Separation of Volatile Fluorides by a Combination of Transpiration and Gas Chromatographic Techniques Edgars Rudzitis Argonne National Laboratory, 9700 South Cass Acenue, Argonne, Ill.
I N CONNECTION with calorimetric studies of fluorine compounds, it became necessary to separate mixtures of gaseous inorganic fluorides and t o determine detection limits for the components. For this purpose a separation concept, previously reported by Cady and Siegwarth ( I ) , has been used in which various features of fractional volatilization and gas chromatography are combined. A n experimental technique has been developed which enables the separation of mixed volatile fluorides as well as determination of their limits of detection. The main advantages of the technique are (a) no involved column preparations are needed, and (b) a n allmetal system is used, which is ideally suited to handling corrosive gases. EXPERIMENTAL Apparatus. The all-metal system, except for some valve parts made of fluorothene, consisted of a helium source in series with two sample cells, a separation column, a detector, traps, and a flowmeter. The components were connected by l/s-inch 0.d. copper tubing and Whitney No. 8 valves. The sampling system consisted of two separately chargeable cells of 0.2- and 15-ml volume. This arrangement enabled the preparation of gas mixtures of less than lop4 mole for calibration purposes in ratios as low as 1 part in lo5 while still remaining in a conveniently measurable pressure range of between 0.1 and 100 torr. The description of the apparatus which is shown in Figure 1 is as follows. The separation column was made of 1/4-inch 0.d. copper tubing, originally 180 cm in length. The tubing was filled with 10-20 mesh copper shot and formed into a spiral 30 cm long by 5 cm in diameter. The lower end of the spiral was connected t o the detector, a Gow-Mac 470 Series 30C micro thermal conductivity cell. The power supply and bridge control unit, Gow-Mac Model 999-Dl, were operated a t 9 mA, and the output was recorded in the 1-mV scale of a two-pen potentiometric recorder. Downstream from the detector were several U-shaped traps made of 1/8-inch 0.d. nickel tubing. A helium pressure of approximately 900 torr was maintained in the column, its flow being regulated by a valve which preceded the detector. Commercial He, nominal purity 99.9 was used without further purification. The temperature along the column was monitored by five iron-constantan thermocouples a t approximately 30-cm intervals and was recorded on a multipoint recorder. The temperature of the thermocouple a t the lowest point of the spiral served as the reference temperature and was recorded (10-mV scale) together with the detector output. Procedure. The helium flow rate is adjusted t o approximately 10 ml/minute. (When high sensitivity is required, a flow rate of -5 ml/minute is advisable; when a peak tends t o overload the detector, a high rate of flow (>>lo ml/minute) should be used for a few minutes.) With the output valve closed (to avoid back-suction of air), the column is placed in a close-fitting Dewar vessel and cooled for approximately 30 minutes with the lower one fourth of the column submerged in liquid nitrogen. During this time the sample cell(s) are
z,
(1) G. H. Cady and D. P. Siegwarth, ANAL.CHEM.,31, 618 (1959).
RECCR3ER
Figure 1.
Gas separation apparatus
valved off from the rest of the system, evacuated, and filled with gas(es) to be analyzed. After temperature equilibration has been achieved, the helium flow is restored by opening all the valves, the liquid nitrogen is dumped, and the cold Dewar vessel is replaced. A t this time, typical thermocouple readings along the column, beginning with the reference thermocouple, are -195", -180", -160", -120" and -75" C . As the column gradually and uniformly warms up a t approximately 1" C per minute, the gases, which were condensed along the column according t o their vapor pressures, repeatedly volatilize and condense as the helium sweeps them into the colder (lower) portion of the column. This results in fractionation of the gases, the fractions finally being swept past the coldest point and into the detector. The component gases can be trapped for identification or recycle without interrupting the experiment. The Dewar vessels are placed around the U-traps, and the Dewar furthest from the detector is filled with liquid nitrogen. As soon as the most volatile component is swept past the detector, as indicated by a peak on the recorder, the next to the furthest Dewar is filled with liquid nitrogen in order t o condense the next most volatile component, and so on. When the experiment is completed, the traps are valved off in the same order. Because the separation column warms u p slowly, there is ample time for these operations. For example, with a PF3-PF5 mixture, the peaks appear approximately 15 minutes apart. RESULTS AND DISCUSSION
Because the separation is based on a continual condensation volatilization process which is solely dependent on temperature, it is convenient to use the term "appearance temperature" in the same manner that the term "retention volume" is employed in gas chromatography. Appearance temperature can be defined as the reference temperature at which the initial detector response to a gas is recorded. The peaks obtained are sharp, and the appearance temperatures well defined. The appearance temperature of selected gases, found with the above described apparatus, is given below (in "C): Ns, 0 2 , F2 NF3, CF4 Xe PF3 BF3, SiF4, C2F6 PF j
-195 - 175 - 170 - 160 -155 -145
c o z , SF6 SeF6, SF1 TeF6 POF3 S2Fio
-140 - 135 -130 -115 - 100
In connection with our calorimetric studies, the following mixtures were separated and the minor component (in parentheses) was detected a t a concentration of approximately VOL. 39, NO. 10, AUGUST 1967
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1 part in lo4 in a total amount of approximately mole: PFs-(PF3); CFd-(CzFc); PF3-PF6-(NF3); S F ~ - ( S ~ F I OXe); PF3-PF5-(POF3). Because of the proximity of appearance temperatures, the detection of SF, in the presence of SF6 was limited to approximately 1 part in 100. The component gases of a mixture can be identified by determination of the appearance temperature of the individual gases-Le., by calibration of the column-or by trapping out the components after they pass through the detector and analyzing them by infrared scan or other analytical means. The empirical nature of the method makes it difficult t o correlate exactly the appearance temperature with other variables such as temperature gradient, rate of warming, flow rate of helium, and length and geometry of the separation column. Also, the separations become increasingly difficult as the gap between appearance temperature narrows. Hence, no attempt was made to fix appearance temperatures more precisely than to the nearest five degrees. The vapor pressure of the substances a t their respective appearance temperatures, except those appearing a t -195" and -175" C, are below 1 mm and n o meaningful volatility correlation can be made.
