Improved Identification of Spilled Oils by Infrared Spectroscopy Carol P. Anderson University of Connecticut, Groton, Connecticut 06340
Timothy J. Killeen University of Connecticut, Storrs, Connecticut 06268
John B. Taft and Alan P. Bentz” U S . Coast Guard Research and Development Center, Groton, Connecticut 06340
Light oils are notoriously difficult to match to their sources after environmental weathering. A new accelerated laboratory-weathering procedure, combined with either log-ratio or vector pattern recognition, gives markedly improved identification of spilled light fuel oils by infrared spectrosCOPY.
Introduction In 1955, Rosen and Middleton ( 1 ) suggested that infrared spectra might be sufficiently characteristic to identify oils. Reviews by Brown ( 2 ) and Bentz (3,4) traced the evolution of the use of infrared techniques to identify oils that have been weathered by comparing them with likely sources. Numerous weathering studies, including major efforts by Frankenfeld ( 5 )and Brown ( 6 ) ,served to provide detailed information as to the effects of weathering on infrared spectra. This information, coupled with an interlaboratory study, led to the development of a standard ASTM method for the infrared analysis of waterborne oils (7), as well as a standard field and laboratory procedure for use by the Coast Guard (8). These techniques were, for the most part, qualitativedepending on visual inspection of overlayed spectra. The original ASTM procedure did give an option for using peak ratios according to a procedure recommended by Kawahara (9). Others began to look at pattern-recognition techniques which could be employed to give more objective interpretations and take into account the weathering effects. Mattson (10 ) and Brown (6) looked at various statistical pattern-recognition schemes. Brown’s log-ratio technique was applied to digitized data from over 1200 infrared spectra, some 900 of which were from weathered oils. Of the 13-21 peaks he used for comparison, Brown found that if all of the log-ratio values lay within 10%of the average, the oils were from a common source; i.e., he allowed a 10%“weathering window” (6). In our experience, the log-ratio technique (using 18 peaks) worked well with all but the very light oils, because of the fact that the magnitude of weathering changes was greater than differences between similar light oils. In an effort to improve the results, we applied weighting factors to the 18 peaks, such that the peaks which changed least with weathering were given the highest factor. A marked improvement was noted, but there were still problems with the light oils ( 11). A new vector pattern recognition technique was developed by Killeen et al. (11,12) for the purpose of identifying weathered oils from both their fluorescence and their infrared spectra. To compare a weathered spill to its source by this method required a third sample, namely, a weathered-source sample. The spectral data from the latter were used with the data from the source to generate a weathering-response surface to obtain the closest match to a weathered oil. The need for a weathered-source sample gave impetus to an already expressed need for a standardized weathering method (13)to enhance matching capabilities of all analytical methods. Thus, a good laboratory-weathering technique would fulfill both the requirement for a weathered suspect to 1230
Environmental Science & Technology
facilitate matching a spill visually and also the requirement for the vector pattern recognition technique. The development of such a laboratory-weathering procedure was the subject of this investigation. Secondly, we wanted to develop a method using 1 mL, or less, since real-world samples of spills are generally of limited quantity. A third, equally important objective was to see whether comparisons could be improved for the fast-weathering light oils. The vector technique also lent itself to quantitative analysis of mixtures; this application was evaluated during this study to see whether it would be possible to compensate for the presence of a second oil or perhaps an oil dispersant.
