Analysis of n-Paraffin Oxidation Products in the Presence of Hydroperoxides 6. D. Boss, R . N. Hazlett, and R . L. Shepard' Chemistry Division, Code 6780, Naval Research Laboratory, Washington, D. C. 20375
The analysis of n-dodecane oxidation products is presented in a plan which determines hydroperoxides at low levels and accounts for or avoids the interference of hydroperoxides in the analyses of other oxidation products. In ester analysis by ethanolic KOH saponification, accurate results are obtained when no reflux solvent is added, provided sufficient H 2 0 is present and hydroperoxides are first reduced by triphenylphosphine. Fully activated silica gel, which is very effective in quantitatively concentrating polar products by column chromatography, does not cause decomposition of the sec-hydroperoxides. High speed liquid-liquid chromatography is used to separate and collect alcohols, ketones, and hydroperoxides for further analyses. Dodecylhydroperoxides decompose in a gas chromatograph to yield equal amounts of the corresponding alcohols and ketones when a stainless steel flash vaporization port is used. A boric acid precolumn is used in the gas chromatograph to trap alcohols in order to allow analysis of the ketones separately, once the hydroperoxides are reduced. Very low levels of olefins are readily determined by gas chromatography after a mild bromination procedure.
There are two logical approaches to the study of hydrocarbon oxidation kinetics and mechanisms. In one approach, the complex reactions of intermediate products can be studied by isolating specific species from the other oxidation products. This first method ignores the solvent effects of other product molecules on reaction kinetics. The second approach, the study of individual reactions in the presence of the complex oxidation mixture, employs a sophisticated analytical scheme ( 1 - 5 ) and tracer or referencing techniques (6). Two different problems in using the latter approach are considered here: interferences from hydroperoxide decomposition and detection of low levels of intermediate products including isomers. This work is oriented to an oxidized n-dodecane system stressed at temperatures in the range of 90 to 200 "C in a borosilicate glass reaction chamber with excess air for up to 36 hours. This report discusses the analysis scheme diagrammed in Figure 1. Oxidation products are detected in aliquots from the oxidation product mixture, though, in each case, silica gel column chromatography can be used to separate the polar from. the nonpolar aliquot molecules for greater sensitivity (7). The effect of hydroperoxides on each of the analyses and separations is considered. Present address, Celenese Fibers Co., P.O. Box 1414. Charlotte.
N.C. 28201. (1) D. M. Brown and A. Fish, Proc. Roy. Soc., Ser. A, 308,547 (1969) (2) F. R. Mayo, Accounts Chem. Res.. 1, 193 (1968). (3) N. M . Ernanuel, E. T. Denisov, and Z . K . Maizus, "Liquid-Phase Ouidation of Hydrocarbons," Plenum Press, New York, N.Y., 1967. (4) B. D. Boss and R. N. Hazlett, Can. J. Chem., 47,4175(1969). (5) A. W. Dawkins, € o r , Chem. News, Normal Paraffin Suppl.. Dec 2, 1966,p 49. (6) B. D. Boss and R. N. Hazlett, Amer. Chem. S o c . , Div. Petroi. Chem., Prepr., 14(2),All4 (1969)
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EXPERIMENTAL Apparatus. Gas-liquid chromatography (GLC) was performed on a GC-4 (Beckman Instruments Company, Inc., Fullerton, Calif.) equipped with a dual flame ionization detector and an 18-8 stainless steel flash vaporization port. All chromatography data were quantified and tabulated by a Hewlett-Packard (Avondale, P a . ) 3360A integrator/computer system programmed for correction of base-line drift. Identification of the major GLC peaks was accomplished by pyrolysis gas chromatography (8, 9) and will be reported soon. For high speed liquid-liquid chromatography (HSLC) experiments, a Model 820 liquid chromatograph (Du Pont Instrument Products Division, Wilmington, Del. See ref. 10) equipped with 1-m long, 2.2-mm i.d. stainless steel columns was used. The immobile phase was Carbowax 400 bonded to 37-75 pm porous surface glass beads (Waters Associates. Framingham, Mass,) sold as Durapak. The mobile phase, dry n-heptane, must be saturated with immobile phase or some other suitable polar material to modify the polarity. The elution detector was a Du Pont UV absorption monitor using the 254-nm Hg emission line. Reagents. The n-dodecane and n-heptane were obtained from Phillips Petroleum Company (Bartlesville, Okla.) as 99.7% pure grade. Chemicals for GLC standards were obtained from Chemical Samples Company (Columbus, Ohio) as 99% pure. All chemicals were tested for volatile impurities by GLC, and hydrocarbons were passed through silica gel to remove traces of polar impurities. The silica gel, 100-200 mesh grade 923 (Davison Chemical Company, Baltimore, Md.), and other chemicals were obtained as standard ACS reagent grade.
