in the following manner.

Circular irradiation areas of 40 mm were used foreach sample; this region being con®ned within the50 mm grid spacing of the thin section support.

Using white-light, a grid area was selected that con-tained the maximum amount of recognizable vitri-nite. Individual areas were photo-oxidized for 1, 5,10 or 15 min and a record of the change in lumines-

cence was recorded.

Carbon micro-NEXAFS measurements

Carbon NEXAFS data was acquired using thescanning transmission X-ray microscope (STXM)and microspectrometer located at the X1A beamline at the National Synchrotron Light Source at

Brookhaven National Laboratory. The X-raysource is an undulator on the 2.5 GeV electron sto-rage ring. In its current con®guration the STXMs

monochromator is capable of 0.3 eV energy resol-ution. Adjustable entrance and exit slits ensure thespectral purity of the monochromatic beam.

Focusing the monochromatic X-ray beam is accom-

plished with a Fresnell phase-zone plate objective

and an order-sorting aperture yielding a spatial res-olution of 55 nm. Micro-spectroscopy using theSTXM necessitates scanning the monochromatorwhile simultaneously moving the sample along the

optical axis to maintain focus using a stepper motordrive. The actual analysis area is consequently100�100 nm due to small positioning errors.

Additional details regarding the design and speci®-cations of the STXM at X1A are available else-where (e.g. Jacobsen et al., 1991).

Measurement protocol involved acquiring anabsorption spectrum spanning the energy range of280±300 eV, this was then corrected for background

absorption by subtraction of a spectrum obtainedwithout the sample. The data are presented as inabsorption mode as the ÿlog(T/T0), where T and T0

are the transmittance of the sample and back-

ground, respectively. Reproducibility from scan toscan is excellent, provided that background spectraare obtained just before or following each sample

spectrum. The inevitable decay in intensity of I0,the synchrotron generated X-ray beam intensity, issu�ciently slow as not to compromise the spectral

quality given total acquisition times on the order of60 s. A typical spectrum spans a wavelength rangeof 3 AÊ ; 512 points are acquired across the spectrumwith a dwell time of 100 ms. Energy calibration is

a�orded through comparison of the position of the1s±p* and Rydberg transitions of CO2 (Ma et al.,1991) bled into the irradiated volume.

RESULTS

Luminescence alteration

Figures 1±3 show typical alteration patterns forthe three di�erent maturity vitrinites. The least

mature vitrinite (DECS-26, Fig. 1) exhibits strong``positive'' alteration with nearly a factor of 6increase in luminescence intensity in 15 min. The

vitrinite of intermediate maturity (DECS-23, Fig. 2)exhibits a very strong negative alteration within sec-onds of irradiation. The luminescence intensitydrops by a factor of 2 within three minutes. Strong

positive alteration progressively recovers the inten-sity within the span of 15 min. The most maturevitrinite (DECS-13, Fig. 3) experiences a progressive

loss in luminescence intensity within 10 min of ex-posure. There is a suggestion of weak ``positive''luminescence alteration at t>10 min. The initial

luminescence intensity of the three vitrinites ranks

Table 1. Analytical properties of samplesa

Sample ID Seam Age Rank % C % H % O + S(di�) % Re¯ectanceb

DECS-26 Wyodak cretaceous subB 76.18 6.17 16.63 0.29DECS-23 Pittsburgh carboniferous hvAb 84.64 5.82 8.00 0.73DECS-13 Sewell carboniferous mvb 88.82 4.98 4.68 1.35

aDerived from channel samples.bMean maximum in oil.

C-NEXAFS analysis 443

Fig. 1. Change in the luminescence intensity during irradiation (390±490 nm) in air measured at 600 nmfrom low maturity vitrinite (% R= 0.29) (DECS-26).

Fig. 2. Change in the luminescence intensity during irradiation (390±490 nm) in air measured at 600 nmfrom intermediate maturity vitrinite (% R= 0.73) (DECS-23).

G. D. Cody et al.444

as DECS-23>DECS-13>DECS-26; after 15 minexposure the ranking shifts such that DECS-

26>DECS-23>DECS-13. In a N2 atmosphere noalteration takes place (Davis et al., 1990); thus thesechanges are clearly the result of photo±oxidation.

C-NEXAFS spectroscopy of vitrinite

The ®ne structure in the C-NEXAFS regionspans the energy range of 283±290 eV; above

290 eV transitions tend to be very broad and over-lap with the absorption edge(s). Table 2 presents acompilation, extracted from the literature, of tran-sition energies and associated organic functionality

relevant to the present work. For the most part C-NEXAFS exhibits good selectivity, in that speci®cenergy regions tend to correspond to carbon in dis-

crete organic functional groups. However, regionsof overlap do exist, for example near 288 eV, wheretransitions associated with carbonyl substituted aro-

matic compounds and aliphatic carbon overlap.Also, transitions near 287 eV associated with ali-phatic ketones overlap with hydroxylated aromaticcarbon.

