Improved Techniques for Corrosive Fluoride Gas Chromatography W. S . Pappas and J. @. Million Laboratory Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Corp., Nuclear Division, Oak Ridge, Tenn. Several new techniques are described which broaden the applicability of gas chromatography to corrosive fluoride gases. Columns prepared with Teflon-6 support have been applied. A new column utilizing fluorocarbon oil instead of the conventional chlorofluorocarbon oil as the liquid phase is described, with application to inorganic and organic fluoride compounds. The loss of small amounts of certain reactive fluoride gases is found to be caused principally by sorption on system surfaces, rather than on the column packing. Replacement of metal tubing with Teflon tubing reduces sorption losses, markedly improving detectability limits for these constituents. A two-step column technique speeds up analysis, without fraction storage, when late elutors are present.
APPLICATION of gas chromatography to reactive fluorine compounds is limited in the choice of suitably resistant column materials. Columns serviceable with such compounds have been restricted principally to chlorotrifluoroethylene (Kel-F, Halocarbon, MFL, Fluorothene, or Hostaflon) oils on Kel-F or Teflon supports in metal tubing (1-6). For some constituents, poor resolution, nonlinear response curves, and unexplained losses are observed. This report describes several recent innovations made during application of corrosive gas chromatography to a wide range of problems. These include a new column based on fluorocarbon oil providing an alternate for the chlorofluorocarbon oil previously used, techniques for more sensitive analysis of reactive fluoride compounds, and a steppedcolumn arrangement which speeds up an analysis without storage of fractions. Specially designed corrosive gas chromatographs (4, 9 constructed of fluoride-resistant materials and utilizing a brass gas-density detector were employed in the studies. All GLC columns were prepared with 40-60 mesh Teflon-6. The columns were prepared by the method recommended by Kirkland (8), except that the volatile solvent, F-113, was removed by vacuum distillation using a rotary evaporator. FLUOROCARBON LIQUID PHASE
Columns prepared with perfluoroalkane oil (approximate formula, CZ1Fd4) (9)have been applied to inorganic and organic fluorine compounds. This provides a suitable alternative to
I- c - c --I FLUOROCARBON KEL-F Figure 1. Structure of two oils the chlorofluorocarbon (Kel-F type) oils conventionally used for reactive fluorine compounds. Chemical structures of the two oils, the fluorocarbon ( C S ~ Fand ~ ~ the ) Kel-F oils, are compared in Figure 1 ;the Ktl-F monomer is characterized by the presence of a chlorine atom in place of a fluorine atom. One necessary property of a satisfactory liquid phase is that it is essentially nonvolatile at the column operating temperature. The useful temperature range for fluorocarbon oil is comparable to that of the popular Kel-F No. 10 oil. Thermogravimetric scans, Figure 2, for the two oils indicate an upper temperature limit of about 100 “C for both oils before loss occurs. DTA scans, Figure 3, show approximately the same boiling point range for the fluorocarbon and Kel-F No. 10 oil, further demonstrating the similar thermal-volatility characteristics of the two oils. Another requirement of a liquid phase is that it be inert toward the sample constituents. Experience indicates that columns prepared with the fluorocarbon liquid are stable to fluorine and reactive fluoride compounds, after pretreatment with chlorine trifluoride to remove moisture. HALOCARBON COOLANT Ah’ALYSIS Halocarbon coolants are employed in heat exchangers to remove the heat of compression in the gaseous diffusion plants of the U. S. Atomic Energy Commission. Efficient operation of gaseous diffusion systems requires the inline measurement of halocarbon coolants that may leak into the uranium hexafluoride process gas stream. The coolants are C-114 (dichlorotetrafluoroethane), C-437 (trichloroheptafluorobutane), and (2-816 (perfluorodimethyl cyclohexane). HEATING RATE: 20 “Cimio.
(1) J. F. Ellis and G. Iveson, “Gas Chromatography 1958,” D. H.
