Anal. Chem. 1991, 63, 1182-1184
1182
1
and C were each one extracted and analyzed in triplicate by this procedure. The total amounts of LAS we measured were 125, 118, and 8 pg/L for samples A, B, and C, respectively. Hence, no significant adverse effect occurred during storage of the Carbopack cartridges. Registry No. Water, 1132-18-5.
A
c11
LITERATURE CITED (1) Marcomini, A.; Giger, W. Anal. Chem. 1987, 59, 1709-1715. (2) Waters, J.; Garrigan. J. T. Water Res. 19811, 17, 1549-1582. (3) Kikuchl. M.; Tokal, A.; HoshkJa, T. Water Res. 1086. 20, 643-850. (4) (;reenberg, A. E.; Comners, J. J.; Jenklns, D. StanUa~IM S W for the Examhation of Water end Wastewater, 18th d.;Amerloan PUMlc Health Association: Washington, D.C., 1985; Section 512 A. (5) Kunihto. K.; Nakae, A.; Muto, 0. BuwkIKagnku 1975, 24, 188-192. (6) Takano, S.; Yagi, N.; Kunlhko, K. Vakagaku 1975, 24, 389-394. (7) Nakae, A.; Tsuij, K.; Yamaunaka. M. Anal. Chem. 1980, 52, 2275. (8) Tayloc, P. W.; Niclcless, 0. J . Chromatog*. 1979, 178. 259-269. (9) Saito. T.; Higashi, K.; Hagiwara, K. Z . Anew. Chem. 1982, 313,
-u
0
4
8
1
2
0
4
8
1
2
t i m e (min)
Flguro 3. Chromatograms obtained from (A) 10 mL of a sewage influent and (B) 50 mL of a fhal effluent. The total LAS concentratkns were 4.83 and 0.204 mg/L, respectively.
problems are eliminated. The small-volume cartridge could be sealed and shipped to the laboratory for elution and chromatographic analysis. In order to assess the feasibility of on-site sampling by a Carbopack cartridge, a river water sample containing LAS was divided in three aliquots. One was extraded in triplicate with Carbopack cartridges that were air-dried for 5 min before storage (sample A). A second aliquot was added to 1?% formalin to prevent microbial degradation of LAS (sample B), and a third aliquot was left unaltered (sample C). Both cartridges and water samples were stored for 30 days a t room temperature. Thereafter, the three Carbopack cartridges were submitted to the remaining part of the procedure for HPLC quantification, while samples B
21-23. (IO) Kkuchi, M.; Tdtai, A.; Yoshide, T. Water Res. 1088, 20, 843-850. (11) Marcomini, A.; Capri, S.; Giger, W. J . chromatog*. 1987, 403, 243-252. (12) MathljS, E.; De k W U . H. Tenslde SUhC&nb, Deterg. 1987, 24 (4), IQ3-IQQ. .- - .- -. (13) Thwman, E. M.; Wllloughby, T.; Barber, L. 6.; Thorn, K. A. Anal. Chem. 1987, 59, 1798-1802. (14) . . castles. M. A.; Moore. B. L.; Ward. S. R. Anal. Chefn. 1989. 61. 2534-2540. (15) Bacabni, A.; *elti, 0.;Lagena. A.; Petronio, B. M.; Rotatori, M. Anal. Chem. 1980, 52, 2033-2038. (16) Borra, C.; Di Corcia, A.; Marchem, M.; Samperi, R. Anal. Chem. 1988, 58. 2048-2052. (17) Battlsta, M.; Di Corcia, A.; Marchetti, M. Anal. Chem. 1989. 67, 935-939. (16) DI Corcia, A.; Marchetti, M.; Samperi, R. Anal. Chem. 1989, 61, 1383-1367. (19) Campanella. L.; Di Corcia, A.; Samperi, R.; Gambacwta, A. Meter. Chem. 1082, 7. 429-43s. (20) Andreoiini, F.; Borra, C.; Caccamo, f.; DI Corcla, A.; Samperl. R. Anal. Chem. 1987. 59, 1720-1725. (21) Di Corcia, A.; Marchetti, M. Anal. Chem. 1991, 63, 580-585. (22) Hand, V. C.; Williams, G. K. fnvlron. Scl. Technol. 1987, 21. 370-373. (23) Nakae, A.; Tsuij. K.; Yamaunaka, M. Anal. Chem. 1981, 53, 1818. (24) Linder, D. E.; Allen, M. C. J . Am. OII Chem. Soc. 1982, 59, 152. (25) Marcomini, A.; Busettl, S.; Sfriso, A.; Capri. S.; La Noce,T.; Uberatorl. A. Proceedings of the Sixth Internatknai Symposium on Organic MIcropoliutants in the Aquatic Environment, Lisbon 1990; p 125.
