Characteristicso...

Vol.3　No.4　Dec.1999JOURNAL OF SHANGHAI UNI VERSITY
R O ,YONG K ,f ,S f L f S ,S U y,z R ,S ,IN Characteristics of the Three -Dimensional Fluorescence
Spectra of Fossil Fuels
YONG Ke -lan
(Scho ol of Life Scienc es ,Shangha i U niv ersity)
L Jing -ci
(Scho ol of Chemistry a nd Chemical Engineer ing ,Shangha i U niv ersity)
A bstr act We have found some fluorescence characteristics of fossil fuels based on the contour maps of the three -dimensional fluorescence spectra of their non -quenching samples.The common fluorescence characteristic is that the main peaks of various fossil fuels are located in the vicinity of excitation/emission wavelength pair 228nm/340nm.
The diversity of fluorescence characteristics can be represented with several indexes α,K ,F and R ,and these index -es provide measurable parameters for division of fluorescence fingerprints of fossil fuels.The fluorescence fingerprints of fossil fuels can be divided into five models named O ,B ,Q ,P and G that are corresponding to condensate oil,light oil,heavy oil,coal and natural gas,respectively.The technique has a potential application in study of environment pollution on crude oil and geochemical exploration of fossil fuels.
Key words three -dimensional fluorescence spectra,fossil fuel ,fingerprint
1　Introduction
　Fossil fuels are the solid ,liquid and gas minerals that
contain carbon and hydrogen elements and can be used as fuel,including coal ,petroleum ,natural gas and source rock .There are abundant aromatic hydrocarbon compounds in fossil fuels.Because of its great sensitivity for the detection of aromatics ,fluorescence spectro metry is enjoying increasing po pularity as an analytical tech -nique for qualitative and quantitative analyses.It is be -coming an important tool for the detection of fossil fuels especially at low concentration levels.However ,varia -tions in composition of the complex mixture of hydro -carbons comprising fossil fuels are insufficient to cause marked changes in fluorescence emission spectra [1,2].Conventional fluorescence methods suffer from non -selectivity and are generally ineffective in spectral struc -tural elucidation of variant fossil fuels.Fossil fuels can not be resolv ed by conventional fluorescence methods which should be regarded as only an aid to identification and are limited to use as an exact diagnostic tool.　Three -dimensional fluorescence(TDF)spectrum anal -
ysis overcomes these shortcomings.This technique has been used for environment contaminant study on crude oil and geochemical exploration [3～5].TDF is essentially a mapping of the fluorescence intensity as the simultane -ous function of excitation wavelenth(Ex)and emission wav elength Em .The display is a stereograph (sho wn in Fig.1),or a three -dimensional representation com-prising a series of closed ,continuous ,equal -intensity contours that it can be called fingerprint.The conto ur map is a graphic pattern in which spectral peaks,anomalies and symmetries can be readily observed.In many cases ,by knowing signature pattern of specific compounds,it can be immediately discerned or de -duced.In the case of complex mixture in which ov erlap -ping patterns prevent the identification of individual components,considerable information is still available.Classes of constituent compounds can often be identi -fied ;differences between closely similar compounds can be noted ;changes in a particular mixture sampled at different times or under different conditions become readily apparent.　TDF can provide much more informations than rou -T y I f f y ff f y f eceived ct.271998
e -lan Asso.Pro .chool o i e ciences hanghai niverisit 20Cheng hong oad hanghai 201800CH A
tine scanning.his is high potential application in mul -ti -component anal sis.t is avorable or selectivit and is e ective in component structural elucidation .A num-ber o scientists have obtained good results stud ing os -
sil fuels with three -dimensional spectra [6～9]
.
　Fig.1　The stereograph of three -dimensional fluorescence
spectrum of crude o il from Oman
　In this paper ,we studied fluorescence properties of fossil fuels and proposed common and div erse fluores -cence characteristics of various fossil fuels.We have found the models and characteristics of fingerprints are directly related with the kind of fossil fuels.
