Technical Note pubs.acs.org/ac
A Practical Cryogen-Free CO2 Purification and Freezing Technique for Stable Isotope Analysis Saburo Sakai*,† and Shinichi Matsuda‡ †
Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan ‡ Izumo Web Ltd., 1-3-3 Imachi-cho, Izumo 693-0002, Japan ABSTRACT: Since isotopic analysis by mass spectrometry began in the early 1900s, sample gas for light-element isotopic measurements has been purified by the use of cryogens and vacuum-line systems. However, this conventional purification technique can achieve only certain temperatures that depend on the cryogens and can be sustained only as long as there is a continuous cryogen supply. Here, we demonstrate a practical cryogen-free CO2 purification technique using an electrical operated cryocooler for stable isotope analysis. This approach is based on portable free-piston Stirling cooling technology and controls the temperature to an accuracy of 0.1 °C in a range from room temperature to −196 °C (liquid-nitrogen temperature). The lowest temperature can be achieved in as little as 10 min. We successfully purified CO2 gas generated by carbonates and phosphoric acid reaction and found its sublimation point to be −155.6 °C at 0.1 Torr in the vacuum line. This means that the temperature required for CO2 trapping is much higher than the liquid-nitrogen temperature. Our portable cooling system offers the ability to be free from the inconvenience of cryogen use for stable isotope analysis. It also offers a new cooling method applicable to a number of fields that use gas measurements.
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GasBench II) use Nafion semipermeable membranes without vacuum and cryogenic liquids. In recent decades, cryocoolers have developed dramatically to achieve cooling temperatures approaching 80 K (−196 °C). This allows applications such as the cooling of infrared sensors, cryotherapy, and cryogenic cooling in space.8,9 Here, we first apply an electrically operated cryocooler to a practical cryogenfree CO2 purification system for stable isotope analysis. This approach is based on portable free-piston Stirling cooling technology that controls the temperature around the vacuum line to an accuracy of 0.1 °C in a range from room temperature to the liquid-nitrogen temperature, which can be achieved in as little as 10 min.
table isotope analysis of light elements has served as a powerful tool since J.J. Thompson first separated neon isotopes by mass spectrometry in 1913.1 In Earth and planetary sciences, light-element isotope studies have contributed greatly to the understanding of Earth and planetary systems, climates, and history. In particular, the isotopes of oxygen and carbon in geological materials have revealed changes in past temperature, ocean salinity, sea level, ice-sheet volume, and carbon cycles of the Earth, all of which are important for predicting the future climate of the Earth.2 For the preparation of isotope measurements of oxygen and carbon, cryogenic methods using liquid nitrogen have been generally used to separate and purify the CO2 sample gas. This separation procedure is based on the differences between the sublimation temperatures of CO2 gas and contaminants such as H2O, using liquid air (−194 °C), liquid nitrogen (−196 °C), and mixtures of dry ice and alcohol (approximately −72 °C) and n-pentane (approximately −131 °C).3−7 The −80 °C cryocool probe in dewar of alcohol and −80 °C cryocool around tubing have also been used as other CO2/H2O distillation methods. Automated CO2 gas purification systems are commercially available, such as heated/LN2 cooled traps on the Thermo Kiel I, II, III, or IV and the IsoPrime MultiPrep. However, these systems require the continuous supply of liquid nitrogen, which quickly empty the liquid nitrogen tank and limits the time of a continuous run (∼48 h per 180 L dewar). As with other routes of water removal, continuous flow carbonate prep systems (e.g., © XXXX American Chemical Society
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EXPERIMENTAL SECTION Cryogen-Free CO2 Purification System. Our cryogenfree CO2 purification system was established based on a portable free-piston Stirling cooling technology (Figure 1). The system is divided into two main units: a calcium−carbonate reaction unit and a CO2 purification unit. The carbonate reaction unit consists of a thermoblock (ND-M01; Nisshinrika, Ltd.), a phosphoric metering pump (PiP1CTC-LF; Fluid Metering, Inc.), and a needle assembly for carbonate reaction Received: February 13, 2017 Accepted: March 22, 2017 Published: March 22, 2017 A
DOI: 10.1021/acs.analchem.7b00544 Anal. Chem. XXXX, XXX, XXX−XXX
Technical Note
Analytical Chemistry
Figure 1. Schematic diagram of the CaCO3 reaction and CO2 purification system based on free-piston Stirling cooling technology for stable isotope analyses. (a) Reaction of CaCO3 powder with phosphoric acid is performed at 90 °C. All vacuum lines and diaphragm valves are vacuum tight, and gas purification is operated by computer control. (b) Cooling plate of the free-piston Stirling cooler is the cooling source as the cold-trap tubing in the aluminum alloy attached via the indium sheet is cooling down. The heating wire is used to heat the cold-trap tubing. The vacuum-tight chamber helps to achieve high cooling efficiency by means of the heat insulation effect. All components except the cooling plate are custom-made.
