Concerning the Application of FT-IR to the Study of Coal: A Critical Assessment of Band Assignments...

layers on the wafer surfaces. We have not attempted a detailed investigation of a three-layer (oxide-silicon-oxide) specimen due to the variability in such parameters as oxide thicknesses and indices of refraction. However, we have observed that the amplitude of the tertiary interferograms does depend upon the actual reflectance of the wafer surface.

The observed interferograms can be corrected before transformation in order to eliminate Fabry-Perot fringes. Hirschfeld and Mantz 5 have developed a procedure for eliminating the effects of secondary interferograms by removing the interferometer signal from the region in which the amplitude of the extraneous interferogram is greater than the amplitude of the main interferogram. The signal in this region is then replaced by the signal from the same region of an interferogram obtained with a different specimen thickness.

We used this procedure for the secondary and the tertiary interferograms in determining the carbon content of a 0.869-mm thick silicon wafer by measuring the intensity of the substitutional carbon absorption line at 607 cm -1 at room temperature. A high-purity 2.0-mm thick vacuum float-zoned wafer was used as a reference. Fig. 3 shows the effect of the tertiary interferogram. In Fig. 3A, both the secondary and tertiary interferograms have been removed using the Hirschfeld-Mantz procedure. Fig. 3B is the difference between the spectrum shown in Fig. 3A and a spectrum obtained from an interferogram which was corrected only for the secondary interferograms. The carbon concentration determined from Fig. 3A is [C] = 1.256 x 1017 atoms - cm -3. The

magnitude of the error derived on the basis of Fig. 3B is approximately 1015 atoms - cm -3. Since Czochralski silicon wafers generally contain from 10 TM to 1017 carbon atoms - cm -3, this error can range from 1 to 10% of the carbon concentration.

II. CONCLUSION

We have demonstrated the existence of tertiary interferograms and have shown that they are produced by multiple reflections both between the surfaces of a specimen and between the wafer and the mirrors of the interferometer. We have also demonstrated a method for correcting the observed interferogram in order to measure the specimen absorptivity more accurately. The magnitude of the error introduced by not correcting for tertiary and secondary interferograms increases as the specimen thickness is reduced. Since the trend in the semiconductor industry is toward measuring impurity contents of thinner wafers, the potential errors introduced by both the secondary and tertiary interferograms will increase.

1. D. G. Mead and S. R. Lowry, Appl. Spectrosc. 34, 167 (1980). 2. R. J. Bell, Introductory Fourier Transform Spectroscopy, (Academic Press,

New York, 1972). 3. M.J.H. van de Steeg, H. W. H. M. Jongbloets, J. H. M. Stoelinga, R. W. van

der Heijden, R. J. M. van Vucht, and P. Wyder, Infrared Phys. 20, 121 (1980). 4. T. Hirschfeld, Appl. Opt. 17, 1400 (1978). 5. T. Hirschfeld and A. W. Mantz, Appl. Spectrosc. 30, 552 (1976). 6. F. R. S. Clark and D. J. Moffat, Appl. Spectrosc. 32, 547 (1978). 7. T. Hirschfeld, Appl. Opt. 16, 1905 (1977). 8. H. W. H. M. Jongbloets, M. J. H. van de Steeg, E. J. C. M. van der Werf, J.

H. M. Stoelinga, and P. Wyder, Infrared Phys. 20, 185 (1980).

Concerning the Application of FT-IR to the Study of Coal: A Critical Assessment of Band Assignments and the Application of Spectral Analysis Programs

PAUL C. PAINTER, RANDY W. SNYDER, MICHAEL STARSINIC, MICHAEL M. COLEMAN, DEBORAH W. KUEHN, and ALAN DAVIS Polymer Science Section, Department of Materials Science and Engineering (P.C.P., R. W.S., M.S., and M.M.C.) and Coal Research Section, The College of Earth and Mineral Sciences (D. W.K. and A.D.), The Pennsylvania State University, University Park, Pennsylvania 16802

The problems assoc ia ted w i t h the appl icat ion of F T - I R to the character izat ion of coal structure are crit ical ly discussed. The controvers ies concerning band as s ignments are considered and i t is concluded that the strong 1600 cm -1 band can be ass igned to an aromatic r ing stretching mode that in mos t coals is inten- s ity enhanced by the presence of phenol ic g r o u p s . T h e applica- t ion of computer rout ines to the determinat ion of O H and CH g r o u p s is considered. Establ i shed criteria for curve f i t t ing are appl ied to the problem. Qualitat ive identi f icat ion of funct ional

Received 4 October 1980; revision received 22 February 1981.

Volume 35, Number 5, 1981

g r o u p s is achieved, but consis tent quant i tat ive m e a s u r e m e n t s will r e q u i r e a d e t e r m i n a t i o n o f the re lat ionship be tween the ext inct ion coeff icients of reso lved bands .

Index Headings: Infrared; Instrumentat ion, FT-IR; Coal, infrared studies.

INTRODUCTION

Infrared spectroscopy is an important and extensively used analytical tool for determining the structure of

APPLIED SPECTROSCOPY 475

organic materials. Most of the fundamental work on applying this technique to coal characterization was performed in the 1950s and 1960s and has been reviewed in a number of authoritative articles. ~-4 Despite the depth of this work there remains some controversy concerning assignments, particularly the strong characteristic band appearing near 1600 cm -1, and attempts to obtain quantitative data have achieved incomplete success, partly due to the lack of suitable model compounds. However, the major limitation in applying traditional infrared methods to materials such as coal is the overlap and superposition of the absorption bands of such complex multicomponent systems.

The introduction of computerized Fourier transform infrared (FT-IR) spectrometers has opened up new pos- sibilities for the spectroscopic characterization of coal and coal derived materials. There are several advantages of FT-IR compared to dispersive instruments, discussed in detail in a number of reviews. 5-7 Essentially, the use of an interferometer rather than a system of gratings and slits results in a higher energy throughput to the detector. This, coupled with the ability of such internally calibrated computerized systems to co-add a large number of interferograms, results in markedly superior spectra, particularly in the energy limiting situations encountered in coal studies. The resulting multiplexed spectrum can then be scale expanded by the computer to display subtle features without undue interference from background noise. However, in coal studies it has been our experience that the most significant results can be obtained by using the on-line minicomputer to extract additional information from the spectra, for example, by subtracting spectra or curve resolving. In this labor~ttory we have developed FT-IR methods for the analysis of mineral matter in coal s-12 and also applied this technique to studies of liquefaction products 13 and the changes that occur upon carbonization TM and oxidation. 15' 16 Solomon17. is has examined the relationship between coal structure and ther- mal decomposition products. This author, in an attempt to obtain a greater insight into structure, initially corrected the spectra of a number of coals in order to account for particle scattering and water content. Each spectrum was then fitted to a selected set of 26 Gaussian bands whose individual widths and positions were held constant so that only their relative intensities were allowed to vary. A quantitative measure of aliphatic and aromatic C--H content was determined using the area under the peaks between 3000 and 2800 cm -1 and between 900 and 700 cm -1.