Figures 4±6 present stacked plots of C-NEXAFSspectra of each vitrinite sample following 0, 1, 5,and 10 min (0, 5, 10, and 15 for the DECS-13) ex-

posure time to 390±490 nm irradiation in air. Priorto discussing the spectral changes resulting fromphoto±oxidation, consideration of the unaltered C-

NEXAFS spectra is warranted.

Each unaltered spectrum (Figs 4±6, t= 0 min) is

dominated by a strong absorption band near

285.5 eV corresponding to a 1s±p transition of pro-

tonated and alkyl-substituted aromatic carbon or

ole®nic carbon. Clearly evident in Fig. 4 is a sharp

absorption band at 288.5 eV corresponding to the

1s±p* of carboxylate carbon (note that the 3 eV

increase in energy in the respective 1s±p* transitions

is related to the electron withdrawing nature of oxy-

Fig. 3. Change in the luminescence intensity during irradiation (390±490 nm) in air measured at 600 nmfrom high maturity vitrinite (% R= 1.35) (DECS-13).

Table 2. Compilation of relevant C-NEXAFS transition energiesand assignments

eV Transition(s) Functionality

283.7 1s±p* quinonea

284.9±285.51s±p* protonated/alkylated

aromatic and PNAb,c

285.8±286.41s±p* carbonyl substituted

aromaticd

287.1±287.4

1s±p* aromatic C±OHa,Ketone±C.Oaliphatice,f

287.7±288.31s±p* aromatic carbonyl

C.0d

287.6±288.2 1s±3p/s* CH3, CH2, CHg,h

288.2±288.6 1s±p* COOHe,i

289.3±289.5 1s±3p/s* C±OH, alcohole,f

aFrancis and Hitchcock, 1992.bHitchcock et al., 1987.cRobin et al., 1988.dHitchcock et al., 1992.eIshii and Hitchcock, 1988.fSham et al., 1989.gHitchcock and Ishii, 1987.hHitchcock et al., 1986.iIshii and Hitchcock, 1987.

C-NEXAFS analysis 445

gen). The intensity of absorption due to this tran-

sition is much weaker in the DECS-23 and DECS-

13 vitrinites (Figs 5 and 6) re¯ecting the lower

intrinsic carboxyl content, consistent with the an-

ticipated functional group chemistry for their re-

spective maturity levels. Comparing Figs 4±6, it is

apparent that the 1s±p* transition of the aromatic

carbon at 285.5 eV becomes progressively more pro-

nounced with increasing vitrinite maturity, as might

be expected in consideration of the trends in aroma-

ticity with maturity. This increase is accentuated,

however, by a reduction in the relative concen-

tration of hydroxylated aromatic carbon. The

lowest energy 1s±p* transition of hydroxylated aro-

matic carbon experiences a 1.7 eV shift towards

higher energy (Francis and Hitchcock, 1992), again

due to the electron withdrawing nature of the

neighboring oxygen. This 1s±p* transition is evident

as a shoulder in the region near 287 eV in each

spectrum; it is noteworthy that the 1s±p* transitions

of aromatic and hydroxylated aromatic carbons

appear comparable in intensity in Fig. 4 (DECS-26)

re¯ecting the much higher concentration of hy-

droxylated aromatic carbon in this low maturity

vitrinite. However, it must further be noted that the

oscillator strength of the 1s±p* transition of hy-

droxylated aromatic carbon is almost a factor of

two greater than that of protonated or alkylated

aromatic carbon (Francis and Hitchcock, 1992).

Absorption bands associated with sp3 hybridized

carbon such as methyl, methylene, alcohol, and

ether carbon are less apparent in these C-NEXAFS

spectra. Core level transitions to bound states as-

sociated with carbon in these functional groups do

occur; the excited state is generally considered to be

a state involving mixed 1s±s* (C±H) and 1s±3p

(Rydberg-like) characteristics (StoÈ hr, 1991). In the

case of aliphatic carbon the 1s±s* (C±H)/3p tran-

sitions are observed near 288 eV (Hitchcock and

Ishii, 1987), alcohols and ethers have transitions

near 289.5 eV (Table 1) (Ishii and Hitchcock, 1988;

Sham et al., 1989). Both transitions are di�cult to

resolve in the present spectra due to relatively low

concentrations and signi®cant band-overlap with

neighboring transitions.