PROCEDURE, RESULTS, AND DISCUSSION Peroxide Measurement. The accurate measurement of low levels of hydroperoxides (the primary oxidation product) in small aliquots is crucial to oxidation research. The method optimized here, which is colorimetric KI analysis ( I I ) , uses a mixture of isopropanol, H20, acetic acid, and KI to effect the reaction of hydroperoxides with 21- to yield 12. In this work, the resulting 13- is detected colorimetrically a t a selected wavelength which depends upon the sensitivity desired and the absorption characteristics of interfering species. The apparatus shown in Figure 2, which supplies a deoxygenated NZ gas purge for the septum sealed colorimeter tubes and for the acetic acid, is required to exclude 0 2 since the 0 2 reacts with KI to form 12. The tubes containing the KI reagent are purged 5 minutes before injecting the sample, then for 2 more minutes prior to injection of deoxygenated acetic acid. Delay greater than a few minutes before injecting the acetic acid results in undesirable sample degradation by side reactions. During the color development period, samples must be stored in the dark to prevent light induced production of 12, but purging during this period is unnecessary. Time for development of the full color intensity varies with the specific hydroperoxide, the reagent composition, and the temperature used. Precautions which reduce interfering absorption are (7) R. A. Brown, M . I. Kay, J. M . Kelliher, and W. A. Dietz, Anal. Chem., 39, 1805 (1967). (8) H . Groenendyk, E. J . Levy, and S. F. Sarner, J. Chromatogr, Sci., 8,599 (1970). (9) C.Merritt, Jr., and C. DiPietro, Anal. Chem.. 44,57 (1972). (10) ti. Fe1ton.J. Chromafogr. Sci., 7, 13 (1969). (11) F. W. Heaton and N. U r i , J . Sci. FoodAgr., 9,781 (1958).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 4 , DECEMBER 1973
CHEMICAL ANALYSIS FOR
I
I
I
ACIDS 8 ESTERS
t-
TEST
KI
FOR
I
I
I
1
\1.
L
I
I
PEROXIDE
I
r\
SlLiCA GEL COLUMN CHROMATOGRAPHY I
POLAR I
I
HY DROfARBONS
PROPUCT
HYDROPEROXIDES
ISOMERS
ISOMERS
ISOMERS
Figure 2. Apparatus for determining hydroperoxides by tion using small aliquots
ISOMERS
Figure 1. Flow chart illustrating the separation and/or analysis of major n-paraffin oxidation products
soaking septa in isopropanol before use and taking care to prevent introduction of metallic contamination from the Nz manifold or brass needle hubs into the acidic medium. If the sample absorbs in the near UV, the spectrophotometer wavelength may be selected a t some wavelength other than the 350 nm 13--absorption maximum in order to eliminate or reduce such interference, but a decrease in sensitivity will result. The chemicals used in preparing the KI reagent must be tested for suitability, and specific lots of ACS reagent grade KI and isopropanol should be selected which yield, relative to isopropanol, a blank solution with 90-100% transmission. The dodecylhydroperoxides can be detected by direct analysis in n-dodecane solvent a t a concentration as low as (0.2 f 0.03) X 10-3M, but prior concentration on silica gel can greatly increase this sensitivity. At higher concentrations. the analysis precision is (10.0 f 0.15) X 10-3M and (300. 2.) X lO-3M. The maximum sensitivity is determined by the solubility of the aliquot in the KI reagent, and the specific wavelength chosen. Reducing the H2O and KI content of the reagent greatly increases the amount of oxidized hydrocarbon that will dissolve. A 13% KI solution in 80/20 isopropanol/HzO will dissolve an aliquot of up to 1% of its volume, but by using a 3% KI concentration in a solvent of 90/10 isopropanol/H2O, an aliquot of over 10% will dissolve. This latter reagent is recommended even though the color development period becomes 30 minutes a t 65°C to determine dodecylhydroperoxides. Since this procedure requires only small aliquots, an oxidation system can be sampled frequently without disturbing the reaction conditions. Different classes of peroxides can be distinguished by extending the above procedure to the methods of Mair and Graupner (12), and Howard and Ingold (13).A recent kinetic approach to peroxide analysis (14) could also be used here to distinguish among peroxides by measuring the light absorption as a function of time. Ester Saponification. Esters are a major product of paraffin oxidation, but the mechanism of formation is not clear (1, 4 , 5). The standard method of the American Society for Testing and Materials (ASTM) is test D94-71, saponification by color indicator titration using phenol-
*
(12) R. D. Mair and A. J. Graupner, Anal. Chem., 36,194 (1964). (13) J. A. Howard and K. U. Ingold, Can. J. Chem., 45, 785 (1967). (14) J. P. Hawk, E. L. McDaniel, T. D. Parish, and K . E. Simmons, Anal. Chem., 44, 1315 ( 1 9 7 2 ) .