The papers of Solomon are the first to report the use of the data handling capabilities of FT-IR in an attempt to obtain quantitative results. In several earlier studies with dispersive instruments, for example the work of Durie et al.19 and Tschamler and Ruiter, 2° similar quantitative measurements of aliphatic and aromatic C--H contents have been made using the integrated intensity of the same peaks as Solomon. 17' 18 However, there are significant differences in the value of the factor, k, relat- ing the ratio of the integrated areas to the corresponding ratio of aliphatic to aromatic hydrogen, HjH~r . Solomon determined a value of 1.06, whereas Durie et al .; 19 determined values ranging from 0.52 to 0.86. Although it was certainly more difficult for this latter group to obtain

476 Volume 35, Number 5, 1981

data with the same degree of accuracy as can be determined with modern instrumentation, their measurements cannot be easily dismissed. Calibration of peak areas to hydrogen content was obtained using proton magnetic resonance studies on extracts representing approximately 40% of the original coal. On the basis of the range of values of k obtained, Durie et al. 19 suggested that any use of infrared spectroscopy to obtain H,~/Har values be treated with caution.

Solomon17, ~s has used a different method to obtain extinction coefficients. Essentially, total hydrogen content from elemental analysis and hydroxyl content from measurements of the area of the O--H stretching band near 3450 cm -1 were used in conjunction with the peak areas of aliphatic and aromatic bands to obtain a plot from which extinction coefficients can be determined. In general, this approach appears to be sound, but there are a number of problems. One difficulty is general to all infrared methods that have been used so far: What errors are introduced by summing peak areas over a number of bands, each of which has an individual extinction coefficient, and essentially averaging such coefficients for the total area? Other problems involve the use of curve resolving techniques and methods for correcting the spectra for mineral matter and scattering.

The work reported so far concerning the application of FT-IR to the characterization of the structure of coal demonstrates that it is potentially a powerful analytical technique. Consequently, it should be useful at this point to assess critically the problems in using this method and attempt to determine how these problems might be solved. In this communication we will pay particular attention to band assignments and the application of certain computer routines. Although this paper presents no fresh insights into the structure of coal, we believe it provides an essential foundation for future FT-IR studies.

I. E X P E R I M E N T A L

All spectra were recorded on a Digilab 15B FTS system. The instrument is internally calibrated by a He--Ne laser so that the frequency scale is accurate to 0.2 cm -1. Spectra were recorded by co-adding 400 "scans" (interferograms) at 2 c m - 1 resolution.

The importance of careful and consistent sample preparation cannot be overemphasized, particularly for quantitative work. We have discussed this point in some detail in our work concerning the analysis of mineral matter in coal. 8-n Essentially, if samples are to be prepared by dispersion in an alkali halide matrix it is necessary to undertake an initial grinding study. We have determined that for most coals approximately 1.3 mg of material in 300 mg of KBr produces optimum results. The coal and KBr are ground together in a Perkin-Elmer Wig-L-Bug. Fig. 1 shows a plot of the integrated absorption of the aromatic C--H out-of-plane bending modes (between 900 and 700 cm -1) of a maceral concentrate (vitrinite) plotted as a function of grinding time. It can be seen that optimum absorption is reached after 25 s grinding time. Based on such initial studies all samples were prepared by grinding for 30 s. It should be noted that different results are obtained with different grinding equipment. For example, an old model Perkin-Elmer Wig-L-Bug

700

600

; 500

300

200

l oo

Z~ A

lt] 20 31) 60 50 60

t ime (aecands)

Fro. 1. Plot of the integrated absorption in the region 900 to 700 cm-' in the spectrum of a coal sample vs grinding time in KBr pellet preparation.

required approximately 20 m i n grinding time to prepare coal samples for analysis.

II. RESULTS AND DISCUSSION

In this study we are primarily concerned with the analysis and assignment of bands due to the organic component of coal. In the infrared spectra of many coals there are also prominent bands that can be assigned to various minerals. Consequently, in order to illustrate some of the problems involved in infrared studies of the organic structure we will initially consider as an example the spectrum of a largely mineral matter free maceral concentrate, vitrinite, shown in Fig. 2. The bands that appear in this spectrum are typical of most medium rank coals. Because the spectrum is, in effect, a composite of the contributions of various functional groups, there is only a limited amount of formation that can be obtained from a direct examination. Brown ~1 demonstrated some years ago that the ratio of the peak heights of aliphatic C--H and aromatic C--H stretching modes near 2920 and 3030 cm -~, respectively, varied according to coal rank. Unfortunately, there are a number of problems with using simple peak height measurements to quantitatively determine these groups. It can be seen from Fig. 2 that the aromatic C--H mode is a weak broad peak that absorbs significantly over the entire 3100 to 3000 cm -1 range. The aliphatic C--H mode at 2920 cm -~ is characteristic of CHe groups and also CH~ groups attached directly to aromatic units. However, CH~ groups attached to other species usually absorb near 2960 cm -1, whereas isolated C--H groups absorb weakly near 2900 cm -~. One advantage of FT-IR is the availability of sophisticated programs that are applicable to problems of this kind. In initial experiments we decided to apply a program that calculates the integrated absorption in any specified spectral region. Thus we obtained a measure of the number of aromatic C--H groups by determining the peak area between 3100 and 3000 cm -1 and a measure of the total aliphatic C--H by determining the integrated absorption between 3000 and 2700 cm-'. These figures cannot be translated into aromatic to aliphatic C--H ratios without a knowledge of the absorption coefficients. However, the ratio of these areas for a number of vitrinite concentrates is plotted against percent reflectance in Fig.

3. We anticipated some sort of relationship between the two properties, since both vary in a systematic fashion with rank. The preliminary results plotted in Fig. 3 strongly suggest a linear relationship, an intriguing and potentially very useful result. Similar plots can be obtained using the out-of-plane aromatic C--H bending modes between 900 and 700 cm -~, as reported by Durie et al.19 and Solomon 17' ~s

This type of measurement (integrated absorption) is not unique to FT-IR and has been applied in a number of previous coal studies, often in conjunction with proton magnetic resonance measurements, in an attempt to obtain useful structural data. The presence of an on-line computer merely serves to make the measurements more convenient and probably more accurate. Nevertheless, the availability and convenience of on-line data process- ing equipment is essential if we are to gain a deeper insight into coal structure through the application of

Sm,c.