2　Experiments
　Apparatus and software
　Fluorescence spectra were obtained with a Perkin Elmer LS -50luminescence spectrophoto meter .Slit widths were set 5.0nm.The luminescence spectropho -tometer was interfaced to the microcomputer.A printer and a color plotter were connected to the microcomput-er ,respectiv ely.FLDM and LC software packages con -trolled the instrument ,data collection and data process -ing .　Reagents Hexane or petroleum ether was used as solvent with further purification before being used .　Samples Crude o il samples were collected or donated by vario us oilfields.The coal samples were obtained from Xuzhou coalfield and Huainan coalfield .The natural gas samples were collected in Sichuan oilfield .The source rock sam-ples were o btained from Xinjiang oilfield .　Sample treatment Crude o il samples were solved and diluted in hexane.Coal samples and source rock samples were ground and x x T y S K ,x f Methodology Sequential scans of the emission spectra (50scans)were carried out between 300and 500nm at different excitation wav elengths ranging from 200to 300nm.Spectra were recorded at a scanning speed of 240nm/min.
　Concentrations of the sample so lutions were chosen and controlled in a linear response with fluorescence in -tensities to av oid possible quenching effects and particu -lar energy transfer in highly complex systems of fossil fuels.
3　Results and Discussions
　TDF spectra of various fo ssil fuel samples collected from China and abroad have been detected.In order to eliminate any possible interference and avo id quench ef -fect ,the solute must be diluted appropriately.Accord -ing to the contour maps of non -quenching fossil fuel samples ,we have found the fluorescence characteristics of various fossil fuels.
　(1)Common fluorescence characteristic of vario us fossil fuels
　The contour maps show that the common fluorescence peak of various fossil fuels is located in the vicinity of excitation /emission wav elength pair 228nm/340nm (±2nm).The peak is the maximum fluorescence in -tensity peak(i.e.the main peak)for all fossil fuels ex -cept natural gas.In the case of natural gas ,the peak lo -cated in the region around excitation/emission wave -length pair 228nm/340nm is not the maximum fluo -rescence intensity peak ,but the second peak .　So me typical contour maps of fossil fuels are illustrat-ed in Fig.2.Contour maps of the samples related to fos -sil fuels ,such as oilfield water and soil abov e reservoir ,have the same common fluorescence characteristic.But contour maps of non -fossil fuel samples such as river mud and other samples have not this common fluores -cence characteristic.The conto ur map of river mud is also shown in Fig.2as contraposition .
　Von d er Dick et a l .[6]
experimented with two crude oil samples from offshore wells.Various steps in the 10-4～10-7g/cm 3
concentration range were used to control a linear response of fluorescence intensities with T y x f x f 3y f 3f y y B y 322Jour na l of Sha nghai Univer sity
e tracted with he ane.he natural gas s ample was ab -sorbed b absorbent EP-PA cartridge then the car -tridge was washed with he ane and the s olution was col -lected or detection.
concentrations.he pointed out that ma imum luo -res cence intensities occur at an e citation wavelength o 20nm and corresponding emission at the v icinit o 40nm or t pical res ervo ir h drocarbons.ut the did
not pro ve that it was a common fluorescence characteris-tic of crude oils and other fossil fuels.Based on experi-mental results of fifty crude oil samples and one thou-sand fossil fuel samples,we come to a conclusion that the co mmon fluorescence characteristic of all fossil fuels is the main peak located in the vicinity of excitation/ emission wavelength pair228nm/340nm(±2nm)
.
(a)
(b
)
(c)(d )
(e)
(f)
Fig.2　Classification of typical fingerprints of fossil fuels and non-fossil fuel
(a)crud e oil fro m Oman,mo del Q,(b)crude o il from Brunei,mo del B,
(c)crud e oil fro m Xinjian g of C h i n a,model O,(d)nat ural gas from Sichuan o f C h i n a,model G,
(e)co al fro m Huainan o f China,model P,(f)mud o f Yang zi riv er of China(non-fos sil fuel)
　(2)Diversity of fluoresc ence characteristics of vario us
fossil fuels
　Besides the common characteristic there is distinct di-
versity in fluorescence properties of various fossil fuels.
We have investigated comprehensively the fingerprints
f f f y
f f f y f f
W f f f
can be divided into three models named O,B and Q.
