R
I
95 A2*
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90 c .
I
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-75-
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-70-
-70-
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-55-
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Figure 17. Successive blank recordings for AI 3092-A line with and without CC14 vapor introduction (see text)
tem, but no methane was introduced during subsequent use. The purification effect for aluminum by the introduction of carbon tetrachloride vapor (as described earlier) is illustrated in the recorder tracings shown in Figure 17. For each run, the furnace was switched on for 12 sec and a maximum temperature of 2640 "C was reached. In the first series (AI-A~),without carbon tetrachloride vapor but after the methane pyrolysis treatment, a large blank signal is observed for aluminum which decreases only slowly with successive heatings. In the next series (B1-B4), a very small amount of carbon tetrachloride vapor was introduced and the blank signals decrease at a much faster rate. The peaks before and after the aluminum peak are from smoke forma-
tion by the carbon tetrachloride during the heating and cooling periods. When the tube is now heated again without carbon tetrachloride (Ci-C,) the signals are initially higher than that of the last treatment (B4), but less than Ai-&, and decrease more rapidly. This illustrates that the generated chlorine promotes removal of the aluminum in a chloride form but also depresses its dissociation. With another tube having a high blank, similar to AI-A~, carbon tetrachloride a t a higher concentration is added (D1 and Dz) and the aluminum signal is absent on successive heating (El and Ez), indicating complete purification. The slightly negative peaks observed are due to an optical effect caused by the expansion of this rather long tube. These experiments were repeated for titanium and a similar purification phenomenon was observed with the use of carbon tetrachloride. Sensitivities (1%absorption) of 0.001 pglml and 0.06 pg/ml were found for aluminum and titanium, respectively, using 25 pl samples and a 2640 "C tube wall temperature. For aluminum, 25 p1 of silver fluoride solution was added separately as a fluorinating agent (100 pg Ag/ml) and the sensitivity increased 1.4-fold. For titanium, 25 p1 of calcium fluoride (30) (saturated solution) was added separately, resulting in a sensitivity increase of 1.3-fold. Using silver fluoride, no enhancement was observed for titanium, presumably due to the low decomposition temperature of silver fluoride. RECEIVEDfor review February 11, 1974. Accepted August 23,1974. Presented in part a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1974. This was was supported in part by the Robert A. Welch Foundation and in part by the National Science Foundation. (30) J. Szava and K. Szava-Benocs, Banyasz, Kut. Intez. Kozlern., 11, 339 (1967); Chern. Abstr., 69, 1 0 8 4 6 3 ~ (1968).
NOTES
Pyrolysis Treatment for Graphite Atomization Systems S. A. Clyburn,' Tibor Kantor,2 and Claude Veillon3 Department
of Chemistry, University of Houston, Houston, Texas 77004
By introducing a mixture of methane and an inert gas into graphite atomization systems, and operating the heated element at temperatures above about 2000 "C, a layer of pyrolytic graphite is deposited on the element surfaces. This layer is very dense, hard, impermeable to gases, nonporous and resistant to oxidation. For graphite atomizers operated intermittently, such as the conventional rod and Pressent address, Varian Instrument Div., Los Altos, Calif. 94022.
Present address, Institute for General and Analytical Chemistry, Technical University, Budapest 1111,Hungary. ,3 Present address, Biophysics Research Laboratory, Harvard Medical School, Peter Bent Brigham Hospital, Boston, Mass. 0211 5 . Author to whom reprint requests should be sent.
tube systems using discrete, small samples, the useful heated element lifetime is greatly extended. Since the process can be repeated many times to restore the heated element, the lifetime becomes relatively indefinite. For graphite atomization systems operated continuously at high temperature, the gradual deterioration of the heated element can be eliminated completely by the addition of a small amount of methane to the inert gas fed into the system. This pyrolysis treatment can be performed continuously, by introducing a low flow of methane which just compensates for the deterioration. This is especially important in graphite atomization systems used continuously with continuous sample introduction. Here the methane just com-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
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MO
280
260
5
24
220
200
Figure 1. Effect of methane on furnace current at constant voltage (see text for explanation of curves)
pensates for the element deterioration caused (primarily) by the residual water vapor introduced along with the desolvated sample aerosol. This technique has been used very successfully by Kantor e t al. ( I ) in atomic absorption spectrometry, and by Clyburn e t al. (2) in atomic fluorescence spectrometry.