Desty, Ed., Butterworths, London, 1958, pp 300-9. (2) A. G. Hamlin. G. Iveson. and T. R. Phillios. ANAL.CHEM., 35, 2037 (1963). (3) I. Lvsvi and P. R. Newton. ibid.. 35.90 (1963). 3. G. Million, C. W. Weber, and P.R. Kuehn, U.S. At. Energy Commission Rept. K-1639 (1966). (5) E. L. Williamson, C. M. Johnson, T. J. Mayo, and W. R. Rossmassler, ibid., KY-485 (1965). (6) V. H. Dayan and B. C. Neale, “Advanced Propellant Chemistry,” R. F. Gould, Ed., ACS Publications, Washington, 1966, pp 223-30. (7) J. G. Million, W. S. Pappas, and C. W. Weber, 155th National Meeting, ACS, San Francisco, April 1968. (8) J. J. Kirkland, ANAL.CHEW,35,2003 (1963). (9) L. Spiegler, “Preparation, Properties, and Technology of Fluorine and Organic Fluorine Compounds,” C. Slesser, Ed., NNES VII-I, McGraw-Hill, New Yosk, 1951, p 492. \
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Figure 3. Differential thermograms for liquid phases Figure 4 compares the separations provided by a Mel-F No. 10 oil column and a fluorocarbon oil column for the coolants and uranium hexafluoride. The significant finding here is the reversal of the order of elution of C-816 and C-437 from the two columns. The chlorofluoro-coolant has a higher affinity, Figure 5, for the chlorofluoro column while the fluoro-coolant shows more affinity for the fluorocarbon column. The two compounds are in a different chemical series ; however, their boiling points are fairly close, 102 "C for C-816 (10) and 99 "Cfor C-437 (11). FLUORINATINGAGENTS FOR URAMUh8 FUEL RECOVERY
Several concepts to recover uranium from spent reactor fuels by fluoride volatility methods are under study (12). Some approaches involve fluorination to uranium hexafluoride with fluorine, directly or after conversion of oxidic fuels to lower fluorides with hydrogen fluoride. In one process the uranous oxide is converted to urano-uranic oxide (U,Oa) prior to the fluoride volatility step. Bromine pentafluoride and chlorine trifluoride are being studied for use as fluorinating agents in an effort to separate uranium from plutonium by selective fluorination of uranium with the interhalogen, plutonium being recovered as plutonium hexafluoride in a subsequent step by fluorination with fluorine (12). Gas chromatographic techniques are being developed for volatility process analysis and control. Constituents of particular interest are bromine pentafluoride, chlorine tri(10) Ibid., p 484. (11) W. T. Miller, Jr., ibid., p 673. (12) J. J. Barghusen and W. J. Mecham, Reactor Fuel Processing, 9,157-64, 168, Summer 1966.
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Figure 4. Separation of coolants and uranium hexafluoride 10-foot by 1/4-insh 0.d. columns; 20% oil; 60 "C; Air carrier, 80 ml per minute
fluoride, fluorine, hydrogen fluoride, uranium hexafluoride, and reaction products. The Analytical Loss Problem. In the chromatographic determination of certain reactive fluoride gases using halocarbon columns in nickel tubing, a loss of small amounts of the reactive gas is customarily observed. Calibration curves have been nonlinear, Figure 6, and detection limits relatively high. Peaks are characteristically skewed, and elution times are concentration-dependent. Prolonged and repeated treatment of the column with the pure constituent gives only a temporary improvement in detection limit. Different degrees of losses have been observed with columns seemingly consisting of the same materials and having the same history. The peak magnitude for a given fluoride sample is a function of the column's immediate history of exposure to the fluoride gas. Point A (above the curve) of Figure 6 was obtained shortly after a larger sample, while point B (below the curve) was obtained after a lapse of several minutes in the sampling sequence. Point C corresponds to the value obtained after several repetitive determinations of the same sized sample. Sample loss is also observed by peak area measurements. No signal is observed for samples containing less than 3 pmoles of chlorine trifluoride, 0.8 pmole of bromine pentafluoride, or 18 pmoles of hydrogen fluoride. CHLORINE TRIFLUBRIDE.For chlorine trifluoride the loss problem is virtually eliminated by simply replacing the nickel tubing with Teflon PTFE tubing. Figure 7 shows the marked improvement obtained. The new calibration curve (at 40 "C)
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VOL. 40, NO. 14, DECEMBER 1968
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Figure 6. Halocarbon column in nickel tubing
is linear with a “zero” intercept; peaks are more symmetrical; elution time is fixed; and the lower detection limit is at least 10-fold better. Apparently the problem was caused by chemisorption of small amounts of chlorine trifluoride by the nickel fluoride surfaces of the column tubing. BROMINE PENTAFLUORIDE. Similar comparative results, Figure 8, were found with bromine pentafluoride; however, the loss was not as great as that found for chlorine trifluoride. HYDROGEN FLUORIDE. The high affinity of hydrogen fluoride toward almost any surface is well known. Initial samples are often completely sorbed prior to reaching the detector. Pretreatment with large samples of pure hydrogen fluoride ( 4 ) or use of a carrier gas spiked with hydrogen fluoride (5) are previous methods used to attack this problem. Figure 9 shows calibration curves obtained for hydrogen fluoride with halocarbon columns in nickel and Teflon tubing by the repetitive sampling technique. Detection limits are much better with the Teflon tubing. The loss still found with the Teflon-tubed column may be caused by sorption on metal fluoride surfaces of the sampling valve, the connecting nickel tubing, or the brass gas-density detector. When conducting the tests, the nickel-tubed column was examined first, after which this column was quickly replaced with the Teflon-tubed column. Without any pretreatment, an initial 25-pmole sample of hydrogen fluoride, using the untreated Teflon column, gave the same response as 21
