Journal of Scientific & Industrial Research Vo1.59, April 2000, pp 300-305

Determination of Synchronous Fluorescence Scan Parameters for Certain Petroleum Products

Digambara Patra, K Lakshmi Sireesha and A K Mishra*

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036 Tel (044) 4458251, Fax (044) 2352545, E-mail : mishra @acer.iitm.ernet.in

Received: 21 October 1999; accepted: 25 January 2000

Synchronous fluorescence scan (SFS) technique is a promising tool for polycyclic aromatic hydrocarbon analysis. For

analytical purposes the SFS parameters that are needed to be optimized are I1A (A" - \,) and synchronous fluorescence

maximum. In the present work, conventional petroleum products available in Indian market like kerosene and petrol, and added lubri cant namely 2T oil have been studied with respect to their SFS parameters. For these multifluorophoric systems conven­tional electronic absorption and fluorescence spectroscopy is of limited use. The effect of dilution (5 - 100 per cent, v/v in cyclohexane) on the SFS parameters has been exami ned. In these multifluorophoric systems , excitation energy transfer results in shifting of synchronous fluorescence maxima with increasi ng concentration of the petroleum product. The correlation of thi s shift with concentration shows the possibility of using it as an analyt ical method to quantify the petroleum products in the

environment.

Introduction

Petroleum products usually contain a host of poly­cyclic aromatic hydrocarbons with high fluorescence ef­ficiency. Fluorescence studies of crude oi ls have pro­vided valuable insight into their fundamental and dy­namic propert ies l.2. Fluorescence obtained from these crude oils comes from the different polycyclic alfOmatic hydrocarbons present in these systems. In spite of their complexities and differences, crude oils often exhibit systematic behaviour, particularly with fluo rescence properties. Total fluorescence scanning gives a ystem­atic spectral fingerprint for different crude oils obtained from different sources3

. Therefore, fluorescence prop­erties of crude oils and related organ ic matter have been used in various app lications. Evoluti on of organic ma­terial in source rock can be monitored by fluorescence4.5.

Ral ston et al. 6 have shown the dependence of fl uores­cence quantum yield with the excitation wavelength, which is similar for different crude oils. Downare and Mullins? have observed that energy transfer produces large red shifts and large widths in the fluorescence emis­sion spectra for long wavelength excitation of different crude oils. They have also found that the fraction of emission resulting from coll isional energy transfer rela-

* Author for correspondence

tive to nascent emission is almost independent of oil types and quantum yields are lower for higher fluorophore concentrations (heavy crude oils) and higher for light crude oils.

Use of fluorescence for characterization of differ­ent crude oils has led to the development and commer­cial use of new kind of fluorescence instrurnentsx. 'II. Out of these, synchronous fluorescence scan gets preference because of its narrower spectral band, high selectivity and simplified spectrum. The technique was initially developed by Lloyd" to identify many polynuclear aro­matic hydrocarbons and refined petroleum products. In this technique, both exci tation and emiss ion monochro­mators are scanned simu ltaneously, keeping a constant wavelength difference inbetween them. Parameters needed to be opt imized in the synchronous fluorescence

scan based analytical methods are ~A and A SFSlna • .

Taylor and Patterson l 2 have analyzed the exc ita­tion resolved synchronous fluorescence of aromatic com­pounds and fuel oil. Lloyd 13 has successfu lly character­ized petroleum products of high relative molecular mass for forensic purposes using SFS. He has also character­ized rubbers obtained from tyre prints using this tech­nique ' 4 . John and Soutar l5 have identified d iffe rent types of crude oils obtained from different sources using thi s technique. They have studied the effect of factors \ ike

PATRA et al.: PETROLEUM PRODUCTS 301

3.0~----------------------------------------------~----------~

QJ 2.0uco.D•...oVl.0

<t 1.0

500400We ve length( nm )

600

Figure I-Absorption spectra or (I) Kerosene (Neat) -. (2) Petrol (Neat) -, and0) ::T nil (i':cat) -.-

wavelength increment. concentration, temperature, andfrequency bandpass on synchronous fluorescence spec-tra of different crude oils. It is found that there is a lilliedifference in the synchronous fluorescence spectra ofdifferent crude oils obtained from different sources,which shows a small variation in the relative amounts ofaromatic components in each crude oil. Shaver andMct.iown" have successfully analyzed the complex coalliquid samples using lifetime synchronous spectrum.

