kv. and 10 ma. in order to obtain a counting rate not exceeding 3000 c./s. Taking the number of counts accumulated for a period of 40 seconds, the value of R , including the background, was obtained. LIEASUREMEXTS OF I,, &, I,, and &,. Without disturbing the precisely fixed position of the goniometer, the readings of Zy,Qy,I,, and &, were then recorded, including their backgrounds. BACKGROUNDMEASUREMENT OF ABOVEVALUES. Since it was observed in the previous tracings that there was no interference at 12.2’, the goniometer was set at this angle for background measurements. Taking into account the statistical correction involved in these measurements, the background values corresponding to each measurement were recorded in the same sequence. TELLURIUhf DETERMIXATION. After correction for the background readings, the above measurements give, in counts per second, R = 2591, I , = 842, I , = 21, Qy = 2641, Q, = 1920. These values substituted in Equation 7 yield T, = 0.733, T , = 0.694, q(T,) = 1.162, +(Tu)= 1.192. Since the weight fraction of tellurium in TeOl equals 0.80, all experimental ratios shown in Table 111 and used in Equation 6 were multiplied by this dilution correction factor, C, giving the final value, C,, which is 2.16% for tellurium in the total sample. DISCUSSION
This investigation was originally attempted to develop a general x-ray spectrographic method for the analysis of po\Tder samples only. However, the aboye applications yield the conclusion that a broader field of analytical problems can be covered. I n fact, if alteration of the original
state or chemical composition of the sample is not objectionable, which often is the case, many advantages are worth considering in the eventual application of this technique to a given problem. It would be irrelevant to attempt comparison of this technique with those applied by means of equipment fundamentally different in principle from the x-ray spectrograph. However, definite advantages might be found in some cases over other conventional techniques used in x-ray spectrography. The matrix effect being experimentally taken care of by the transmission measurement introduced in the general equation, time-consuming calibration curves for each element in matrix of different composition are not required. Extensive tests for simulating comparable matrix of standards and tested samples become unnecessary. Tedious calculations of correction factors for compensating the matrix effect are replaced by readily available experimental values. The range of sensitivity is bound to a compromise which can be established by the analyst between the minimum dilution of the sample in the diluent and the resulting transmissibility of the pellet. The degree of sensitivity is easily predictable for a given sample with the help of Figure 4 and calculations of its mass absorption coefficient by a method published in 1961 ( 5 ) . The precision and accuracy rest only on the care taken in producing homogeneous pellets, and selecting orders of emission lines such that peak background ratios permit optimum readings. Birks (3) gives a statistical treatment of x-ray measurements.
Therefore, assuming that the physical state of a given sample meets the requirements of Equation 6, the quantitative determination of its elements is reduced to the emission, transmission, and weight measurements of the tested and standard pellets and to the emission of the Radiator sample only. ACKNOWLEDGMENT
The authors thank L.S. Birks, X-ray Optics Branch, C. S. Naval Research Laboratory, Washington, and also K. Heinrich, Department of Commerce, Washington, for their valuable advice and interest in this work and express their appreciation to Claire Powers for her help in the experimental work. LITERATURE CITED
(1) Beattie, H. J., Brissey, R. M., ANAL.
CHEM.26, 980 (1954).
(2) Berth, E. P., Longobucco, R. J., Advan. X-Ray Anal. 5 , 447-56 (1962). (3) Birks, S., “X-Ray Spectrochemical
4;
Analysis, Interscience, New York, 1959. (4) Claisse F., iVoreZco Reptr. 3 , 3 (1957). (5) Leroux, J., “Advances in X-Ray Analysis,” W. M. Mueller, ed., pp. 153-60, Vol. 5, Proceedings of 9th Annual Conference on Applications of XRay Analysis, Denver, 1960, Plenum Press, New York, 1961. (6) Leroux, J., Norelco Reptr. 4, 107 (1957). (7) Leroux, J., Lennox, D. H., Kay, K., ANAL.CHEM.25. 740 (1953). (8) Leroux, J., hahmud, ’M., Appl. Spectru. 14, 131 (1960). (9) Liebhafsky, H. A., Pfeiffer, H. G., Winslow, E. H., Zemany, P. D., “XRay Absorption and Emission in Analytical Chemistry,” Wiley, New York, 1960. RECEIVED for review May 25, 1965. Accepted October 15, 1965.
