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Talanta 52 (2000) 457–464

The use of synchronous luminescence spectroscopy inqualitative analysis of aromatic fraction of hard coal

thermolysis products

Aniela Matuszewska *, Maria Czaja

Silesian Uni 6ersity, Faculty of Earth Sciences, Department of Geochemistry, Mineralogy and Petrography, 60 , Bedzinska str.,

41-200 Sosnowiec, Poland

Received 2 June 1999; received in revised form 7 March 2000; accepted 7 March 2000

Abstract

The synchronous luminescence method was used in qualitative analysis of aromatic fraction of low-temperature tar

from hard coal. The spectra obtained by this method are simpler than spectra obtained with the use of conventional

emission luminescence method. The synchronous luminescence analysis requires the selection of respective Du

parameter values. This parameter is a constant difference between position of excitation and emission monochroma-

tors during measurement. From literature, the Du parameter value of 23 nm was first used here. The characteristic

emission ranges of spectra obtained indicated (by comparison with spectra of standards) degree of condensation of

aromatic compounds present in investigated mixtures. It was also possible to identify some individual compounds.

However, this identification could be more effective with the use of the respective value of Du parameter for each

particular component of the mixture. This manner of analysis was used here, e.g. for investigating aromatic fraction

containing phenanthrene (identified previously by gas chromatography method) among other compounds. The

spectrum recorded at Du value characteristic for phenanthrene (53nm) presents a rather simple shape with a maximum

at 346 nm attributed to phenanthrene after standard and literature data. © 2000 Elsevier Science B.V. All rights

reserved.

Keywords: Synchronous luminescence; Aromatic compounds; Coal thermolysis

www.elsevier.com /locate/talanta

1. Introduction

The development of methods of analysis of

aromatic components in complex mixtures of nat-

ural and industrial origin is very important from

the point of view of cognition, application and

environmental protection. The methods of analy-sis usually used, require extensive fractionation of complex mixtures. The analytical methods not

requiring a separation of mixture at all or requir-ing only a partial fractionation of complex mix-

tures would be undoubtedly very advantageous.Some techniques of luminescence method can

comply with these needs. Among these, the emis-* Corresponding author. Fax. +48-32-2915865.

0039-9140/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 3 6 9 - 6

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A. Matuszewska, M . Czaja / Talanta 52 (2000) 457–464 458

sion technique was of use, particularly to the

investigation of aromatic compounds existing in

natural and industrial products [1–8]. The objects

of investigations were polycyclic aromatic hydro-

carbons, which raises interest because of their

mutagenic and carcinogenic properties, beginning

from four condensed rings [9].

The emission technique gives us the chance to

analyse the organic mixtures without their previ-ous separation and because of this, has been

called a ‘spectral fractionation method’ [1]. This

analysis consists of selective excitation of the mix-

ture by radiation of wavelengths specific for inves-

tigated components of mixture. Very complex

mixtures, however, require more steps of analysis

in order to identify one component. In this case, a

greater number of spectra have to be recorded

with the use of different excitation wavelengths

characteristic for investigated compounds. This

fact, and because of a relatively low resolution of conventional luminescence emission spectra, were

the reason for another, more effective technique,

becoming more attractive as the ‘spectral fraction-

ation method’, namely the synchronous lumines-

cence spectroscopy.

The synchronous luminescence technique is

used more and more frequently in the investiga-

tions of aromatic components of mixtures of vari-

ous origin [1,9–22]. Thanks to the considerable

simplification of the spectrum, this technique

seems to be of greater analytical importance thanconventional emission techniques, particularly

when very complex mixture of compounds are

submitted to investigation.

The synchronous luminescence technique con-

sists in the record of spectrum at the constant

difference between the positions of emission and

excitation monochromators (Du).

In this work, the value of Du=23 nm was

mainly used in the analysis of fractions under

investigation. This value was used in a series of

works, as the effective parameter in the analysis of petroleum products and other mixtures of aro-

matic hydrocarbons of various origin [20– 22].