However, con vent ional petroleum products likepetrol, kerosene, and lubricant like ~T oil have neverbeen subjected to evaluation of their synchronous fluo-rescence spectral properties. The present study is aimedat cvalu.uion of synchronous fluorescence parametersfor petrol. kerosene. and 2T nil that arc commonly avail-able in Indian market. The effect of dilution on thesemultifluurophoric systell1~~ h:!s been studied.

Experimental Procedure

Kerosene, petrol licadd), and 2Tui! (Ser'.()i werecollected 1':-011; th.: Ilh';;i market in CIll'!111:!i Par.iculatcmatter in the products was removed bv ccnuuuu.uion at·WOO rplll. Cyclohcxanc (HPLC grade. Runbax y) \\'asused as solvent after purification and was subjected toblank experiments to CnSUI\_: its tluorimemc purity. Aknown volume of pure sumpl« \vas pipcued (Jut and wasmade up to the corrcsponding-conccnrrat ion (in per cent)with cyclohe x.mc.

-

Absorption spectra were recorded on a Shirnadzuspectrophotometer. Fluorescence spectra were obtainedon a Hitachi FASOO spectrofluorimeter. For fluorescenceas well as synchronous fluorescence measurement thescan speed was 240 nrn/s and PMT voltage was at 7()OY. Excitation and emission slit width were Snrn. Excita-tion source was 100 W Xenon lamp. The corrected fluo-rescence spectru 111 was recorded in the range ~()() - (I()O

nm and synchronous fluorescence spectra were plottedin the excitation scale. The spectral shifts were measuredby taking the first derivative of synchronous fluorescencespectra. Three independent experiments were performedforvarnples obtained from different sources

Results and Discussion

Generally. petroleum products like petrol. kc'I'O'l'IlCand ~T oil contain a large amount of organic 1I1nk-.:uic-s.which absorb light strongly in LJV range. 1 he .ibsorp-lion SIXL'lra of neat samples of petrol. kerosene ,ll1<i :2'1'oil arc shown in Figure I.

Fiuure I shows that petrol :lr,J kcroseru: :ib:-'(Irhstr\)il~:h' in (il''' UV range from ~O() It' _~'i()!1n'. \'.!l;.:rl':i,2T ui] absorb ~;tro!1gly ill the rangt' 2(){) to ci~()I~;!1. 1-1<.1\\-

'';\IT in the :Ii)\CIlL'l' of any <tructurc. ab:-;"lpl!UI1 SP,'L'-trornetry ha:; no analytical uxc for the,,' s)'\!ems

Conventional Fluorescence or Petroleum Pro.hut-.

Kerosene shows tile cui-off line or cxc itntion IV:!It'-

length ill the ran~L' J.5() to .150nl1l. :\(J chall~c \\ :l.' (lh-

302 J SCI INO RES VOL 59 APRIL 2000

J~) r------------------------------'

.~ c: .!l 2500 .5 0) u 5 2fOO

~ g 1500

C 0)

.::: HID ,g 0)

0:::

Kerosene Emission spectra

Wavelength (nm)

Fi gure 2-F1 uorescence emi ssion spectra of 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 per cent kerosene in cyclohexane and kerosene (Neat, 100 percent ). A" = 330nm

served in the emission maximum of kerosene in the ex­c itation range 250 to 330nm . The excitation wave length was fixed at 330nm, for which the emi ss ion intensity was maximum. No fluorescence emission was observed when petrol was excited in the wavelength below 300nm and above 400nm. 2T oil shows a excitation maximum of 470nm and emi ss ion max imum at around 51 Onm. The conventi'onal fluorescence spectra of kerosene, a repre­sentative sample, at various dilutions (5 - 100 per cent, v/v in cyc lohexane) is given in Figure 2. The spectra are generally broad with some degree of structuring. An enhancement of fluorescence intensity with a red shift of emission maximum is observed with increasing con­centration. The plot of variation of flu orescence inten­sity at 344nm with concentration is shown in Figure 3(a).