Each spectrum is dominated by a broad region of

absorption extending from 290 eV to higher ener-

gies. As mentioned above, this is principally the

absorption edge(s) of carbon, i.e. the ionization

threshold(s) where the energy of the X-ray photon

is su�cient to completely remove the core electron

from the in¯uence of the core±hole. The intensity

of this absorption is, however, modulated by a

superposition of broad 1s±s* transitions and, at

Fig. 4. Stack plot of C-NEXAFS spectra of low maturityvitrinite (DECS-26) following irradiation (390±490 nm) inair for 0, 1, 5, and 10 min. The arrows highlight regions ofthe spectra where there are signi®cant changes in intensitycorresponding to photochemical oxidation with exposure

to 390±490 nm irradiation in air.

Fig. 5. Stack plot of C-NEXAFS spectra of mediummaturity vitrinite (DECS-23) following irradiation (390±490 nm) in air for 0, 1, 5, and 10 min. The arrows high-light regions of the spectra where there are signi®cantchanges in intensity corresponding to photochemical oxi-dation with exposure to 390±490 nm irradiation in air.

G. D. Cody et al.446

higher energies (>320 eV), photo±electron scatter-

ing (e.g. EXAFS).

The unaltered spectra (Figs 4±6, t= 0 min), exhi-

bit clear maturity dependent trends, i.e. in aromati-

city and losses in organic oxygen containing

functionality. As will be described below, maturity

dependent di�erences also exist in the photochemi-

cal oxidation behavior with progressive reaction

time as detected in the time resolved C-NEXAFS

spectra of each sample.

Beginning with the least mature vitrinite (DECS-

26), the following salient features are noted. The

most prominent change in the C-NEXAFS spectra

is growth in intensity of at 288.5 eV. There are also

clear increases in intensity at 287.1 eV and 290 eV.

These increases are o�set by a large decrease in

intensity at 288 eV and a slight decrease at

286.3 eV. The intermediate maturity vitrinite

(DECS-23) exhibits similar trends (Fig. 5); most

prominent, again, being the increases at 288.5 eV

and 290 eV, and 287.1 eV and a decrease intensity

in the region around 288 eV. However, there is also

a subtle increase in intensity in the region around

286.3 eV.

The photochemical alteration chemistry of the

most mature vitrinite (DECS-13) departs signi®-

cantly from the trends described above. Although

there is a clear increase in the intensity of the 1s±p*

transition (288.5 eV) corresponding to the for-mation of COOH functional groups, an equally

pronounced increased in intensity at 286.3 eV isalso evident. Contrary to the behavior of the twoless mature vitrinites, this sample exhibits an

increase in intensity at 288.1 eV and a decrease inintensity near 287.2 eV.

Assignment of the 290 eV transition

Assignment of the relatively prominent absorp-tion band at 290 eV, clearly evident in samplesDECS-26 and 23, is interesting and serves to

demonstrate the molecular aspects of C-NEXAFS.The high energy and narrow cross-section of the290 eV absorption band suggests a 1s±p* transition

from carbon with a relatively large 1s ionization po-tential. The clear correlation between the intensityof the COOH 1s±p transition and the transition at290 eV suggests that is related to carboxyl function-

ality. However, an electronically isolated COOHgroup, e.g. in the case of formic or acetic acid hasonly one unoccupied p orbital, hence a single strong

1s±p* transition is expected and observed (e.g. Ishiiand Hitchcock, 1987, 1988; Sham et al., 1989).Electronic coupling between the p electrons of the

carboxyl and other p systems would be required togive rise to additional higher energy 1s±p* tran-sitions.In the case of vitrinite, electronic coupling

between the carbonyl of the COOH groups andaromatic p systems may reasonably be expected dueto the relatively high aromaticity of these macro-

molecules. The e�ects of this coupling can bereadily shown via molecular orbital calculations.Figure 7 presents the results of extended HuÈ ckel

molecular orbital calculations (EHMO) for aceticacid, benzeneacetic acid, benzene, and benzoic acid.With the exception of benzene, each calculation

simulates the e�ects of core level photoexcitation ofthe carboxyl carbon using the equivalent ionic corevirtual orbital model (EICVOM) (Schwarz, 1975;Tossell, 1991). This method has been applied exten-

sively to facilitate interpretation of C-NEXAFSdata (e.g. Ishii and Hitchcock, 1988; Francis andHitchcock, 1992; Hitchcock et al., 1992) and, quali-

tatively, does an excellent job at reproducing thecore-level spectra.In the case of benzeneacetic acid there is virtually

no orbital interaction between the COOH and phe-nyl groups evident by the nearly identical HuÈ ckelenergies of the aromatic p orbitals in benzene andbenzeneacetic acid. This is hardly surprising as the

COOH group is rotated out of the plane of the phe-nyl group due to steric factors, nor is there any po-tential for conjugation with the ring. One observes

in the C-NEXAFS spectrum, therefore, a single 1s±p* corresponding to the COOH group; essentiallyindistinguishable from that of acetic acid. In the

case of benzoic acid, however, there is rather exten-

Fig. 6. Stack plot of C-NEXAFS spectra of high maturityvitrinite (DECS-13) following irradiation (390±490 nm) inair for 0, 5, and 15 min. The arrows highlight regions ofthe spectra where there are signi®cant changes in intensitycorresponding to photochemical oxidation with exposure

to 390±490 nm irradiation in air.