I " 1 " " I " [ I 7 - r - m . . . . . . . . . I , ~ -1 - - [ - - - ~ - -~ I ' ' ' 3(;000 32000 33(~0 2Ll100 20100 [ ~ 0 0 I 2 0 0 B~O cm ' l

Fro. 2. FT- IR spectrum of a vitr inite concentrate.

.o

o

.2

T o

o

\ L~

S"

7 -

$

0.7 o'.i Q',~ ~',0 ,'., l'.~ ,'.~ ,i~ Reflectance {%)

FZG. 3. Plot of the integrated absorption between 3000 and 2700 cm -~ (aliphatic CH) divided by the integrated absorption between 3100 and 3000 cm -1 (aromatic CH) vs percent reflectance of a set of vitrinite concentrates.

APPLIED S P E C T R O S C O P Y 4 7 7

more sophisticated routines such as curve resolving. For example, it should theoretically be possible to resolve the aliphatic C- -H stretching bands between 3000 and 2800 cm -1 into components that can be separately assigned to CH3, CH2, and CH groups. Maddams 22 has recently reviewed curve resolving methods and outlined the procedures that should be applied in order to avoid erroneous or misleading results. In this paper our primary aim is to discuss this and other critical aspects of the use of FT-IR in coal studies. For expository purposes it is therefore convenient to divide the rest of this discussion into four sections. First, we will at tempt to clarify the remaining controversies concerning band assignments. We will then turn our attention to a consideration of methods for correcting or adjusting spectra to account for mineral matter and particle scatter. Finally, we will consider the use of curve resolving methods in the context of measuring functional groups, specifically O- -H and C--H.

A. B a n d Ass ignment s . The assignment of the most characteristic bands in the infrared spectra of coals can be considered well-established. 1-4 These assignments are listed in Table I and the most important have been noted in Fig. 2. There remains a degree of confusion concerning two regions of the spectrum, between 1000 and approxi-

TABLE I. B a n d as s ignments for the infrared spectra of coals.

Aliphatic and aromatic groups Oxygen containing function groups

Wave number (cm-') Assignment Wave number (cm ') Assignment

3030 Aromatic C--H 2950 sh CH~ 2920~ Shliphatic --CH 2850J [CH2 and CH~

1600 Aromatic ring stretch

1490 sh Aromatic ring stretch

1450 CH~ and CH:~ bend, possibility of some aromatic ring modes

1375 CHa groups

900-700 Aromatic C--H out- of-plane bending modes

860 Isolated aromatic H 833 (weak) 1,4 Substituted aro-

matic groups 815 Isolated H and/or 2

neighboring H 750 1,2 Substituted, me, 4

neighboring H

3300 Hydrogen bonded

1835 C=O, anhydride 1775-1765 C~--O, ester with electron

withdrawing group attached to single bonded oxygen

O II

Ar--O--C--R 1735 C~O, ester

1690-1720 C~O, ketone, aldehyde and, --COOH

1650-1630 C=O highly conjugated O

II eg Ar-~C--Ar

Approximately 1600 High conjugated hydrogen bonded C~O

1560-1590 Carboxyl group in salt from --COO

1300-1110 C--O stretch and O--H bend in phenoxy structures, ethers

1100-1000 Aliphatic ethers, alcohols

mately 1350 cm -1 and the near 1600 cm -1. Solomon 17' is has resolved a number of bands in the former region of the spectrum and assigned them to ethers. However, a close examination of the literature concerning coal spectra 1-4 and group frequencies 23 demonstrates that assignments in this region are complicated and it is often not possible to assign bands to specific functional groups. Essentially, vibrations involving C--O stretching (presumably in both ethers and phenols), C--C stretching, O- -H bending and CH2 bending appear in this region of the spectrum. Close lying vibrational energy levels having the same symmetry properties are allowed to mix, so that each mode can take on some of the character of the other. There is the further possibility or even probability of mechanical coupling between, for example, adjacent C--C and C--O stretching vibrations in ethers and between C--O stretching and O--H bending motions in phenols. Consequently, it is likely that bands between 1000 and approximately 1350 cm -1 cannot be described in terms of simple motions of specific functional groups or chemical bonds, but instead have a complex, poorly defined, mixed character.

There remains an interesting degree of controversy concerning the assignment of the 1600 cm -1 band. There are two possible assignments. The band may be due to an aromatic ring stretching vibration and/or a chelated conjugated carbonyl structure, as in acetylacetone. 1-4 In most review articles the question is left dangling, although Friedel I considered the assignment to a carbonyl structure more logical. In a FT-IR study of coal liquefaction products 18 we observed an increase in the intensity of the 1600 cm -1 band with phenolic OH content, as did Solomoni7. 18 in an extensive study of a number of coals. Unfortunately, this data could be interpreted in terms of both assignments. Ring modes could be intensity enhanced due to the presence of phenolic OH or it could be argued that the correlation simply reflects that in coals with a higher phenolic OH content there are more chelated conjugated carbonyl groups. If the controversy is to be resolved, the previously cited evidence in favor of the carbonyl assignment has to be explained. The prin- cipal evidence is: (1) Fujii 24' 2s observed that the intensity of the 1600 cm -1 band decreases after methylation and reduction reactions. (2) The absence of appropriate model compounds in which a strong band near 1600 cm -1 can be assigned to an aromatic ring mode. In the spectra of most aromatic materials the 1600 cm -1 band is weak.

We disagree with the assignment to a carbonyl structure on a number of grounds. First, conjugated hydrogen- bonded structures similar to acetylacetone have more than one characteristic absorption band. In effect, functional groups of this type have four nearly equivalent bonds (two CO, two CC) and prominent modes near 1600, 1500, 1450, and 1260 cm -1 should be observed. 23 In the spectra of coals, bands are observed at these frequencies but are relatively weak and broad and can be clearly assigned to other functional groups. Furthermore, upon methylation of the - - O H groups involved in hydrogen bonding to a highly conjugated carbonyl, we would expect a shift from about 1600 to 1650 cm -1, where carbonyl groups in, for example, quinone type structures absorb strongly. 23 However, upon methylation Fujii 2455 observed a band appearing at much higher frequencies, near 1700


cm -1. Conversely, Durie and Sternhel126 observed no change in the intensi ty of the 1600 cm -1 band upon acetylation, in sharp contrast to the methyla t ion studies of Fujii.

