Determination of Petroleum Wax Odor by Gas Chromatography

Shell Oil Co., Houston Research Laboratory, P. O. Box J 00, Deer Park, Texas 77536. A gas chromatographic determina- tion of petroleum wax odor has be...
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Determination of Petroleum W a x Odor by Gas Chromatography LARRY R. DURRETT Shell Oil Co., Houston Research laboratory, P. 0. Box 100, Deer Park, Texas 77536

b A gas chromatographic determination of petroleum wax odor has been developed which can be easily correlated with the olfactory rating of wax samples as defined by an odor panel. In addition to simply rating the wax with respect to overall odor, the gas chromatographic method determines the concentrations of the specific compounds from which the odor is derived. A sample of vapor above molten wax is withdrawn from a closed system with a hypodermic syringe and thence injected into a conventional packed-column gas chromatograph, which is equipped with a hydrogen-flame ionization detector. Since the flame ionization detector does not respond to air or water vapor, only the volatile organic components present in the wax are detected. Thus, a “finger-print’’ of the volatile components above the molten wax is obtained.

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lack of it, is one of the more important properties of petroleum waxes, particularly if the wax is to be used in food packaging. Petroleum wax odor is usually determined by the olfactory senses of a panel of experts. Such a panel generally categorizes the odor of wax as a “solvent” odor, an “oxidized” odor, or an “undefinable” odor. Waxes are rated according to the scale: (1) no odor, (2) no objectionable odor, (3) objectionable odor, and (4) very bad odor. A wax sample is passed a t odor ratings of 1 and 2 and failed a t ratings of 3 and 4. The rating a particular wax receives is the average of the ratings of the individual panel members. Since individuals may vary markedly in response tJodifferent types and levels of odor, the average result may represent individual ratings of 1 through 4. Additionally, a wax which has a very bad odor tends to saturate the olfactory senses for a period of time, thus invalidating subsequent ratings. Hence, a more objective test to supplement or replace the subjective judgment of an odor panel would appear to be highly desirable. Peterkin and Loveland (4) have developed a colorimetric test which measures the amount of carbonyls (only aldehydes and ketones) stripped from

molten wax. The carbonyl test results are reported to show an 80% correlation with the ratings of a wax odor panel. However, this method is obviously limited to waxes in which the odor is caused by aldehydes and ketones; furthermore, these contaminants are assumed to make equal contributions to odor. In recent years, gas chromatography has been utilized in practically every scientific field of endeavor. These applications have run the gamut from the analysis of extremely complex mixtures to fundamental studies of solution thermodynamics and reaction kinetics. In view of the very high sensitivity of the detecting devices employed in gas chromatography, applications to trace analysis, particularly to studies of various flavors and aromas, have been numerous. A gas-liquid chromatographic (GLC) method has now been developed for the determination of petroleum wax odor. This method, which is based on the assumption that wax odor, as detected by human olfactory senses, is attributable to relatively volatile compounds which are present in the vapor above molten wax, is the subject of this paper. Apparatus. The apparatus employed for the determination of wax odor consisted of a conventional packed-column gas chromatograph equipped with a hydrogen-flame ionization detector. The column employed was a 6-foot length of 3/leinch stainless steel tubing (0.12-inch i.d.) containing 5% w. SE-30 silicone gum rubber on 60- to 80-mesh firebrick, The column was operated a t 100’ C. and argon was used a t the mobile phase a t an exit flow rate of 75 ml. per minute. The column effluent was directed into a hydrogen-flame ionization detector. A polarizing potential of 300 volts negative was applied to the detector jet and the collecting electrode, located 7 mm. above the jet, was essentially at ground potential. The detector output signal was amplified with a Keithley Model 410 micromicro ammeter. The electrometer output was recorded on a 5-mv. potentiometric recorder and was also fed to a voltage-to-frequency integrator (1) for peak-area measurements. These components have been described previously ( 3 ) . The detector background current was 1 X lo-” amp. and its noise level amp. The apparatus was 1 X