C-NEXAFS analysis 447

sive electronic interaction between the carbonyl and

aromatic p systems; leading to the prediction of

three 1s±p* transitions resulting from photoexcited

carboxylate carbon. In Fig. 7 these states are evi-

dent by the presence of charge density centered on

the carboxylate carbon. The lowest energy tran-

sition would be to the ÿ10.912 eV orbital (24a) and

should exhibit a strong absorption band near the

same energy as the 1s±p* transitions of acetic or

benzeneacetic acid (288.5 eV). A relatively intense

second 1s±p* transition is predicted 3 eV higher

(corresponding to the ÿ7.890 eV (26a) orbital). The

absolute energies and transition intensities, as pre-

dicted by the magnitude of the total charge density

matrix element at the ``excited'' carbon, are not

expected to be highly accurate using the EHMO

method; rather the signi®cance of these calculations

is that they predict a second, moderately strong,

1s±p* transition.

In a published spectrum of ethylbenzoate a sharp

transition is observed lying 1.4 eV above the

288.5 eV COOR (where R = ethyl) 1s±p* transition

(Hitchcock et al., 1992). This transition was

assigned to a second 1s±p* transition of the substi-tuted aromatic carbon with large contributions

from a second 1s±p* transition associated with theCOOR group; a point clari®ed using the EHMO-EICVOM approach (Hitchcock et al., 1992). It is,

therefore, likely that the second 1s±p* transitionpredicted in Fig. 7 corresponds to the 289.9 eVabsorption band observed in Hitchcocks spectrum

of ethylbenzoate.Assignment of the 290 eV resonance to carboxyl

substituted aromatics is a conservative interpret-

ation of the present C-NEXAFS data. If correct, itrequires that the majority of COOH groups to bederived from photooxidation of benzylic carbon, asopposed oxidation of carbon at positions b, g, andbeyond along aliphatic side chains of aromaticrings.

Trends in photochemical evolution of the C-NEXAFSspectra

The spectral trends observed in the C-NEXAFSspectra are the result of photochemical oxidation.

In order to highlight the chemistry and kinetics it is

Fig. 7. Results of EHMO calculations employing the EICVOM method to include the e�ects of thephotoexcited core electron. For acetic acid, benzeneacetic acid, and benzoic acid, the photoexcited car-bon is the carboxylate. Only the ground state UMOs of benzene are shown. The diameter of the circles,highlighting the localization and sign of the UMOs, correlate with the magnitude of the 2p AO coe�-

cients obtained from the total charge matrix, centered on each atomic center.

G. D. Cody et al.448

useful to deconvolute each spectrum to reveal the

time rate of change in absorption intensity related

to the formation or decomposition of a speci®c

functional group. Due to the obvious band overlap

in each spectrum, peak deconvolution is clearly not

a simple or straight-forward task. In particular,

there is di�culty involving overlap of the C (1s)

absorption edge(s) at energies near and above

290 eV. There are also numerous other low intensity

absorption bands that contribute to the spectra, e.g.

higher order p* transitions in aromatic compounds

as well as weak 1s±3s and ±4p transitions associated

with sp3 hybridized carbon (Hitchcock et al., 1986;

Francis and Hitchcock, 1992). Accepting these di�-

culties, the following conservative approach has

been adopted.

Individual absorption bands are de®ned by con-

sidering all peaks and shoulders from t = 0 to 10

or 15 min; the spectra are then ®t to yield minimum

residuals assuming Gaussian shaped absorption

bands (Fig. 8). Absolute peak intensities are relative

to the total carbon within the illuminated volume;

variations in thickness and/or density from sample

to sample makes an assessment of the number den-

sity of carbon atoms in the illuminated volume di�-

cult. All intensities, therefore, are normalized

relative to the intensity of the 1s±p* transition of

aromatic carbon. Provided that the concentration

of aromatic carbon is invariant with reaction pro-

gress, changes in the normalized intensities of the

component bands is a linear function of functional

group concentration. Consideration of the spectra

shown in Figs 4±6, reveals that the intensity of the

1s±p* transition of aromatic carbon is nearly con-

stant relative to the total carbon content (approxi-

mately evident by the absorption intensity on ``top''

of the C (1s) absorption edge, i.e.0293±296 eV).