RECEIVED for review November 13, 1990. Accepted March 4,1991.
Robust, High-Efticiency, Hlgh-Capacity Cryogenic Trap Carl A. M. Brenninkmeijer
DSIR Physical Sciences, Nuclear Sciences Group, P.O. Box 31 312,Lower Hutt, New Zealand Cryogenic traps are frequently used for removing condensable gases from a gas mixture (1). In essence, such traps consist of a piece of U-shaped tubing immersed in a cold bath. The function is based on the condensation of the vapor to be removed on the cold wall when the gas flow passes through the tubing. A t sufficiently low flow rates, the residual concentration of a condensable component is determined only by its vapor pressure in relation to the total pressure. However, in most applications, the effectiveness is less, and multiple loops are employed. A quantitative description of the actual trapping is rather complex (2) and may not be complete. The trap I describe here was developed for removing COz and N20at liquid nitrogen temperature from a flow of air for the isotopic analysis of atmospheric CO. The metal trap replaces about eight loops of glass tubing and has several advantages. The limited effectiveness of a single-loop trap is due to at 0003-2700/9 110383-1182$02.50/0
least two factors. First, the condensed material may be dislodged from the wall by the gas stream and become resuspended in the outflow. Also, as can be easily observed in the case of condensing water vapor, fog formation can occur with a similar result. Second, trapping is limited by the rather slow process of diffusion to the cold wall. Adding more loops increases the overall efficiency, but at the cost of a higher consumption of coolant. Since most traps are constructed in glass (3), a rather fragile construction results. Stevens et al. ( 4 ) introduced a glass capillary type of trap, the increased efficiency of which is based on the decreased distance over which the condensables have to diffuse. Also, a larger surface mea is exposed. Horibe et al. (5) introduced a trap with a glass frit and comment on its high efficiency for trapping water in a single pass. The present design is based on similar considerations, and the detailed construction is shown in Figure 1. 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 11, JUNE 1, 1991
-
INLET
Cabn bdlowr
I
vmthg hole
- ghrr %re
wmhor
c
4 50"
-1. CK#)8sectkmofaRusslenddltrap. ThethreeglassSoxhkt thlmMes are fltted on the threaded insert wlth several windings of thin tinned copper wire (not shown). After fitting the thimbles, the insert is screwed into the upper flange and subsequently the bottom flange is welded. A glass fiber washer provides a seal between the Insert and the top flange.
The trap is constructed of stainless steel, and the assembly is welded. To minimize consumption of coolant, commercially available, thin-walled stainless steel bellow sections (Cajon) form the thermal barrier between the gas-processing system and the trap submerged in liquid nitrogen. The mechanical flexibility of the bellows provides an additional advantage. At the outlet, sheathed thermocouple wire acting as a heater element (6) is wound around the bellows to prevent too much cooling of the Viton O-ring seals and outlet valve. Although not shown in Figure 1,it is advisable to provide a safety relief valve in the system because under certain conditions high pressurea can arise when using liquid nitrogen. Most elegantly, such a safety feature can be implemented by using Vacutaps, which are spring-loaded vacuum-operated glass taps (7).It must also be noted that when air is processed, cold traps submerged in liquid nitrogen must be operated at pressures below 0.3 bar to prevent the liquification of oxygen. We use pressures of 100-200 mbar. The trapping of condensables takes place on the three glass fiber elements in a Russian doll arrangement. The glass fiber thimbles are commercially available (Schleicher and Schiill) and are commonly used in a Soxhlet extraction apparatus. The fiber material is borosilicate glass, 0.75-1.5 pm in diameter. An inorganic binder is used, and the surface area is 2 m2/g. The fiber element is characterized by a retention of over 99% particles of >1 Ctm. The thimble shape offers mechanical robustness in relation to pressure gradients, while
1183
