ANALYTICAL CHEMISTRY, VOL. 50, NO 9, AUGUST 1978
LITERATURE CITED (1) (2) (3) (4) (5) (6)
R. E. Dessy and J. A. Titus, Anal. Chem., 45, 124A (1973). T. J. Williams, Chimia, 27, 669 (1973). R. E. Dessy and J. A. Titus, Anal. Chem., 46, 291A (1974).
(7) R. E. Dessy and J. A. Titus, Anal. Chem., 49, llOOA (1977). (8) H. V. Maimstadt, C. G. Enke. and S. R. Crouch, "Electronic Measurements for Scientists", W. A. Benjamin Inc., Menio Park, Calif., 1974, pp 226-262. (9) D. G. Larson, P. R. Rony, and J. A. Titus, Am. Lab., 7 (6), 76 (1975).
P. Lykos, Ed., ACS Symposium Series, Voi. 19 (1975). J. M. McQuiiian and D. C. Waiden, Comput. Networks, 1 (5) 1 (1977).
W. Greene and U. W. Pooch, Computer, 10 (11) 12 (1977).
1405
for review February
24~
Accepted
279
1978.
Variable-Temperature Cryogenic Trap for the Separation of Gas Mixtures David J. Des Marais Extraterrestrial Biology Division, Ames Research Center, NASA, Moffett Field, California 94035
Cryogenic traps offer a clean, simple, and relatively rapid means for resolving simple gas mixtures. This report describes a continuously variable-temperature cold trap which can both purify vacuum-line combustion products for subsequent stable isotopic analysis and isolate t h e methane and ethane constituents of natural gases. Murakami and Okamoto ( I ) , Crosmer e t al. ( Z ) , and Stump and Frazer ( 3 )have previously described variable-temperature cold trap designs for specific applications. I feel the trap described here embodies all the important capabilities of these former designs, yet is generally easier and less expensive t o build and operate.
EXPERIMENTAL Figure 1 is a schematic drawing of the cold trap. The trap is U-shaped and consists of two 0.635-cm (0.25-in.)diameter by 8-cm long pieces of stainless steel tubing (C) welded to a 30-cm segment of Cajon flexible tubing (G)(part no. 321-4-X-12, Cajon Company. 32550 Old South Miles Road, Cleveland, Ohio 44139). A 20-cm long, welded stainless steel canister (D) surrounds the lower portion of the trap, generating a 3-mm annular air space between the canister and the flexible tubing. Asbestos-insulated 25-gauge chrome1 A resistance wire (E) is wrapped around the flexible tubing and brazed to Fiberglas-insulated copper wires which emerge from the enclosure via a 0.635-cm (0.25-in.) stainless steel tube (F) and a vacuum epoxy feedthrough (B). A 30-gauge chromel-alumel thermocouple (J)is brazed to the bottom of the U-trap, and the wire leads also emerge from the housing via the steel tube (F) and the feedthrough (B). During use, the canister is almost totally immersed in liquid nitrogen. A rubber stopper (A) is inserted in the aperture near the vacuum feedthrough (B) to prevent the condensation of liquid oxygen inside the enclosure. The stopper is preferred to a more permanent closure for reasons of safety. Should a leak permit condensation to occur inside the canister, the stoppered aperture would serve as a pressure release valve for the confined gas. In operation, a gas mixture to be resolved enters the evacuated U-trap at room temperature. The trap canister is then almost totally immersed in a liquid-nitrogen bath, cooling the U-trap as heat flows across the 3-mm annular air space between the flexible tubing and the canister wall. As the trap cools, the gas mixture components condense sequentially according to their relative vapor pressures. As Table I shows, about 12 min is required for the bottom of the U-trap to attain liquid-nitrogen temperature. Measurements made using traps with additional thermocouples located at positions G and H (Figure 1)describe a thermal gradient along the length of the trap. This gradient reflects the steady-state balance between heat flow from the flexible tube t o the canister, and heat flow into the tube from the warmer upper portions of the trap assembly. As discussed by Crosmer et al. ( 2 ) ,such a thermal gradient promotes the fractionation of gas mixture components. After the bottom of the trap (J) has attained liquid-nitrogen temperature, the passage of current through the resistance wire (E) warms the U-trap and permits the distillation of successive gas components at trap temperatures optimal for their resolution. Table I lists the power requirements as a function of temperature for a properly constructed trap. The consumption
Table I. Variable-Temperature Trap Thermal Behavior and Power Consumption time (min)
after temp. ("C) at location ___ LN, G H J immersion
power input
(W 0 0 0
25 a
0 0 0 1
a
a
a
163 - 153 - 137 - 103 - 40 +9
--
25
25 a a a a
190 - 167 - 153 - 125 - 78 -- 38 -
- 50 - 108 - 160 - 190
195 - 175 - 165 - 145 --llO - 75 -
0 1 2 4 8
12 b b
2 b 4 b 8 12 b Not measured. The trap usually requires about 1 rnin t o increase its temperature 30 'C and attain a new steady-state thermal profile. rate of liquid nitrogen is typically low, even at "warmer" trap temperatures. For example, when trap thermocouple J is at -110 "C, 8 W of heater power is being balanced by a liquid-nitrogen evaporation rate of approximately 4 mI,/min.
