INSTITUTE OF LASER MEDICINE

Prof. Dr. P. Hering   

Both nonradioactive carbon isotopes find today growing applications. ¹³C is widely used as a marker in medical diagnostic, biology and NMR. ¹²C is interesting for material science and technology; e.g. pure ¹²C diamond has a 1.5 times better heat conductivity (at 300K) than the normal crystal, so it is the best heat conductor at all. At present ¹³C and ¹²C are produced by means of the low temperature distillation of CO. It takes place in a sequence of huge liquid nitrogen cooled columns, which need several months to reach the separation equilibrium. We develop instead a compact and flexible enrichment technique, based on the isotopically selective multiphoton dissociation of CHClF₂ by short pulses of a CO₂ laser: 
 
 

CHClF2 + n hn		CF2 + HCl
CF2 + CF2		C2F4

Under the appropriate conditions, including a suitable laser wavelength around 9.6 mm, the rare ¹³CHClF₂ molecules (1.1%) absorb the CO₂ laser light much better and dissociate with about 100 times higher probability comparing to ¹²CHClF₂. Thus the product C₂F₄ is 100 times enriched with ¹³C and shows an isotope ratio ¹³C/¹²C of 1:1. The rest CHClF₂ is enriched with ¹²C. Addition of NO₂ to the irradiated mixture leads to the oxidation of the dissociation product: 
 

CF2 + NO2

COF2 + NO

 
 

Transversely Excited Atmospheric pressure (TEA) CO₂ lasers with pulsed discharge are commonly used to induce the molecular dissociation. We have instead modified an industrial CO₂ laser with a continuous discharge, combining it with a fast mechanical chopper as the modulator of the laser resonator quality (Q-switch). At a pulse repetition frequency of 10 kHz it emits short (250 ns) pulses of 15-20 mJ energy. As compared to TEA laser such a system is distinguished by its nearly Gaussian beam profile, much higher reliability and longer lifetime. Therefore the photon costs, which play an essential role in the commercialization of the process, are about an order of magnitude smaller. 
Other decisive factor is an efficient utilization of the photons. In order to attain the long absorption path length at the high intensity we employ a multipass cell with two refocusing mirrors (Fig. 2). 

Our demonstration facility can run day and night automatically. A personal computer collects all the important dissociation, gas and laser parameters and sends commands to the  controllers, electrical valves, cooled gas trap, laser and other equipment. A central element of the control system is a quadrupole mass-spectrometer, which measures on-line the concentrations of different isotopomeres in the dissociation cell. 

1.1 mol of ¹³C has been enriched in a nearly continuous experiment during 12 days with the production rate R₁₃ of 5 mmol/h (corresponds to 1.5 g ¹³C/24 hours). That is the highest ever enriched with a laser method quantity of a rare isotope. Using 48 mJ pulses from a more powerful oscillator-amplifier system we have demonstrated R₁₃ = 24 mmol ¹³C/h in a short experiment. 

11 mol of ¹²C have been enriched from the natural 98.9% up to  99.993% of isotope purity at R₁₂ = 87 mmol ¹²C/h (R₁₂ >> R₁₃ because of 89 times higher ¹²C natural abundance). 

Fig. 3. Theoretical scaling curves and  actual results for ¹³C (a) and ¹²C (b) enrichment rates. 
* high-power oscillator-amplifier system „MIDAS“, 48 mJ/pulse, in cooperation with  U. Bielesch, J. Schäfer, J. Uhlenbusch, Inst. f. Laser- und Plasmaphysik, HHU Düsseldorf 

 

The dissociation yield and so the enrichment rate grow nonlinear with the laser pulse energy. A more powerful 5 kW (cw) CO₂ laser, which is standard nowadays in industrial applications, would provide according to our estimations 170-190 mJ per pulse at 8 kHz. With this laser and by the appropriate scaling of the multipass cell up to 90 g ¹³C/24 h could be produced (1.4 kg C₂F₄/24 h with ¹³C:¹²C ratio of 1:1). Such an enrichment is fairly adequate for the most application. If the higher ¹³C content is however desirable, C₂F₄ could be transformed back to CHClF₂ (probably UV-induced reaction with HCl) and enriched further up to 99% with 10 times weaker CO₂ laser beam and 140-150 times smaller gas throughput than at the first stage. C₂F₄ and especially COF₂ could be also easy converted to CO and enriched further by a traditional low temperature distillation. The final enrichment from 50 to 99% requires in the conventional method much less expenditure than the fist step and could be done in a very compact facility. 