Semitheoretical Approach to Interelement Correction Factors in Secondary X-Ray Emission Ana lysis GEORGE ANDERMANN Chemistry Department, University of Hawaii, Honolulu, Hawaii
b A semitheoretical method for obtaining exponential correction factors for interelement effect calculations is based upon utilizing ratios of the differences in mass absorption coefficients. Its chief advantage is elimination of the usual requirement for special standards. Furthermore, there are no limitations on the number of constituents. The method appears to b e particularly useful for the analysis of soft region elements.
S
calculations in secondary x-ray emission analyses have been attempted by numerous investigators. Among the early conPECTROCHEMICAL
82
ANALYTICAL CHEMISTRY
tributors were Gillam and Heal (3)) Koh and Caugherty (61, Noakes (9), and Beattie and Brissey ( 8 ) . In 1955, Tingle (18) presented a paper which later gave rise t o the empirical exponential correction factor approach as practised by Hasler and Kemp (4) and Kemp and Andermann ( 5 ) . Also in 1955, Sherman published his theoretical derivations for x-ray emission analysis (IO), and in 1959 he provided a simplified version (11). In 1958 Mitchell (8) published a completely empirical approach, which evidently provides still another useful method for evaluating x-ray data. More recently Marti (7) was successful in analyzing stainless
steel with a n “influence factor” approach based on absorption coefficients. The method described below is based on the above-mentioned exponential correction factor approach (4, 5 ) . The exponential correction factor approach previously used was somewhat cumbersome for a complicated multicomponent system, since the method was completely empirical. With the empirical approach it was necessary either to have a large library of standards or analyzed samples, or to undertake the expensive manufacturing of a small set of very carefully prepared and analyzed standards. The new semitheoretical approach is not limited in the
same manner. Another serious limitation of the empirical approach, a weakness common to all empirical approaches, is that it cannot predict the direction of the changes for new interelement effects. The basic limitation to this method, which is a limitation to all methods, is that there be a complete inventory of all elements present. Still another limitation is that true enhancement effects are not now taken into account.
THEORY
When the method of empirical exponential correction factors of Hasler and Kemp (4) and Kemp and -4ndermann (5) is used, Equation 1 provides the determination of element 1 in a multicomponent system containing elements I , 2, 3, 4, 5 . . . . . . .
where ( I I ) Trefers to the true intensity, the intensity of a system composed only of elements 1 and 2, and refers to the measured intensity of element 1 in a system containing other elements. In Equation 1 elements 3, 4, 5 , etc., replace element 2. The replacement of element 2 a t a fixed concentration of element 1 constitutes the so-called interelement effect upon the determination of element 1. In Equation 1, coefficients k refer to the specific correction factors and terms C to the concentrations involved. As an example of the old method, if one wants to obtain the value of k3 it is necessary to hold the concentrations of elements 4, 5 , 6, etc., constant. The general practice has been to plot In ( I J T / ( I J Magainst C3. The slope of such a plot provides a value of k3. One of the difficulties of this method has been the rather arbitrary choice for drawing ( I J T . It is self-evident that the more points one has, the more readily one can draw (Il)*.After obtaining the most suitable value of k3, the old method provides a way of obtaining the replacement of element 2 by 3 if the determination of 1 is of interest. By these means, the empirical approach provides, one step a t a time, an evaluation of the various interelement effects. This article shows how one simple assumption can provide a useful insight into interelement effects due to absorption. The basic requirement of the method was to write a relationship which would permit the eventual establishment of a pseudo three-component system. In other words, the following relationship could be considered for the determination of element 1:
(IdT
(IdM x exp [F(aC3
=
+ PCI + rCd1
(2)
where F had to be the empirical constant, and values of C Y , p, and y had to be based upon some theoretical considerations. Equation 2 is rigorously identical to Equation 1, provided
(8b)
- PZ)
(8c)
=
k 3 ~ 5
Fp
(3b)
ks
Fr
(3c)
