Determination of 15 N/ 14 N and 13 C/ 12 C in Solid and Aqueous Cyanides Craig A. Johnson U.S. Geological Survey, Box 25046, MS 963, Denver, Colorado 80225 The stable isotopic compositions of nitrogen and carbon in cyanide compounds can be determined by combusting aliquots in sealed tubes to form N 2 gas and CO 2 gas and analyzing the gases by mass spectrometry. Free cyanide (CN - aq + HCN aq ) in simple solutions can also be analyzed by first precipitating the cyanide as copper(II) ferrocya- nide and then combusting the precipitate. Reproducibility is (0.5‰ or better for both δ 15 N and δ 13 C. If empirical corrections are made on the basis of carbon yields, the reproducibility of δ 13 C can be improved to (0.2‰. The analytical methods described herein are sufficiently ac- curate and precise to apply stable isotope techniques to problems of cyanide degradation in natural waters and industrial process solutions. Cyanide (CN - aq ) is used in the mining industry, particularly at mines where gold and silver are extracted from ores by heap leach cyanidation, 1 and also in the photoprocessing and electro- plating industries. Cyanide is also released naturally by certain bacteria and fungi and by the degradation of cyanogenic glycoside- or lipid-producing plants. 2 The chemistry of cyanide is quite complex due to the fact that it can form a wide variety of compounds and complexes. 3 Because cyanide is highly toxic, its degradation both in natural waters and in industrial process solutions has been and continues to be the subject of intensive study. 4,5 The purpose of this report is to present an analytical method that allows stable isotope techniques to be applied to the study of cyanide degradation. The method permits analysis of 15 N/ 14 N and 13 C/ 12 C in solid cyanide salts and in free cyanide (CN - aq + HCN aq ) contained in simple aqueous solutions. The same method holds promise for isotopic analysis of complexed cyanide by first carrying out conventional reflux-distillation for acid dissociable complexes 6 and then analyzing the CN - aq collected in the alkaline trap. To date there has been only one cyanide-related study employing stable isotope techniques. In that study, 7 15 N-labeled HCN was used in a soil utilization experiment, and nitrogen isotope analyses were performed on ammonia (NH 3 ), one of the cyanide degradation products. There have been no direct analyses of the stable nitrogen or carbon isotopic compositions of cyanides. EXPERIMENTAL SECTION Solid cyanide salts are prepared for analysis by combusting them to form CO 2 and N 2 gases. The combustion is carried out in 9-mm-o.d. Vycor tubes. Prior to use, the tubing is cut into 22- cm lengths, sealed at one end, and baked at 500 °C in a muffle furnace overnight. Sample numbers are then inscribed on the tubes with a diamond pencil. Two grams of cupric oxide wire (Baker, 1820-01), previously baked for 4 h at 600 °C and sieved, is loaded into the tube, followed by a quantity of sample corresponding to ∼75 μmol of CN, followed by 1 g of copper shot (20-30 mesh, Baker 1720-01). Samples should be finely pow- dered to maintain near-quantitative yields. The tubes are attached to a vacuum line and pumped for a minimum of 4 h to a final pressure of ∼10 -6 Torr. They are then sealed at 15-cm length and placed in a muffle furnace that has been preheated to 500 °C. The temperature is raised to 850°C, held for 15-30 min, and slowly lowered to room temperature. To extract and purify the CO 2 and N 2 , a sealed tube is placed in a Pyrex tube cracker and attached to a vacuum line like that shown in Figure 1. After the line is evacuated, the traps are cooled to liquid nitrogen temperature, and valve 3 is closed. The tube is cracked, and CO 2 is allowed to condense in the traps. The manometer reading is then recorded and converted to the N 2 yield using a prior pressure-gas quantity calibration. Valve 3 is then opened, and the N 2 gas is condensed onto ∼100 mg of type 5- Å molecular sieves ( 1 / 8 -in. pellets, Linde) in either a stopcock-sealed Pyrex bottle or a 6-mm-o.d. Pyrex tube. When the transfer is complete, the stopcock is closed or the tube is sealed using a torch. (1) Mudder, T. In Proceedings of Conference on Cyanide and the Environment; van Zyl, D., Ed.; Geotechnical Engineering Program: Colorado State University, Fort Collins, CO, 1985; pp 3-10. (2) Conn, E. E. Annu. Rev. Plant Physiol. 1980 , 31, 433-451. (3) Smith, A.; Mudder, T. The Chemistry and Treatment of Cyanidation Wastes; Mining Journal Books Ltd.: London, 1991. (4) Enzminger, J. D. Water Environ. Res. 1993 , 65, 407-410. (5) Turney, W. R.; Thomson, B. M. Water Environ. Res. 1993 , 65, 4010-4013. (6) U.S. Environmental Protection Agency. Test methods for evaluating solid wastes, 3rd ed.; SW-846, U.S. Government Printing Office: Washington, DC, 1986; Vol. 1C, pp 9010A-1-9010A-15. (7) Strobel, G. A. Soil Sci. 1967 , 103, 299-302. Figure 1. Diagram of the vacuum line used for gas extraction. The line is constructed from Pyrex tubing and Kontes O-ring valves. Vacuum is supplied by a mercury diffusion pump backed by a mechanical pump. Anal. Chem. 1996, 68, 1429-1431 This article not subject to U.S. Copyright. Published 1996 Am. Chem. Soc. Analytical Chemistry, Vol. 68, No. 8, April 15, 1996 1429