Pattern-Recognition Techniques Used in this S t u d y In order to conduct more objective comparisons of oils, we employed computerized quantitative techniques by using 18 carefully selected data points ( I 1). Table I lists the data points used, as well as some weighting factors which will be discussed later. Two techniques have been used for our purposes. The first is the log-ratio method of Brown (6);the second is the vector method of Killeen et a1 ( 1 2 ) . Log-Ratio Method. The values of the percent difference in the peaks are sorted into ranges of 0-5,5-10,lO-15,15-20, 20-50, 50-100, and >loo. Replicates of the same oil (a definite match) will have all peaks lie within the 0-5 range. To allow for weathering, a wider variation is permitted. Brown et al. (6) found that the weathered oils generally match with all peaks in the 0-10 range. Vector Method. Briefly, the vector method considers the digitized spectra as 18-dimensional vectors (n-dimensional if n digitized data points are used). The data are normalized to compensate for differences in sample thickness, etc. The vectors of the suspect and one, or more, weathered samples of that suspect span a hyperplane in 18-space. That is, the weathered vector(s) and the unweathered vector generate a “weathering surface” for the suspect. The spill sample may not lie on this surface; the orthogonal projection of the spill vector on the “weathering surface” gives the closest vector on the surface which is designated the “computer-simulated weathering” vector. The angle between the spill vector and its projection on the surface represents the “closeness” of the spill to the suspect. The position of the projection will indicate whether the spill is more, or less, weathered than the suspect. This method is an extension of a comparison technique used in fluorescence spectroscopy (12). The effectiveness of this technique is highly dependent on a reasonable simulation of real-world weathering. Weighting-Factor Approach Peak-by-peak examination of the log-ratio data for many oils identifies peaks with greatest variability. Changes in the No. 6 oil were virtually nil in the 700-900 cm-1 region but were very pronounced in the 900-1300 cm-l region. (This latter difference disappeared from the infrared spectrum with deasphalting.) Peak variability depends to some extent on the oil type due to compositional differences, such as higher ali-
This article not subject to U.S. Copyright. Published 1980 American Chemical Society
phatic/aromatic ratios. According to our initial premise, the peaks that were relatively stable were assigned weighting factors of 0.9 or 1.0. Smaller weighting factors down to 0.3 were given to the peaks that varied the most. See Table I for the weighting factors used for each wavenumber. Table I1 shows the improvement in the degree of match when using the weighting factor, as a function of oil type. The No. 6 oil has all peaks within the 0-5% range (originally two outliers); the No. 4 oil improves from 15 to 17 in the 0-5% range; the No. 2 goes from 9 to 12 in the 0-10% range; and the crude improves from 9 to 15 within the 0-10% range. Although the weighting factor gives noticeable improvement in comparing all oils with their weathered counterparts, it is unsatisfactory for light oils.
Laboratory Weathering Many investigators have artifically weathered oils. The weathering systems have varied from a sophisticated indoor apparatus with recycling sea water, simulated wind, and sunlight ( 5 )to simple wading pools ( I 4 ) as well as a system which confined an oil in the actual marine environment (6). The sample size varied from 1 L (at The Research Institute of the Gulf of Maine in 2000-L outdoor tanks) to microliter quantities (15). Brown et al. (6) conducted a 2-year study comparing outdoor weathering with laboratory weathering. One of the most successful, rapid laboratory-weathering procedures was developed in his laboratory (16)in which 2 mL of unweathered oil is shaken with sea water in a test tube for 1 h at 35 “C,the water is removed, and the sample is placed under vacuum (0.05 torr) for 1 h. It was our intent to see whether this technique might be improved upon by modifications which would simulate oxidative weathering as well as loss of the lighter components. The visual effects on the infrared spectrum of exposing an oil to the aquatic environment are clearly seen if spectra of unweathered and weathered oils are examined. Figure 1shows the formation of carbonyl a t 1710 cm-l, the downward displacement of the base line in the 1300-900-~m-~ region, and the numerous changes within the 900-700 cm-l region. It should be noted here that the changes in a light oil due to weathering are comparable in magnitude to the differences between individual light oils. The heavier oils do not show such distinctive differences upon weathering. However, a n y weathering changes can increase the difficulty of making identification of a spill source unless compensation is made
for the weathering. In visual interpretation, the analyst must mentally compensate for known weathering effects. In the log-ratio method, the 10%allowance can be made for weathering, or weighting factors can be used in which those data points are weighted the least which change the most with weathering. Alternatively, a good simulated weathering will permit identification with all 18 peaks within 5% as seen later in Table 111. With the vector technique, a laboratory-weathered sample is a necessary part of the comparison. For convenience in interpretation, the results are graphically displayed as shown in Figure 2. The problem is not simple, since the natural “weathering” of an oil involves complex processes which cause chemical and physical changes in the hydrocarbon mixture. Evaporation, dissolution, oxidation, polymerization, and biodegradation are some of the processes whose combined effects are termed “weathering.” The laboratory-weatheringstudies were set up to evaluate the following variables: oil type, time, light, temperature, solubility, and air flow as well as film thickness. Polymerization and oxidation are temperature-dependent, light-induced processes. We studied infrared, short-, and long-wave ultraviolet light sources and found that the best simulation was effected with a long-wave ultraviolet source (about 360 nm, such as is used for TLC illumination).