Figure 3(b) and Figure 3(c) depict the vari ation of fluorescence intensit ies for petrol and 2T o il , respec­ti ve ly, at various concentrat ions. The regu lar increase in intensity observed in Figure 3(a) for kerosene makes it possible for it to be used as a calibration curve. How­ever the curves for petrol and 2T oil pass through a maxi­mum value and decrease in intensity with concentrati on. Therefore, an intensity based calibration at higher con­centrat ion from 50 - 100 per cent could work for kero­sene but fails for petro l and 2T oil.

SYllchronous Fluorescence Scan of Petroleum Products

Optimization of I1A (Aex - Acm) was done for eac h of t.he petroleum product by obtaining the synchronous

fluorescence spectrum at different I1A in the range 5 -

O~L-~I--L-~I--L-~I~~~I _~_~I

o w ~ ro M ~

Concentraoon (in %, VN)

Figure 3- Plot of Ouorescence intensity with concentration of (a) Kerosene, (b) Petrol and (c) 2T oi l in cyclo exane (in per cent , v/v). The A" and A,m used for kerosene are 330nm and 344nm, fo r petrol 325nm and 347nm and , for 2T oil 470nm and 5 10nm, respective ly

100nm at every 5nm interva l. Figure 4 gives the repre­

sentative SF spectra for kerosene at d ifferent I1A. Comparison of these spectra with the conventi onal

fluorescence spectrum shows that there is a s ignificant band compress ion (less FWHM) in the SF scan. The

maximum intensity was found at I1A = 15nm for kero­

sene and was chosen as optimized I1A. Similar optimi­zation was done for petrol and 2T oil. The optimi zed

I1A fo r petrol and 2T o il were 40nm and 35nm, respec­ti vely. The synchronous flu orescence spectra of kero­sene at various dilutions (in cyclohexane) are shown in Figure 5.

The overall shi ft of I Onm on going from 5 per cent kerosene to neat kerosene is comparab le to the shi ft of maximum observed in conventional fluorescence spec­trum (Figure 2). However the narrower SF bands make it possible to observe the variation of SF spectra l sh ift with concentration more clearl y. The f irst deri vat ive of

PATRA et at.: PETROLEUM PRODUCTS 303

~,-----------------------------------~

310 320 330 340 350 360

Wavelength (nm)

Figure 4-Synchronous fluorescence spectra of kerosene (neat) at vari­ous i'lA (\, - \,). The values are shown in the fi gure

Keros

3000

S-~ 2000

i?:-'iii c 1000 Q)

E Q) 0 0 c Q) 0 800 C/)

1st derivative synchronous floorescence spectra

~ 600 a kerosene(neat) ::l

;;::: 400 C/) ::l 200 a c e 0 .r:: 0 -200 c >. (f) -400

-600

300 310 320 330 340

Wavelength (nm)

Figure 5-Synchronous fluorescence spectra and first derivative syn­chronous fluorescence spectra of kerosene at various con­centrations in cyclohexane (in per cent , v/v). i'lA = 15nm

synchronous fluorescence spectrum provides a more accurate value of the max imum . Representative first derivative spectra for kerosene are given in Figure 5 from which the value of SF maxima were obtained. Figure 6(a) shows the shift of these SF spectral maxima with concentration .

It is observed that there is a shift of spectral maxi ­mum to longer wavelength. The spectral shift is more rapid in the lower concentration range as compared to the higher ranges. The overall variation, however, is quite regular. It may be noted that the shoulder that appears towards shorter wavelength in 5 per cent solution , rap­idly diminishes in intensi ty. SF spectra of petrol (Fig­ure 7) and 2T oil (Figure 8) show similar trends.

co E

330 a. 328 I_1-1- 1- 1

326 1-1- 1--324 y,/ -+-Ierosene

1/ 322 r 320~~--r-''--'I--~-'-I-r-'tr-'-~,~

b. 310

300

-~290 ~ = Q.)