12
libera-
(1) Nz source, (2) Cu wire packed quartz tube at 410 “C to scavenge 02,(3) on-off valve (one of many needed for multiple samples), (4) Nz inlet needle, (5) Nz exhaust needle, (6) spectrophotometric tube fitted with rubber septum, (7) reservoir fitted with syringe top for dispensing acetic acid (Labindustries, Berkeley, Calif.), (8) dispensing needle, (9) N2 exhaust hole
phthalein, a procedure designed for moderately high levels of esters in a hydrocarbon liquid. Besides the ethanolic KOH, methyl ethyl ketone is used as a solvent in this procedure to assure that the ethanolic KOH and the hydrocarbon to be tested will form a one-phase system with a fixed reflux temperature. In Table I, the ASTM method is applied to samples with low ester levels. Tests 1-4 clearly show that the added methyl ethyl ketone participates in the reaction, while dioxane as a solvent (test 5 ) also gives poor results. In tests 1 and 2 very high blanks were encountered. These can be attributed to the large volume of added methyl ethyl ketone as shown by tests 3 and 4 where the acidity of methyl ethyl ketone is determined by making it the only sample in experiments with no added acids, esters, or other solvents. The other tests in Table I demonstrate that for low levels of esters dissolved in ndodecane, no solvent is needed other than the ethanolic KOH and a small amount of HzO. Complete saponification requires 120 minutes for “dry” ethanolic KOH and less than 30 minutes if Y4 to 1% of H2O is added. Fifty per cent excess KOH is needed to ensure complete reaction, and the reaction is complete even when the reflux mixture contains two phases. Accuracy of 1% or better is obtained for samples containing 0.25% of ester, though greater sensitivity can be obtained if the silica gel concentration procedure is used as described below. When an oxidation mixture is being tested for ester content, the possible interference of hydroperoxides must be considered. To examine this possible interference, a sample of tert-butylhydroperoxide in n-dodecane and a sample of oxidized n-dodecane were prepared. Saponification equivalents were determined on each sample as prepared, and then after reduction of hydroperoxides with 10% excess triphenylphosphine (TPP) (15) which reduces the hydroperoxides to the equivalent alcohols. It is clear from the results in Table I1 that during saponification about 25% of the hydroperoxides present reacted to neutralize the alcoholic KOH. After TPP reduction of the hydroperoxides, the correct ester concentrations were determined. This method is accurate despite the presence of (15) D. E. Van Sickle, J. Org. Chem., 37, 755 (1972).