WAVLLENOTH
I
(MICRONS)
7
0
e
0
IO
I4
I2
IO i
NO. 2
FUEL OIL
MEAT OUTDOOR
2
DAY1
J l 2000
’
i 1800
’
l 1600
*
1400
i
~
It00
W A V E NUMB LR (CY
Flgure 1. An
l
’ 1000
l
’ 040
,
I 000
-I1
unweathered No. 2 fuel oil compared to the weathered
sample.
Table 1. Weighting Factors for the 18 Peaks Used for Comparison wavenumber, cm-’
1304 1165 1145 1070 1032 955
wf
wavenumber, Cm-‘
wf
wavenumber, cm-’
wt
0.8 0.5 0.3 0.3 0.5 0.6
918 890 87 1 849 832 810
0.7 1.o 0.9 1.o 1.0 0.4
790 782 766 744 722 700
0.5 0.4 0.5 0.7 0.6 0.4
Table 111. Effect of Laboratory Weathering on Comparison of No. 2 Oils by Log-Ratio to Outdoor WX Sample A range, %
oil A
oil B
Oil
0-5 5-10 >10
18 0 0 2.04
1 13 14 23.01
8 2
av
c
oil D
4 3 11 17.02
a 12.24
oil E
5 5
a 13.65
Table II. Log-Ratio Comparisons of Different Oils Weathered vs. Unweathered with and without Weighting Factors No. 6 range,
0-5 5-10 >10 av
YO
No. 2
NO. 4
crude
WlO
with
WIO
wlth
WIO
with
WIO
with
16 2 0 3.24
18 0 0 1.60
15 3 0 2.94
17 1 0 2.07
4 5 9 12.75
8 4 6 7.07
8 1 9 8.23
6 9 3 7.09
~~
Volume 14, Number 10, October 1980 1231
Lighter components of oil are lost in the weathering process by dissolution in water and by evaporation. The effects of dissolution were studied by equilibrating the oils with aqueous media which included water taken from Long Island Sound, synthetic sea water, and distilled water. The effects of evaporation were studied by using air flow over an oil film and by vacuum treatment. We found that the best simulation of losses caused by evaporation and dissolution could be achieved by passing an air current of 2.6 m/s (equivalent to a 5-knot wind) over an oil film. During the development of the laboratory-weathering procedure, the changes produced in the weathering experiments were monitored by computer plots of the 18 absorbances, normalized to the highest value which was given an arbitrary value of 1.0. This provided a ready, visual presentation for comparison to help us optimize the weathering parameters by minimizing the differential between the two curves, as shown in Figure 2 . In this figure, the upper set of curves (prepared by connecting the 18 absorbances) shows that the nonoptimum laboratory-weathered oil was considerably more weathered than the outdoor-weathered oil. The lower set shows the close match between an oil weathered under recommended laboratory conditions and the outdoorweathered oil. The evolution of the weathering apparatus began with an empirical approach to the application of heat, light, water, and air to the oil. The final configuration of the apparatus, shown in Figure 3, uses neat samples of unweathered oils, without water, applied as a thin film, and irradiated with long-wavelength ultraviolet light. Air circulation is provided by a fan. With the lamp set at a distance of 10 cm, the effective energy intensity was ca. 15.6 mW/cm2. An electric cooling block was found to be necessary to regulate the temperature of the sample a t 20-25 OC. This technique can weather samples of approximately 1 mL in a 7.5-cm diameter petri dish down to as little as 30 mg on an infrared salt plate. In this apparatus, 4-5 h of exposure to UV at room temperature was necessary to obtain weathering changes comparable to 1or 2 days in the natural environment. The No. 2 fuel oil depicted in Figure 1 was weathered outdoors; differences from the unweathered oil are immediately apparent and would render identification difficult. Figure 4 shows a comparison of the same outdoor-weathered oil with the oil