methane flow have been observed. With the furnace system (and apparatus) described in reference ( 2 ) , background emission measurements were made in the observation region with and without the methane flow. In the 2000-6000 8, mgion, the only differences observed were very weak OH, Cp, and CH bands in the presence of the methane. Above about 2100 "C, a small blue flame appears above the furnace opening, presumably due to combustion of the H2 (from the pyrolysis) and perhaps CO (from the oxidation) with entrained air in this region. These 2 atomization systems (1, 2 ) have proved to be extremely stable over long periods of time. The pyrolysis treatment described here has contributed significantly to this stability and has greatly reduced the need to replace the heated graphite elements with the attendant recalibra-
tion. The constant surface replacement and nonporous coating have greatly reduced cross-contamination and memory effects. This pyrolysis treatment should prove to be applicable and advantageous with virtually any type of heated graphite atomization system.
RECEIVEDfor review February 11, 1974. Accepted August 23, 1974. Taken in part from the M.S. thesis of S.A. Clyburn, University of Houston, May 1974. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1974. This work performed a t the University of Houston, Houston, Texas, was supported in part by the National Science Foundation and in part by the Robert A. Welch Foundation.
Rapid Thermogravimetric Estimation of Oil Stability H. J. Nieschlag, J. W. Hagemann, and J. A. Rothfus Northern Regional Research Laboratory, Peoria, Ill. 6 1604
D. L. Smith Trane Company, La Crosse, Wis. 5460 1
Many laboratory procedures have been devised to estimate, or predict, the keeping quality of vegetable oils. Most methods subject an oil sample to conditions that attempt to accelerate normal autoxidation. Testing is usually continued until the sample is either rancid or in an oxidized state approaching rancidity. Older techniques promoted deterioration by maintaining samples a t elevated temperatures in contact with air; periodically samples were either tested for weight gain ( 1 , 2 ) or examined organoleptically for rancidity (3).The end-point has also been determined by chemical and/or spectral analyses either for peroxide or other oxidation products ( 4 - 6 ) . Deterioration has also been measured in terms of viscosity changes upon heating (7). In other procedural variants, air is either bubbled through the oil (8) or oxygen is pressurized in contact with the sample (9).
For years the "Active Oxygen Method" (AOM) was the most widely accepted method in the United States ( 8 ) ,but the modified ASTM oxygen bomb method ( 9 ) , which is faster and more reproducible, is gaining general acceptance ( I O ) . Newer methods emphasize microanalytical procedures such as gas chromatography (11,12) and differential scanning calorimetry (13,1 4 ) to estimate oil stability. (1) H. S. Olcottand E. Einset, J. Amer. OilChem. Soc., 35 161 (1958). (2) H. S. Olcott and E. Einset, J. Amer. Oil Chem. Soc., 35, 159 (1958). (3) N. T. Joyner and J. E. Mclntyre, Oilsoap, 15, 184 (1938). (4) "Official and Tentative Methods of the American Oil Chemists' Society," W. E. Link, Ed., Method Cd 8-53, 3rd ed., 1973. (5) L. K . Dahle and R. T. Holman, Anal. Chem., 33, 1960 (1961). (6) L. K. Dahle, E. G. Hill, and R. T. Holman Arch. Blochem. Blophys., 98, 253 (1 962). (7) G. Fuller, M.J. Diamond, and T. H. Applewhite, J. Amer. OilChem. Soc., 44, 264 (1967). (8)"Official and Tentative Methods of the American Oil Chemists' Society," W. E. Link, Ed., Method Cd 12-57, 3rd ed., 1973. (9) W. M. Gearhart, E. N. Stuckey, and J. J. Austin, J. Amer. Oil Chem. SOC.,34, 427 (1957). (10) F . A . Norris, Kirk-Othmer€ncycl. Chem. Techno/., 8, 795 (1965). (11) P. K. Jarvi, G. D. Lee, D. R. Erickson, and E. A. Butkus. J. Amer. Oil Chem. Soc., 48, 121 (1971). (12) C. D. Evans, G. R. List, R. L. Hoffmann,and H. A. Moser, J. Amer. Oil Chem. Soc., 46, 501 (1969). (13) C. K. Cross, J. Amer. OilChem. Soc., 47, (6), 229 (1970). (14) R. L. Blaine, Amer. Lab., 6 , 18 (1974).