The discrepancies of these results can be simply resolved. The work of Fujii involved the s tudy of a low- rank coal (C 77.9%), whereas Durie and Sternhell examined bi tuminous coals (C 84.0 to 89.9%). In low-rank coals there is usually a significant concentrat ion of COO- groups which absorb near 1580 cm -1 and contr ibute to the intensi ty of the 1600 cm -1 band. Upon methyla t ion (but not acetylation) the C O 0 - group is converted to an ester tha t absorbs near 1700 cm -1, as recently observed by Liot ta 27 in a s tudy of the alkylation of coal. Fur ther evidence for this in terpreta t ion of the results of Fujii is provided by a simple FT- IR study of a lignite (PSOC 624, C 73%). The infrared spectra of this coal before and after washing with 1N HC1 are compared in Fig. 4. A difference spect rum obtained by subtracting the spectrum of the acid washed material from tha t of the paren t coal is shown in Fig. 4. Positive (above the base line) bands in the spect rum represent functional groups tha t are removed upon washing, whereas negative bands represent groups tha t appear. Accordingly, the positive 1580 cm -1 and negative 1710 cm -1 bands demonst ra te a conversion of charged carboxyl groups COO- to the carboxylic acid form, COOH. We can therefore conclude tha t a port ion of the intensity of the 1600 cm -1 band in low-rank coals is due to the presence o f - - C O 0 - groups tha t absorb near 1580 cm -1. I t should also be noted tha t we have detected - - C O O - groups in oxidized coals of higher rank.15. 16

Finally, if the band at 1600 cm -1 is to be assigned to an aromatic ring stretching mode we have to explain why this band is intense in the spectra of coals bu t relatively weak in the spectra of most aromatic materials. Brown 2~ made the observation tha t the 1600 c m - 1 band is much more intense in the spectra of phenols, presumably because the presence of the oxygen a tom results in a larger change in dipole moment during the vibration. However, we also have to consider tha t most model compounds are low molecular weight materials. If aromatic s t ructures are linked in a polymeric s t ructure we would expect the infrared spectrum to be influenced by factors such as conjugation and vibrational coupling. We have recently

1610

1580

1900 600cm "1

Fro. 4. Top, FT-IR spectrum of a lignite; middle, FT-IR spectrum of the same sample washed with 1N HC1; bottom, difference spectrum.

A 29~0 1610

t ' i '° o,,

3000 ;200 2800 2H00 2000 I600 1200 800 cml

FIG. 5. Top, FT-IR spectrum of an English subbituminous coal. Bot- tom, FT-IR spectrum of a synthetic resin; phenol, dihydroxynaphtha- lene-formaldehyde copolymer.

synthesized polymeric materials tha t we believe will be good models for coal structures. 2s Fig. 5 compares the infrared spect rum of a phenolic resin, a phenol-dihydroxynaphtha lene- formaldehyde copolymer, to tha t of an English subbi tuminous coal. Although there are differences in the relative intensities of certain bands (e.g., the 1260 C- -O stretching characteristic of phenolic groups) there is a striking correspondence of the main spectral features, part icularly the characterist ic 1600 cm -1 band. Consequently, we can conclude tha t a strong 1600 cm -~ band can be assigned to an aromatic ring stretching mode in s t ructures in which there is the possibility of intensi ty enhancement due to the presence of phenolic groups or due to the linkage of aromatic entities by methylene and possibly e ther bridges. 2 In addition, in low-rank or oxidized coals there is an absorpt ion near 1580 cm -~ due to COO- groups. Evidence previously cited in favor of the assignment of the 1600 cm -I band to a conjugated hydrogen bonded carbonyl can be explained by the presence of carboxyl s t ructures in the coal.

B. C o r r e c t i o n s o r A d j u s t m e n t s t o t h e S p e c t r a . In order to obtain quant i ta t ive measurements of the func- t ional groups present in coal by FT- IR it is necessary to first account for the minerals tha t m ay be present. Sol- omon17, ~8 repor ted tha t coal spectra were corrected by subtracting the contributions of kaolinite and illite and scaling the spectra to give the absorbance for 1 mg of coal dmmf (dry mineral mat te r free). However, it is often the case tha t the spectra of mineral components are dominated by clays such as kaolinite and illite, s' 9 but these may consti tute only 20 to 30% by weight of the mineral ma t t e r present. Fur thermore , pyri te does not have absorpt ion bands in the mid-infrared region used in coal studies, so tha t the contr ibut ion of this mineral would not be measured by simple subtract ion procedures. Th e most accurate method for determining mineral content and adjusting the spectra to account for all the materials tha t may be present is to first obtain the low- tempera ture ash and then adjust the spectrum to the equivalent of 1 mg of organic material using the fractional amount of ash in the coal so determined. The spectrum of this ash can then be directly subtracted from tha t of the coal so as to eliminate the bands of the mineral constituents. 9 Occasionally, organic sulfur and nitrogen can be fixed as inorganic sulfate and ni t ra te in the ashing process, bu t these minerals can readily be detected and quanti ta t ively measured by FT- IR methods. '~' 12

The second adjus tment to coal spectra tha t concerns


us, since we believe it may lead to spectral artifacts, is the correction of the base line for scattering. In his seminal paper on the infrared spectra of coal, Brown 2~ discussed the origin of the sloping background and concluded that scattering was not the only source. Micro- scopic examination of various coals showed no significant variation in average particle size, but there was a general trend to higher background absorption with increasing rank. Reviewing the work of Brown and other similar studies, Dryden 2 concluded that at least in higher rank coals part of the background can be attributed to the "wings" of electronic absorption bands extending into the infrared. The background absorption may therefore vary with frequency in a nonlinear fashion. In fact, if we examine the spectrum of the vitrinite sample shown in Fig. 2, a substantial sloping base line is apparent between 3800 and approximately 1850 cm -1, but the spectrum flattens out and apparently slopes in the opposite direc- tion between 1800 and 500 cm -1, as shown in Fig. 6. This type of variation in background is apparently not unique to coal. Maddams 22 has pointed out that various workers have used parabolic functions to fit base lines. Clearly, if we straighten the base line according to the slope in one region (e.g., between 3800 and 1850 cm-~), as shown in Fig. 6, there will be a distortion in the remaining part of the spectrum. Solomon ~7' is used this approach and obtained spectra similar to those reproduced here, with the absorbance minima between 1000 and 500 cm -~ clearly raised above the new (straight) base line. In subsequent curve resolving an extremely broad band (half width approximately 200 cm -~) centered near 700 cm -~ was determined. This band was tentatively assigned to a carbonyl group and was stronger than the characteristic C ~ O stretching mode near 1700 cm -~. However, the characteristics of the 700 cm -~ band do not correspond to any known carbonyl group frequency. We suggest that it may be an artifact of the method used to straighten the base line. The bottom spectrum shown in Fig. 6 displays the result of using two separate sloping straight base lines, illustrated in the top spectrum, to adjust the spectra between 3800 and 1850 cm -1 and between 1850 and 500 cm -~, respectively. In this adjusted spectrum there is clearly no broad (200 cm -~ half width) underlying absorption centered near 700 cm -~.