However, the order of the appearance temperatures generally conforms with that of the respective boiling points. Although the appearance temperatures have no absolute meaning, it was found that their relative order, as listed above, remained unchanged even when other variables were changed by as much as a factor o r two. Further evidence of this unchanging relative order is the close agreement of the appearance temperature differentials for the two gases, CF4 and COZ,common to this work and that of Cady and Siegwarth (I); despite the marked dissimilarities (geometry of equipment, warm-up rate, etc.) of the two studies, these differentials were 35" C and 39" C, respectively. ACKNOWLEDGMENT
The author thanks Murray Barsky and Milton Ader for their cooperation and helpful discussions. RECEIVEDfor review March 31, 1967. Accepted May 22, 1967. This study was done under the auspices of the U. S. Atomic Energy Commission under Contract No. W-31-109eng-38 through the University of Chicago.
Separation and Determination of Aniline and the Toluidine, Xylidine, Ethylaniline, and N-Methyltoluidine Isomers by Gas Chromatography of Their N-Trifluoroacetyl Derivatives Ray A. Dove Research Dicision, The Goodyear Tire & Rubber Co., Akron, Ohio 44316
INDUSTRIALLY IMPORTANT ARYL AMINES derive from the nitration and subsequent reduction of appropriate hydrocarbons. Complexity of the final amine mixture turns o n the composition of the original hydrocarbon mixture and also the selectivity in the nitration step. It is not surprising, therefore, that commercial xylidine materials may contain, in addition to the six isomeric xylidines, varying levels of the three ethylanilines, the three toluidines, and also aniline. The method described below permits the separate estimation in a common mixture of 19 aryl amines including those mentioned above plus N-methylaniline, N,N-dimethylaniline, N-ethylaniline, and the three N-methyltoluidines. The amines are chromatographed as their N-trifluoroacetyl (TFA) derivatives in a single 70-minute run using a column prepared from a blend of Carbowax 20M and Apiezon-L. No procedure could be found in the literature having a comparable qualitative scope or the quantitative applicability of the one presented. Gas chromatography (GC) has been used relatively little in the analysis of complicated amine mixtures. Furthermore, GC (where used) has often emphasized the separation rather than the quantitative aspects of the analysis. This, in part, stems from the highly polar nature of the amine bases and, consequently, their unfavorable chromatographic behavior. Efforts t o make amines more amenable t o G C include the use of inert column supports (Teflon, silanized Celite, glass beads), specialty stationary phases, and coated KOHtreated supports. Perhaps the most effective route to reliable amine analysis via GC entails prior derivative formation. 1 188
ANALYTICAL CHEMISTRY
Among such derivatives, the TFA amides are probably the most popular. The paper by Vanden Heuvel, Gardiner, and Horning (1) discusses the general merits of several amine derivatives for GC work. Morrissette and Link (2) illustrate the favorable attributes of the T F A derivatives t o the GC of fatty amines. Parsons and Morath (3) were able to resolve the three isomeric toluidines using a Ucon oil on a NaOH-treated support. F & M Scientific Division (4) reports the separation of the isomeric xylidines on a Carbowax-KOH column. Funasaka, Kojima, and Igaki (5), using a column packing of dodecyl benzene sulfonate and diglycerol on Celite, show distinct peaks for the xylidine isomers and report some semiquantitative data. EXPERIMENTAL
Apparatus. An F & M Research Model 810 gas chromatograph equipped with a hydrogen flame detector was used. A stainless steel column, 18 feet x 0.125 inch with a 0.02-inch wall thickness, was packed with a substrate 9.5% Apiezon-L (1) W. J. A. Vanden Heuvel, W. L. Gardiner, and E. C. Horning, ANAL.CHEM., 36, 1550 (1964). (2) R. A. Morrissette and W. E. Link, J . Gas Chromatog., 3 , 67
(1965). (3) J. S. Parsons and J. C. Morath, ANAL.CHEM.,36, 237 (1964). (4) "Facts and Methods," Vol. 6, No. 6, F & M Scientific Division,
Hewlett-Packard Corp., Avondale, Pa., 1965. (5) W. Funasaka, T. Kojima, and H. Igaki, ANAL.CHEM.,36, 2214 (1964).