Our method employs a commercially available thermogravimetric system in which changes in sample weight are monitored continuously while the sample is temperature programmed a t a reproducible rate in a pure oxygen environment. This empirical procedure is essentially a return to the simple technique of heating an oil in an oven and weighing it periodically. Now, however, with a highly sensitive recording electrobalance, weight changes that occur during heating can be permanently recorded; this was never feasible before. In our hands, thermogravimetric measurements give more sensitive indications of sample transitions than differential scanning calorimetry (13). Temperature programming also offers advantages over an isothermal procedure: less analysis time is required and a wider range of sample reactivities can be accommodated. Analysis time, including cool-down, is about 40 minutes and only a 2-11 sample is consumed.
EXPERIMENTAL A Perkin-Elmer TGS-1 thermobalance was used in conjunction with its associated DSC-1B differential scanning calorimeter. Sample weight changes sensed by the electrobalance were plotted on a strip-chart recorder equipped with an events marker. Furnace temperature was displayed on the digital readout of the DSC-1B and marked on the recorder chart by the events marker. It is necessary to calibrate the thermobalance furnace so that deviations from the temperature displayed on the digital readout are known. The apparatus was calibrated at 5 'C/min using boiling points of carbon tetrachloride (77 "C) and xylene (139 "C) and the magnetic transition of alumel (163 "C). During calibration, nitrogen was passed through the balance at 30-35 ml/min. The volatile solvents were heated in sealed pans with pin-hole vents. A small vent effects a more abrupt weight change a t the boiling point, Each flat pan lid, before being sealed to a sample pan, was placed on thin paper over a hard surface and pierced at its center with a sharp needle. The combination of paper and hard surface limits the penetration of the needle through the lid and produces a tiny hole. The balance was calibrated to weigh a maximum of 10 mg and the controls were set so that full-scale of the 10-inch recorder chart represented a weight of exactly 100 pg. All samples were analyzed with an oxygen flow of 35-40 ml/min
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14. DECEMBER 1974
2215
Table I. Oxidative Stability of Crambe Oil Samples, Thermomavimetric Parameters. a and Ultraviovlet Absorbance at 233 nm Sample
" ' s a7t h0 ec rcm a '
Figure 1. Thermogravimetric curve for a typical crambe oil sample. See text for description
RESULTS AND DISCUSSION A thermogravimetric curve for a typical oil is shown in Figure 1. The line from A to B indicates loss of solvent (if present) as the sample is held isothermally at 70 O C . Temperature programing is started when the recorder no longer registers any change in weight, as a t B. The straight line from B to Ti represents a static condition that is analogous to the "induction period" of classic procedures except that 2216
Cb
191 182 176.5 161.5 162.5 151 151 136.5 117.5
i' Increasing Temperature. T e ~ ~ ~ ~ 70" ~ l u 10r e-250'C at 5'/minute Pi'ogram
through the system. Approximately 1.7 mg (2 pl) of crambe oil was added to a tared aluminum balance pan. The pan was then placed in the furnace a t room temperature and the exact sample weight was determined. After centering the recorder pen on the chart paper, the temperature was increased to 70 "C and held a t this temperature to remove any trace of solvent that might be present. As soon as the base line had stabilized, indicating that any volatile material had been removed, the sample was heated a t 5 "C/min to 200-250 "C. Temperatures obtained from these scans were corrected using the calibration curve. The 5 OC/min heating rate was selected because it was the fastest program that provided adequate differentiation. An arbitrary chart speed of 0.5 in./min produced acceptable curves. Normally a sample gained weight at first, then finally lost enough weight a t 200-250 "C so that the recorder pen was no longer on the scale of the chart paper. At this time, the scanning rate was increased to the maximum of 80 OC/min and kept at this rate until the furnace reached a temperature of 500 "C to thermally clean the pan. The sample oxidized rapidly a t this temperature; the furnace was held a t 500 "C until the tare of the aluminum sample pan was within 1-2 wg of its previous weight. The apparatus was then cooled in preparation for the next