was generally operated a t a full-scale recorder deflection corresponding to 2 X lo-” amp. Analytical Procedure. The procedure utilized in this determination approaches the ultimate in simplicity. A 1-pint paint container is filled to within one inch of the top with the wax sample (ca. 300 grams). This quantity results in a vapor space of approximately 100 ml. above the wax. The container, into the lid of which a hole has been drilled to accommodate a silicone septum, is then sealed and placed in a water bath at 190’ F. for about one hour (the time required for the wax to melt and the volatile components to equilibrate will depend, of course, upon the melting point of the wax and the bath temperature). A sample of vapor (exactly 0.50 ml.) above the molten wax is then withdrawn from the system with a Hamilton No. 750 hypodermic syringe and thence injected into the chromatograph. Thus, a “fingerprint” of the volatile constituents in the vapor space above the wax is obtained. For simply rating the odor of a particular wax by this method, the quantity of wax sample in the container is not, within reason, extremely criticalLe., one can estimate one inch from the top of the container with the naked eye with sufficient accuracy. If, on the other hand, an accurate determination of the volatile constituent concentration in the wax is desired, then one must place the same quantity of wax in all containers, including the standards with which the instrument is calibrated. In the latter case, the sample should be weighed or calibrated containers should be used. The 1-pint paint containers were utilized simply because wax samples are routinely taken in such containers and because sensitivity presented no problem. Should additional sensitivity be required, the vapor space above the wax could be decreased by increasing the wax level in the container, and a larger vapor sample could be withdrawn with the hypodermic syringe. In view of the low noise level of the detector, the sensitivity could also be increased tenfold by operating a t a full-scale recorder deflection corresponding to 2 X 10-12 amp. rather than 2 X amp. Identification of Volatile Components in Waxes. A large number of wax samples containing a wide spectrum of types and levels of odor were examined with the GLC “fingerVOL 38, NO. 6, MAY 1966

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printing" technique. The volatile constituents were identified from retention data on the previously mentioned silicone gum rubber column and from similar data on a 40-foot benzyl cyanide-silver nitrate column which was operated at 25' C. The compounds which have been found in the various waxes studied include C r C s paraffins and monoolefins methyl ethyl ketone and toluene (dewaxing solvents), and, in the case of waxes having an oxidized odor, C r C s aldehydes. The identities of these compounds were confirmed with an additional gas chroma-

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ANALYTICAL CHEMISTRY

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and olefins are not resolved on the silicone gum rubber column at 100" C. Although olefins make much larger contributions to odor than do paraffins, the fact that these contaminants are not resolved is not considered critical since the paraffin/olefin ratio in some five samples which were examined with the benzyl cyanide-silver nitrate column was always between 1.5 and 2. Moreover, in most instances, these components are not present in sufficient concentration to make a significant contribution to the overall odor. Effect of Bath Temperature and Wax Sample Residence Time. The temperature of the water bath in which the waxes are melted must be maintained relatively constant since one measures the contaminant concentration in the vapor above the molten wax rather than the concentration in the wax proper. The

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Figure 2. Effect of temperature on toluene concentration in vapor Sample: 1 3 8 ' F. m.p. wax containing toluene; rampie size, 0.5 ml. of vapor

effect of temperature on the contaminant concentration in the vapor is illustrated in Figure 2 with a 138'-140' F. m.p. wax containing 40 p.p.m. toluene. It can be seen that if the calibration of toluene was performed at 190' F. and then samples analyzed at 200' F., the resulting error would be approximately 17y0 or 1.7% per degree change in bath temperature. Although the magnitude of the temperature effect will depend on the contaminant in question, the bath temperature should be precisely controlled for most accurate results. Another important parameter which must be considered is the time required to melt the wax and to equilibrate the volatile components present. This time requirement will depend on the bath temperature, the nature of the sample container, the quantity of wax sample, and its melting point. The time requirement is seen in Figure 3 to be approximately one hour in the case of 300-gram sample of 145'-150' F. m.p. wax containing about 65 p.p.m. toluene at a bath temperature of 190' F. In this experiment 300 grams of sample was placed in each of two containers. These samples were then placed in the bath, one with and the other without the lid so that the physical state of the wax could be observed. Vapor samples (0.50 nil.) were withdrawn from the closed system at 5-minute intervals and injected into the chromatograph until the height of the toluene peak ceased to change. The physical state of the sample was observed during the experiment and is indicated in Figure 3. Although a period of one hour is sufficient for waxes with melting points as high as 150' F., the high-melting point waxes will require somewhat longer periods of time. Repeatability and Accuracy. The repeatability and accuracy with which the volatile components concentration in wax can be determined by the method described herein depends

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on how well a number of factors are controlled, since the method employs the quantitative injection approach to quantitative analysis. These factors are: (1) the quantity of wax in the sample container and thus the vapor space above the sample, (2) the bath temperature and sample residence time, and (3) the ability of the analyst to inject repeatedly a constant-volume vapor sample. To ascertain the degree of repeatability and accuracy which may be expected with the method, a 300-gram Table