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 14, DECEMBER 1973
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Table I.Saponification Tests Saponification conditions Ester concentration. mmoles, I.-’ Sample dissolved in n-dodecane 1. Methyl caprate 2. Methyl myristate 3. M E K 4. M E K
5. Methyl undecanoate
laurate laurate 8. Methyl laurate 9. Methyl caprate 10. Methyl caprate 11. Methyl caprate 12. Methyl caproate 13. Methyl caproate 14. Methyl myristate 15. Methyl myristate 6. Methyl 7. Methyl
Sample ml
Ethanolic
3 3 1 .o 30. 3
10 ml 0.083N 10 m l 0 . 0 8 3 N 5 ml 0.126N 5 ml 0.126N 5 ml 0 . 1 0 0 N 5 mi 0.082N 5 ml0.082N 5 mi 0.082N 5 ml0.082N 5 ml 0.082N 5 ml 0 . 0 8 2 N 5 ml 0 . 1 1 1 N 5 ml O . l l l N 5 ml 0.111N 5 ml 0.126N
1 1 1
1 1 1 1 1 1 1
Solvent, ml
KOH
10 M E K 10 M E K ...
...
...
...
...
... ...
... ...
... 3
, . .
...
...
... , . .
3 3 3
, . .
10
...
tert-Butylhydroperoxide Oxidized in n-dodecane, n-dodecane. rnmoles I. mmoles I
292. 0.0 69.6 69.6
Silica gel columns% ti20 ( w / w )
-’
25.6 5.5 18.1 12.6
After TPP reduction Hydroperoxides Acids Saponification Actual Esters
0.0
0.0 5.5
0.0 0.0
12.0 6.5
0.0
Ester added, TPP reduction Hydroperoxides Acids Saponification Total esters detected Total esters present
0.0 0.0 23.7 23.7 24.0
0.0 5.5 35.2 29.7 30.5
TPP and TPP oxide as demonstrated by the third experiment in Table I1 where ester, added to each of the original samples, is accurately measured. In the light of these results, previously published tests for esters in the presence of hydroperoxides must be re-examined. Silica Gel Column Chromatography. Activated silica gel is known to absorb polar molecules and retain these molecules quantitatively despite elution by large volumes of hydrocarbon solvents (7). In the present work, research grade hydrocarbons have been purified down to the very low electrical conductivity of 0.01 picomho m - l from the “as-received’’ conductivity of 1.0 picomho m Silica gel column chromatography can be used to quantitatively separate polar from nonpolar oxidation products. First the oxidation product mixture is added a t the top of the column, followed by pentane to elute nonpolar molecules. Addition of methanol then strips off the polar molecules. Stable molecules are quantitatively concentrated by this procedure ( 7 ) , but the technique must be closely examined to evaluate the effect of highly catalytic gel surfaces on the easily decomposed hydroperoxides. Accordingly, four silica-gel columns are prepared: one with fully activated gel and three with silica gel deactivated by varying 2390
60 60 60 60 15 15 60 120 120 60 60 60 12 12 30
As
As
prepared
determined
Blan k
143.7 f 0.8 65.4 f 0.5
138. f 7 62.2 f 4 13.0 13.3 25 f 5 27. f 1 58. f 1 64.6 f 0.5 78.6 f 0.5 76.7 f 0.5 69.7 f 0.5 106.5 f 0.7 102. f 0.7 63.3 f 0.5 145. f 0.8
206. 206.
0.0 0.0 42.2 f 0.2 64.9 f 0.4 64.9 f 0.4 64.9 f 0.4 77.7 f 0.5 77.7 77.7 106.5 106.5 65.4 143.7
f 0.5 f 0.5 f 0.7 f 0.7 f 0.5 f 0.8
23.
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Table I I I. Dodecylhydroperoxides Recovered by Silica Gel Column Chromatography
Elution of column
Before TPP reduction Hydroperoxides Acids Saponification Apparent Esters
3 3 3 3
... 20 Dioxane
Table II. Saponification with Hydroperoxides Present
-’
Hz0, drops
Reflux time, min
0%
1 . 10 ml n-Heptane 10 ml !?-Heptane 15 ml Methanol 10 ml Methanol 10 ml Methanol 5 ml Methanol 5 ml Methanol 5 ml Methanol
2.2 0.0 63.8 87.4 83.5 13.3 5.7
Total recovery Total added at start Recovery, YO
255.9 256. 100.
2. 3. 4. 5. 6. 7. 8.