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Figure 7. Comparison of halocarbon columns in Teflon and nickel tubing
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pmoles gave after treatment with hydrogen fluoride, indicating a loss within the untreated Teflon-tubed column of only 4 pmoles. Results of the following experiment, Figure 10, indicate that most of the loss of hydrogen fluoride occurred at surfaces separate from and downstream of the Teflon column. Samples of hydrogen fluoride run prior to the chlorine trifluoride drying treatment gave the normal negative peak, using dry air carrier with the gas-density detector; however, the first hydrogen fluoride sample injected after the chlorine trifluoride elution gave a positive peak at the hydrogen fluoride peak position. Chemisorbed chlorine trifluoride, downstream of the column, was apparently displaced by hydrogen fluoride which forms a stronger complex with the fluoride surface. If chlorine trifluoride had been displaced from within the column, it would have been retained longer by chromatographic action of the column. These experimental results suggest that a hydrogen fluoride-spiked carrier might minimize adsorption of other fluoride compounds in the fashion that water has been used to reduce adsorption of other polar compounds (13). In an effort to eliminate completely the loss problem, an allplastic system, principally of Teflon, is currently being designed, including the detector. (13) H. S. Knight, ANAL.CHEW,30,2030 (1958).
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Figure 8. Calibration curves for bromine pentafluoride in halocarbon columns
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Figure 9. Calibration curves for hydrogen fluoride in halocarbon columns
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FLUORINE. The loss problem has not been observed with fluorine. For fluorine analysis in the presence of permanent gases, the salt converter technique (14, 15) has proven satisfactory. In this method fluorine is converted to chlorine by a pre-column sodium chloride reactor ; the chlorine is easily separated chromatographically from the permanent gases. Bromine Pentafluoride System. In the evaluation of fluorinating agents for the uranium volatility process, bromine pentafluoride is reduced principally to bromine rather than to bromine trifluoride (12). Therefore, the major analytical emphasis in off-gases is in determining bromine, bromine pentafluoride, and uranium hexafluoride. Chromatographic separation provided by a fluorocarbon oil column is compared with that provided by Kel-F No. 10 oil in Figure 11. Fluorocarbon oil provides good resolution of all of the constituents in about 10 minutes, whereas no useful separation is effected with Kel-F oil in that period, in tests made at 50 "C. Furthermore, at 50 "C, Kel-F oil poorly resolves bromine from bromine pentafluoride even with a longer column in twice the time. At 40 "C, some separation was found, but it was still not analytically useful. Therefore, further studies were confined to the fluorocarbon column. CALIBRATION. Figure 12 shows calibration curves for bromine, bromine pentafluoride, and uranium hexafluoride. With Teflon tubing, all are straight lines with ''zero" intercepts. Chlorine Fluorides System. The reaction of excess chlorine trifluoride at 40 "C with dehydrated U308 yields uranium hexafluoride, chlorine monofluoride, chloryl fluoride (CIOS), and to a smaller degree, chlorine. For a simulated off-gas
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Figure 11. Separation of bromine fluorides and uranium hexafluoride
mixture, the separation provided by the fluorocarbon column is compared to that with a Kel-F oil column in Figure 13. Elution orders are generally the same; however, chloryl fluoride is not resolved from chlorine trifluoride by the fluorocarbon column. A range of conditions and longer times still did not provide useful separation on the fluorocarbon oil; therefore, the Kel-F oil column is the best choice here. Of general interest in the volatility fuel recovery field is the relatively new compound, chlorine pentafluoride (12, 16). In a single test, a separation factor of 1.14 was found for chlorine pentafluoride relative to chlorine trifluoride on a column of 2075 Kel-F No. 10 oil on Teflon-6 at 40 "C. TWO-STEPCOLUMN.Uranium hexafluoride is eluted several minutes after chlorine trifluoride in the G C examination of these off-gases. A two-step column technique, Figure 14, has been devised which speeds the analysis for late elutors, (16) D. F. Smith, Science, 141, 1039 (1963).