In order to highlight the time dependent changes

in absorption intensity, di�erences in normalized

intensities (Rn=In/I1s±p*) are presented as a func-

tion of time, e.g. DI = RntÿRn

t = 0. This approach

is conservative as it records the time dependent

di�erential spectral intensity placed in the

context of formation or destruction of speci®c or-

ganic functional groups. Note that the di�erential

spectral intensities have not been scaled to re¯ectdi�erences in the oscillator strengths of di�erent

transitions.

Figures 9 and 10 reveal that both samples DECS-

26 and 23 exhibit grossly similar spectral trends

with reaction progress. The dominant change, in

both samples, results from the formation of COOH

functionality and a loss in aliphatic functionality;

i.e. gains at 288.5 and losses at 288.1 eV, respect-

ively. The reactions responsible for such spectral

features presumably are of the type:

Where R is most likely to be a phenyl group based

on the discussion above regarding the 290 eV

absorption band. Whether or not one would be

able to detect the intermediate peroxide is related to

both its stability and the magnitude of the tran-

sition dipole moment of the core level transition.

This transition is presumably a 1s±s*/3p type and

would be expected to lie in the vicinity of similar

transitions for primary (Ishii and Hitchcock, 1988;

Sham et al., 1989) and secondary alcohols (Cody,

unpublished data), i.e. near 289.5 eV and possibly

as high as 290 eV. This being the case, it is likely

that the absorption due to peroxides would lie

under the sharp 290 eV absorption band and con-

tribute to its apparent intensity.

In consideration of this possibility, it is interest-

ing to note the relatively fast growth of the 290 eV

absorption band in DECS-26 (Fig. 9) at the shortest

times, followed by slow growth over the range of

the experiment (this is particularly evident in Fig. 4,

t= 1 min). The same absorption band in Sample

DECS-23 (Fig. 10) exhibits considerably slower

Fig. 8. Example of deconvoluted near edge region of thecarbon 1s spectrum. Each C-NEXAFS spectrum was ®twith seven Gaussian line shapes. The energy and half-widths of these bands were held constant while the ampli-tude is varied to achieved minimum residuals. No e�ortwas made to correct the intensities for the presence of the

C(1s) ionization threshold(s).

C-NEXAFS analysis 449

growth kinetics over the same time interval. In both

samples, however, the initial kinetics of the COOH

formation is slower than that for the growth of the

290 eV absorption band; an observation that is con-

sistent with the hypothesis that the intensity in the

region around 290 eV band corresponds to a num-

ber of transitions, including one derived from a

reaction intermediate (e.g. a peroxide) on the way

towards forming COOH, e.g. reaction (1).

The extent of COOH formation in both samples

DECS-26 and DECS-23 is comparable after 10 min

irradiation. However, DECS-23 di�ers signi®cantly

from DECS-26 in the extent to which intensity at

288.1 is lost; sample DECS-26 loses more than

twice the intensity after 10 min exposure.

Both DECS-26 and 23 exhibit an increase in

intensity at 0287 eV. Certainly one means to do

this is through the formation of phenol and/or ali-

phatic ketone (Table 2) via some photochemical

reaction. It is clear that one could write equation (1)

such that the products are a ketone and water, as

opposed to the acid. We can discount such an

attack of the benzylic carbon in these two samples

as this would lead to the formation aryl ketones

which have their C.O 1s±p* transitions near 288 eV

(Table 2). Note, that in the case of the most mature

sample, DEC 13, there is evidence of aryl ketones

as described below.

A well known reaction that forms phenol and

acetone is the oxidation of cumene with peroxide

(March, 1992), e.g.

In vitrinite the relevant reaction would be

equation (3) where O2 attacks the b carbon of the

b±C5 linkage, the predominant linkage in lignin

derived aromatic macromolecules and low and

intermediate rank coals (Hatcher et al., 1994).

Note, that both the CAR±OH and CAL.O 1s±ptransitions lie near 287 eV (Table 2), making this

reaction a suitable candidate to explain the spectral

features in the lower maturity samples.

Fig. 9. Variation in the di�erential spectral intensity with exposure time. Sample DECS-26, low matur-ity vitrinite. Di�erential intensity at 288.5 eV (open square) corresponding to the 1s±p* transition ofCOOH groups, 290 eV (open triangle) corresponding to a second 1s±p* transition of COOH groupsand peroxide, 287.2 eV (solid circle) corresponding to aromatic C±OH 1s±p* and aliphatic carbonyl1s±p, 286.3 eV (solid square) corresponding to carbonyl substituted aromatic 1s±p*, and 288.1 eV (open

diamond) corresponding to aliphatic 1s±3p/s* and aromatic carbonyl 1s±p.