a large geometrical surface area is realized. The prototype Russian doll trap which I tested had only one, smaller size thimble. It appeared that adding thimbles increased the efficiency, in contrast to looptype traps, where increaaingthe length of the submerged part has little effect. Practical considerations led to the present 3-in-1 Russian doll trap. The performance of the Russian doll trap has been evaluated in a system used for separating CO from air for 13C,'80, and 14Canalysis. This system is largely based on the method given by Stevens et al. (4) and works as follows. Air, at a flow rate of 4 L min-I, is stripped of its C02,N20,and H20content by means of molecular sieve and loop-type traps in liquid nitrogen. After selective oxidation of the CO content (40-200 ppb), the resulting C02 is trapped at liquid nitrogen temperature. N20, which is initially present a t a level of about 280 ppb, has to be removed completely because of its interference with mass spectrometric I3C/l2C and 1e0/160 measurements (8,9). h i d u a l traces of N20, equivalent to as little as 0.1% of the CO-derived C02, can be detected. This has been used to measure the N20 reduction factor achieved with a Russian doll trap. The reduction factor for C02 obtained with the Russian doll traps has been measured volumetrically with the amount of C02recovered. Any excess C02over what is expected on the basis of the CO content is due to C02 breakthrough. When using air from which only H20 had been removed, a Russian doll trap as described can remove at least 350 cm3 of C02from 1m3of air at a flow rate of 4 L min-'. A reduction factor of a t least loo0 is achieved. For N20,a reduction factor of about 300 is obtained. The difference may be due to the higher vapor pressure of NzO. The reduction factors are comparable to those of a conventional glass trap of eight loops (8m"mi.d. tubing) of 450-mm length, submerged over 300 mm in liquid nitrogen. Apart from being robust, the Russian doll trap has the advantage of a large capacity; conventional traps cannot cope with 350 cm3 of COP Furthermore, the consumption of liquid nitrogen is less. The Russian doll trap requires 0.7 L h-', compared to 1.2 L h-' for the eight loop traps. The pressure drop across the Russian doll trap is larger but well below 10 mbar. Removal of the condensed vapors is achieved by back-flushing with air that has been dried with molecular sieve. A remarkable feature of the Russian doll trap is its capacity to remove ethane. When using molecular sieves and loop traps in our systems, we could often detect very small traces of ethane in our CO-derived C02 samples, depending on the initial purity of the air sample being processed. These traces disappeared completely when using Russian doll traps. This trapping of ethane, which has a significant vapor pressure a t liquid nitrogen temperature, could indicate that at least part of the trapping action is based on adsorption. It is known that cooled glass has a certain cryopump action (IO),and therefore, we tested the Russian doll traps for possible retention of CO, but none was detected. Complete evaluation of the working of traps of the Russian doll type will require considerable effort. A t present, the trapping process is not completely understood. However, thie should not hinder further applications because the Russian doll trap is superior to the frequently used glass loop trap for cleaning up gases. The Russian doll trap is also extremely efficient in removing water vapor, although no data are presented here due to the considerable problems of quantifying minute amounts of water. LITERATURE CITED (1) Brennlnkmeljer, C. A. M. Anal. chem.1982, 54, 2622-2623. (2) Qraf, W. Ph.D. Thesls. Technical University of Miinkh, FRG, 1979. (3) Lowe. D. C.; Brennlnkmeijer. C. A. M.; Tyler, S. C.; Mugokencky. E. J. J . oeophs. Res., In press.
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ANALYTICAL CHEMISTRY, VOL.
63,NO. 11, JUNE 1. 1991
(4) Stevens, C. M.; h u t . L.; WalWng, D.; Venters. A. Eerlh Plemt. Scl. Lett. 7072, 76, 147-?65. Y.; -m, K.; Takakuwa, Y. J . Cbophys. Res. 1073, 78.
(5)
m,
2626-2629. (6) kennkJtmdJer, C. A. M.;Hemmlngaen, I. J . phys. E . Sci. Instrum. 1088, PI. 502-503. (7) kenntnkmd~#,c. A. M.; ~ourmn,M. L. AMI. them. l o w , 57, 960. (8) Crab. H.; K d b C. D. (kochkn. Cas"h&n. Acfa 1063, 27,
549-551. (9) Mook, W. G.; Jongsma, J. T~#us 1067, SB, 96-99. (10) Haller. F. 6. Rev. Scl. I n s m . 1964, 35, 1356-1357.
RECEIVED for review December
1991.
3,1990. Accepted March 1,