If coal spectra are to be curve resolved or if spectra of materials having different backgrounds are to be compared accurately, it may prove necessary to adjust the

FiG. 6. Top, FT-IR spectrum ofvitrinite concentration with estimation of base line positions; middle, spectrum corrected for single, sloping straight line spectrum; bottom, regions of spectra corrected separately for background.

base line. However, the spectra reproduced in Fig. 6 indicate that such procedures are more dependable if applied separately to specific local regions of the spectrum.

C. The De te rmina t i on of H y d r o x y l Func t iona l Groups. There have been a number of attempts to measure the - -OH content of coal by infrared spectroscopy, either by direct measurement of the intensity of the O--H band near 3450 cm -117, is, 29 or by measuring the intensities of characteristic bands introduced by chemical reaction, e.g., acetylation. 26 ()sawa and Shih 29 determined a relationship between the specific extinction coefficient of the 3450 cm -1 absorption and the hydroxyl content which was subsequently applied in FT-IR studies of coal by Solomon) 7' ~s There are a number of problems with this approach. The extinction coefficient reported by Osawa and Shih relates to the peak height of the 3450 cm -~ band. Solomon ~7 apparently applied this coefficient to the area under this absorption. Assuming the coefficient was adjusted to reflect areas rather than peak heights, it is still difficult to see how accurate results can be obtained. Not only were different methods used to estimate the base line, but Solomon applied the same extinction coefficient to the summed areas of four separately resolved bands. Furthermore, in plots of optical density at 3450 cm -~ vs weight of coal sample 29 there is an intercept at positive values of optical density. Osawa and Shih 29 proposed that this "residual" absorption was due to traces of water remaining in the KBr disc, despite the care taken in the preparation and subsequent drying of the sample, a° FriedeP has discussed at some length the omnipresence of water in KBr preparations and noted that heating to 175°C is required to remove completely water bands, which nevertheless reappear upon cooling. Breger and Chandler 31 reviewed this problem and also reported that they could not eliminate the persistent water band near 3450 cm -~, despite numerous attempts at dehydration. These authors also noted that the band may not only be due to water, but also hydroxyl groups substituted in the alkali halide lattice, as previously proposed by Durie and Szewczyk. ~2 We have performed similar work in this laboratory and found residual absorption near 3450 cm -1 in samples heated at 120°C in vacuum for two days, in contrast to the result reported by Solomon 17 that this absorption could be eliminated by heating overnight at 120°C. However, in our studies we found the residual absorption to be weak, 0.02 absorbance units (for 300 mg of KBr). Nevertheless, this is enough to introduce a significance systematic error into absorbance measurements, since many coal samples have an absorption of approximately 0.15 absorbance units near 3450 cm -~ in the spectra of "dried" pellets (1 mg of coal in 300 mg of KBr).

There are other problems in simply using the optical density of the 3450 cm -1 band to determine quantitatively the number of OH groups. In addition to water absorbed by the KBr discs, water is bound to the coal itself in a complex manner that is apparently a function of rank. The broadening of the hydroxyl band due to hydrogen bonding (the degree of which could vary from coal to coal) and the possible presence of NH groups are also factors that conspire to make the interpretation of quantitative measurements suspect. Finally, it is not always


desirable to heat coal samples before analysis. Coking coals, for example, are easily oxidized and we have detected the formation of carbonyl groups and loss of aliphatic C--H groups at low levels of oxidation (1 h at 150°C). ~6 Consequently, this band may be useful in the qualitative comparison of the hydroxyl group content of difference coals, providing that the KBr pellets are prepared in an identical manner, but for quantitative measurements we should consider other procedures.

In view of these factors, we suggest that it will ulti- mately prove more useful to measure OH groups in coal by a combination of FT-IR and chemical procedures. Durie and Sternhel126 reported an infrared study of acetylated coal 20 years ago. Although some useful linear plots were obtained, the method was complicated by the overlap of the acetyl bands with those of the original coal. This made the determination of base lines and the measurement of peak intensities subject to possible error. The problem is illustrated in Fig. 7, which compares the infrared spectrum of an Arizona HVC coal (PSOC 312) to that of the same sample subsequent to acetylation. FT-IR is capable of solving many problems of this type (band overlap) by simple spectral subtraction. Fig. 7 also shows the difference spectrum obtained by subtracting the spectrum of the original coal from that of the acetylated product. The characteristic acetyl bands are now relatively well-resolved and it is a straightforward task to draw an appropriate base line and measure peak heights, or even make integrated absorption measurements of, for example, the 1370 CH3 mode. We will discuss the details of the use of acetylation reactions in conjunction with FT-IR in a separate publication. Here we wish to point out the potential of FT-IR for discriminating between different types of OH groups in the original coal, since this brings us to the sensitive subject of curve resolving.

Chemical methods for determining the degree of acetylation measure total (reactive) OH and do not distinguish between types of OH groups. Acetylation of coal results in the formation of esters:

O

Acetic Anhydride II COAL--OH ) COAL--O--C--CH~

Alkyl esters normally absorb near 1740 cm -~, but when an electron withdrawing group such as an aromatic entity is attached to the single-bonded oxygen this band is shifted to approximately 1770 cm-1. 21 Consequently, the strong band near 1765 c m -1 in the difference spectrum

I ' ~ O £ 3 1 7 .

I

,r ,o A ....

.... A / \ I]°"..,

. . . . . - ~ - ~ c ~ ~ • i , . , i , , . i . , . i . , . - m ~

~@~0 t800 160@ 1400 t200 t0@0 80@ c~ I

Fro. 7. Top, FT-IR spectrum of an acetylated Arizona coal; middle, FT-IR spectrum of the original coal; bottom, difference spectrum.

shown in Fig. 6 can be assigned to acetyl groups that have reacted with phenolic OH, whereas the shoulder near 1725 cm -1 was initially assigned to acetyl groups that have reacted with alkyl OH (although the frequency is low compared to other alkyl esters). If these bands can be resolved a measure of the relative proportions of these groups could be obtained. Our initial attempts to accom- plish this aim illustrate some of the problems involved in curve resolving.