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sample of the previously mentioned wax base stock was contaminated with 40 p.p.m. toluene. The sample was placed in the water bath, which was held at 190' + 1' F. for one hour prior to vaporsample withdrawal. Subsequently, six determinations were performed. The results of these determinations are tabulated in Table I1 and indicate that repeatability of 2-3% and accuracy of 4 5 % can be obtained. Comparison of GLC and OdorPanel Ratings of Waxes. I n addition to calibration of the gas chromato-

Repeatability and Accuracy Data

(Sample: 138°-1400 F. m.D. wax contaminated with 40 D.D.m. _ - toluene) Tolucne % Deviation Toluene peak concentration From average From added height, inches determined, p.p.m. 2.29 2.19 2.18 2.10 2.25 2.23 Average 2.21

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Table 111.

Comparison of GLC and Odor-Panel Ratings" of Various Waxes GLC Odor-panel Wax, Odor attributed to m.p. C3-C5 Toluene, Individual OF. HC, p.p.m.b p.p.m. Rating A B C D E Average 124-126 2 3 1 1 2 1 1 1 1 130-132 6 2 1 2 1 2 1 1 1 130-132 250 ... 4 3 4 4 4 4 4 132-136 12 60 2 2 3 1 1 2 2 134-138 10 50 2 2 2 1 2 1 2 138-140 1 2 1 1 2 1 1 1 1 139-143 15 10 1 1 1 1 1 1 1 145-150 ... 150 3 3 4 4 3 3 3 145-150 ... 125 3 2 2 3 3 2 3 150-160 ... 4 1 1 2 2 1 1 1 158-1 62 20 3 1 1 3 1 1 1 1 172-1 80 ... 5 1 2 3 2 2 2 2 a Rating system-1, no odor; 2, no objectionable odor; 3, objectionable odor; 4, very bad odor. b Ca-Cs paraffins and monoolefins-paraffin/olefin ratio-1.5-2.

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graph and the odor panel with the synthetically prepared standards, some 30 commercial wax samples, which had been rated by the odor panel, were examined with the GLC technique to determine whether the concentrations of the various volatile compounds could be correlated with the wax-odor panel rating. These data indicated that a correlation was indeed feasible. Twelve wax samples ranging from very low melting point to very high melting point were taken randomly in an attempt to obtain samples with varying levels of odor. These samples were examined by the GLC technique and accordingly given an odor rating. Subsequently, the same samples were rated by a wax odor panel. A comparison of the odor ratings of these samples is given in Table 111, which also categorizes the compounds to which the odor is attributed. These data demonstrate that petroleum wax odor can be determined by gas chromatography. Additionally, and of equal importance, the source of

the odor can be identified and thus indicate the course of action which may be required to reduce the odor. The gas chromatographic method offers a repeatable, accurate determination of the volatile components present in petroleum waxes, the results of which can be correlated with the ratings of a wax-odor panel. Although approximately one hour is required for melting and equilibrating the sample, the GLC determination proper requires less than five minutes. Generally, a batch of eight to ten samples were melted simultaneously so that the time requirement per determination was of the order of ten minutes. Since the method determines only the volatile component concentration in waxes, the success of the wax odor correlation obviously depends on how well the determination is initially calibrated with wax-odor panel ratings. In this regard, it is, of course, desirable to examine a relatively large number of samples containing various types and levels of odor. Since individuals do vary in response to different types and

levels of odor, it is also advisable to include as many odor “experts” as possible on the panel during the initial calibration. ACKNOWLEDGMENT

The author thanks M. J. O’Neal, Jr. and G. P. Hinds, Jr. for their critical examinations of the manuscript. LITERATURE CITED

(1) Davis,. C. E., Riggs, W. A,, “An

Electromc Integrator-Digitizer for Gas Chromatography,” 13th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 5-9, 1962. (2) Dorsey, J. A., Hunt, R. H., O’Neal, M. J., Jr., ANAL.CHEM.35, 511 (1963). (3) Dnrrett, L. R., Taylor, L. M., Wantland, C. F., Dvoretzky, I., Zbid., 35, 637 (1963). (4) Peterkin, M. A., Loveland, J. W., Petrol. Refiner 40, 133 (1961).

RECEIVEDfor review July 12, 1965. Accepted March 10, 1966.

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