The value of Du=23 nm was chosen as an effi-

cient value to take a fair middle course between

the sensitivity and resolving parameters. The use

of this value of Du parameter makes it possible to

estimate the range of condensation degree of aro-

matic compounds occurring in an organic mix-

ture. It is also possible in this case to identify

some individual compounds.

More effective for selective analysis of individ-

ual components of mixture is, however, the most

favourable choice of Du parameters for particular

compounds. This procedure makes possible, the

analysis of complex mixtures of compounds with-out their previous fractionation or after partial

separation into several fractions [1].

For identification of a particular compound,

the value of the Du parameter is chosen most

often as a difference between effective radiation

wavelength in excitation spectrum and the wave-

length corresponding to the most intensive maxi-

mum of luminescence spectrum of the analysed

compound. In this case, at the simultaneous

movement of both monochromators, the men-

tioned most intensive maximum will be recordedand the spectrum will be considerably simplified

[1]. The number of bands depends on the chosen

value of Du.

In this work, the conventional emission lu-

minescence spectra were recorded as well as syn-

chronous luminescence spectra of relatively

narrow aromatic fractions of heavier low-temper-

ature tar distillation (to 270°C) product obtained

from gas-flame coal. The preliminary character of

investigations, in the range presented here, was a

reason for the partial separation of mixtures withthe use of thin layer preparative chromatography

(TLC) into polar, aliphatic and aromatic frac-

tions. A secondary fractionation was then also

made by the TLC method of aromatic fraction

into narrower subfractions containing concen-

trates of aromatic rings of different degree of

condensation: 2–5 rings, dominating in successive

fractions, respectively. It seems that the procedure

used here of deeper fractionation should be per-

formed in the case of introductory investigations

of mixtures not analysed earlier to facilitate thechoice of respective analytical parameters, espe-

cially Du values. The preparative thin layer chro-

matography method seems to be particularly

suitable because it requires no special apparatus

and is relatively simple. The separation procedure

should obviously be carried out taking into ac-

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count other provided concurrent procedures of

analysis.

2. Experimental

2 .1. Preparation of samples

The object of our investigations was an aro-matic fraction of low-temperature tar from gas-

flame coal taken from Rybnik Coal District

(Poland). The tar was obtained by the low-tem-

perature carbonization process in the temperature

range of 20–520°C, under atmospheric pressure.

The tar was distilled afterwards up to 270°C and

the residue obtained was separated by thin layer

chromatography (TLC) into aliphatic, aromatic

and polar fractions. The aromatic fraction was

then submitted to secondary separation by TLC,

into four narrower fractions isolated under theUV lamp (254 nm) in accordance with the various

fluorescence colours (pale-violet, dark-violet,

green, yellow).

After elution of particular fractions from silica

gel scraped off from TLC plates and consequent

solvent evaporation, the samples were prepared

for analysis using capillary gas chromatography

and synchronous luminescence techniques as de-

scribed below.

2 .2 . Methods of in6estigations

The TLC preparative method was used for the

separation of the sample into aliphatic, aromatic

and polar fractions. The mobile phase was n-hep-

tane. Merck plates (20×20 cm) covered with a

silica gel layer (thickness: 2 mm) were used. The

aromatic fraction was identified on the TLC plate

in the light of a UV lamp (254 nm) as an area

showing various fluorescent colours: from pale-vi-

olet to yellow. The silica gel from this area was

scraped off from the TLC plate, and then intro-duced after crumbling into glass column on thin

layer of cotton wool (extracted earlier by

methylene chloride) and a layer of 1 cm neutral

aluminium oxide (for chromatography). The elu-

ent used was n-heptane evaporated afterwards

from eluate in a Buchi evaporator.

The secondary development by TLC, of an

isolated aromatic fraction was performed on the

Merck plates, with a silica gel layer thickness of

0.2 mm. The mobile phase used was n-heptane.

The separation was made into four groups of

aromatic compounds, in accordance with various

fluorescent colours visible in the light of a UV

lamp (254 nm). The procedure used for the recov-

ery of these fractions from TLC plates was thesame as described previously, but this time four

separate areas were scraped off from the TLC

plate. Each of them showing other fluorescent

colours: pale-violet (fraction 1), dark-violet (frac-

tion 2), green (fraction 3) and yellow (fraction 4).