Figure 2. Cooling performance of the cryogen-free cooling system and gas recovery stability. (a) Cold-trap tubing can be cooled from room temperature to liquid-nitrogen temperature in as little as 10 min. Notably, the cooling time from room temperature to the sublimation point of CO2 gas in the vacuum system is only 8 min 15 s. (b) Evolved CO2 gas generated from weighed CaCO3 powder that reacts with phosphoric acid is highly correlated, which means that our cryogen-free cooling system recovers CO2 gas properly. (c) R18 (18O/16O concentration) and R13 (13C/16O concentration) from 50−300 μg of carbonate material shows high reproducibility. R18 = 0.98523, 0.00023 (1SE, n = 10); R13 = 1.00325, 0.00028 (1SE, n = 10).
that is installed in a stainless-steel casing. The piston repeatedly compresses and expands helium gas to cool the cold head (cooling plate; Figure 1b) of the extended part of the casing. By fixing a heat-transport direction from expansion space (cold head) to compression space (located between the piston and displacer) through a regenerator, the cooling plate (Figure 1b) can be cooled to the temperature of liquid nitrogen (−196 °C) in as little as 10 min from room-temperature conditions (Figure 2a). The 1/16 in. diameter stainless-steel tubing (cold trap tubing; Figure 1b) for gas trapping is embedded in aluminum alloy (alloyed by Satokin Co., Ltd. and Newest Corp.) that is adhered to the cooling plate of the Stirling cooler via an indium sheet. A heating wire is placed on top of the aluminum alloy to allow heating of the cold-trap tubing. A thermistor is embedded in the aluminum alloy to monitor and regulate the temperature to within 0.1 °C by a temperature regulator unit. The entire
(JASCO International Co., Ltd.). The CO2 purification unit consists of a stainless-steel vacuum-tight purification line with leak-tight diaphragm valves (Series 99; Parker Hannifin Corp.), a capacitance manometer (722B; MKS Instruments, Inc.), and a portable free-piston Stirling cooler (FPSC; product number: SC-UF01; Twinbird Corp.; 88 mm diameter, 238 mm length), all of which are connected to a dry pump (DeoDry 15E; Kashiyama Industries Ltd.). The purified CO2 sample gas is introduced to the analytical instrument for stable isotopic analysis. In this study, we performed the measurements with a tunable infrared laser differential absorption spectrometer10,11 (TILDAS; Aerodyne Research, Inc.). The FPSC, a type of cryocooler that relies on compression and expansion of a gas to bring about temperature changes,11 has two major moving parts (the piston and the displacer) that oscillate linearly along the same axis within a single cylinder B
DOI: 10.1021/acs.analchem.7b00544 Anal. Chem. XXXX, XXX, XXX−XXX
Technical Note
Analytical Chemistry cooling unit is enclosed in a vacuum-tight chamber that is connected to the dry pump to achieve high cooling efficiency by means of the heat insulation effect. Sample Preparation and Analysis. Carbonate material with a weight of 50−300 μg (in-house standard calibration by NBS-19, the IAEA standard for oxygen and carbon isotopes in carbonate) is loaded into a 1.5 ml reaction vial, and the septum cap is screwed on. The vial is set in the temperature-controlled thermoblock at 90 °C (Figure 1a). A double-hole-type needle is inserted into the reaction vial. Air in the vial is evacuated from the upper needle hole. When a vacuum condition (0.1 Torr) is achieved, the evacuation of the vacuum line is closed, and three drops of phosphoric acid (∼50 μL) are introduced from the lower hole by a fluid metering pump for reaction with the carbonate (CaCO3 + H3PO4 → CaHPO4 + H2O + CO2). Once the acid has been delivered, the fluid pump reverses its flow to draw back the acid that remains inside the needle volume. The needle remains in the sample vial for 10 min, which is sufficient time for the reaction to go to completion. The generated CO2 gas and water, and the other trace gases, are trapped in the cold-trap tubing at −196 °C. After pumping over the frozen sample to remove noncondensable gases, the trapped CO2 is released into the line by warming the Stirling cooling unit to −115 °C,12 which continues to trap the water and other contaminants. The sublimated CO2 volume is then measured with the manometer. The stable isotopes of the purified CO2 gas are measured by TILDAS, which is modified for batch analysis.10,11,13
Figure 3. Relationship between temperature change of the cold-trap tubing and CO2 gas pressure. The results clearly show the sublimation point of CO2 gas in the vacuum line to be −155.6 °C.
isotope analysis and also offers a new approach for the use of cooling methods in a number of fields of gas measurement.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +81 46 867 9960. Fax: +81 46 867 9775. E-mail:
[email protected].
