2431
Anal. Chem. 1985, 57,2431
Variable Cold Temperature Bath for Preparing Gas Samples for Isotope Ratio Mass Spectrometry Meyer Feld and James S. Rounick* Isotope Department, The Weizmann Institute of Science, 76100 Rehovot, Israel Researchers measuring 13C/12C and 1a0/160 ratios of COZ often cryogenically purify the COzbefore analysis. A common procedure is to remove water on a vacuum line with a cold trap. This is a particularly important step when 1%/160 ratios are to be determined on C 0 2 because of a possible isotope exchange between the oxygen of the sample Cog and the contaminating water which can occur in the ionization chamber of the mass spectrometer and lead to erroneous results. In many stable isotope laboratories the differential freezing properties of liquid nitrogen (bp -195.5 ”c) and a dry iceacetone slurry (-78 “C) are used to cool a trap which can separate COz from water on vacuum systems. In our laboratory we have found that use of these mixtures often does not provide the adequate separation needed because the trap is not cold enough, allowing minute quantities of water to contaminate the COz sample to be measured. This contamination can cause up to a 2% shift in 6l80 values of some samples. Consequently, we have developed a simple, portable device for use on a vacuum line which can accurately cool a trap to fl “C at temperatures ranging from room temperature to liquid nitrogen temperature. We have used this apparatus at -95 “C and have found it invaluable in “cleaning up” COz samples and it has decreased our laboratory’s variability in determinations considerably. DESCRIPTION OF T H E APPARATUS Any temperature bath works on the principle that when the bath reaches the required temperature the source of energy (cooling in this case) is switched off. With our device, instead of switching off the source of cooling, it is simply isolated (Figure 1). This is achieved by having a silvered, doublewalled vacuum vessel (A) containing stirred alcohol (B) as our bath. Normally, the vessel is open to atmospheric pressure via a solenoid valve (C). In order to avoid ice formation inside the vessel walls, rubber tubing is attached to the atmospheric inlet valve and immersed in the liquid nitrogen so only “clean air” (nitrogen gas) enters the vessel. In this configuration the bath is cooled by liquid nitrogen kept in an outer Dewar flask (D). Heat is transferred from the bath through the air in the walls of the vessel to the liquid nitrogen. The temperature in the bath is measured by a thermocouple (E) which is attached to a commercial temperature controller (F) which controls the solenoid valve. When the set temperature is reached the solenoid valve is switched and the air in the vessel is pumped out through the vacuum line’s existing pumps (G) (diffusion and rotary pumps). When a good vacuum is achieved between the vessel walls, the bath begins to heat up, the temperature-controlled solenoid valve is opened, and air enters vessel A, beginning the cooling cycle again. While several variable temperature cryogenic traps have been described previously (1, 2 ) , we believe our trap offers increased flexibility by using a different, yet simple, principle of operation. The trap is completely portable and can be attached to any existing vacuum line without modification. It functions as a cold Dewar on a laboratory jack and requires
0003-2700/85/0357-2431$01.5010
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Figure 1. Simplified cold bath: (A) silvered, double-walled glass vessel; (8)stirring motor and rod; (C) solenoid valve; (D) outer Dewar flask; (E) thermocouple; (F) temperature controller; (G) pumping system (D.P., diffusion pump; R.P., rotary pump).
only attachment to the existing vacuum line’s pumping system. The bath can be installed in 15 min and with a volume of 500 mL of alcohol in the inner Dewar can be cooled to -95 “C in 20 min. Samples can then be processed as with any conventional Dewar-cooled trap a t a rate of about 1 sample/min. The trap is easily and rapidly cleaned (degassed) by simply lowering the assembly and pumping out the vacuum trap. Further, unlike other systems, our bath allows a flowthrough cleaning of gas samples which in conjunction with the ability to change the volume of alcohol reservoir varies its heat capacity. Thus, gas samples of variable sizes can be processed. While the bath is invaluable for cleaning water from COz samples as discussed above, it has further applications. For example, it could be used to separate other gases on a vacuum system, which require temperature differentials not readily available with conventional bath mixtures (e.g., COzand HzS). Furthermore, its use is not limited only to vacuum systems. It can also be used as a cold bath for a variety of other chemical applications. ACKNOWLEDGMENT We thank Joel Gat for stimulating the design of this apparatus. Registry No. 02,
13C, 14762-74-4;C, 7440-44-0; lSO, 14797-71-8; 7782-44-7; COZ, 124-38-9; HZO, 7732-18-5.
LITERATURE CITED (1) Brenninkmeijer. C. A. M. Anal. Chem. 1982, 5 4 , 2622-2633. (2) Des Marais. David J. Anal. Chem. 1978, 5 0 , 1405-1406.
RECEIVED for review February 21, 1985. Accepted May 22, 1985.
0 1985 American Chemical Society
