Identification of Reclaimed Oils by Statistical Discrimination of Infrared Absorption Data ANDREW UNGAR Armour Research Foundation of Illinois Institute of Technology, Chicago, 111. ANTHONY M. TROUOLO Department o f Chemistry, University of Chicago, Chicago, 111.
b A function, Q, of the infrared absorbances of a lubricating oil at 14 wave lengths, is an excellent discriminator between virgin and reclaimed oils. The effectiveness of the test is due primarily to the fact that virgin oils are refined from a single or small number of crudes, whereas reclaimed oils have such a diverse origin that the difference in complexity of the two groups results in statistical properties that can be used to discriminate between them. A double logarithmic transformation of the virgin oil Q values appears to have a normal distribution, truncated at the lower end at a value of approximately 294. This value i s taken as the dividing line between virgin and reclaimed oils. The discriminating criterion was principally aimed at making the possibility of misidentifying a virgin oil negligible. Using such a criterion, 85% of the samples of known reclaimed oils were correctly identified.
D
INFRARED ABSORPTION DATA
ing oil processing plants throughout this country now use a t least some reclaimed oil in their base stock. As this practice seems certain to continue, perhaps even on a larger scale, a method of distinguishing virgin oils from reclaimed oils is desirable as protection against misrepresentation. A preliminary investigation indicated that the presence of engine metals, other inorganic elements, or oxidized organic compounds or, indeed, any changes in composition that occur during engine use or during reclaiming could not be expected to characterize reclaimed oils. This does not mean that changes do not occur during engine use, but that these changes are not consistent and may be compensated to a variable extent, depending on the degree of reclaiming. Further work showed, however, that oils of these two types could be differentiated by their infrared spectra. This differentiation is due, not to changes during engine use or reclaiming, but to the fact that reclaimed oils are composite oils, so diverse in origin that they display, in a statistical sense, average properties. This method, in other words, detects the multiplicity of crude sources in the reclaimed oil in contrast with the single crude source, or small number of crude sources, of the virgin oil.
recent years, the armed services as well as many large bus, truck, and taxicab operators have found it economically advantageous to reclaim used motor oil. This practice has spread to the collection, reclaiming, and sale of crankcase drainings from service stations. Many lubricatURIKG
r
Infrared absorbance data werc obtained from 73 virgin oils and 54 rcclaimed oils a t 14 wave lengths in the sodium chloride region ( 2 to 15 microns) on a Perkin-Elmer Model 12C infrared spectrometer modified with a “double pass” optical system. The wave lengths used were 7.66, 8.68, 9.33, 9.68, 10.36, 10.60, 10.70, 10.91, 11.23, 11.42, 11.77, 12.28, 13.87, and 14.35 microns. The spectra from 2 to 15 microns n-ere obtained from samples placed in a sodium chloride demountable cell with a 0.101111111. spacer. Constant cell path thickness was ensured by frequent calibration with a Bradford oil having an absorbance of 0.175 a t 10.36 microns (1). The absorbances were calculated by means of the base-line method. Spectra typical of a virgin and a reclaimed oil together with the base lines are illustrated in Figures 1 and 2. Questions regarding the maintenance of long-term calibration require further work. For instance, the requirements for a stable standard or standards, to maintain calibration a t all the chosen wave lengths, have not been explored. The transfer of this method to 0tht.r instruments would require the axailability of such a standard. To obtain some idea of the distribution of the absorbances, the smoothed frequency polygons shown in Figures 3 and
I.100
\ I1 V
---*
4.700 ,800 ,900 1.0
I
1
I
7
8
9
I I I I 10 I1 12 13 WAVE LENGTH, MICRONS
I 14
1.5 (D
15
Figure 1. Typical infrared absorption spectrum of virgin oil showing base-line method of calculating absorbances
I
7
8
9
10
I1 12 13 WAVELENGTH. N I C R O N S
I
14
1.5
,I
Figure 2. Typical infrared absorption spectrum of reclaimed oil showing base-line method of calculating absorbances VOL. 30, NO. 2, FEBRUARY 1958
187
40
20
,050
.IO0
.I50
,050
0
.IO0
ABSORBANCE
Figure 3.
0
,100
.050
,200
.I50 ABSORBANCE
Distribution of infrared absorbances of lubricating oils
4 were prepared. These graphs suggested that a t almost every wave length the modes of the two distributions were, within the limits of error, indistinguishable. I n each case the range of values for the reclaimed oil absorbances n.as smaller than the corresponding range for the virgin oils. I n short, there appeared to be two families of unimodal distributions with coincident means and small and large rariances, respectively.