G. D. Cody et al.450

In the case of DECS-26, there is a small decrease

in intensity at 286.3 eV with time (Figs 4 and 9);

DECS-23, on the other hand, exhibits essentially no

change in this spectral region with time. This

absorption band is reasonably assigned to a 1s±p*transition of aromatic carbon substituted with a

carbonyl (Hitchcock et al., 1992) either a aldehyde

or ketone. Aldehydes are likely to be present in low

maturity vitrinites, e.g. syringealdehyde and/or

vanillin are commonly observed in immature wood

derived organic samples (Hedges et al., 1985).

Losses in intensity at 286.3 eV would result from

autooxidation of the aldehyde by O2 ®rst to a per-

oxyacid and ®nally to the aromatic acid. This reac-

tion would also lead to a reduction in intensity near

288.2 eV due to the loss of the carbonyl's 1s±p*transition and presumably accounts for the greater

loss in this spectral region for sample DECS-26

over sample DECS-23.

Sample DECS-13 exhibits substantially di�erent

spectral trends than the samples described above.

First, the total extent of reaction, i.e. gain or loss in

intensity of a given absorption band, is considerably

less than either DECS-26 or 23. For example, the

maximum amount of COOH intensity developed is

nearly a factor of four less than DECS-26 (Fig. 9).

A similar observation of a reduction in the extent

of reaction with increases in vitrinite maturity has

been reported by Mitchell et al. (1996) using FTIR.

Secondly, the most signi®cant growth in intensity

is near 286.3 eV. As discussed above, intensity in

this spectral region indicates the formation of car-

bonyl substituted aromatic carbon; either aldehyde

or ketone. Given that aldehydes are unstable under

oxidizing conditions, a reasonable reaction that

would yield these spectral trends involves the oxi-

dation of the benzylic carbon in a hydroaromatic

system to the ketone, e.g.

An additional consequence of such a reaction

would be changes in intensity at 288 eV correspond-

ing to the gains in the aromatic carbonyls 1s±p*absorption and losses in the benzylic carbons 1s±s(C±H)/3p absorption. It is clear from Fig. 10 that

growth in intensity at 288.1 eV parallels that at

286.3 eV, consistent with the formation of aryl±ke-

tones. That the intensity at 288.1 eV grows with

reaction progress indicates that the oscillator

strength of the ketones 1s±p transition must be

greater than that of the 1s±s* (C±H)/3p transition;

a point that has been veri®ed experimentally (Ishii

and Hitchcock, 1988).

Finally, the highest maturity vitrinite exhibits a

signi®cant loss of intensity in the vicinity of 287 eV

(Figs 6 and 11). This trend is the opposite of what

was observed in the lower maturity vitrinites (Figs 4,

5, 9 and 10). The energy of this transition is too

low to be assigned to aliphatic carbon (the principal

organic functional group lost via photochemical

oxidation of the lower maturity vitrinites) and

must, therefore, be attributed to some other func-

tional group. As discussed above, obvious possibili-

ties are hydroxylated aromatic carbon and aliphatic

ketone. The oxidation of hydroxyquinone structures

to quinone would result in a reduction intensity

near 287 eV. Quinones, however, exhibit a strong

absorption band at 283.6 eV; no such absorption is

observed in any of the samples (Figs 4±6). Invoking

an aliphatic ketone is problematic as this most

mature vitrinite has both a low oxygen content and

high aromaticity. A more probable explanation is

that polycyclic aromatic compounds, phenanthrene

in particular, are oxidized to aromatic acids (e.g.

van Krevelen, 1993). Phenanthrene has relatively

intense 1s±2p* and 1s±3p* transitions at 286.9 and

287.4 eV, respectively (Cody, unpublished results).

Conversion of phenanthrene or other phene type

polycyclic aromatic compounds to single ring aro-

C-NEXAFS analysis 451

matic acid compounds would result in a reductionof the size of the p orbital manifold leading to a re-

duction in absorption intensity in spectral regionaround 287 eV.