analysis. After a number of samples were run, the sample pan gradually accumulated a weighable residue and occasionally either had to be cleaned with acetone or replaced. Conjugated unsaturation was determined by ultraviolet (UV) absorbance a t 233 nm. Sample absorbances were measured in spectral-grade cyclohexane solution in a 1-cm cell using a Beckman DK-PA spectrophotometer. The absorbance of a 1% solution was calculated for comparison with the thermogravimetric data. Crambe oil samples of different ages were selected to represent the widest possible range of sample condition. Three of the freshest oils were prepared in our pilot plant during December 1972. Sample A was solvent extracted (hexane was used for this and all other solvent extracted samples) while C and D were both press oils. Sample B was commercial oil obtained from Ashland Chemical Co., Mapleton, Ill., September 1971. The oldest oil, sample E, was a commercially refined and bleached oil prepared in February 1964 via a mixed press-solvent process. I t was kept in refrigerated storage under nitrogen and proved to be in remarkably good condition considering its age. Sample F was a commercially prepared crude degummed crambe oil produced in October 1972. I t was a rather dark oil and was treated in the laboratory with activated carbon to improve its color. The most deteriorated oil, sample I, was a refined and bleached oil produced commercially in November 1965 by a mixed press and solvent extraction process. It had been stored at ambient temperature and was slightly rancid as judged by its odor. Sample G was from this same production run but had been carefully kept under nitrogen in refrigerated storage. A 1:l mixture of these two oil samples was prepared for use as sample H.
T,,
Tt'
cc
196.5 189.5 189 178.5 181.5 169 175.5 171.5 161.5
I\eight (lain,
0.09 0.14 0.23 0.35 0.46 0.24 0.59 0.73
0.67
A?33d
2.1 2.1 1.8 1.6 3.7 3.5 6.3 9.6 13.4
Sample mixtures of A and I 4
87.5 75
50 37.5 25 12.5
S" 9 5 5 Confidence limits
189.5 182 170 158.5 149.5 137.5 1.9 2.3
196.5 191 182 177 173.5 168.5 1.4
1.6
0.10 0.19 0.28 0.44 0.53 0.64 0.035 0.042
3.6 4.9 7.5 9.5
11.1 12.2
0 Each value is the mean of three determinations. b Initiation temperature. Transition temperature. d Absorbance of a 1% solution at 233 nm. e Standard deviation. Calculated from pooled variance for all sample and mixture replicates.
the temperature is increasing; in these other methods only time changes. We call point Ti the "initiation temperature"; it is that temperature a t which the rate of oxidation increases rapidly as shown by a gain in weight. This change is similar to the weight gain that signals the end of the "induction period" in isothermal methods. T t is the "transition temperature," the point of maximum gain in sample weight. As temperature increases beyond T t , the sample continually loses weight until the recorder pen is no longer on scale ( E )and the run is finished. The distance "C" measures weight gain in micrograms, and the slope of the line between Ti and Tt is a measure, in ,ug/"C, of the rate of reaction of sample with oxygen in excess of weight loss through volatilization of oxidation products. A characteristic volatilization rate can be determined from the slope of the line between T t and D. With crambe oil, best precision was realized with samples between 1.5 and 2.0 mg. Samples less than 1.5 mg did not completely coat the pan and samples larger than 2.5 mg showed less than optimum percentage weight gains. To facilitate reaction with oxygen, a high surface area-to-weight ratio is desirable. All of the samples listed in Table I weighed between 1.7 and 1.8 mg. Sample volatilization could affect Ti and T t if it causes excessive weight loss before 200 "C. To test this possibility, samples of D and I were heated a t 5 "C/min in nitrogen instead of oxygen. At 200 "C, sample D had lost only 0.2% of its weight and sample I, 0.5%. Boiling points determined on samples D and I in sealed pans with pin-holes were 404 and 403 "C, respectively. Neither the volatility nor boiling point of crambe oil is affected much by sample age nor do these properties have an appreciable effect on Ti or T t . UV absorbance a t 233 nm ( A 2 3 3 ) measures the amount of conjugated unsaturation present in an oil. I t also corre-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1 4 , DECEMBER 1974
and Tt. For such an oil, which we have yet,to see, the point D in Figure 1 would seem to be a most important parame-
100-
906 8OF 10C
- 600
50. w
-=
40+ 3020101
:IO
-/e'
120
~
L
130
140
/:,;.'