These models represent condensate oil,light oil and
heavy oil respectively.The fingerprints of coal and nat-
ural gas can be divided into other two models named P
and G(Fig.2).The fingerprints of vario us source rock
B Q,
F x,f
f f f
323 Vol.3　No.4　D ec.1999YONG K.L.:　Characteristics of the Three-D imensional Fluorescence...
o i t various crude o il samples and one thousand other
oss il uel samples or divers it o luorescence character-
istics.e hav e ound that the ingerprints o crude oils
s amples can be divided into model and model ac-
cording to their nature.or e ample the ingerprints
o the source rock samples o condensate oil are o model
B.The fing erprints of source rock samples of light oil may be of model B or model Q,and that of heav y o il are of model Q.
　In order to describe this viewpoint more clearly,we take the main peak as a center,and draw two lines par-alleled to X and Y axes respectively(Fig.2).The fin-gerprint is divided into four regionsⅠ,Ⅱ,ⅢandⅣby the two lines.The main peak is called T1,the peak ap-pearing in regionⅠis called T2,the peaks appearing in regionⅡare called T3and T4,the peak appearing in regionⅢis called T5.Besides appearing of T1,we de-fine the fingerprint with peaks appearing in regionⅡas mo del O,the fingerprint with peaks appearing in region Ⅰand regionⅡas model B,the fingerprint with peaks appearing in regionⅠas model Q,the fingerprint with a row of peaks appearing in regionⅠas model P,and with the peaks appears in regionsⅡandⅢas model G. Further investigating on the relation of fingerprints of various fossil fuels,we have found that the mod els of O,B,Q,P and G were represented condensate o il,light oil,heavy oil,coal and nature gas respectively.
　In order to represent their div ersities in quantity,we pro po sed the following indexes:
　(1)α—an index about contour strike of fingerprint It is measured by the angle formed by the line paral-leling X-axis and the link line o f peaks(link line be-tween the points representing peaks in the contour, shown in Fig.2).
　(2)K—an index about the slope of main peak
　It shows the variant rate at which highness of main peak decreases with the increase of wavelengths.The rate can be obtained by dividing the fluorescence intensi-ty of main peak by the numbers of equal-intensity con-tours in emission wavelength range from340～380nm.
　(3)F—an index of fluorescence intensity of charac-teristic wavelength pairs.
　(4)R—an index of fluorescence intensity ratio of two characteristic wav elength pairs.
　α,K,F and R provide measurable indexes for divi-sion of fluorescence fingerprints of crude o ils.By these index es,the fingerprints of various fossil fuels can be di-vided into five models named O,B,Q,P and G.The ranges of these indexes are shown in Table1.
　F f f f f
Table1　Range of indexes of various typical fossil fuels
Fo ssil fuel
Mod el of
fing erprint
α/(°)
α1α2
K R
Condensat e
o il
O100～13060～800.65～0.80>6 Light o il B55～6570～1100.55～0.70 2.5～6 Heav y o il Q45～60———0.40～0.60<2.5 Coal P30～45———0.65～0.75 2.5～6 Nat ural gas G100～130180～195———<1 fuels,a common characteristic was observed that the maximum fluorescence peak of them is located in the same position around the excitation/emission wave-length pair228nm/340nm(±2nm).The common characteristic of crude oils in TDF spectra shows clearly that two-or three-ring aromatics are the main compo-nents in crude oils under the experimental conditions de-scribed above.This characteristic exists in other fossil fuel samples,too.But non-fossil fuel samples do not have this characteristic,so we can obtain an unambigu-ous distinction from various fluorescence po llution ing a set of indexesα,K,F and R to repre-sent the diversity of fluorescence characteristics of vari-ous fossil fuels,we can divide fingerprint models of vari-ous fossil fuels into O,B,Q,P and G.It can be used as the scientific basis for classification of fossil fuels.These results would contribute to diagnostic fluorescent abnor-malities of various sources,such as natural and artificial abnormalities,and provide the solid basis for correlation of oil/o il and oil/source rock.There is potential applica-tion in study of environment contaminant on crude oil and geo chemical exploration.
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324Jour na l of Sha nghai Univer sity
4Conclusions
rom a number o real ingerprints o various ossil
ment19924:2in Chinese。