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 A sample of vapor above molten wax is withdrawn from a closed system with a hypodermic syringe and thence injected into a conventional packed-column gas chromatograph, which is equipped with a hydrogen-flame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.
0
DOR, or
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
* 745
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-
None ....
2-Methylbutene-1 2-Methylbutene-1 2-Methylbutene-1 Toluene Toluene Toluene Toluene To1u ene Toluene To1u en e n-Pentane
n-Pentane
...
i
10 100 1 10 50 100 150 200 300 10 100 1 0.1 1 1 1
_1 1 1 3 2 2 2 2 2 2 3
1_ 2 3 4 1 2 2 3 3 3 4
14
746
ANALYTICAL CHEMISTRY
I
I
I
I
I
I
I
PEOOO
I
-
I¶
zc'
24000 vr
10
4;:
-
5
-
Y
-
Y
EOOOZ
- 4000 I
1 _1 1 1 2 2 3 4 1 1 1 1 2 2 2 2 3 3 3 2 4 4 1 1
1_ 2 3 4 1 1 4 3 3 3 4 3
2
1
1
~
Propionaldehyde 3 3 n-Butyraldehyde 4 2 n-Butyraldehyde 4 4 4 4 4 n-Valeraldehyde 4 3 Acetone 1 1 2 1 2 Acetone 10 1 1 2 2 2 Methyl ethyl ketone 1 1 2 2 Methyl ethyl ketone 10 2 2 2 Ethyl alcohol 1 1 1 1 1 Ethyl alcohol 10 2 2 2 1 Acetic acid 1 1 3 Acetic acid 10 3 4 Rating system: 1, no odor: 2, no objectionable odor; 3, bad odor. 0
I
1 1
1 1 2 3
3 3
3 3
3
2
3
2 1 1 1
1 2 1 3
1 3
1 1 2 4 1 1 2 2 3 3 4 1 1 3 3 4 3 1 2 2 2 1 2 1 3
1 2 objectionable odor; 4, very
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
7.00
I
I
I
,
1
I
I
I
I
r' 5.00
:c 14.00
E
170
180
190
BATH TEMP.,
PO0
1.00
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
3.00
PI0
OF,
0 ~,o,~-04-~-O-O-0 /"
1
0
I
- 0 4
0
10
40 p.p.m.
I
PO
30
I
I
I
40
50
60
I
70
1
I
EO
90
IO0
HEATING PERIOD, MIN.
Figure 3. Time required for 145'-150° melt and toluene present to equilibrate
F.
m.p. wax to
Bath temperature, 190' F.; sample sire, 0.5 ml. vapor A. ca 20% melted; B. ca 50% melted; C. ca. 80% method; D. wax
completely melted
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
II.
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
43 41 41 39 42 42 41
+5.0 0.0 0.0 -5.0 +2.5 +2.5 2.5
+7.5
+2.5 +2.5 -2.5 +5.0 $5.0 4.2
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.
VOL 38, NO. 6, MAY 1 9 6 6
8
747
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.
Versatile Combination Ozone and Sulfur Dioxide Analyzer FERDINAND SCHULZE’ E. 1. du Pont de Nemours & Co., Inc., Wilmingfon, Del. A versatile combination analyzer has been devised to overcome mutual interference due to coexistence of ozone and sulfur dioxide in the atmosphere. The measurement principle involves liberation of iodine from an iodide solution by ozone in one channel, and consumption of iodine by sulfur dioxide in a second channel. The iodine concentration is measured amperometrically. The system is standardized by means of coulometrically generated iodine. Sodium iodide reagent is circulated continuously by a two-channel metering pump through a bed of nylon fiber which removes free iodine quantitatively; thus, reagent is regenerated, no replenishment is required, and integrity of the reagent is ensured. Ozone is preferentially removed from air entering the sulfur dioxide channel by filtering through a bed of ferrous sulfate crystals. Sulfur dioxide is removed from the ozone channel by filtering through a bed of quartz chips soaked in a solution of chromium trioxide in aqueous phosphoric acid. The important advantages of the instrument are rapid response, high sensitivity, ease of standardization, and capacity for unattended operation.
748
ANALYTICAL CHEMISTRY
T
coexistence of ozone and sulfur dioxide in the atmosphere, particularly around urban areas, creates many perplexing problems relating to health, plant growth, and environmental durability of industrial products. Thermodynamic considerations indicate that ozone and sulfur dioxide should react with each other, leaving only an excess of one or the other. However, the extreme dilution of these gases in the atmosphere permits them to coexist for kinetic reasons. There is a question, therefore, regarding the environmental effects of ozone and sulfur dioxide in the absence or presence of the opposite member. This situation is aggravated by the fact that coexistence of ozone and sulfur dioxide complicates the determination of these substances. The complication arises from the fact that, whereas liberation of iodine is the basis for numerous manual and instrumental analytical procedures for ozone, sulfur dioxide interferes in these procedures by reducing iodine ( 3 , 4 , 6 ) .By the same token, analytical procedures based on reduction of iodine by sulfur dioxide will be in error due to presence of ozone. Experience has revealed several shortcomings in a number of the manual and continuous recording procedures based HE
on iodine liberation for monitoring the ozone content of the atmosphere. Inability to calibrate unequivocally in terms of ozone concentration is most frustrating. Other disadvantages are lack of sensitivity, sluggish response of continuous analyzers, uncertainty regarding the integrity of potassium iodide reagent, and the necessity of frequent attention which makes unattended operation for long periods infeasible. Although many of these difficulties may be problems of experimental technique, the basic problem is the interference of sulfur dioxide in ozone determination. Meaningful information about the atmospheric environment requires simultaneous monitoring for ozone and sulfur dioxide concentrations. Similar problems exist in control of ozone and sulfur dioxide concentrations in environmental chambers. Improved ozone- and sulfur dioxiderecording analyzers have been devised, and a combination unit capable of monitoring both gases simultaneously is the subject of this paper (6). The application of this device for concentration control in environniental chambers is an obvious modification. 1 Present address, 3205 Fordham Road, Wilmington, Del.