1
,
150
160
_
Temperature
_
110
180
190
200
"C
Figure 2. Initiation ( T,) and transition ( Tt) temperature vs mixtures of samples A and I (Table I)
lates well with peroxide value (6, 1 3 ) and is an easy way to obtain some indication of an oil's condition. Unfortunately, A 2 3 3 measures only a few of many possible oxidation products present in a degraded oil. Further, A233 and peroxide value only reflect the condition of the sample a t the time of measurement. They give no direct information as to how rapidly the sample might deteriorate. Other methods of determining oil stability, such as the AOM, empirically measure the resistance of a sample to oxidation and thus give an indication as to how long the sample will remain usable in the future. Our results suggest that the thermogravimetric procedure has potential as a means of estimating both the condition of an oil and its ability to withstand further oxidation. The samples in Table I are arranged chronologically with the most recently produced oils a t the top of the table; A 2 3 3 values roughly reflect this order. Sample mixtures in the lower half of Table I simulate sample aging by using known combinations of the oldest oil with the most recent oil. T Iand Tt for these mixtures are plotted in Figure 2 us. the amounts of the two samples in the various mixtures. Mathematical analyses reveal that second-order equations best fit the data for both parameters. T I covers a wider temperature range than T t and thus is the more sensitive indicator of a sample's oxidation state. Statistical data a t the bottom of Table I indicate that T t has less variation with sample replication than T,. It is easy to read the value of Tt since it is measured a t the peak of the weight-temperature curve. T I is more difficult to locate precisely and thus shows a little more variation. Fresher oils gained less weight. On extrapolating from this result, an ideally fresh oil would apparently show no gain in weight, and it would thus be impossible to locate T I
ter. This hypothetically fresh oil can be envisioned as having a temperature value located at the intersection of curves Ti and T t in Figure 2. In other words, the points Ti, T t , and D coincide for the limiting value of "freshness." This value, calculated from the equations for curves Ti and T t in Figure 2, is 203 "C for crambe oil. Mixtures of other samples may generate curve segments different from those of Figure 2, but we could expect that for crambe oil, Ti and T t curves would intersect a t the same limiting value. Sample A, our freshest sample, crossed the extrapolated base line (point D, Figure 1) at 199 "C; older samples have lower values. Ti is still the most useful parameter except for extremely fresh oil. Three months prior to obtaining the data summarized in Table I, samples D and I were analyzed for Ti, T t , per cent weight gain, and A233, respectively, with the following results: sample D, 189 "C, 195 "C, 0.27%, and 1.6; sample I, 129 "C, 165 "C, 0.75%, and 11.0. Thermogravimetric data for both samples definitely indicate sample deterioration when compared to the determinations 3 months later (Table I). A233 values suggest that sample I had degraded, but this parameter gives no indication that sample D had changed. From these limited data, we are led to believe that the thermogravimetric parameters, especially Ti,are more sensitive indicators of sample condition than A 2 3 3 values. We believe our thermogravimetric method, by empirically measuring an oil's resistance to oxidation, provides data which can be useful for estimating the relative keeping qualities of different samples of the same oil. Proof that the method has merit for use with other types is needed as is a thorough comparison with accepted methods such as the AOM (8) or the oxygen bomb method (9).The most difficult problem, of course, is how to conclusively prove the validity of any method for predicting oil stability!
ACKNOWLEDGMENT The authors appreciated the assistance and encouragement of C. D. Evans and G. R. List during the preliminary stages of this work. We are indebted to W. F. Kwolek for help with the statistical analyses.
RECEIVEDfor review June 27, 1974. Accepted August 26, 1974. The mention of firm names or trade products does not imply that they are endorsed or recommended by the Department of Agriculture over other firms or similar products not mentioned.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
2217