(14) C. W. Weber and 0.H. Howard, ANALCHEM., 35,1002(1963). (15) 0. Rochefort, Anal. C'hitn. Acta, 29, 350-7 (1963).
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Figure 12. Calibration curves for bromine pentafluoride, bromine, and uranium hexafluoride
17-foot, 1/4-inch 0.d. columns; 20% oil on Teflon-6; nickel tubing Fluorocarbon oil column: 28" C; Air Carrier, 75 ml per minute Kel-F No. 10 oil column: 3.2 "C; Air Carrier, 60 ml per minute VOL. 40, NO. 14, DECEMBER 1968
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Figure 15. Chromatogram with sodium fluoride column
Figure 14. column
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Chromatogram of a “two-step”
20 % Kel-P No. 10 oil on Teflon-6 at 40 “C with 85 ml per minute of air (initial); nickel tubing Step 1 column: 27-foot by ‘/4-inch 0.d. Step 2 column: 10-foot by ‘/&-inch0.d.
without storing the marginally-resolved early fractions. Shortening of the column immediately after elution of chlorine trifluoride liberates the slower moving uranium hexafluoride, reducing the total analytical time to about one third. This method eliminates separation losses which occur in fraction storage associated with a similar technique reported previously (2). The chromatogram of Figure 14 was obtained with nickeltubed columns. On comparing later results obtained with Teflon-tubed columns, it was inferred that the nickel fluoride surface, while causing the loss of some chlorine trifluoride, contributed to the resolution of the two close peaks, chloryl fluoride and chlorine trifluoride. To obtain the equivalent separation with the Teflon-tubed column required about twice the elution time. CHEMISORPTION COLUMN.Further studies of the chemisorption phenomenon indicated that a column using sodium fluoride as a solid phase might delay the chlorine trifluoride constituent in preference to the reaction products of the vola-
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tility process. Brief tests were conducted with a 10-foot by I/k-inch 0.d. column packed with Harshaw technical grade I/*- X I/*-inch pellets. Chlorine, chlorine monofluoride, and chloryl fluoride are eluted early, Figure 15, with other nonretained gases. Uranium hexafluoride and hydrogen fluoride off-gases are fixed as double salts and retained by the column. Chlorine trifluoride, therefore, is the only component transiently detailed by the column and it is resolved from the “composite impurities” peak in about 2 minutes. Although the sodium fluoride colmnn shows promise in providing a more specific analysis for chlorine trifluoride without significant loss, further work is required. Divalent metal fluorides tested, such as magnesium and copper, indicate some loss of the chlorine trifluoride sample. The authors are examining the possibility of spiking a halocarbon column with a metal fluoride (one which does not irreversibly complex any of the sample constituents) in an effort to improve the separation of chloryl fluoride from chlorine trifluoride. ACKNOWLEDGMENT
The authors acknowledge the encouragement of C. W. Weber in this program and his helpful comments concerning the manuscript. R. D. Rivers and S. B. Woodfin assisted in the experimental work.
RECEIVED for review April 22, 1968. Accepted August 30, 1968. Work performed at the Oak Ridge Gaseous Diffusion Plant operated by Union Carbide Corp., Nuclear Division, for the U. S. Atomic Energy Commission. Presented at the 155th National Meeting, ACS, San Francisco, March 31April 5, 1968.