The first problem is the choice of mathematical function most suitable for characterizing the observed band shape. In a review of the literature, Maddams 22 has pointed out that the evidence in favor of Lorentzian band shapes in the spectra of homogeneous materials is very strong. Gauss-Lorentz sum and product functions have often been used, as with dispersive instruments a shift from Lorentzian to Gaussian band shapes was found with increasing slit width. Although FT-IR instruments do not have a set of slits we decided to use a sum function, since band shapes can also be affected by factors such as interactions between the components of a complex system. The function chosen is the sum of a Gaussian and Lorentzian band with equal half widths in the proportion f t o (1 - f):

A = fAo exp(- ln212(X--- X°) 72~ aX,/ J J (1)

( 1 - f)Ao + 1 + [2(X - Xo)/AX,/2] 2

where A is the peak height, X is the wave number coordinate of the peak, and X~/2 is the bandwidth at half height. Initial estimates of these parameters are input to a program that fits the parameters for a set of bands to the experimental spectral profile by a least squares op- timization procedure, as described by Fraser and Su- zuki. 33 In an initial analysis of the carbonyl region of the difference spectrum shown in Fig. 7, three bands were defined for curve fitting at the initial estimated positions of 1770, 1725, and 1605 cm -~. Although a "good" fit in least squares terms was obtained the results are probably meaningless. Fig. 8 shows the difference spectrum between 1900 and 1550 cm -1 together with the three component bands determined by the program to give the best fit to the data. A strong, very broad band near 1725 cm -1 is the major component and appears stronger than the band assigned to acetylated phenolic-OH groups near 1765 cm -~. This result does not make sense in terms of what is known of the structure of coal. It is thought that phenolic OH groups are by far the major hydroxyl component. Although a large difference in the extinction coefficients of various types of ester linkages could account for this discrepancy, it is far more likely that the curve resolving procedure is in substantial error. There are two probable sources of such error. First, that we have not included a sufficient number of bands in the analysis; second, that the 1765 cm -~ is significantly asymmetric, as also illustrated schematically in Fig. 8. These initial errors and misconceptions on our part brings us to a key point in curve resolving. It is usually possible to obtain a reasonable fit between a synthesized spectrum consisting of a sum of a sufficient number of bands and an observed spectral profile. However, if we are to have


1 I ! t90~ t8~0 Iq00 t800

Fzc. 8. Top, spectral profile of the difference spectrum shown in Fig. 6 between 1900 and 1550 cm-' fitted to three curves; bottom, spectral profile considered with an asymmetric major peak.

confidence in the results it is desirable or even essential to have an initial knowledge of the n u m b e r of bands, a considerat ion reviewed in some detail by Maddams . 22 Consequently, we suggest tha t the curve resolving me thod utilized by Solomon, '7' is a fit of complete coal spec t ra to a set of 26 Gaussian bands, could lead to the type of p rob lem we encountered in our initial a t t e m p t a t curve resolving.

T h e qualifications outl ined above indicate tha t in order to obta in a reasonable analysis of the carbonyl region in coal difference spect ra we need to establish two procedures. The mos t impor t an t is a me thod for the determinat ion of the n u m b e r of bands in a profile. For certain problems it might also prove necessary to be able to curve fit a symmet r i c band shapes.

M a d d a m s 22 discussed two approaches to the determinat ion of the n u m b e r of bands contr ibut ing to a spectra l profile. The first, factor analysis, has been adap ted to F T - I R ins t ruments by Antoon and co-workers. 34 How- ever, this procedure de te rmines the n u m b e r of components, so tha t if a par t r icular species has more than one absorpt ion band in the spectral range under consideration, a factor analysis will underes t imate the number of bands in tha t region. The o ther me thod reviewed by M a d d a m s 22 is the use of der ivat ive spectroscopy, which has been recognized for m a n y years as a means of en- hancing the appearance of minute shoulders in spectra. 35 T h e derivat ive of digital data can be computed by a Newton-Gregory interpolat ing polynomial {seventh order). 36 The appl icat ion of this me thod to the carbonyl region of the difference spec t rum of an ace ty la ted coal (PSOC 272, Ken t ucky no. 9, HVB coal) is shown in Fig. 9. T h e spec t rum and its second derivat ive are bo th presented in Fig. 9. The min ima in the second derivat ive profile correspond to the posit ions of bands and shoulders. 22 The presence of mos t of the bands can be con- f i rmed by inspection of the spectral profile. The only quest ionable ass ignment is of a band near 1747 c m - ' , in which the second derivat ive min ima is of the order of magni tude of the noise level. However, if the difference


spec t rum is examined carefully an inflection near 1747 cm -1 can be observed. M a d d a m s 22 has pointed out tha t the value of the eye and brain should not be underest i- m a t e d in such judgements . Less subject ive evidence, however, is provided by examinat ion of other coals in which the components have different relat ive intensities. For example, the difference spec t rum obta ined f rom an ace ty la ted lignite is shown in Fig. 10. T h e presence of a band near 1740 cm -t is clearly indicated f rom the second derivat ive curve shown in Fig. 10. I t is satisfying to observe tha t bands in approx imate ly the same posit ion are obta ined f rom spect ra of coals of different rank. The small differences in f requency are to be expected in mater ia ls tha t vary in chemical structure.

In addit ion to providing initial values of the peak posit ions for the least squares ref inement , the second der ivat ive curves can be used to obtain an initial es t imate of the width a t hal f height f rom a measure of the distance be tween inflection points. I t is essential to have a good initial es t imate of these values if convergence to a good (and meaningful) least squares fit is to be obtained. Figs.

PSOC 272

I

i900 i8'00 i7'00

,67

1631

I~100 cm'1

Fro. 9. Bottom, difference spectrum between 1900 and 1550 cm ' obtained from acetylated PSOC 272 coal; top, second derivative of spectrum.

1721

1661 I

1632

I

lso~ ,e'oo 1'~oo t~'o~ ,m.1 Fro. 10. Bottom, difference spectrum obtained from an acetylated lignite; top, second derivative of spectrum.

PSOC 272 1 7 6 9 1 7 6 9 1 7 6 9

i

SKE~[D RARnSHAP[ FA[TOR (1769 c~ I I~̂ Nl~) SK[t4[O BANliSHAP[ FACTOR (l?~b9 CH ~l S^ellJ) RKEWEP lIANDRHAflli FAC]OI~ (i75~ c~ "18^ND) I~=1) e=O*l a'O,2

FIG. 11. Resolution of the difference spectrum obtained from acetylated PSOC 272 coal in the region 1900 to 1550 cm-' into five bands. From left to right the 1769 cm-' has an increasing degree of asymmetry as defined by the factor B (see text).

11 and 12 show the curves resolved in the carbonyl region of the difference spectra of the bituminous coal and lignite considered above. Fig. 11 also shows the effect of describing the 1769 cm -1 band as asymmetric according to:

A__Aoexp_ ln2 {ln(I + 2B)(X-Xo)/AX1/2} 2 B (2)

where B is a factor that determines the degree of asymmetry, as described by Fraser and Suzuki. 33 It can be seen that the effect of asymmetry is to reduce the intensity of the 1745 cm -1 band. However, it has been empha- sized that in studies of this type band shapes should not be skewed unless there are a priori grounds for doing so. 22 Consequently, a fit that has symmetric bands is to be preferred. This aspect of the problem is presently being checked by a study of model compounds, and the results, together with evidence supporting a complete assignment of the resolved bands, will be presented in a future publication. It is pertinent to point out here, however, that the band near 1720 cm -1 can be assigned to residual acetic acid, so that previous methods of measuring OH groups, for example using 14C-labelled acetic anhydride, could be in error. The bands near 1773, 1740, and 1660 cm- ' can be assigned to acetylated phenolic

\ OH, alkyl OH, and N - - H groups, respectively. Accord-

/ ingly, this preliminary work indicates that FT-IR should not only be useful in determining total OH through measurements of the intensity of the CH3 and C--O bands, but also has the clear potential for discriminating between types of OH groups and also measuring N - - H groups through an analysis of the C ~ O stretching region, providing that curve resolving methods are applied with circumspection.