~ C. C>

~460 is e440

-'= <..>

~20 -1-lO/1g2T~1

x 400

~ ~ ro 00 ~

Concentrat~n (in %, vlv)

Figure 6-Plot of first derivative synchronous fluorescence maximum (ASFS"'OX) with concentration (i n cyclohexane in %. v/v). (a) Kerosene (i'lA = 15nm), (b) Petrol (i'lA = 40nm) and (c) 2T oi l (i'lA = 35nm). In Figure 6(c) the (x-x-- x) line refers to the shi ft of shorter wavelength band and the (*---*­-*) line refers to the shift of longer wavelength band

The extent of synchronous fluorescence spectral shift for petrol was 20nm and for 2T oil it was much more, i.e., 72 nm. For 2T oil, it is observed that the syn­chronous fluorescence spectrum at lower concentration is essentially an overlapping two band system; the shorter wavelength band which is the predominant band at 5 per cent rapidly decreases in intensity with increasing con­

centration and at ~ 20 per cent only longer wavelength band persi sts. Figure 6(c), therefore, shows the shift of shorter wavelength band maximum up to 20 per cent and that of longer wavelength maximum above that con­centration.

The other important observation that is made is that the intensity of SF maximum does not vary much with concentration for kerosene but for petrol and 2T oil , in­tensity keeps on decreasing with increasing concentra­tion. This trend was already observed for conventional fluorescence spectral intensity too (Figure 3), The lin­earity of electronic spectral parameters like optical den­sity and fluorescence intensity with concentration is usu-

304 J SCI INO RES VOL 59 APRIL 2000

1800 PerceoIBgc of petrol in cyclahexane (vlv)

1600

:f 1~

1200

" ~ 1000

b! 1Il0 e

~ ~O

'" > .f.! 400

~ 200

0 270 300 330

Waveleq;th (mn)

Figure 7- Synchronous fluorescence spectra of 5, 10, 20, 30,40, 50, 60,80 per cent petrol in cyclohexanc and petrol (neat, 100 per cent). 6."A = 40nm

all y expected only at quite low concentration ranges of the sample. In the chosen concentration range for our study (5 - 100 per cent) intensity based estimation tech­niques are not ava ilabl e. However the regul arity of shift of SFS max imum offers such a poss ibility. The narrow spectral bands of SF technique makes it poss ible to ob­tain the spectral shifts to a fairly high degree of accu­racy. Therefore spectral shift based calibrati on curves given in Figure 6 provide reliable estimation method for petroleum products at high concentration ranges. Even the error ranges of data, such estimations fo r kerosene and petro l for which the overall spectral shi ft is low, can be made withi n 10 per cent accuracy. For 2T oil the ac­

curacy is much better at ± 1.5 per cent.

Almost every petroleum product contains various polycyc l ic aromatic hydrocarbon fl uorophores. Low­boiling products like petrol and kerosene predominant ly contain one- and two-ring sys tems l7 whose absorption and tluorescence spectrum appears at hi gher energy « 350 nm). A higher boiling product like 2T oil c ntains polycyclic aromatic hydrocarbons of various ring sizes ranging from one-ring system to 4- and 5-ring sys tems . Non-radiative resonance energy transfer between mol­ecules occurs by a Foster type mechanism when the emiss ion spectrum of the energy donor overlaps with the acceptor absorption spectrum IX. The effi ciency of such energy transfer is gi ven by the formula:

I-:.T = (R·~ J K 211 ·4 a) x 8.7 1x 1023 s·',

where R is the distance between the donor and acceptor in A, J is the overlap integral which expresses the de-

~ 2000 '-'

.~

M .13 1.lOO

~ 8 U ~ 1000

t::: .,

j JOO

~ 0

.-------------------, 2Toil

no 400 co Waveleq;th (001)