44%
90%
147%
HvdroDeroxides recovered, umoles
0.0 29.0 39.9 21.0 94.4 . . .a
0.0 17.1 9.7 94.1 85.5
18.8 25. 3.4 . . .a
..,a
0.0
Column cloqqed
amounts of HzO as in Table 111. This specific silica gel requires 14.7% HzO (w/w) to effect complete deactivation (26). Each packed column is 15 cm long by 2 cm wide, with a void volume of 20 cm3. A fifth column of activated silica gel is used as a control with only pure n-dodecane added as a sample (not in Table). Ten milliliters of oxi1 pmoles of hydrodized n-dodecane containing 256 peroxides is added to each of the other four columns. The hydroperoxides recovered a t each elution step are determined. Clearly, from Table 111, only fully activated silica gel quantitatively retains the hydroperoxides, but this same column does not cause sample decomposition. Methanol elution results in quantitative recovery of the dodecylhydroperoxides. T o obtain a sample of the isolated polar products, it is necessary to remove the methanol. Neither salting nor adding H20 to the methanolic solution was successful separation methods because of emulsion formation, but a highly concentrated polar product mixture was readily prepared by methanol evaporation. In any case, it is necessary to filter the mixture to remove traces of silica gel before removing the methanol. High Speed Liquid-Liquid Chromatography. The major polar products of the polar fraction from n-dodecane oxidation (dissolved in methanol) were separated by
*
(16) S. G. Perry, R. Amos, and P. I . Brewer, “Practical Liquid Chromatography,” Plenum Press, New York, N.Y., 1972.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
I
I
I
I
I
i
RETENTION TlME(min) Figure 4. GLC chromatograms of oxidized n-dodecane
RETENTION TIME (mid Figure 3. Chromatogram from HSLC of the polar products from
n-dodecane oxidation Chromatographic detector I S a UV spectrophotometer at 254 nm The nheptane mobile phase is flowing at 2 8 cm3 m i n - ' , 830 psi, and 18 "C See text also
HSLC as shown in Figure ?. When the peaks labeled dodecanones, dodecanols, and dodecylhydroperoxides were collected, only the hydroperoxide peak showed a positive test during KI analysis. Infrared absorption spectra and GLC chromatograms were obtained for each peak to provide positive identification. The hydroperoxide infrared spectrum contained no carbonyl absorption, indicating that no acids, esters, or ketones were collected at the same time. This technique is especially valuable because it provides purified hydroperoxides a t the milligram level from a single 10-mg injection, the limit for adequate resolution a t these flow rates. A 50-mg sample was successfully separated when collection of the partially resolved sample from the liquid chromatograph was followed by solvent evaporation, re-injection, and evaporation a second time. Purified hydroperoxides collected by this separation were used in experiments described below. Gas-Liquid Chromatography of Hydroperoxides, Alcohols, and Ketones. There are six possible isomers of the straight chain dodecanols, dodecanones, and dodecylhydroperoxides. During GLC analysis, the alcohol and ketone isomers have been observed to exhibit overlapping elution times ( 4 ) . In addition, the analysis is complicated by the possible decomposition of the hydroperoxides in the GLC injection port. The complexity of the analysis depends on whether individual isomers are to be determined, or a total for each functional group is desired. Two successful techniques can be applied to separate the alcohols and ketones: silylation and boric acid reaction in a precolumn. Silylation was accomplished by injecting a mixture of hexamethyldisilizane, trimethylchlorosilane, and pyridine into the GLC port a t some time ( A t ) after injecting a sample containing alcohols ( 4 ) . Alcohols react to form silyl ethers which have greater retention volumes than the alcohols and show an increase in flame ionization detector response. For the dodecanols, the response increase was measured as 133.2 f 0.2%. By measuring the total area of the flame response for the alcohols and ketones without silylation and then the response for ketones plus silyl ethers after silylation, the total alcohols and ketones can be calculated: the alcohols from the increase in the area and then the ketones as the residual
The upper trace shows the dodecanols and dodecanones unresolved. In the lower section, a boric acid precolumn (see text) has removed the alcohols leaving the numbered dodecanone isomers. Chromatographic conditions (see text also): 100-ft DC-550 support-coated open tubular column with 5 cm3 min-' H e at 145 "C. Flame ionization detector with 40 cm3 min-' HZ, 220 cm3 min-' air, and 116 cm3 min-' H e carrier make-up gas
Table I V . Isomers Formed during n-Dodecane Air Oxidation at 200 "C Position of the functional group
c-1 c-2 c-3 C-4, 5, 6
Isomer Present, YO Dodecanones
Dodecanols
1.1 f 0.5 24.5 f 0.7 19.0 k 0.7 55.4 f 0.5
25.3 f 0.7 18.2 f 0.7
... 55.5 f 0.5
Dodecylhydroperoxides
0.9 24. 19. 57.