Chemical correlations with luminescence alterationbehavior

It is reasonable to assume that the di�erences inluminescence alteration behavior exhibited by thesethree vitrinites would correlate with the di�erencesin photochemical oxidation behavior as detected

via C-NEXAFS; however, such a correlation is byno means necessary. The luminescence of vitrinitesmay very well be controlled by the presence of di-

lute lumophors, whereas the photochemical oxi-dation detected using C-NEXAFS in this studyand FTIR in other studies probes the chemical

structural changes a�ecting the majority of organicfunctional groups. Nevertheless, the strong positiveluminescence alteration of samples DECS-26 andDECS-23 appears to correlate well with the

growth of carboxylic acid functionality upon ex-posure. The simplest explanation for this corre-lation is that some of the COOH bearing products

are lumophors; however, this explanation is notsupported by molecular spectroscopy. The additionof COOH groups to aromatic or aliphatic mol-

ecules does not signi®cantly alter their lumines-cence at these excitation wavelengths. Interactionbetween the COOH group and the aromatic pelectrons (e.g. Fig. 7) does lead to a shift inabsorption and ¯uorescence maxima to longerwavelengths. However, in the case of single ringaromatics this shift is small and is accompanied by

a pronounce decrease in luminescence intensity(Ja�e and Orchin, 1962).A more plausible explanation is that the increase

in luminescence intensity with time results from aparallel reaction involving a considerably more di-lute species. Such a compound would, evidently, be

undetectable via C-NEXAFS, but with photochemi-cal oxidation would yield a lumophor in su�cientquantities as to manifest signi®cant changes in the

luminescence intensity at 600 nm. For example, intheir study of the luminescence alteration of modelcompounds, Eberhardt et al. (1992) observed thatanthracene and pentacene both exhibited positive

alteration in the region near 600 nm upon exposureto 410 nm light in O2. The most probable expla-nation for this alteration is the formation of

quinones through the breakdown of trans-annular(epi-) peroxides. Trace quantities of anthraquinone

were detected by Hayatsu et al. (1978) in their

study of UV irradiation of solutions containing coal

particles. The extent to which such chemistry isoperating in the present samples must be very

minor, however, as there is no evidence of growth

in absorption intensity corresponding to quinones

(Table 2) in Figs 4±6. Furthermore, if such reac-tions were primarily responsible for positive altera-

tion, then one would reasonably expect a greater

extent of reaction in the most mature vitrinite

(DECS-13), which clearly contains a higher concen-tration of polycyclic aromatic hydrocarbons.

Alternatively, there may be a more subtle connec-tion between the formation of COOH groups and

positive luminescence alteration. For example, an

increase in the molar volume of the macromolecular

system due to an increase in concentration ofCOOH groups is likely. Such an increase would

result from reactions such as equation (3), which

leads to cleavage of network linkages in the vitrinite

macromolecule, hence dilation of the macromolecu-lar network. An increase the luminescence intensity

would, therefore, result from a reduction in ``con-

centration quenching''. The anticipated e�ect would

be a blue shift in the emission peak maximum withreaction progress; a trend observed by Eberhardt et

al. (1992) for two di�erent vitrinites exhibiting

``dual'' alteration. It is noteworthy that all of the

blue-light exposed regions of the ultra-thin sectionsexhibited some degree of warping, suggesting

volume expansion, perhaps the result of the for-

mation of COOH groups.

Identifying a spectral correlation with ``negative''

alteration is surprisingly di�cult. For example, the

most pronounced change in negative luminescencealteration behavior is observed in DECS-23 (Fig. 2)

where the intensity drops by nearly a factor of 2

in one minute. A study of the C-NEXAFS data

presented in Figs 5 and 10; however, reveals noobvious changes in the bulk carbon chemistry that

might account for such a large and rapid change in

the luminescence yield. It is probable, therefore,

that the negative alteration involves a photo±oxida-tion reaction of a dilute lumophor, not detectable

amidst the bulk carbon chemical signature recorded

in the C-NEXAFS spectrum.

In the Eberhardt et al. (1992) study of lumines-

cence alteration the principal reaction that con-

trolled the ``negative'' alteration in intermediate andhigh maturity vitrinites was interpreted to be due to

the formation of an epiperoxide through the reac-

tion of anthracene and singlet O2, e.g.

G. D. Cody et al.452

Fig. 10. Variation in the di�erential spectral intensity with exposure time. Sample DECS-23, mediummaturity vitrinite. Di�erential intensity at 288.5 eV (open square) corresponding to the 1s±p* transitionof COOH groups, 290 eV (open triangle) corresponding to a second 1s±p* transition of COOH groupsand peroxide, 287.2 eV (solid circle) corresponding to aromatic C±OH 1s±p* and aliphatic carbonyl1s±p, 286.3 eV (solid square) corresponding to carbonyl substituted aromatic 1s±p*, and 288.1 eV (open

diamond) corresponding to aliphatic 1s±3p/s* and aromatic carbonyl 1s±p.