.I50
ABSORBANCE
-
I
RECLAIMED OILS VIRGIN OILS
I
0 20
7.66 p
~a 0 0 0
ro2 5
,100
.050
,250
,200
,150
,300
ABSORBANCE
0Y -
o
40
a301
.
STATISTICAL ANALYSIS
The composite character of the reclaimed oils has been suggested as a basis for distinguishing between them and virgin oils. There is a large body of statistical literature, called discriminatory analysis, concerned with assigning individuals to their true class by means of measurements of a number of characteristics. The techniques grew out of problenis of classification in taxonomy, paleontology, and related fields. Most of the literature, howerer, deals with the case in which, for each measured characteristic, the means of the competing classes are different, and the covariance matrices are the same. This is the reverse of the case under consideration. Smith (3) has described a method which makes allon-ance for heterogeneous variances. It is not feasible, however, t o calculate Smith's discriminatory function for 14 variables without a n electronic com188
ANALYTICAL CHEMISTRY
in. in-
4 :0. 2
___---,300
,400
,500
,600
,700
ABSORBANCE
Figure 4. Distribution of infrared absorbances of lubricating oils
puter, and it was not possible during this study to undertake such a computation. For this reason, a fresh study was initiated to find a simple statistical test, based on infrared absorbance measurements, which would discriminate between reclaimed and virgin oils. Q Function. A preliminary statistical analysis of the d a t a confirmed the conclusions drawn from the distribution curves (Figure 3): There was a significant difference between the means of
the virgin oil values and the reclaimed oil values a t only six of the 14 wave lengths (see Table 1)) whereas the variance of the reclaimed oil values was significantly the smaller a t every wave length. With the aid of these facts, there was formulated a function, Q, defined as
n-here i indexes 14 wave lengths 2, = absorbance of oil at wave length i T , = mean absorbance of reclaimed oils a t wave length i (Table I). Because of the narrow spread of the reclaimed oil data and the wide dispersion of the virgin oil data, the numerators of the individual terms of the summation will be, in general, smaller for the reclaimed oils than for the virgin oils. As the scale of magnitude of the absorbances varies over a wide range from wave length t o wave length, the contributions of the individual terms n-ere equalized by dividing by r,. The Q values for all virgin oils and reclaimed oils studied are listed in Table 11. It is apparent that the Q function is capable of excellent, though not absolute, discrimination. Discussion of Results. T h e question may be asked: Why do t h e two groups of oils exhibit distinctive difference in variances? The answer undoubtedly lies in t h e relative complexities of the oils. It is known t h a t , primarily for economic reasons, most virgin oils are refined from a single or a small number of crude sources. Reclaimers. on the other hand, escept those involved in fleet operations, must, of necessity, combine oils froni a large number of sources for their operations. Statistical th(,ory predicts that the observed phenomenon will occur under these circumstances. The essential idea is that if the characteristic, I', of a population of objects has a random distribution with mean p and variance u2, random samples of size A' from this population ail1 be distributed with mean p and variance u2/-Y. I n the present instance each reclaimed oil is effectively a sample of N (large) crude sources and hence the variances of the measured characteristics are inversely proportional t o the number of virgin sources in the reclaimed mixture. The actual situation is more complex than this, because of a variable N and other similar factors, but the essential effect remains the same. Some experimental results confirmed these ideas. Fortunately, information was available regarding the formulation of VI29 and 7"30, which had given apparently anomalous results. They are of the same brand, 10 and 30 weight. respectively. The brand is one of several sold by a large refiner and is known to be formulated from a large variety of crude oils. A number of other brands of the same refiner were tested; all the Q values were within the main body of the virgin oil group in Table 11. Supporting evidence for the proposed model also was obtained when a known virgin oil was used in a n automobile for 2000 miles, drained, and reclaimed
in the laboratory. The Q value of the original oil, 343, differed by a small amount from the value of 311 obtained from the reclaimed used oil. Table 1. Mean Values of Absorbances of Lubricating Oils at Several Wave Lengths -
Wave Length, Microns 7.66 8.68" 9.33 9.68 10.36 10 60" 10 70 10 91 11 23 11.42 11.7'ia 12.2@ 13.87" 14.35a
a
Mean Value Virgin Reclaimed oils oils 0.068 0,090 0.068 0.049 0.111 0 038 0 037 0 028 0 058 0.059 0.025 0.086 0.449 0.023
0.072 0,071 0.054 0,050 0.060 0 03i 0 035 0 023 0 058 0.059 0.027 0,090 0.469 0.034 Significant difference between mean
values of initial virgin and reclaimed oil samples, as determined from Welch's ( 5 ) modified t test (2-tail criterion, 0.06 level of significance).