Application of the method is straightforward. To get f in Equation 5 it is only necessary to know the mass absorption coefficients of elements 2, 3, 4, 5 , etc.. . Element 2 is chosen somewhat arbitrarily (generally one of the major constituents), and the differences in p are listed with respect t o p2. The ratio values r4, r5, r6, etc., are readily computed. The concentrations supplied are multiplied by the appropriate r values. The adjusted concentrations are added with proper regard to sign. Once this is done, a pseudo threecomponent system is obtained, and the formerly used techniques of the empirical approach are utilized. A hypothetical working curve is drawn that gives the proper ( I l ) r for all samples. For each sample the term In [ ( I l ) T / ( I l ) . U ] is plotted against the adjusted total concentration value, to give the value of f. What is significant is that, in contradistinction to the old empirical method, all the elements and all the samples are used in the semitheoretical approach in obtaining a representative value for f, which is then used to convert each individually measured intensity for all the interferences in a single calculation. The first application of this method occurred for the determination of SiOzin glass. This is difficult and inaccurate by any method-for example, the determination of 7001, SiOz in glass presents an almost insoluble problem to an optical emission spectroscopist. The reason why previous x-ray attempts failed can be understood if Table I and Figure 1 are inspected. Table I shows the concentration ranges of the 15 constituents for the 10 samples investigated. Figure 1depicts the unconverted SiKa intensity for these samples as measured on an ARL VXQ. The ranges of concentration were of such nature that the old empirical exponential factor approach could not be applied. The scatter of points in Figure 1 could not be blamed on lack of precision, for the author obtained a precision of =t0.14% SiOz a t a level of 70% Si02 using a 4-minute exposure. As Figure 1 shows, three samples show depressed signals: A, B, and C. Sample A has 0.87. BaO, sample B has 3% BaO, and sample C has 12.7% BaO. However, it would be impossible to obtain a correction factor for BaO only by simply using the empirical
=
x
- C3 + (P4 - PZ) c4 + (p3 + [(pa
~(2)
I.(Z)C5
*
(4)
Filter replacement is governed by the differences in p , the mass absorption coefficients, as shown in the Appendix. In Equation 4 the empirical constant, F’, is employed rather than F because it can be shown readily (see Appendix) that for a simple filter problem, terms such as area, total mass, etc., must be introduced before standard absorption equations are reduced to relationships involving concentrations. It is now convenient to normalize the relationship shown in Equation 4. (IdT = ( I J M
x
+
[f(~3C3
+ TICS)1
~4C4
(5)
where
and
- PZ)/(P3
- P Z ) = k3r4
k4 =
(Z1)’U
f
k6 = f b 5
F2)/(c(3
(84
APPLICATION
exp (F’
exp
k4 = f(p4 -
=f
(34
and so on. Equation 2 states that it is possible to define a single constant, F , which can be obtained empirically for any given multicomponent system. Implicit in this relationship is the assumption that F can be a constant only for a given instrument. How does one obtain the values of CY, p, and y? If one poses the problem from the point of view that infinite depth is available for analysis, and that getting a corrected intensity for the outgoing beam is the real problem left, then assuming that F can incorporate the effect of matrix variation upon the incoming beam, one is merely left with a simplified filter problem for the outgoing beam. In other words, in the determination of element 1, the replacement of element 2 by element 3 is simply a filter replacement issue. Under these conditions the following expression may be written: =
k3 = fr3
= Fa
k3
(ZdT
In other words, the correlation between the empirical and the semitheoretical approach is :
=
F’
(P3
- P2)
(6)
VOL. 38, NO. 1, JANUARY 1966
0
83
0
0 I
1
I
I
I
60
65
70
75
80
IM ( I SiKa)
Figure 1. Uncorrected determination of Si02 in glass A.
Sample contains 0.8% BaO Sample contains 3% BaO Sample contains 12.7% BaO
B. C.
approach, because the intensity of the sample with 0.8% BaO (sample A) is just as much depressed as the intensity of sample B with 3% BaO. If, on the other hand, the intensities are corrected for the interelement effects due to the light elements only, by using the
Table I.
Concentration Ranges in Glass
Constituent
Bab Fez03 KzO A h 0
Table Fused
53-80 0-17 0.5-16 0-14 0-5 0.2-15 0-13 0-0.5 0-7 0 -1 0-0.5 0-1 0-0.5 0-1 0-0.1
II. Concentration Ranges in Finished Cement on Ignited Basis“
Constituent
Concentration range, yo
A1203
1.64- 3 . 2 1 30.87-34.06 9.69-12.16 0.18- 2.31 0.58- 1 . 8 7 0.76- 1 . 3 6 0.20- 0 . 6 5 0.07- 0 . 2 9 0.02- 0.08 0.14- 0.16 0.02- 0.23 0.02- 0.50
CaO SiOz MgO FelO3
so;
(LizB40,-50.0)
NanO
XrizO3
Ti02
a
Concentration range, 5%
Pa05 KzO To get approximate concentration on
unignited basis, values in table should be multiplied by 2.