f 0.3 f1 f1 f1
area, assuming the contribution to each from thermal degradation of hydroperoxides is known. The second technique is the insertion of a boric acid precolumn into the GLC flow stream after the injection port where the precolumn can be maintained a t about 210 "C (27). The precolumn was found to be 99.5+% effective in removing the alcohol from a mixture of 2% 4-dodecanol and 2% 4-dodecanone in n-dodecane. With alcohols stripped from the gas stream, the individual ketone isomers can be determined to the extent that they are resolved. The 4-, 5 - , and 6- isomers are not resolved (as seen in Figure 4), but it has been reasoned that they are formed in equal amounts ( 4 ) . The contribution of hydroperoxides to the ketone peaks by thermal degradation in the GLC inlet can be eliminated by using the previously mentioned TPP reduction to alcohols which are then subtracted from the gas stream by the precolumn. Thus, dodecanone isomers have been determined as shown in Table IV. To complete the analysis of the individual isomers of the hydroperoxides and alcohols and to account for the contribution from thermal degradation of hydroperoxides to the alcohols and ketones seen, it is necessary to prepare a hydroperoxide sample free from alcohols and ketones. A mixture of isomeric dodecylhydroperoxides in n-heptane was obtained by HSLC from an oxidized n-dodecane as described above. Injection of the sample (0.71 f 0.01 mole YO) into a gas chromatograph with an 18-8 stainless steel injection port at 207 "C results in a chromatogram of (17) F E Regnier and J C Huang, J Chromatogr S o ,8 , 2 6 7 (1970)
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Table V. Olefin Determination by Bromination (1 5 "C) and G LC Sample
1 2
3 4 5 6 7
a Figure 5. GLC chromatogram of an oxidized n-dodecane where (0.044f 0.002)Mdodecene isomers have formed. The chromatogram is of the dibromo derivatives of the 6 possible isomers (labeled) as formed by "gentle" bromination. Chromatograph conditions as in Figure 4
dodecanols and dodecanones a t a total concentration of 0.69 f 0.01 mole %. Similar results were obtained a t 170 "C. Separate analysis of the ketones and of the alcohols obtained from the hydroperoxide decomposition are made by the silylation technique mentioned above using various values of I t . The resulting chromatograms of silyl ethers and ketones show no changes for At's between 5 seconds and 2 minutes, demonstrating that hydroperoxide decomposition is rapid in the injection port area. The total alcohol/total ketone ratio is measured typically as 1.04, 1.00, and 0.98 at the standard temperature of 207 "C. The results are used to subtract the contribution of hydroperoxides to the chromatograms of oxidation product mixture with or without silylation, knowing the hydroperoxide concentration. When injecting GLC samples on-column, hydroperoxides are exposed to the catalytic effects of the packing and adsorbed liquid. Under these conditions, hydroperoxide decomposition yields different relative amounts of the alcohols and ketones. When individual isomers are being examined rather than total alcohols and total ketones, it is necessary to know the actual isomer distribution of the hydroperoxides in order to subtract the contribution of decomposed hydroperoxides from the chromatograms. This is accomplished by detailed analysis of the partially resolved chromatograms of the pure hydroperoxide sample with and without silylation and/or by reducing the hydroperoxides with TPP and examining the alcohol peaks. In Table IV, the isomer distribution is listed for the dodecylhydroperoxides formed during n-dodecane oxidation under the indicated conditions. The detailed analysis of the alcohol isomers is the remaining problem. This is obtained from the chromatograms of silylated and unsilylated oxidation mixtures, taking into account the contribution from the ketones and hydroperoxides determined above (see Table IV). The isomers of the alcohols, ketones, and hydroperoxides are nearly identical, apparently controlled by the slow step of oxidation, the formation of the initial alkyl free radical. Gas-Liquid Chromatography to Detect Olefins. A satisfactory method for detecting very low levels of olefin (