Finally, it is interesting to note that the value of f in Eq. (1) was determined to be 0.98 (i.e., the band shape was essentially Gaussian) for the bands resolved for PSOC 272 (symmetric bandshapes) shown in Fig. 11. In contrast, the value of fdetermined for the bands resolved in Fig. 12 for the lignite was 0.27. One interpretation of this result is that we are examining a more homogeneous

1 7 7 3 [ Lignite

• 4 5

i I I

1.90¢ tSe¢ 1]00 t6¢¢

FIG. 12. Resolution of the difference spectrum obtained from the acetylated lignite into six bands.

population of acetylated products in the lignite than in the PSOC 272 coal, since a symmetric distribution of absorbing species, each with a Lorentzian band shape, could result in a Gaussian profile. However, this type of interpretation of band shapes is essentially speculative. The key point is that differences in band shape are found from coal to coal and even in different spectral regions of the same coal. It would therefore appear preferable to curve resolve regions of coal spectra separately by an iterative procedure rather than assume that all bands have the same shape.

D. The D e t e r m i n a t i o n o f C - - H Groups . In a number of studies attempts have been made to obtain structural information by measuring the peak heights or integrated intensities of bands assigned to aromatic and aliphatic C- -H modes. 1-4 As we mentioned above, there is some variation in values of extinction coefficients determined by different groups. 17' 21-40 In addition, Retcof-


sky ~7 calibrated the C--H aromatic out-of-plane vibrations of pyridine and carbon disulfide extracts of the same coal using the results of proton magnetic resonance studies, but obtained different curves for the two sets of samples. These differences could be due to variations in the values of extinction coefficients for diverse coals. However, there is a remarkable similarity in the spectra of coals and their extracts or liquefaction products. TM In addition, the main features of the spectra of coal do not show significant changes as a function or rank, varying only in the relative intensities of some features and the appearance of prominent bands due to carbonyl and carboxylic acids in low-rank (or oxidized) material. (An- thracites could be considered an exception to this gen- eralization because the presence of an intense background absorption has to be regarded as a key feature. Nevertheless, superimposed on this background are the familiar features observed in the spectra of most coals.)

On the basis of this similarity we would intuitively expect only minor differences in the values of the extinction coefficients of the characteristic C--H group frequencies. We suggest that the observed variation could be due to the procedures used to make most measurements. For example, Retcofsky 37 and Durie e t a l . ~4 considered the integrated absorption between 680 and 920 cm -~ in which there are typically three major aromatic C--H out-of-plane bending modes, near 850, 810, and 750 cm -~. This implicitly assumes that the extinction coefficients of these three bands are the same or that the relative intensities of the three bands remains unaltered from coal sample to coal sample. The latter possibility is clearly not true when a range of coals from different sources is examined. There are significant differences in the relatively intensities of the out-of-plane C--H modes that reflect different degrees and types of aromatic sub- stitution. Consequently, it would be more accurate to define the absorbance in the range 920 to 680 cm -~ as the sum of (usually) three components, each with a distinct extinction coefficient. In fact, von Tschamler and de Ruiter 2° argued on statistical ground that the extinction coefficients for the 850, 810, and 750 cm -1 band are approximately in the ratio 1:0.75:0.4. Consequently, one aim of future FT-IR work should be to establish such relationships. Curved resolved bands in, for example, the aromatic C--H out-of-plane region, could then be summed according to a weighting factor determined from the relationship between extinction coefficients.

The same argument applies to measurements in the aliphatic C--H stretching region in which it has become conventional to measure the integrated absorption between 3000 and 2800 cm -t as a measure of total aliphatic C--H concentration. This region of the spectrum has to be considered as the sum of contributions from three components, CH, CH2, and CH3. If the spectral contri- bution of these groups could be separated, then not only would we have obtained a greater insight into the structure of coal and its variation with rank, but more consistent aliphatic to aromatic C--H ratios might be obtained by taking into account differences in the extinction coefficients of these groups.

In order to obtain a meaningful resolution of the bands in the C--H stretching region, we followed the procedures outlined above for analysis of the carbonyl region

of the difference spectra of acetylated coals. Fig. 13 shows a scale-expanded plot of the aliphatic C--H stretching region of the spectrum of the vitrinite concentrate considered in Fig. 2. Also displayed in Fig. 13 is the second derivative plot, clearly indicating the presence of five bands. With an appropriate initial choice of band position and width at half height obtained from this plot, the C--H stretching region can be resolved into the five components shown in Fig. 14. At this point it is tempting to make the obvious assignment of the 2956 and 2864 cm -t modes to assymetric and symmetric CH3 stretch, the 2923 and 2849 cm -t bands to the asymmetric and symmetric CH2 stretch, and the 2891 cm -t band to lone C--H groups. However, we have to consider one more limitation to curve resolving methods. The identification of peaks in a profile by second derivative techniques will obviously depend upon the relative intensities of the peaks involved and their separation relative to their half widths. It has been estimated that two-component peaks will be distinguishable only if they are separated by an amount comparable to their semi-half widths. 22 If we carefully consider established group frequencies in conjunction with what is known of the structure of coal, we have to conclude that the 2923 cm -1 band is a composite of two contributions, the asymmetric CH2 stretching mode and the asymmetric CH8 stretching mode of methyl groups attached directly to aromatic rings. However, the position of the symmetric CH3 stretching mode appears to be less sensitive to local environment, appearing near 2865 to 2875 cm -~ for methyl groups attached to alkyl chains or aromatic rings. 23 Consequently, if appropriate extinction coefficients can be determined it should prove possible to use the symmetric CH2 and CH~ modes to determine these groups and there is then the further possibility of using the 2956 and 2923 cm -~ bands to determine the distribution of methyl groups. (The intensity of 2923 cm -1 band would have to be considered as the sum of the contributions from CH2 and CH8 groups.)

P S M C 52

2 9 2 3 2891 ~2956 ~ , 2 8 6 4

2849

i i

Fro. 13. Bottom, scale-expanded aliphatic C- -H stretching region of the spectrum of a vitrinite concentrate; top, second derivative of the spectrum.