84
ANALYTICAL CHEMISTRY
semitheoretical approach it is possible to obtain true empirical correction factors for BaO only. The truly empirical correction factor for BaO turns out to be 0.0138, whereas the semitheoretical correction factor is calculated to be 0.0127. Considering the marked difference of BaO compared to some of the other constituents, the agreement between the empirical and the semitheoretical correction factors for BaO is very satisfactory. Whereas the uncorrected x-ray analysis, as shown in Figure 1, is completely meaningless, the corrected x-ray analysis of Figure 2 represents an average deviation of 10.2570 SOz. This level of accuracy in this concentration range of Si02 represents a relative error of 10.36% of the amount present. Perhaps an even more significant aspect of the above investigation was the fact that the above found semitheoretical correction factors were “blindly” applied to a set of data obtained a few years prior to the above experiment. The results were gratifying. For that group of seven samples an average deviation of 10.28% Si02 was obtained a t a level of 70% SiOz. Next, the same semitheoretical correction factors were applied to SiOz determination in fused, finished cements, and an accuracy of =kO.l% SiO, was obtained a t a level of about 25% SiOz. This accuracy actually approached the total precision of the method (1). Several other attempts were made immediately to evaluate the new method for the determination of other soft region elements. One of the more successful is illustrated below. In a recent article on the analysis of finished cements ( I ) , fairly successful analyses were shown on some very carefully analyzed standards. To evaluate the semitheoretical approach for fused finished cement analysis the data for some of the more important constituents were reduced in a rigorous manner. For example, the average deviation of Alto3 determination was presented by the author and his coworker as 10.12% A1203. Applying the semitheoretical method, and taking ignition losses into account t o obtain a proper material balance, the average deviation of A1203 determinations becomes *0.034% & 0 3 , which in this concentration range represents a relative error of 10.64%. The uncorrected AlzOa determination on the ignited basis is presented in Figure 3. The corrected A1203determination is shown in Figure 4. Table I1 illustrates the variation of the 12 constituents in the fused finished cements. Although the improvement in the determination of the rest of the constituents is not as marked as in the case of Al2o3,there is some improvement in each case. Table I11 shows the com-
I T (ISiKol)
Figure 2. Corrected determination of Si02 in glass SiOz-COO system K = 1.45C~1~0, 1 . 2 5 c ~ , o ~.OOCN,,O -k 0.83C~,,o, 0.75Cuno f 0 . 1 8 C ~- O.68~i,0 - 0.48Cp2o, - O . ~ ~ C B , O ~ 0.37Cs0, 0.1 8CX,O
+ +
+
-
parative data. What is truly significant is that the new mean error (average deviation) values are either well within the total precision of the method or very close to it. When the average deviations are as low as those shown in Table 111, errors in chemical analysis become important. Perhaps the most critical test of whether or not the semitheoretical approach would be applicable in the hard region would be its application to steel analysis, such as stainless steel. The empirical correction factors of ARL research workers in the past had been obtained in a rather arduous manner. It is possible to calculate in a few minutes semitheoretical correction factor values that are close to the old empirical values, as shown in Table IV. In each case nickel has been adopted as the reference element that is replaced by the interfering element. In view of the crudeness of this evaluation, and the definite possibility of errors in the experimental values, the agreement is satisfactory. This limited experiment on stainless steel analysis also confirms Marti’s claim that the influence factors are primarily due to absorption ( 7 ) . CONCLUSIONS
The author was able to test the applicability of the semitheoretical approach on a few other analytical problems. In every instance the results, even in the hard region were a t least as good as the uncorrected values, or better. In these cases, the empirical approach could not be applied because of the unavailability of proper standards. The over-all impression of the author is that it is too early to state whether or not this method is as
Figure 4. Corrected determination of A1203 in fused finished cements on ignited basis
Figure 3. Uncorrected determination of A1203 in fused finished cements on ignited basis
AI208 - CaO system K' = -3.0 [2.38C~,o f 1.48C~,,o, 1.46C~.,o 1.36f 0.44C~i0, 1.06(CLi2~,0, - 50.0) - 1.oOcSi0, CM~O 0.83Cp,05 - 0.6OCs0, 0.27C~,o - o.oZCsr~]
-
See ( I ) 1 Ignition loss