weathered in the laboratory; the vastly improved coincidence of the curves is readily seen-particularly over the standard 18 points selected. In this case, 4.5 h at 22 "C in our apparatus achieved the equivalent of 2 days of outdoor weathering. Achievement of this close a match for the No. 2 fuel oil is particularly gratifying, since light oils are notoriously difficult to identify after weathering. We reported this specific problem of identifying light oils using log-ratio or vector pattern recognition techniques at the 1978 Pittsburgh Conference ( 1 1 ) . One of the most significant achievements reported in the present paper is the fact that laboratory weathering gave a marked improvement in matching light oils.
-
2
I
1
LLBORLTORV WILTHIRIYC OUTDOOR WfATHtRlYC
, "
i i
'
l
'
l
'
-4
l
'
i
'
l
'
1330 1260 1190 1120 1050 980
l
'
910
l
'
l
'
840 770
l
700
WAVENUMBER (RECIPROCAL CENTIMETERS) Figure 2. Comparison of outdoor-weathered and laboratory-weathered infrared spectral data: (upper set) poor simulation; (lower set) suggested laboratory-weatheringconditions.
U V
LAMP
SAMPL.E FILM PETRI DISH
ALUMINUM COOLING BLOCK
ItMP.
STIR.
Figure 3. Weathering apparatus.
WAVELENQTH
0
0
?
(MICRONS)
8
9
IO
IO
14
I@
1
rl
Results with Laboratory Weathering
Since the magnitude of change with light oils is so profound on weathering, the question arises as to whether all light oils might not approach the composition of any one weathered oil. To see whether this is true, five laboratory-weathered fuel oils (A-E) were compared to one weathered outdoors for 1day (oil A). The results shown in Table 111clearly show that only the true source approached the outdoor-weathered sample when it was weathered in the laboratory. Table IV shows log-ratio results of comparisons between a spilled No. 2 oil and two suspected sources. Before our artificial weathering technique was employed, the true source, 1232
Environmental Science & Technology
ao
PO00
WAVENUMBER (CM-')
Flgure 4. A laboratory-weathered No. 2 fuel oil compared to the 2-day weathered sample.
A, did not match the spill as well as did suspect B. After artificial weathering, suspect A is correctly identified as the spill source, while B is no longer as close a match. There remained the question as to whether there were any significant improvements in matching by using our proposed weathering technique over that of Brown et al.’ (16).Brown and his group volunteered to use their technique to weather the same samples that we were testing. We then analyzed their laboratory-weathered samples, on our infrared spectrophotometer, by using the same digitizing hardware and comparison software. The results are shown in Table V, where the number of peaks lying within Brown’s 10%“weathering window” are tabulated for comparisons made with oil which had been weathered outdoors for 1 and 2 days. The table clearly shows that laboratory weathering improves the degree of match for the light oils. Our present results show a slight edge over Brown’s, particularly with the No. 2 fuel oils. The true merit of our method becomes clear when we consider the 5% window as shown in Table VI. The trend is that Brown’s method of laboratory weathering shows also general improvement over unweathered samples, and our proposed method again generally gave better results than Brown’s. It must be emphasized that this generalization is made for the proposed method for the light oils only, since this was the thrust of this investigation, and our heavy-oil data base is limited.