In the foregoing facts lie the strength and the limitations of the method: An oil reclaimed from used oil originating from a single or very few crude sources will be identified as a virgin oil. Because of economic factors, this occurrence will be infrequent, except in the case of fleet operators. On the other hand, virgin oils refined from a large variety of crude oils, such as V'29 and V'30, will be identified as reclaimed oils. Except in very large and complex refining operations, the production of such oils is uneconomical, and should consequently present only a minor hazard of incorrect identification. Furthermore, the extensive sampling program makes it seem reasonable that no other such oil exists. -4 program of surveillance, which would be a necessary adjunct t o the use of this method, should detect the appearance of comparable oils, and in such cases collateral information would usually be available, as in the case of VI29 and V'30. Samples. The samples of virgin and reclaimed oils were collected by experienced personnel throughout t h e
Table II. Values of Q Function for Various Lubricating Oils" XO.
Q
NO.
Q
VI12 v 25 V' 9 v 3 VI10 VI11 V'20 V'21 V'13
2086 1442 1332 1254 1132 1105 1053 1052 1036
'5 14
506
988
.
v 5 VI28 V 20 V' 5 V '24 v 33 v 34 V'16 V' 7
,505 ...
497 485 482 48 1 481 478 47i
NO. R'25 R 11
Q
R' 7
250 249 248 247 24 1 230 227 224
R'l9 R 8 R 28
R 9
R' 2
452
VI23 VI25
926 888
V 28
423 409
R 2 R 27
208 202
V 16
649 646 640 634 632 624 610 (io3 600 596 572 570 562 560 557 554 553 548 536 535 529 518
7"14 VI26 V 13 R 19 v 37 v 9 R 4
377 369 361 358 363 330 321 315 314 313 312 310 307 307 307
R 14 R -3 R '26 R' 9 R'18 R 21
186 186 177 176 175 174 168 163 159 158 145 143 138 132 131
V'22 V' 4 v 10 v 1 V'l6 v 39 V' 6
v 8 V 19
v 35 V 32 I' 41 v 21 v 2 V 29 v 7 V 31 v'
3 R 6 IT 6 V' 1
~~~
v 3s VI32 R 24 V '3 1 V'l7 V 40 R 1 R'30 R 10 R 25 R 23 R 12 R 22
259 255
R' 1 R'll R'14 R'12 R' 5 R'22 R'23 R' 3 R'10 R'21 R'15 VI29 R' 8 R'17 R' 6 R 28
130 ~. .
129 126 119 112 111 104
V and R identify virgin and reclaimed oils, respectively. Unprimed and primed letters identify members of an initial and a later sample, respectively. Corresponding code numbers in two samples do not necessarily designate identical brands.
VOL. 30, NO. 2, FEBRUARY 1958
189
United States. Enough supplementary information was obtained a t the same time to identify definitely the nature and source of the sample. The samples of virgin and reclaimed oils were collected in a manner to provide as representative a sampling as possible. It is estimated that well over 92% (by volume of consumption) of virgin oils are included-certainly all the crude sources in the United States (and some outside), The percentage of virgin oils represented in the sample was estimated by determining the lubricating oil refining capacity of the refineries known to be associated with given brands, as listed in a recent report on United States operating refineries (9). The refineries associated with a number of brands were not known, so it is reasonable to assume that the 92% estimate based on known associations is conservative. Smith (4) has classified crude oil sources on a geographical basis into eight areas: (1) California, (2) Rocky Mountains, (3) West Texas, (4) Gulf Coast, (5) Mid-Continent, (6) Mississippi, ( 7 ) Michigan, and (8) Appalachian. All areas have a t least one refinery associated with a brand included in the virgin oil sample. Although the presence of a refinery in a geographical area does not guarantee that the crude oil refined is from the same area, considerable effort was made during the sampling to include smaller refiners, because they undoubtedly represent crude sources in the areas in which they are located. It seems a safe assumption, therefore, that all the major crude sources of U. S. lubricating oils have been covered. Thus, it appears very unlikely that enough virgin oils were missed in the sampling to shift the limits and extreme fractions of the distribution of the observed Q values for the virgin oils. Such a shift could occur only if the small, unrepresented segment of the virgin oils consisted almost exclusively of blends which are more complex mixtures of crudes than the virgin oils represented. The stability of the population must also be considered. What guarantee is there that the population will not shift, and in particular, shift downward? Aside from the fact that a continuing survey of the virgin oil population is advisable to spot such a trend in case it should occur, and to improve the statistical basis of the analytical method, certain factors make the shift unlikely. Unless crude sources change in some important way, this factor will be stable; economic factors argue against the complex blending of oil from many areas, except in a few special cases. It is possible, however, that changes in technology might cause a general downward shift of the Q 190
ANALYTICAL CHEMISTRY