The apparatus for gas chromatography was a

FISONS 8000, equipped with a FID detector and

with a capillary column rtx-5 (length: 25 m; i.d.:

0.32 mm, thickness of stationary phase film: 0.2

mm). Helium was used as a carrier gas. The split-

less injection system was fixed.Injection volume was 1 ml of sample solution in

methylene dichloride. The temperature program

was as follows: (1) injection and detector cham-

bers: 280 and 315°C, respectively; (2) the oven

temperature range: 60–315—C: (a) heating from

60–300°C with the rate of 6°C/min, (b)/ isother-

mal heating at 300°C for 15 min, (c) heating to

315°C with the rate of 10°C/min, (d) isothermal

heating at 315°C for 5 min. Table 1 presents the

retention-time values, estimated from standards

used for qualitative GC analysis of the investi-gated fractions.

The spectrofluorimeter was Fluorolog 3-12

Spex from Jobin Yvon. The light source was a

Xe– ozone-free lamp (45 W). The solvent used

was n-hexane. The investigations were performed

at room temperature. To avoid the influence of

oxygen as a luminescence-quenching agent [23],

the solutions prepared for our investigations were

subjected to degasing in the ultrasonic bath before

measurements.

3. Results and discussion

Four fractions of aromatic compounds isolated

with the use of TLC were analyzed by GC. The

chromatograms obtained were interpreted using

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Table 1

The retention time values estimated for standards used in the

qualitative GC analysis of investigated fractions

Retention time (min90.1 min)Aromatic compound

Naphthalene 10.1

12.72-Methylnaphthalene

1-Methylnaphthalene 13.1

14.82-Ethylnaphthalene

15.21,6-Dimethylnaphthalene15.62,3-Dimethylnaphthalene

16.11,2-Dimethylnaphthalene

18.6Fluorene

21.9Phenanthrene

22.1Anthracene

24.12-Methylphenanthrene

25.73,6-Dimethylphenanthrene

Fluoranthene 26.4

27.3Pyrene

31.7Benzo[a]anthracene

Chrysene 32.2

32.4Naphthacene

35.9Benzo[b]fluoranthene36.8Benzo[e]pyrene

36.9Benzo[a]pyrene

37.2Perylene

40.5Dibenzo[ah]anthracene

Picene 40.8

Benzo[ghi]perylene 40.9

Coronene 47.1

the accessible standards of aromatic compounds

of various condensation degree (Table 1). In the

successive fractions 1–4, the compounds domi-

nated with two, three, four and five condensed

rings, respectively. The IR absorption spec-

troscopy method also used for investigation of the

fractions, has shown that, for these structures, the

high degree of alkyl substitution was distinctive:

this was stated on the basis of spectra shape in thewavenumber range of 700–900 cm−1. The intense

maxima at 870, 810 and 750 cm−1 are to be

assigned to out-of-plane vibrations of one iso-

lated, two adjacent and four adjacent aromatic

C– H groups, respectively [19]. This distribution

of hydrogen atoms at aromatic rings and rela-

tively intense bands at 1380 cm−1 (deformation

vibrations in CH3 groups) suggest a considerable

degree of alkyl substitution of aromatic

components.

Fig. 1(a–d) shows the spectra obtained by syn-chronous luminescence technique of fractions 1–4

recorded at the value of parameter Du equal to 23

nm. The analysis of fraction 1 using GC, has

shown a domination of alkyl substituted com-

pounds of the naphthalene type. In the syn-

chronous spectrum obtained, these compounds

are represented by the band with maximum at 329

nm. According to literature [1] data, this band can

originate from the compounds like: 1-methyl-

naphthalene (327 nm), 1,6- or 2,3-dimethylnaph-

thalene (328 nm). (The presence of thesecompounds has been indicated here also by GC

analysis.) Naphthalene and also mono- and

dimethylnaphthalenes, with the various types of

substitution, emit in the range of 322– 342 nm

[1,9,20,21].

In the synchronous spectrum of fraction 1 (Fig.