Vector Method The laboratory-weathering technique provides the required sample of weathered oil for the vector method. Figure 5 shows the simulated weathering vector (curve S)superimposed over a sample of the same No. 2 fuel oil weathered in the natural environment (curve Y). Both are significantly different from the unweathered oil (curve X). These data show that vector simulation is a valid method of identification of sources of light-oil spills using computerized infrared data. Similar vector comparisons for other oil types (No. 4, No. 6 fuel oils and crudes) show that the vector method can compensate successfully for weathering effects.
Table IV. Spill Compared to Its Source and a Similar Oil SPlll (011 A) vs.
suspected source
artificial weathered
range, %
peaks In range ell A Oll d
0-5 5-10 >10 av
9 3 6 8.6
0-5 5-10 >10 av
18 0 0 1.17
8 8 2
6.5 8 6 4 7.4
Analysis of Mixtures The vector method can be adapted to analyze mixtures, for example, in those cases where an oil spill is on uater already contaminated by another oil, or when an oil is mixed with a dispersant used in cleanup, etc. In cases such as these, instead of weathered and unweathered spectra, the spectra selected are those of the components expected to be in the unknown mixture. The spectra of the individual components are used to generate a hyperplane. The distance from this plane to the spill vector is an indication of the closeness of the spill to any possible mixture of the components, while the position of the projection in the plane indicates the proportion of each suspect involved in the spill. Of course, if one component of the mixture is totally unknown, i.e., a reference spectrum is unavailable, then this analysis will be useless. The method has been applied to several known mixtures with promising results. Oil Mixtures. The vector method, applied to a mixture of a No. 2 fuel oil and a No. 6 fuel oil, not only identified these two oils from several possible sources but also approximated the ratio of the two oils in the mixture. This is shown in Figure 6. The accuracy of this identification was particularly striking since one of the possible sources was a No. 4 fuel oil comprising a similar mixture of No. 6 and No. 2 fuel oils. The predicted whereas the actual concentration of component ?, was 83.470, concentration was 82.4% (wt %). This is typical of the several experiments which were run on oil mixtures. Oil/Dispersant Mixtures. The vector method can approximate the composition of an oil/dispersant mixture down to the levels of visual detectability of the dispersant in the spectrum. Oil/dispersant mixtures which have been in contact with water give erroneous results because varying amounts of the dispersant components are removed by the water. Conclusions This study has demonstrated a useful laboratory-weathering technique and an improvement in oil matching by pattern-recognition techniques using either weighting factors or the laboratory weathering. The most dramatic improvement was in the identification of weathered light oils, which have been perennial problems. The technique for accelerated laboratory weathering can be used with small samples of light oils using thin films, dry under a UV light. It has the following attributes: (a) It improves comparisons by the log-ratio method to give excellent results even with light oils. (b) I t provides a weathered sample for the vector method. (c) It promises to be versatile enough to serve as a basis for a standard weathering method for other analytical techniques besides infrared. Preliminary results of the comparison of laboratory-weathered light oils to outdoor-weathered oils by the other standard oil identification techniques, thin-layer chromatography, gas chromatography, and fluorescence, indicate that in all cases the laboratory-weathering technique
Table V. Number of Peaks, Out of 18 Measured, Which Lie within 10% after 1 or 2 Days of Weathering for Various WX Methods Anderson 011
1
A No. 2 FO B No. 2 FO C No. 4 FO D No. 4 FO E No. 5 FO
18 18 18 18 18
z14 18 16 18 18
Brown
none
1
2
1
2
15 18 18 18 18
7 16 13 17 16
9 17 15 16 18
16 8 16 12
4
Volume 14, Number 10, October 1980
1233
Table VI. Number of Peaks, Out of 18 Measured, Which Lie within 5 % after 1 or 2 Days of Weathering for Various WX Methods Anderson
Brown
2
oil
A No. 2 FO B No. 2 FO C No. 4 FO D No. 4 FO E No. 5 FO
18 12 18 18 18
7
18 13 17 10
NO. 2 FUEL OIL
--
Y
9
,.