RESULTS AND DISCUSSION The FPSC has a significant advantage over previous methods of cryogenic distillation in that it does not require any cryogens. The cold-trap tubing in the aluminum alloy connected to the FPSC can be cooled to the liquid-nitrogen temperature (−196 °C; Figure 2a) in as little as 10 min, which means that the CO2 gas purification process is only required to operate for a short time. The CO2 pressure generated from the acid reaction and the weight of the CaCO3 powder are highly correlated (R2 = 0.99; Figure 2b), which indicates proper recovery of the CO2 gas. The oxygen and carbon isotopic ratios of CO2 measured by TILDAS demonstrate a high repeatability (R18 = 0.98523, 0.00023 (1SE, n = 10); R13 = 1.00325, 0.00028 (1SE, n = 10) (Figure 2c). For CO2 to be separated from H2O and other contaminants, a few examples of conventional cryogens are freezing mixtures of liquid nitrogen with dry ice and alcohol (approximately −72 °C) and with n-pentane (approximately −131 °C). In particular, the n-pentane mixture has been used for high purification in ultramicroanalysis to avoid sulfurous anhydride and hydrogen sulfide contaminants.4−7 Our cooling system can achieve any temperature above −196 °C, which cannot be achieved using conventional cryogens. This means that we can control the cooling temperature depending on the purpose. We also evaluated the sublimation point of CO2 gas in our vacuum line. Figure 3 shows the first precise report of the CO2 gas-releasing temperature from solid-state CO2 in a vacuum condition. Experiments using different amounts of CO2 gas show that the sublimation point at 0.1 Torr is exactly −155.6 °C independent of the amount of gas. This means that cooling to just below −155.6 °C is sufficient for freezing CO2 gas. We can shorten the cooling time to 8 min 15 s (Figure 2a). In summary, our portable-sized cryogen-free cooling system releases us from the inconvenience of cryogen use for stable
ORCID
Saburo Sakai: 0000-0002-3809-0254 Author Contributions
S.S. established the overall design and made the components. S.M. built the electronic connections and software for automatic control. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Jin Fujino (Twinbird Corp) for valuable advice on the free-piston Stirling cooling system and Yasuyuki Ohtsuka (Satokin Co., Ltd.) and Seiichiro Saito (Newest Corp.) for generous support in embedding the cold-trap tubing in the aluminum alloy. We also gratefully acknowledge Asahiko Taira, Naohiko Ohkouchi, David Dettman, and Danzhou Yang for their support in establishing this system. Partial funding of this work was provided by grants-in-aid for scientific research by MEXT/JSPS (No. 15H03756) to S.S.
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REFERENCES
(1) Thomson, J. J. Proc. R. Soc. London, Ser. A 1913, 89, 1−20. (2) Zachos, J.; Pagani, M.; Sloan, L.; Thomas, E.; Billups, K. Science 2001, 292, 686−693. (3) Nier, A. O.; Gulbransen, E. A. J. Am. Chem. Soc. 1939, 61, 697− 698. (4) Mizutani, Y.; Oana, S. J. Mass Spectrom. Soc. Jpn. 1973, 21, 255− 258. (5) Wada, H.; Fujii, N.; Niitsuma, N. Geosci. Repts. Shizuoka Univ. 1984, 10, 103−112. (6) Nakamura, T.; Hirasawa, M.; Igaki, K. Radiocarbon 1995, 37, 629−636. (7) Kusakabe, M. Geochem. J. 2005, 39, 285−287. C
DOI: 10.1021/acs.analchem.7b00544 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry (8) Ross, R. G., Jr.; Boyle, R. F. Cryocoolers 14; ICC Press: Boulder, CO, 2007; pp1−10. (9) Radebaugh, R. J. Phys.: Condens. Matter 2009, 21, 164219. (10) Tuzson, B.; Mohn, J.; Zeeman, M. J.; Werner, R. A.; Eugster, W.; Zahniser, M. S.; Nelson, D. D.; Mcmanus, J. B.; Emmenegger, L. Appl. Phys. B: Lasers Opt. 2008, 92, 451−458. (11) Sakai, S. Abstract of 3rd Symposium Solar System Materials, 2015. (12) Ishimura, T.; Tsunogai, U.; Gamo, T. Rapid Commun. Mass Spectrom. 2004, 18, 2883−2888. (13) Ono, S.; Wang, D. T.; Gruen, D. S.; Lollar, B. S.; Zahniser, M. S.; McManus, B. J.; Nelson, D. D. Anal. Chem. 2014, 86, 6487−6494.
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DOI: 10.1021/acs.analchem.7b00544 Anal. Chem. XXXX, XXX, XXX−XXX