1(a)), a second maximum also exists, at u=374

nm. This band probably originates from methyl

derivatives of phenanthrene, which are admixtures

in this fraction. An example of compounds which

emit in this range may be 2-methylphenanthrene(374 nm), identified also by GC, or 9,10-

dimethylphenanthrene (372 nm) as well as 9,10-

dibuthylphenanthrene (372 nm) [1]. Generally,

phenanthrene and mono- and disubstituted alkyl

derivatives of phenanthrene show fluorescence in

the range of 346–394 nm [1,20,21] (Table 2).

Fig. 1. The synchronous luminescence spectra of fraction 1–4

recorded at the value of parameter Du=23 nm: (a) fraction 1

(—); (b) fraction 2 (---); (c) fraction 3 ( – – – ); (d) fraction 4

(. . .).

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Table 2

The positions of characteristic bands in the fluorescence excita-

tion (uexc) and emission (uem) spectra of standards of

phenanthrene, anthracene and some of their alkyl derivatives

[1]

uexc (nm)Compound uem (nm)

346aPhenanthrene 254

356274

282 364384293a

9,10-Dimethylphenanthrene 354257

271 372

394279

285

299

9,10-Dibutylophenanthrene 352257

372271

280 393

289

304

Anthracene 253 377b

383340

399357b

376 402

423

1-Methylanthracene 383256

345 388

361 403

406382

428

432

a

Du=53 nm.b Dl=20 nm.

nation of phenanthrenes, as well as anthracene in

fraction, 2 was stated simultaneously by the capil-

lary gas chromatography method.

Fig. 1(c) presents a synchronous spectrum of

fraction 3, obtained at Du=23 nm. The maxi-

mum at 384 nm may be attributed to ben-

zo[a]anthracene identified in fraction 3 by

GC.This compound and its alkyl derivatives have

the fluorescent range of 384– 455 nm [1,8]. Themaximum at 384 nm may be attributed also to

pyrene. Pyrene and its alkyl derivatives have the

fluorescent range: 372–397 nm [1,8,20].

The synchronous spectrum (Du=23 nm) of

fraction 4 is shown in Fig. 1(d). There is one

maximum at 390 nm. In fraction 4, the domina-

tion of five-ring aromatic compounds was stated

by GC. Among these compounds, the wavelength

of maximal fluorescence most approximative to

390 nm is characteristic for benzo[e]pyrene (389

nm) and dibenzo[ah]anthracene (394 nm). Thesecompounds show fluorescence in the ranges: 389–

410 and 394–422 nm, respectively [1,21].

The more effective identification of individual

compounds, in particular fractions, was made

with the choice of specific value of the parameter

Du for the investigated compound. For example,

the value of Du=53 nm corresponds with

phenanthrene (in accordance with the definition

presented above) as the result of substraction:

Du=346–293 nm [1] (Table 2).

In Fig. 2(a) the spectrum of fraction 2 is pre-sented with the use of the value of parameter

Du=53 nm. The maxima at 346 and 364 nm,

characteristic for phenanthrene confirms the oc-

currence of this compound in fraction 2.

Another confirmation of the utility of this

parameter value is Fig. 2(b) presenting a spectrum

of mixture of phenanthrene and anthracene

recorded with the use of values of Du parameter

characteristic to phenanthrene (53 nm). The max-

ima obtained, cover those obtained in Fig. 2(a)

with an accuracy of measurements of 3 nm.Fig. 3(a) presents synchronous luminescence

spectrum of fraction 2 recorded at Du=20 nm

(value characteristic for anthracene) with the max-

imum at 376 nm. The same position of maximum

shows a spectrum in Fig. 3(b) representing the

mixture of anthracene and phenanthrene recorded

The contribution to the bands at 374 nm may

also be given by anthracene, which has a maxi-

mum at 376 nm in synchronous spectra recorded

at Du=23 nm (Fig. 1(b)). According to Refs.

[1,8,16,20], anthracene and its alkyl derivatives are

characterized by the emission range of 377– 443

nm. The difference of 3 nm between the values of

maximum of the anthracene band, submitted here

for discussion: 377–374nm, may be of instrumen-tal origin.