values. Such a change would be detected only by continuation of the survey. Distribution of Virgin Oil Q Values. I n the preceding discussion, emphasis has been placed on the virgin oil population rather than the reclaimed oils. The reason for this is the requirement that, though the analytical test must identify a high percentage (if not necessarily all) of the reclaimed oils, it is more important to keep the likelihood of misidentifying a virgin oil as being reclaimed negligibly small. Hence interest has been focused on the lower tail (lower values) of the virgin oil Qfunction distribution. When suitably transformed, the data were well fitted by a normal (Gaussian) curve, which exhibited evidence of being truncated a t the lower end. This is illustrated by Figure 5, a plot of the transformed data on normal probability paper. The transformation, T = log (log Q - 2) 1, was used. This is simply a double logarithmic transformation. The function 2” = loglog Q mould accomplish the same thing, but the mechanics of manipulating the data make it more convenient to calculate T-i.e., it becomes unnecessary to carry characteristics in using the log tables. Many types of data which take on values from zero upward, or have some other sharp lower limit, possess a skewed distribution (with the long tail in the direction of the high values). A single logarithmic transformation is usually sufficient to convert these data to a normal distribution. There is no a priori reason to expect the virgin oil Q distribution to be so extremely skewed unless the larger companies, which represent a large portion of the sales market, blend a small number of crude sources in the finished oil. Another possibility is a disproportionately large fortuitous representation of low Q-valued virgin
oils in the sample. In any case, the extreme asymmetry suggests a pronounced lower limit for the distribution. Principally because of the evidence that the lower end of the distribution was cut off, tolerance limits (probabilistic limits on fractiles of a distribution) were not calculated. The point of truncation of the T distribution is estimated to be 0.6700 (Figure 5 ) , which is equivalent to Q = 294. At present insufficient information is available to describe the lower tail of the virgin oil Q population with desirable precision. The statistical esploration of the data was not begun until the program of sampling was well under way; consequently, although it is “representative” in the senses discussed above, it is not a random sample. The virgin oils, as well as the reclaimed oils, were assembled in two groups, some months apart (see note, Table 11). The two samples of virgin oils exhibit some inconsistencies, which are probably the result of the sampling method; there was a conscious emphasis on oils from a single geographical region in the first sample and an emphasis on other oils in the second. While the means of both samples are, within the limits of experimental error, the same, the variance of the second sample is significantly larger than that of the first. In spite of this, reference to Table I1 shows that the lower limits of both samples are approximately the same. Had the evidence for truncation not been encountered, consideration would have been given to setting a probabilistic lower limit on the virgin oil population, and the sampling method would have imposed a limitation on the validity of such a procedure. However, duplication of the lower limit in the two virgin oil samples provides further corroborative evidence for truncation. If the value of Q = 294 is
+
,
- -
____
7-i
-
-
i -1
--
CUMULATIVE FREQUENCY I N X
Figure 5. Cumulative probability distribution of transformed Q values of virgin oil samples
adopted as the critical level below which an unknown oil is identified as reclaimed, 85% of the known reclaimed oils listed in Table I1 are identified as such. SUMMARY
In an analytical method for discriminating between virgin and reclaimed automotive engine lubricating oils, a statistical function, Q, of the infrared absorbances, a t 14 wave lengths, of the oils, serves as the discriminant. The method relies on the differences in complexity of composition of most virgin and reclaimed oils for its successfor a number of technical and economic reasons, most reclaimed oils are a com-
plex mixture of crude sources, whereas individual virgin oils, for the most part, are blended from a few crudes. The Q values of the 7 3 virgin oils examined were (with two exceptions) satisfactorily described by a double logarithmic normal distribution, truncated a t the lower end. The point of truncation is estimated a t Q = 294. Below this value an oil is identified as reclaimed, Of the 54 known reclaimed oils examined, 85y0had Q values below the critical value. ACKNOWLEDGMENT
The authors express their appreciation to W. C. McCrone and R. E. Putscher for their many helpful sug-
gestions, and to R. D. Hites, who obtained the infrared spectra. LITERATURE CITED
( I ) Fred, hl., Putscher, R. E., AXAL. CHEM.21, 900 (1949). (2) Oil Gas J . 54, 215 (1956). ( 3 ) Smith, C. A. B., Ann. Eugenics 13, 272-82 (1947). (4) Smith, H. M., Ind. Eng. Chem. 44, 2577 (1952). ( 5 ) Welch, B. L., Biometrika 34, 28-35 (1947).