applicable in the hard region as in the soft region. In trying to evaluate the validity of the basic assumption that there is a single constant, one does not need to take an inflexible approach. With sufficient experience it might be possible to find a functional relationship between f and the atomic number of the interfering elements. Undoubtedly f is not a true constant, but it appears very likely that it has been sufficiently so for the specific illustrations mentioned above. Although insufficient evidence is on hand to declare the above semitheoretical approach a real salvation for the interelement effects in x-ray analysis, the results to date have been very encouraging. ildditional theoretical and experimental studies are necessary to correlate and further evaluate the many proposed methods for x-ray calculations. ACKNOWLEDGMENT
The author thanks Applied Research Laboratories, Inc., for permission to publish the experimental data reported. APPENDIX I
=
I, exp
(-PAPAX)
A similar expression can be written if the
+
-
Table
111. Improvements in Finish Cement Analysis
f 0 . 034 fO.10 =k0.23 k0.023
A1203
SiOz CaO
B.
Fused
Absolute mean error, 7'" Semitheoretical Previous approach data
Constituent
attentuation is now attributable to a filter of 2 thickness containing element
Fen03
Then
+
f0.12 f 0 .12 f0.29 f 0 . 040
Referred back to unignited basis, defined usually as average deviation. See (I). a
I5 = I, exp
(-PEPEX)
(I-b)
The ratio of IAto IB-namely, I R gives a measure of the intensity change when A is replaced by B, or
Zg = IA/IB = exp
(-PAPA%
+
P5PBz)
(1-4
If the filter is a mixture of A and B, the ratio of intensities, I R ' , is proportional to the weight concentrations of A and B in the filter. To introduce a concentration term, C, in this relationship the usual manipulations are performed-Le., the term ( p a x ) is expressed in terms of the volume, V,and the area, a , of the filter.
Table IV. Correlation between Empirical and Semitheoretical Exponential Correction Factors in Stainless Steel Analysis
InterCorrection factors An- fering Semialysis element Empiricala theoreticalb Cr Mo +0.027 +0.035 +0.046 Cr W +0.045 Fe Mo -0,0125 -0.009 Unpublished data from Applied Research Laboratories. b Calculated from empirical correction factor for Ni interference. 0
Thus
I n general, when a filter containing element A is placed in the path of moncchromatic radiation, according to standard absorption equations ZA
where IA= measured intensity after attenuation by filter A I, = intensity before attenuation by A x = path length P A = density of filter A
-
(1-4
=
(MAx)/V= MA/U
(14) Next both numerator and denominator are multiplied by 100 M T . PAX
PAX
= (100MAMT)/(100MTa) (I-e)
where
M T = total mass of filter Consequently PAX
= C~MT/100a
VOL. 38, NO. 1, JANUARY 1966
(I-f) 85
where C A =
% concentration of A by weight in filter
Similarly p B x = CB.Vl~/lOO a
(1%)
It is now possible to rewrite Equation I-c to yield
(I-h) The relationship in Equation I-h, by itself, is of limited value: Its usefulness becomes significant only when it is realized that the problem posed above is one of equivalent replacement. In other words, using the semitheoretical method, one does not need to know what the theoretical intensity should be for a given multicomponent system; rather one wants to know how the observed intensity is modified as a given concentration of element A is replaced by an equivalent concentration of element B. Under these circumstances CA = CB = C , and the desired relationship is obtained :
Whereas element B replacing an equivalent concentration of A might decrease the observed intensity ratio by lo%, another element such as D or E might increase it in this equivalent concentration exchange. Clearly, fof a multicomponent system the final intensity ratio is a product of the individual equivalent concentration exchange ratios. Reverting back to the symbols used above for the determination of element 1 in a system composed primarily of elements 1 and 2, with the equivalent concentration exchange of element 2 by elements 3, 4, 5, etc., the intensity ratio, ZR’,is given by
which finally yields
Equation I-k demonstrates how the empirical constant, F’, of Equation 4 must contain terms such as area and total mass. Furthermore, it lays the basis for a rigorous justification of the original assertions by Tingle ( I d ) and Hasler and Kemp (4). LITERATURE CITED
(I) Andermann, G., Allen, J. D., AXAL. CHEM.33, 1695 (1961). (2) Beattie, H. J., Brissey, R. M., Ibid., 26, 980 (1954). (3) Gillam, E., Heal, H. T., Brit.J. A p p l . Phys. 3, 353 (1952). (4) Hasler, RI. F., Kemp, J. W., “Methods
for Emission S ectrochemical Analysis”, p. 81, Philadeghia, Pa., 1957. (5) Kemp, J. W., Andermann, G., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1956. (6) Koh, P. K., Caugherty, B., J . A p p l .