In Fig. 1(b) a synchronous spectrum is shown

of the investigated fraction 2. The maximum at

367 nm may be attributed to phenanthrene (364

nm, see Ref. [1]) and maximum at 376 nm, to

anthracene (377 nm, see Refs. [1,16]). The domi-

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at 20 nm. It confirms the occurrence of an-

thracene in the analysed mixture and fraction 2

(see Table 2).

In Fig. 3(a) the weak bands at 325 and 340 nm

exist showing the probable presence of naphthale-

nes, which are admixtures in fraction 2.

For comparison the conventional emission

spectrum of fraction 2 is shown in Fig. 4(a). The

excitation wavelength was uexc=345 nm. Themaxima present there at 383, 388 and 399 nm,

approximately the bands characteristic for 1-

methylanthracene (383, 388, 403 nm [1]) (Table 2).

Fig. 4(b) shows an emission spectrum of frac-

tion 1 obtained at the same value of uexc: 345

nm. The similar shape of this spectrum shows the

emission of compounds of the same type (in frac-

tion 1, anthracenes are present as admixture com-

pounds). This selective excitation by specific uexc

Fig. 3. The synchronous luminescence spectra recorded at the

Du=20 nm of: (a) fraction 2 (— ); (b) phenanthrene and

anthracene mixture (. . .).

Fig. 2. The synchronous luminescence spectra recorded at the

Du value of 53 nm of: (a) fraction 2 (—); (b) phenanthrene

and anthracene mixture (- - -).

confirms possibility of the use of this method, the

so-called ‘spectral fractionation’ of mixtures [1]

without their previous separation into individual

compounds. This specificity of uexc, consists here

in the fact that u=345 nm is one of several

maxima in excitation spectrum of 1-

methylanthracene.

4. Conclusions

The technique of synchronous luminescence, in

accordance with designation: ‘the method of spec-tral fractionation’ [1] may be used to analyse

complex mixtures without fractionation into indi-

vidual compounds. It seems, however, useful to

perform previous chromatographic separation

into narrower fractions, especially, in the case of

very complex natural or industrial products. In

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the preliminary analysis of thermolysis products

of hard coal, performed in this work, the isolation

was made of the group of aromatic hydrocarbons.

Then, the secondary separation by preparative

TLC of the aromatic fraction was performed, into

four narrower fractions, in which, the compounds

dominated with growing condensation degree:

from 2– 5 rings. The content was qualitatively

stated by gas chromatographic and then by lu-minescence spectroscopy.

The synchronous luminescence technique

makes it possible to simplify the spectra and this

property was employed here to analyse a series of

aromatic fractions.

The use, however, of one value of parameter

Du=23 nm (see Refs [20–22]) gives only approxi-

mate information. Refs [20–22] utilised this value

only for identification in mixtures of group of

compounds of various degree of condensation. In

the work presented here, some individual com-

pounds of investigated aromatic fractions have

been identified by synchronous luminescence tech-

nique with the use of Du=23 nm namely: methyl-

naphthalenes (fraction 1), phenanthrenes and

anthracenes (fraction 2) and very probable exis-

tence has been stated of benzo[a]anthracene (frac-

tion 3), benzo[e]pyrene and dibenzo[ah]anthracene(fraction 4). These compounds were also identified

in the fractions under investigation by GC. These

results may confirm the usefulness of the Du

parameter value of 23 nm, to estimation of con-

densation degree range of aromatic components

dominating a mixture.

The condition of more evident results of selec-

tive analysis of mixture components by syn-

chronous luminescence technique is, however, to

find the respective values of parameter Du specific

for particular compounds. In this work, the Duparameter value of 53 nm for phenanthrene and

20 nm for anthracene were tested as effective to

analysis of individual compounds in two-compo-

nent and multicomponent mixtures.

The identification of other components may be

performed after finding by experimental manner

of respective values of Du parameter.

References

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Fig. 4. The fluorescence spectra obtained at Duexc=345 nm

of: (a) fraction 2 (—); (b) fraction 1 (. . .).

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