C₂F₄, COF₂ and CHClF₂ are good starting molecules for synthesis of many interesting compounds, like e.g. in the following known reaction, 
 
 

C2F4 +C2H5OH

CHF2CF2OC2H5  .

 
 

References:
[1]	W. Fuss, J. Göthel, K. L. Kompa, M. Ivanenko, W. E. Schmid, "Multiwavelength Q-switched CO2 laser with continuous discharge.", Appl. Phys. B 55 (1992) 65.
[2]	W. Fuss, J. Göthel, M. Ivanenko, K. L. Kompa, W. E. Schmid, "Isotope selective dissociation of CHClF2 by one- and two-wavelength Q-switched CO2 laser.", Z. Phys. D 24 (1992) 47.
[3]	W. Fuss, J. Göthel, K. L. Kompa, M. Ivanenko, W. E. Schmid, "Simultaneous multiwavelength emission from a high repetition-rate Q-switched CO2 laser.", SPIE 1810: Gal Flow and Chemical Lasers, pp. 45-48 (1992)
[4]	W. Fuss, W. Schmid, "Mechanische Zerhacker für optische Strahlung, insbesondere Laser-Güterschalter, mit Kegelspiegel.", Patent DE 42 07 762 A 1 (1993).
[5]	W. Fuss, J. Göthel, M. M. Ivanenko, K. L. Kompa, W. E. Schmid, K.Witte, "IR multiphoton absorption and isotopically selective dissociation of CHClF2 in a Herriott multipass cell.", Z.Phys. D 29 (1994) 291.
[6]	W. Fuss, J. Göthel, M. Ivanenko, W. E. Schmid, P. Hering, K. L. Kompa, K. Witte, "Macroscopic isotope separation of 13C by a CO2 laser.", Isotopenpraxis Environ. Health Stud. 30 (1994) 199.
[7]	J. Göthel, M. Ivanenko, P. Hering, W. Fuss, K. L. Kompa, "Macroscopic enrichment of 12C by a high-power mechanically Q-switched CO2 laser.", Appl. Phys. B 62 (1996) 329-332.  You can download this paper here:  c12.pdf (55K).
[8]	P. H. Ma, B. Wu, P. Hering, J. Göthel, M. M. Ivanenko, G. C. Chen, J. L. Liu, M. X. Chu, Y. Jing, W. Fuss, "C-13-selective dissociation of 1-chloro-1, 1-difluoroethane via vibrational excitation of the C-C bond.", Chem. Communications 12, 1429-1430 (1996)
[9]	M. M. Ivanenko, H. Handreck, J. Göthel, W. Fuss, K. L. Kompa, P. Hering, "Isotope selective IR multiphoton dissociation of CHClF2 in the presence of NO2.", Appl. Phys. B 65, 577-582 (1997).
[10]	M. Ivanenko, P. Hering, U. Bielesch, J. Schäfer, J. Uhlenbusch, W. Fuss, K. L. Kompa, "Large scale 13C / 12C Separation by IR Multiphoton Dissociation of CHClF2.", Proc. 12th Int. Conf. on LASERS ´97 (New Orleans, Dec. 15-19), Ed. J. J. Carroll, T. A. Goldman, STS Press, pp. 664-669 (1998).  You can download this paper here:  C-separation.pdf (81K).
[11]	M. Ivanenko, P. Hering, "Stabil-Isotopentrennung von 13C- und 12C- für die industrielle Anwendung.", Laser und Optoelektronik (LaserOpto) 30(6) 64-68 (1998).  You can download this paper here: C_la_sep.pdf (85K).
[12]	T. Mitra, "Erforschung der Zerstörschwelle von Spiegeln für die IR-Laserphotochemie in einer Multireflexionszelle", Diploma thesis, Heinrich-Heine-Universität Düsseldorf, Germany, (1998).  You can download this paper here:  Diplom_Mitra (806K).

(The free Adobe(R) Acrobat(R) Reader allows you to view, navigate, and print PDF files across all major computing platforms.)