550

Figure 8-Synchronous fl uorescence spectra of 5, 10,20,40,50,80 and 90 per cent 2T oil in cyclohexane. 6"A = 35nm

gree of spectral overlap between the donor emi ss ion and the acceptor absorption and its units are M ·lcm" K is a factor describing the relati ve orientation in space of the transition dipoles of the donor and acceptor, n is the re­

fracti ve index of the medium and ex is the emi ss ive rate

of the donor which can be ex pre sed as <Pd / 'rd, where

<Pd is the qu antum yie ld of the donor in the absence of

acceptor and 'rd is the li fe time of the donor in the ab­

sence of acceptor. For randomly oriented donor and ac­ceptor molecul es in solution the orientational facto r con­tributes an averaged value of 2/3. T he efficiency is then governed by the spectral overlap integ ral and the mean di stance between the donor and acceptor molecules. Pe­tro le um produ c ts inva ri ab ly containing mu ltipl e fl uorophores absorb ing and emitting in a broad wave­length range, is expected to have extensi ve spectral over­lap of absorption and emiss ion spectra of various spe­cies. The efficiency of energy transfer then becomes only a funct ion of the mean d istance of separation between the donor and acceptor pairs . Thi s di stance decreases with increas ing concentration of the petroleum product sample. In neat samp les, R's of variou s donor and ac­ceptor pairs being minimum the most effic ient and mul­tip le Foster type energy hopp ing would be observed the cnergy being nonradiatively transferred from higher e lec­tronic energy states (lower ring system po lycyc lic aro­matic hydrocarbon) to the lowest energy sink (hi gher ring system polycyclic aromatic hydrocarbons) through multiple hopping. Emiss ion can then be observed mostl y from species emitting at longer wavelengths. With in­creasing di lution the mean distance of separati on in­creases, thus resulting in less efficient re ·onance energy

PATRA et al.: PETROLEUM PRODUCTS 305

transfer which is reflected in progressive blue shift of the emission maximum. This phenomenon which is ob­served for conventional fluorescence spectra (Figure 2) is quite clearly observable for synchronous fluorescence spectrum due to the extensive narrowing of spectral bands. It is appreciable that the identity of individual fluorophores that could be present in the sample and their absolute concentration are unimportant for developing an estimation method based on shift of spectral maxi­mum; what is important is the regularity of the shift with concentration.

Acknowledgement

The authors gratefully acknowledge the financial support extended by Council of Scientific and Industrial Research, New Delhi, to carry out this project.

References Chen R F, Chadwick D B & Lieberman S H, Org Geochem, 26 (1997) 67-77.

2 Quinn M F, 10ubian S, AI-Bahrain F, AI-Aruri S & Alameddine 0 , App! Spectrosc, 42 (1988) 406-410.

3 Gray N R, McMillen S 1, Kerr 1 M, Denoux G & McDonald T 1, Soc Petro/ Eng (SPE), SPE 25990 (1993) 465 - 475 .

4 Wang 1, Wu X & Mullins 0 C, ApI'/ Spectrosc, 51 (1997) 1890-1895.

5 Bezouska 1 R, Wang 1 & Mullins 0 C, App! Spectrosc, 52 (1998) 1606-1613.

6 Ral ston C Y, Wu X & Mullins 0 C, ApI'/ Spectrosc, 50 ( 1996) 1563-1568.

7 Downare T D & Mullins 0 C, AI'p! Spectrosc, 49 (1995) 754-764.

8 Reyes M V, Soc Petro! Eng (SPE) , (December 1994) 300-305.

9 Bright F V, AI'p! Spec/rosc, 42 (1988) 1245-1250.

10 Hertz P M R & McGown L B, App! Spectrosc, 45 (199 1).73-79. II Lloyd 1 B F, Nat Phys Sci, 231 (1971) 64.

12 Tay lor T A & Patterson H H, Anal Chem, 59 (1987) 2180-21 87.

13 Lloyd 1 B F, Analyst, 105 (1980) 97-109.

14 Lloyd 1 B F, Analyst, 100 (1975) 82-95.

15 10hn P & Soutar I, Anal Chem, 48 (1976) 520-524.

16 Shaver 1 M & McGown L B, Appl Spectrosc, 49 (1995) 8 13-8 18.

17 Gruse W A, Stevens D R, Chemical Technology of PetroleulII. Third edn (McGraw Hill , New York) 1960, pp 425 - 486.

18 Lakowicz 1 R, Principles of Fluorescence Spectroscopy , Thi rd edn (Plenum Press, New York) 1986, pp 305 -306.