/ //2"9' / I / \ \ [ 2864

2 9 2 3 I PSMC 52

I I 37)(D(~ 29~q) ~8@@ c~ "I

FIG. 14. Resolution of the aliphatic C - - H stretching region into five bands.

Finally, the 2891 cm -~ could be applied to the determination of tertiary hydrogen.

III. SUMMARY AND CONCLUSIONS

In this communication we have attempted to clarify aspects of the application of FT-IR to the study of coal structure. Correct band assignments are obviously cen- tral to the proper use of this technique, and we have concluded that the 1600 cm -1 band, the most prominent in the spectra of coal, can be assigned to an aromatic ring stretching mode that is intensity enhanced by the presence of substituents, predominantly phenols. In low-rank and oxidized coals the 1600 cm -1 mode overlaps bands due to --COO- groups that appear near 1580 cm -1.

One of the major advantages of FT-IR in the analysis of complex systems is the ready application of sophisticated programs made possible by the necessity of an on- line minicomputer in these instruments. However, these programs have to be applied with considerable caution. Specifically, we attempted to assess critically the problems involved in determining functional groups by curve resolving methods. There is a large body of work in the literature concerning this problem and by applying the results of these studies we can conclude that:

(1) Empirically determined band shapes should be calculated using least squares curve fitting programs. (2) In order to have confidence in the results it is necessary to have a good initial estimate of the number of bands in a profile, their frequency and width at half height. (3) These methods should be applied separately to specific regions of the spectrum.

We have considered the application of these criteria to the determination of OH and CH groups in coal. By combining FT-IR measurements with acetylation procedures it appears feasible not only to measure total

(reactive) OH and NH content, but to distinguish between phenolic and alkyl OH groups by curve resolving methods. Finally, we have argued that the determination of consistent extinction coefficients for aliphatic and aromatic C--H bands for various coal samples would be facilitated by the use of curve resolving procedures and a determination of the relationship between the extinction coefficients of individual components. In previous studies it has been general practice to sum the integrated intensity of a number of bands and hence implicitely assume that each mode has the same extinction coefficient.

A C K N O W L E D G M E N T S

The authors gratefully acknowledge the financial support of the Department of Energy under contract No. DE-AC22-OPC30013 and the Pennsylvania State Cooperative Program in Coal Research.

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Ed., (Academic Press, New York, 1978), Vol. II, p. 75. 5. P. R. Griffiths, Chemical Infrared Fourier Transform Spectroscopy, (John

Wiley and Sons, New York, 1975}. 6. J. L. Koenig, Appl. Spectrosc. 28, 293 (1975). 7. M. M. Coleman and P. C. Painter, J. Macromol. Sci. Revs. Macromol. Chem.

C16(2), 197 (1978). 8. P. C. Painter, M. M. Coleman, R. G. Jenkins, P. W. Whang, and P. L. Walker,

Jr., Fuel 57,337 (1978). 9. P. C. Painter, M. M. Coleman, R. G. Jenkins, and P. L. Walker, Jr., Fuel 57,

125 (1978}. 10. P. C. Painter, R. W. Snyder, J. Youtcheff, P. H. Given, H. Gong, and N. Suhr,

Fuel 59, 364 (1980). 11. P. C. Painter, S. Stepusin, R. W. Snyder, and A. Davis, Appl. Spectrosc. 35,

102 (1981). 12. P. C. Painter, J. Youtcheff, and P. H. Given, Fuel 59, 523 (1980}. 13. P. C. Painter and M. M. Coleman, Fuel 58, 301 (1979). 14. P. C. Painter, Y. Yamada, R. G. Jenkins, M. M. Coleman, and P. L. Walker,

Jr., Fuel 58, 293 (1979). 15. P. C. Painter, R. W. Snyder, D. E. Pearson, and J. Kwong, Fuel 59, 282 (1980). 16. P. C. Painter, M. M. Coleman, R. W. Snyder, O. Mahajan, M. Kamatsu, and

P. L. Walker, Jr., Appl. Spectrosc. 35, 106 (1981). 17. P. R. Solomon. ACS Division of Fuel Chemisltry Preprints 24 #3, 184 (1979)

and Adv. Chem. Ser. to be published. 18. P. R. Solomon, Paper presented at meeting of Chemistry and Physics of Coal

Utilization, Morgantown, West Virginia, 2-4 June 1980 in press. 19. R. A. Durie, Y. Shewchyk, and S. Sternhell, Fuel 45, 99 (1966). 20. H. von Tschamler and E. de Ruiter, Brennstoff-Chemie 9, 280 (1963). 21. J. K. Brown, J. Chem. Soc. 744 (1955}. 22. W. F. Maddams, Appl. Spectrosc. 34, 245 (1980). 23. N. B. Colthup, L. H. Daly, and S. E. Wiberley, Introduction to Infrared and

Raman Spectroscopy. (Academic Press, New York, 1975). 24. S. Fujii, Fuel 42, 17 (1963). 25. S. Fujii, Fuel 42, 341 (1963}. 26. R. A. Durie and S. Sternhell, Austr. J. Chem. 12, 205 (1959). 27. R. Liotta, Fuel 58, 725 {1979). 28. M. Strasinic, M. M. Coleman, and P. C. Painter, unpublished results {1978). 29. Y. O0sawa and J. W. Shih, Fuel 50, 53 (1971). 30. S. Fujii, Y. C)sawa, and H. Sugimura, Fuel 49, 68 (1970). 31. I. A. Breger and J. C. Chandler, Anal. Chem. 41, 506 (1969). 32. R. A. Durie and J. Szewczyk, Spectrochim Acta 13, 593 (1959). 33. R. D. B. Fraser and E. Suzuki, in Physical Principles and Techniques of

Protein Chemistry. S. J. Leach, Ed. (Academic Press, New York, 1973), Part C, p. 301.

34. J. K. Antoon, L. D'Esposito, andJ. L. Koenig. Appl. Spectrosc. 33, 351 (1979). 35. A. E. Martin, Spectrochim Acta 14, 97 (1959). 36. P. C. Gillette, D. Kormos, M. K. Antoon, and J. L. Koenig, Selected Computer

Programs With Application to Infrared Spectroscopy, (Digilab Users Group Conference, 1979).

37. H. L. Retcofsky, Appl. Spectrosc. 31, 116 (1977). 38. R. A. Durie, Y. Shewshyk, and S. Sternhell, Fuel 45, 99 (1966). 39. H. L. Retcofsky and R. A. Friedel, Fuel 47, 487 (1968). 40. J. K. Brown and W. R. Ladrer, Fuel 39, 87 (1960).

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Concerning the Application of FT-IR to the Study of Coal: A Critical Assessment of Band Assignments...

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