RECEIVED for review May 31, 1956. .hecepted October 30, 1957. Work sponsored as a joint cooperative venture by t h e Pennsylvania Grade Crude Oil Association with member and nonmember support.
Spectrophotometric Microdetermination of Copper in Copper Oxidases Using Oxalyldihydrazide GEORGE R. STARK and CHARLES R. DAWSON Department o f Chemistry, Columbia University, New York, N. Y.
b A new method is presented for the determination of the copper in copper oxidases using oxalyldihydrazide and acetaldehyde in ammoniacal solution. The method will determine as little as 0.1 y of copper per mi. in the presence of protein with a precision of 2 to 3%. Manganese(l1)interferes somewhat, but cadmium(ll), nickel(ll), magnesium(ll), iron(ll), zinc(ll), cobalt(il), and calcium(ll) do not. Gelatin, plactoglobulin, and glutathione do not interfere, but bovine plasma albumin does, and this interference is quantitative. The copper content of several samples of ascorbic acid oxidase and tyrosinase has been determined and these data correlated with the activity and total protein content of the samples.
T
copper content of the purified enzyme ascorbic acid oxidase corresponds to 6 atoms of copper per molecule of enzyme (6). During isolation of the oxidase, the copper content becomes an index to the purity of the enzyme (12, 19). In the case of highly purified tyrosinase the copper is directly proportional to both the cresolase (17) and the catecholase (11) activity, and hence can be used as a criterion of enzyme purity. Because the purification procedures now in use provide only very small amounts of highly purified enzyme, and these are obtained only after weeks of preparation, it is essential that a copper determination be accurate and HE
highly sensitive, so as to expend as little enzyme as possible. Depending on the reactions involved, the determination of copper in proteinaceous material may be complicated by the presence of the protein. I n such case, the protein must be destroyed by ashing. This procedure involves much time and tends to reduce the accuracy of the measurement through spattering, vaporization, transfer of solutions, etc. Colorimetric determinations of protein-bound copper have been described utilizing diethyldithiocarbamate, but these often involve ashing (6, I S ) . Micro copper determinations have been described which are not affected by extraneous protein. All of these utilize hydrochloric acid to free the bound copper. A method without any ashing requirements, based on the use of the dropping mercury electrode, determines as little as 1 y of copper per ml. with an average deviation of *3% (1). However, this procedure is timeconsuming and considerable experience is necessary before satisfactory results are obtained. The Warburg method (20) has been in use in these laboratories for some time (12). It utilizes the copper catalysis of the aerobic oxidation of cysteine to cystine, followed manometrically, and is capable of determining 0.1 y of copper in a 2.6-ml. reaction volume. Three to four hours are necessary to perform one determination in duplicate and such duplicates usually have a deviation of * 5 to 10%. Furthermore, the precision is not im-
proved significantly by using larger amounts of copper. Another method utilizing diethyldithiocarbamate has been described which does not involve ashing (3, 8). Gubler and coworkers ( 8 ) have determined as little as 0.2 to 0.4 y of copper in 3.0 ml., and given data on the precision. Peterson and Bollier (18) have described the spectrophotometric determination of serum copper with biscyclohexanoneoxalyldihydrazone, after freeing copper by the method of Gubler and coworkers (8). Gran ( 7 ) has described the use of oxalyldihydrazide for the spectrophotometric determination of low concentrations of cupric ion. This reagent, together with acetaldehyde or formaldehyde in ammoniacal solution, gives an intense blue-violet color with copper(I1). Gran reports that the molar absorbancy index for the copper(I1) - acetaldehyde - oxalyldihydrazide complex is 29,500 a t 542 mp, This unusual intensity of absorption (about three times that of the diethyldithiocarbamate-copper complex) suggests that a copper microdetermination for copper proteins, based on the use of oxalyldihydrazide, might be superior in sensitivity to the older diethyldithiocarbamate methods and easier and more precise than the Warburg method. Gran ( 7 ) has given data for solutions containing only copper and reported that the presence of reasonable amounts of hydrochloric, nitric, and perchloric acids or of sodium sulfate had no VOL 30, NO. 2, FEBRUARY 1958
191