X
wx VECTOR
2
6 15 14 15 18
2 11 8 15 10
1
2
4
1 9 4 6 8
13 5 8
14
Acknouledgment We are indebted to Dr. Chris W. Brown and his group at the University of Rhode Island for laboratory weathering samples that we were investigating to permit a direct comparison of their accelerated weathering test (16) with that reported here.
h
SIM
OIL
UNWX
none
1
Literature Cited
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1
1
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,
,
,
,
,
,
,
,
01.
8 0 0
~
~
~
~
‘
l
‘
~
’
~
’
l
facilitates the identification of the unweathered oil. The vector analysis provides a means of identifying oils and determining composition of mixtures of known components. The combination of the vector method and the laboratoryweathering technique is an especially powerful tool in the analysis of spilled oil.
1234
Environmental Science & Technology
‘
(1) Rosen. A. A,: Middleton. F. M. Anal. Chem. 1955.27. 790-4. (2) Brown, C. W.; Lynch, P . F.; Ahmadjian, M. Appl. Spectrosc. Reu. 1975,9, 223-48. ( 3 ) Bentz, A. P. Anal. Chem. 1976,48, 454A-472A. (4) Bentz, A. P. Adu. Chem. Ser. 1980,No. 185, 55-60. ( 5 ) Frankenfeld, J. W. “Weathering of Oil at Sea”, National Technical Information Service, Final Report AD787789 (CG-D-7-75);16-22, 1973. (6) Brown, C. W.; Lynch, P. F.; Ahmadjian, M. “Identification of Oil Slicks by Infrared Spectroscopy,” National Technical Information Service, Final Report ADA040975 (CG-81-74-1099);29, 36,38, 46ff, 1976. ( 7 ) “Annual Book of American Society for Testing and Materials Standards,” part 31, D3414-79. (8) Chemistry Branch, U.S. Coast Guard Research and Development Center, “Oil Spill Identification System,” National Technical Information Service, Final Report ADA003803 (CG-D-41-75), 1977. (9) Kawahara, F. K. Enuiron. Sci. Technol. 1969,3, 150-3. (10) Mattson, J. S. “Classification of Oils by Application of Pattern Recognition Techniques to Infrared Spectroscopy,” National Technical Information Service, Final Report ADA039387 (CG81-75-1364), 1976. (11) Anderson, C. P.; Killeen, T. J.; Bentz, A. P. “Weighting Factors for Infrared Data Points in Pattern Recognition for Oil Identification,” Abstracts 1978 Pittsburgh Conference, 397, 1978. (12) Killeen, T.’ J., Eastwood, D. and Hendrick, M. S.“Oil Matching l ‘ l l Using a Simple Vector Model,” Abstracts 1978 Pittsburgh Conference, 398, 1978; submitted for publication to Anal. Chem. (13) Bentz, A. P. “The Influence of Weathering on Identification of Spilled Oil Samples,” Proceedings of Workshop on Pattern Recognition Applied to Oil Identification, IEEE Catalog No. 76 CH 1247-6C, 5-14,1977. (14) Scolnik, M. E.; Scott, A. C.; Anbar, M. “Method of Identifying and Determining Source and Age of Petroleum Found in the Marine Environment,” National Technical Information Service, Final Report ADA010704 (CG-D-61-75);44-47,1974. (15) Gruenfeld, M; Frederick, R. “Identification of Milligram Quantities of Petroleum Oils,” AOC Newsletter, U S . EPA, 24, 9, 1975. (16) Brown, C. W.; Lynch, P. F. Anal. Chem. 1976,48, 191-5.
Received for Reuiew July 13,1979. Accepted May 23,1980.
.