Phys. 23, 427-33 (1952). (7) Marti, W., Spectrochim. Acta 18, 1499 (1962). ( 8 ) Mitchell, B. J., ANAL. CHmf. 30, 1894 (1958). (9) Noakes, G. E., ASTM Spec. Tech. Publ. 157. 57 11953). (10) Sherman, J., Spectrochim. Acta 7, 283 (19,55). (li,>dz;d.,-co. 6, 466 (1959). (12) Tingle, W. H., Pittsburgh Conference on Analytical Chemistry and Applied __ Spectroscopy, 1955.
RECEIVEDfor review June 25. 1962. Resubmitted October 6, 1965. Accepted November 17, 1965.
Counter Double Current Distribution with Continuous Recovery for Isolation of Methyl Linolenate R. 0. BUTTERFIELD, H. J. DUTTON, and C.
R. SCHOLFIELD
Northern Regional Research laboratory, Peoria, 111. Counter double current distribution (CDCD) was used to produce unisomerized methyl linolenate (methyl 9,12,15-octadecatrienoate) of 99.9% purity from methyl esters of linseed oil without prior concentration. The rate of production is five times that of countercurrent distribution. Production of the labile lipid illustrates the general applicability of CDCD. In addition, a system is described that continuously recovers the solvent and product. This system completes the automation of the CDCD equipment so that solvent inventory, safety hazards, and labor are reduced.
B
RoblIh.ATIoN-debromination has been the accepted, if not the only, method for the preparation of methyl linolenate (methyl 9,12,15-octadecatrienoate) (4); however, it induces up to 25% of trans isomers in the naturally occurring all-cis compound. Bside from 86
ANALYTICAL CHEMISTRY
the scaling up of chromatographic procedures (7, 8 ) , only liquid-liquid extraction methods have held the possibility of isolating an unisomerized compound. I n large laboratory-scale equipment, up to 96% pure linolenic acid has been prepared on a pound per hour basis (1), and by using countercurrent distribution (CCD) monitored by a continuous flow refractometer (3) high-purity methyl linolenate has been isolated in a batch process a t a rate of 75 grams per week (9)*
With the description of counter double current distribution (CDCD) (6),a continuous method for isolating high-purity linolenate was possible. Continuous and essentially unattended operation became a reality with the addition of a continuous solvent-product recovery system. More than 300 grams of linolenate can be separated in a week from linseed methyl esters. The linolenate has a purity of greater than 99.9% and essentially no trans isomers.
EXPERIMENTAL
Counter Double Current Distribution. Distributions were performed
with a robot-operated 25-tube CDCD apparatus (H. 0. Post Scientific Instrument Co.) with hexane and acetonitrile (9), the immiscible solvent pair. Linseed methyl esters and a methyl linolenate concentrate (81%) were used as feed materials and were diluted 1 to 2 with hexane to reduce viscosity and to improve stabihty to oxidation in the pump reservoir. The ratio of the immiscible solvents, the position of the feed tube, and the rate of feed were varied from run to run. Preparation of Feed Materials. Alkali-refined linseed oil was transesterified with sodium methoxide. Product composition was 58.5% linolenate, 15.7% linoleate, 17.4% oleate, and 8.4% saturates. A methyl linolenate concentrate was prepared by urea crystallization of the linseed methyl esters ( 5 ) . Its composition was 8l.lyO linolenate, 16.470 linoleate, and 2.5% oleate.