EXPERIMENTAL Apparatus. Two graphite atomization systems were utilized, both employing continuous sample introduction. One system was used for atomic absorption measurements with both continuous and discrete sample introduction and is described in reference ( 1 ) . The other atomization system was used for atomic fluorescence measurements and is described in reference (2). Desolvated sample aerosol was introduced into the atomizers from a Veillon-Margoshes system ( 3 ) .Gas flow rates were monitored with rotametertype flowmeters calibrated and used according to the procedures described by Veillon and Park ( 4 ) . Temperature Measurements. Temperatures were measured over the 0-1300 "C range with a chromel-alumel thermocouple connected to a sensitive, multi-range electrometer (Model 150A, Keithley Instruments, Cleveland, Ohio),and over the 800-2900 "C range with an optical pyrometer (Model 8632-C, Leeds and Northrup, Philadelphia, Pa.). Gases. The atomization system (and sample introduction system) used for the atomic fluorescence measurements (2) was operated exclusively with argon, while the other system used for atomic absorption (I) was operated with either nitrogen or argon. The argon and nitrogen had specified purities of 99.99% and 99.9~6,respectively. Initially, bottled methane (99%)was used for the pyrolysis treatment. Identical results were obtained when the natural gas piped into the laboratory was used. This natural gas is approximately 96% methane (the balance consists mostly of ethane, propane, COz, and N2), and was used for all subsequent measurements and investigations. RESULTS AND DISCUSSION When the atomization system used in reference ( I ) was first developed (51, ordinary graphite was used for the heated element. In this particular design, the porosity of the graphite was not a problem from the standpoint of sample loss, but the relatively rapid oxidation caused by the residual water vapor from the sample introduction system was. This oxidation was perhaps aided by the porosity. After only a few hours of operation, the tube element had deteriorated, and its electrical characteristics had begun to drift noticeably after only minutes of operation a t high temperatures. When methane was introduced into the furnace along with the argon and sample aerosol, and the tube held a t a temperature above about 2000 "C, a hard, nonporous layer of pyrolytic graphite built up on the tube sur(1)T. Kantor, S.A. Clyburn. and C. Veillon, Anal. Chem., 46,2205 (1974). (2)S.A. Clyburn, E. R. Bartschmid, and C. Veiiion, Anal. Chem., 46, 2201 I 1 9741. (3)C.~Veilonand M. Margoshes, Spectrochim. Acta, Part B, 23, 553 (1968). (4) C. Veillon and J. Y. Park, Anal. Chem., 42, 684 (1970). (5)M. K. Murphy, S. A. Clyburn, and C. Veillon, Anal. Chem., 45, 1468 (1973).
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face. Following this treatment, the tube was far more resistant to attack and the useful lifetime was extended severalfold. When noticeable deterioration did again occur, the treatment could be repeated and the element restored to its original temperature-voltage-current characteristics. As an additional benefit for this particular furnace design, the tube gradually assumed the proper shape for a long, uniform-temperature, heated zone as the process was repeated several times. One tube was even intentionally broken and the two halves were placed in the furnace and literally welded back together by the pyrolytic coating. The pyrolytic coating formed a t wall temperatures of about 2000 "C and above, which agrees with the data reported in the book by L'vov (6). For both atomization systems (1, 21, when continuous sample introduction is used, the residual water vapor from the sample introduction system ( 3 ) slowly oxidizes the heated element. While the pyrolytic coating is far more resistant to this attack, its lifetime before the characteristics of the heated element begin to change noticeably is still finite. By adding a low flow of methane to the furnace continuously during operation, this gradual oxidation can be offset by the pyrolysis. This is best illustrated by the data shown in Figure 1. These data are for 4 different graphite elements machined to approximately the same initial dimensions, and are for the application of the same constant voltage (5 V) across the leads in each case. The initial temperature is about 2100 "C. Curve A represents the case where no methane is introduced. After about 10 minutes, the current begins to drop, because of a decrease in the cross-sectional area of the graphite element. Curve B represents the case where methane is introduced a t a constant flow rate. The rate a t which pyrolysis is occurring is slightly greater than the rate of oxidation by the residual water vapor, and the current gradually increases because of the increase in the cross-sectional area of the heated element. Curve C is similar to R, except that the methane flow is too low. In Curve D, the methane flow is just sufficient to compensate for the oxidation, and the electrical characteristics (and temperature) exhibit no change over the 2-hour period. Actually, this condition has been maintained over a 30-hour period (3 consecutive 10-hour runs) with no change in the voltagecurrent-temperature characteristics. At that point, the test was discontinued. Some conditions must be met for this extremely longterm stability. The solution aspiration rate, gas flow rates, spray chamber temperature, and condenser cooling water temperature must all remain constant. The last is perhaps most important since the water vapor content of the gas is determined by the saturation vapor pressure a t the temperature of the gas issuing from the sample introduction system. With an argon flow rate of 2.25 l./min, a solution aspiration rate of 2.72 ml/min, a chamber heat input of 288 W and the condenser cooling water a t 15 "C, the exiting gas temperature was about 26 "C. The vapor pressure of water a t this temperature is about 26 Torr, and the methane flow rate necessary to maintain the furnace a t equilibrium was just equal to that required to react with this residual water vapor. The presence of oxidants in the sample solution, such as nitrates, requires a slightly higher methane flow to compensate for this additional oxidation. In practice, the system is very stable and slight drifts in the characteristics due to variations in the parameters are easily compensated for by slight adjustments to the methane flow. No adverse effects due to the introduction of this small (6) B. V. L'vov. "Atomic Absorption Spectrochemical Analysis," Adam Hilger, London, 1970,p 206.
DECEMBER 1974
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
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