Fig. 11. Variation in the di�erential spectral intensity with time. Sample DECS-13, high maturity vitri-nite. Di�erential intensity at 288.5 eV (open square) corresponding to the 1s±p* transition of COOHgroups, 290 eV (open triangle) corresponding to a second 1s±p* transition of COOH groups and per-oxide, 287.2 eV (solid circle) corresponding to aromatic C±OH 1s±p* and aliphatic carbonyl 1s±p,286.3 eV (solid square) corresponding to carbonyl substituted aromatic 1s±p*, and 288.1 eV (open dia-

mond) corresponding to aliphatic 1s±3p/s* and aromatic carbonyl 1s±p.

C-NEXAFS analysis 453

Singlet oxygen is well known as a dienophile andthe cycloaddition reaction above is facile for

anthracene and larger acenes (e.g. Wasserman andMurray, 1979). Our present results cannot supportor reject their interpretation. It can be stated, how-

ever, that there is minimal anthracene and otheracenes, in any of the vitrinite samples studied.Polycyclic aromatic hydrocarbons with three or

more rings have a large manifold of 1s±p* tran-sitions, e.g. 7 for anthracene and phenanthrene. Inthe case of anthracene the C-NEXAFS spectrum is

dominated by a pair of equally intense 1s±p* tran-sitions at 284.9 and 286.2 eV (Cody, unpublishedresults). Anthracene, if present, must be of lowenough concentration as to have its characteristic

spectrum obscured by the chemistry of the moreabundant substituted aromatic constituents.However, even a trace of anthracene may be of

major importance to the luminescence behavior ofthe vitrinite, if it operates as the principal lumophorin high maturity vitrinites (e.g. Eberhardt et al.,

1992).

CONCLUSIONS

The strong dependence of vitrinite maturity onthe kinetics and chemistry of photo±oxidation dis-cussed above is noteworthy and not entirelyexpected. For example, if it were assumed that the

photo±oxidation principally consumed aliphatic car-bon through the synthesis of acids, then one mightreasonably expect a progressive decrease in the

extent of reaction with increased maturity (increas-ing aromatic content) without signi®cant changes inthe kinetics or chemistry. What is observed is sur-

prising. Intensity at 286.3 eV is lost in the lowestmaturity vitrinite and gained in the highest maturitysample; the opposite trend occurs for intensity in

the spectral region near 287.2 eV. Virtually all ofthe major spectral trends (excluding the formationof COOH groups) reverse with increasing maturity.In all samples, benzylic carbon is the most suscep-

tible to oxidation, however, in the lowest maturitysamples the principal products are aromatic acids,whereas in the most mature sample the principal

products are aryl±ketones. In the lower maturitysamples aromatic acids, aliphatic ketones, and hy-droxylated aromatic compounds are formed; ali-

phatic and aldehydic carbon are lost. In the moremature vitrinite, aryl±ketones and aromatic acidsare formed, whereas polycyclic aromatic com-pounds are lost.

These maturity trends in photochemistry clearlyre¯ect the di�erences in molecular structure atdi�erent maturity levels. As the luminescence altera-

tion is clearly the consequence of photochemicaloxidation these results support the use of lumines-cence alteration as an independent probe of thermal

maturity. However, it must be noted that the

photochemical oxidation of vitrinite is a complexand heterogeneous process involving the formation

kinetics of reactive species, e.g. singlet O2, coupledwith adsorption, surface and bulk di�usion, and, ul-timately, reaction with the organic substrate. The

only means to thoroughly understand these reac-tions would be to engage in a systematic study ofsimple compounds, selected to model speci®c reac-

tion centers in the di�erent vitrinites. The resultspresented and discussed above suggest that such ex-periments have merit. Moreover, the results suggest

that the use of microscopic luminescence alterationmeasurements as a microprobe of molecular struc-ture may be reasonable.

Associate EditorÐJ. Hower

AcknowledgementsÐThe authors gratefully acknowledgethe Janos Kirz and the X1A beam line for their help inobtaining the C-NEXAFS data. George Cody gratefullyacknowledges ®nancial support from the Exxon EducationFoundation. Alan Davis and Gareth Mitchell acknowledgesupport from the Department of Energy under grantDE-FG-2293PC93223. Adam Hitchcock and StephenUrquhart very graciously o�ered to obtain carbon (1s)inner shell spectra of methoxybenzene and showed thatthe CAR±OH 1s±p* transition of phenol is, in fact, stron-ger than the CAR±O±Me 1s±p* transition. We are verygrateful for their help, advice, and discussions related tothese and other questions involving C-NEXAFS. TheX1A beam line is located at the National SynchrotronLight Source, a DOE supported facility, located atBrookhaven National Laboratory. Finally, the authorsthank Dr. Ron Wilkins and another anonymous reviewer;their comments and suggestions greatly improved thispaper.

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C-NEXAFS analysis 455