RESULTS AND DISCUSSION The ability of the variable-temperature trap to separate gas mixtures is shown in Table 11. T h e first mixture represents typical vacuum-line combustion products of geochemical samples such as rocks or marine sediments. Separation of the individual gases of this mixture by the cold trap, facilitates the measurement of their abundance and stable isotopic compositions. COz and SO2 aliquots of known isotopic composition were used in this demonstration. T h e gas fractions distilled from the trap were quantified using a mercury manometer, and subsequently analyzed for their purity and isotopic composition using a Nuclide 6-60 RMS mass spectrometer. T h e gas mixture was condensed in the variable-temperature trap in the manner described earlier. The trap was then warmed to -143 "C and the volatilized C 0 2 was distilled for 5 min from the variable-temperature trap to a liquid-nitrogen trap incorporated into the mercury manometer. Despite the substantially greater abundance of SOz in the original gas mixture, the COZ recovered by this distillation was more than 99% pure, and had sustained little isotopic fractionation. T h e t r a p was warmed to -85 "C and the SO, was distilled for 10 min into the mercury manometer. T h e SOz fraction also exhibited a very high purity and had sustained negligible isotopic fractionation. T h e variabletemperature trap was then warmed to room temperature to recover the HzO. T h e somewhat lower recovery achieved
This article not subject to U.S. Copyright. Published 1978 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
Table 11. Variable-Temperature Trap Separation of Gas Mixtures fraction initial mixture
1
3
2
combustion products temp. at pt. J, "C CO,, pmol 6 3c, "loo a SO,, pmol 6
6.4
i
0.2c
0 59
i
1
0 16i 1
34s)
H,O, pmol
-143 6.4 i 0.2 - 0 . 2 i 0.2 0.04 i 0.01
d d
- 85
0.2
i
0.04
d 58i 1 +0.1 * 0.2 d
24 0
z
0.01
d
0.5
i
0.05
d
14i 1
natural gas - 195 - 172 25 5 228 r 5 0.16 i 0.01 0.06 f 0.01 1 0.19 r 0.01 24+1 0.10i 0.01 0.4 0.12 + 0.01 0.26 i 0.01 8.8 * 0.4 0.1 0.15 i 0.01 trace 1.7 t 0.1 6 13c,0/ O G = [("C/'2C)samp~e/(13C/1'C)reference - 111000, where (L3C/IZC)reference is taken to be that of the C O , in the initial mixture. Analogous to 6 I3C in a . Uncertainties expressed as one standard deviation. Not measured.
temp. a t pt. J, "C CH,, wmol C,H,, pmol C,H,, r m o l >C3H8,pmol
228 i 24 i 9.1 i 1.9 i
(1
-'D E-
-F
G-
-H
'J
Figure 1. Schematic drawing of the variable-temperaturecryogenic
trap probably reflects adsorption of this gas onto the unheated glass surfaces of the vacuum line. Table I1 also shows the results of an attempt to separate a natural gas sample. T h e gas fractions were analyzed using a Hewlett-Packard Model j830A gas chromatograph fitted with a flame-ionization detector. A 50-m glass capillary column was coated with n-octanol and temperature programmed from 10 to 100 "C a t 10"/min. As the trap cooled, a Toepler pump circulated the gas mixture through the cold trap. After thermocouple J attained liquid-nitrogen temperature, the uncondensed gas was removed from the vacuum
_-
line and analyzed. This gas was found to be almost pure methane. T h e gas fraction harvested from the trap at --172 "C was 98% ethane. The fraction obtained when the trap was warmed to ambient temperature constituted 95 % by volume of the propane and higher molecular weight components in the original gas mixture. These data show that the variable-temperature trap could facilitate the isolation and isotopic analysis of relatively pure methane, ethane, and higher molecular weight fractions of natural gas. Also, because such a procedure could remove the large methane and ethane components of the gas sample, the gas chromatographic analysis of the higher molecular weight residue would achieve a higher sensitivity for minor components in this residue. The present variable-temperature cold trap design compares favorably with other gas separation techniques in terms of expense, simplicity, and versatility. The trap can be fabricated from easily obtainable materials a t a cost of less than $250. Although a sophisticated temperature-control system could be interfaced with the cold trap, a 20-W power supply and a thermocouple readout device are sufficient for its operation. A typical three-component gas mixture can be separated in about 40 min. The trap can handle 100 pmol amounts of gases such as SOz and HzO, a feat well beyond the capabilities of conventional gas chromatographs. Therefore, although the cold trap resolution is substantially lower than those of gas chromatographic systems, the trap is still quite useful for a variety of useful applications requiring the separation of simple gas mixtures.
ACKNOWLEDGMENT T h e encouragement of I. R. Kaplan is gratefully acknowledged.
LITERATURE CITED (1) Y. Murakarni and H. Okamoto, "Transactions of the Tenth National Vacuum Symposium of the American Vacuum Society", 1963, Macmillan, New York, N.Y., 1963, p 93. (2) W. E. Crosrner, N. C. Thomas, P. H. S. Tsang, and R. J. Duckett, Rev. Sci. Insbum., 44 (7). 837 (1973). (3) R. K. Stump and J W. Frazer, Lawrence Livermore Laboratory, Rept UCRL-50318 (1967).
RECRIVED for review March 24, 1978. Accepted May 8, 1978. This work was supported by NASA Grant No. NGR 05007-221.
