METHOD FOR THE ENRICHMENT OF CARBON 13 BY MEANS OF LASER IRRADIATION

Abstract: A method for the enrichment of carbon 13 by use of multistage laser irradiation. A mixture of CHClF₂ and Br₂ is used as starting material. The mixture is irradiated with laser rays emitted from a carbon dioxide gas laser to obtain CBr₂F₂ enriched with carbon 13. This CBr₂F₂ enriched with carbon 13 is once more irradiated with laser rays emitted from a carbon dioxide gas laser to produce C₂Br₂F₄ which is further enriched with carbon 13. Alternatively, the product CBr₂F₂ of the first laser irradiation is once more irradiated together with O₂ to produce COF₂ which is further enriched with carbon 13.

Description: This invention relates to a method for the enrichment of carbon 13 by use of laser rays and particularly to a method for the enrichment of carbon 13 by means of laser isotope separation by way of infrared multiple-photon decomposition. Natural carbon comprises isotopes of mass numbers 12 and 13, the ratios of which are 98.9% and 1.1%, respectively. Conventional methods for the enrichment of carbon 13 are based on low temperature distillation of CO, but these methods have the disadvantages that it is necessary to use a large amount of poisonous gas and a large scale apparatus, and that the manufacturing costs are high. Therefore, it is desirable that carbon 13 is separated safely and cheaply by use of laser irradiation. Prior art methods for the enrichment of carbon 13 by laser irradiation will now be explained. A working substance such as CF₃X (wherein X is Cl, Br or I), or CF₂HCl is irradiated with laser rays emitted from a CO₂ laser. In the infrared multiple photon decomposition of CF₃X, the final product is C₂F₆. In the infrared multiple photon decomposition of CF₂HCl, the final product is C₂F₄. The final products are enriched with carbon 13. In this method, if a practically significant high yield is intended, the ratio of carbon 13 after the enrichment is only 80% and does not reach the ratio obtained in the conventional enrichment methods using low temperature distillation of CO. In the method described in JP-A-60/132629 (1985), C₂F₆ is irradiated in the presence of Br₂ with pulsed laser rays emitted from a CO₂ laser with adequate wavenumber and fluence. As a result of the photo-dissociation and the subsequent reaction, CF₃Br is produced which is enriched with carbon 13 to a ratio of 20% to 30%. Next, the product CF₃Br is separated and once more irradiated with pulsed laser rays emitted from a CO₂ laser under particular conditions to induce an infrared multiple photon decomposition. The product of the second infrared multiple photon decomposition is C₂F₆, in which the ratio of carbon 13 is increased to 90%. Based on the knowledge obtained from the above investigations, we invented a new method for the enrichment of carbon 13 by use of multistage laser irradiation. Namely, the laser irradiation is used several times in order to gradually increase the concentration of carbon 13 because if laser irradiation is used only once, the degree of enrichment does not exceed a limitation. An object of this invention is to provide a method fulfilling the following requirements which are important from the view point of practical use, i.e. that the working substance easily causes the multiple photon decomposition, the working substance is largely and cheaply obtainable, the product of the first infrared multiple photon decomposition is directly usable in the second decomposition and the materials obtained during the decomposition process are reusable. The present invention relates to a method for the enrichment of carbon 13 by use of a multistage laser irradiation. The present invention is characterized in that a mixture of CHClF₂ and Br₂ is used as the starting material. The mixture is irradiated with laser rays emitted from a CO₂ laser to obtain CBr₂F₂ enriched with carbon 13. This CBr₂F₂ enriched with carbon 13 is once more irradiated with laser rays emitted from a CO₂ laser to produce C₂Br₂F₄ which is further enriched with carbon 13. Alternatively, the product CBr₂F₂ of the first laser irradiation is once more irradiated together with O₂ to product COF₂ which is further enriched with carbon 13. Fig. 1 schematically shows the reaction process according to this invention. When natural CHClF₂ is irradiated with pulsed laser rays emitted from a CO₂ TEA Transversely excited atmospheric pressure laser, CF₂ and HCl are obtained by infrared multiple photon decomposition. The laser rays must be mildly focused by a lens in order to induce the decomposition. If the wavenumber of the laser rays is about 1030 to 1050 cm⁻¹ and the fluence is below 10 Jcm⁻², the product C₂F₄ is enriched with ¹³C. This result is explained by the following reaction mechanism. CHClF₂ + nhν → CF₂ + HCl CF₂ + CF₂ → C₂F₄ The reaction (1) represents the process that CHClF₂ absorbs a large number of photons. This causes decomposition. Under the above described irradiation conditions, molcules including ¹³C are selectively decomposed and CF₂ radicals including a large amount of ¹³C are generated. According to the coupling reaction of radicals (2), C₂F₄ enriched with ¹³C is produced. When a large amount of Br₂ exists in the reaction system, the following reactions will occur: CF₂ + Br₂ → CBrF₂ + Br CF₂ + Br₂ → CBr₂F₂ CBrF₂ + Br₂ → CBr₂F₂ + Br CBrF₂+ CBrF₂ → C₂Br₂F₄ Br + Br → Br₂ The CF₂ radicals enriched with ¹³C react to CBrF₂ radicals in the reaction (3) or to CBr₂F₂ in the reaction (3′), so that the reaction (2) is completely prevented. In consequence of the subsequent reactions (4) and (5), CBr₂F₂ and C₂Br₂F₄ are obtained as products. The relative ratio thereof varies depending on the laser irradiation conditions and the added amount of Br₂. But the yield of CBr₂F₂ is much greater than that of C₂Br₂F₄. The obtained CBr₂F₂ is highly enriched with ¹³C. As a result of the ¹³C selective infrared multiple photon decomposition of the mixture of natural CHClF₂ and Br₂, (a) CHClF₂ depleted of ¹³C, (b) CBr₂F₂ enriched with ¹³C, (c) C₂Br₂F₄ enriched with ¹³C, (d) HCl, and (e) unreacted Br₂ exist in the reaction system. In order to separate each composition, a low temperature distillation method can be used in view of the distinct boiling points points (CHClF₂ - 40.8°C, CBr₂F₂ 24.5°C, C₂Br₂F₄ 47.3 °C, HCl - 85°C, Br₂ 58.8°C). Alternatively, if the amount to be processed is small, the separation can be caused by a preparative type gaschromatograph having a column packed with silica gel. As a result, CHClF₂, CBr₂F₂ and C₂Br₂F₄ are almost completely separated. In the infrared absorption spectrum of natural CBr₂F₂, a strong absorption band is recognized at 1095 cm⁻¹. This is due to stretching vibration of the ¹²C-F bond. The wave number of the absorption band due to the stretching vibration of the ¹³C-F bond is lower than that of the ¹²C-F bond by 20 to 30 cm⁻¹, that is, 1065 to 1075 cm⁻¹. Therefore, in the infrared multiple photon decomposition of CBr₂F₂ there is a large isotope effect concerning carbon. When natural CBr₂F₂ of 666Pa (5 Torr) is irradiated with pulsed laser rays at a wave number of 9P(28), that is, 1039.37 cm⁻¹ and a fluence of 3 J cm⁻² generated by a CO₂ laser, C₂Br₂F₄ is produced according to the following reaction mechanism as a main product, and the abundance ratio of ¹³C reaches 40%. CBr₂F₂ + nhν → CBrF₂ + Br CBrF₂ + CBrF₂ → C₂Br₂F₄ Br + Br → Br₂ There is a great selectivity for ¹³C in the photochemical decomposition process of the reaction (8). Therefore, if the CBr₂F₂ products obtained by the first laser irradiation are further irradiated with laser rays emitted from a CO₂ laser, the second stage infrared multiple photon decomposition causes the final product to be highly enriched with ¹³C. On the other hand, when CBr₂F₂ is irradiated together with O₂, the final product becomes COF₂ which is also enriched with ¹³C. According to this invention the enrichment of carbon 13 is quite easily accomplished by multistage laser irradiation, and carbon 13 can be obtained in high yield. Since the starting material CHClF₂ is largely and cheaply manufactured and substances obtained during the reaction process are reusable, this invention brings a great advantage when it is industrially used. CO₂ is adequate as starting material for synthesising organic compounds labeled by ¹³C. When the second stage infrared multiple photon decomposition occurs in the mixture of CBr₂F₂ and O₂ and the product is subsequently treated with water, CO₂ enriched with ¹³C is directly obtained. The specific nature of this invention, as well as other objects, uses and advantages thereof, will be clear from the description and the accompanying drawing. Fig.1 is a flowchart illustrating steps to obtain a final product enriched with ¹³C according to this invention. EXAMPLE A CO₂ TEA laser was calibrated to oscillate and emit laser rays at the 9P(22) line of about 4J/pulse. The wavenumber of the laser rays was 1045.02 cm⁻¹. The laser rays were focused by a long focal lens and a mixture of CHClF₂ of 6.6 kPa (50 Torr) and br₂ of 1.3 kPa (10 Torr) was irradiated therewith, which mixture was enclosed in a reaction cell of about 3 m in length and 5 lit in volume with windows of NaCl at opposite ends. The number of irradiated pulses was restricted so that about 0.5% of CHClF₂ was decomposed. The product containing carbon was mainly CBr₂F₂. A small amount of C₂Br₂F₄ was also obtained. The irradiated gas was taken into the preparative type gaschromatograph with a column packed with silica gel and of 17.5 mm in diameter and 3.5 m in length, and CBr₂F₂ was separated. The results of mass spectrometry for the CBr₂F₂ are as follows: In the mass spectroscopic analysis of CBr₂F₂, the ion signals of CBrF₂⁺ are strongest. The isotope species thereof are ¹²C⁷⁹Br¹⁹F¹⁹F⁺ (m/e=129), ¹³C⁷⁹Br¹⁹F¹⁹F⁺ (m/e=130), 12 C⁸¹Br¹⁹F¹⁹F⁺ (m/e=131) and ¹³C⁸¹Br¹⁹F¹⁹F⁺ (m/e=132). In the case that the laser rays were focused by a lens of 3 m in focal length, the ratio of ¹³C in the CBr₂F₂ molecule reached 55%. Next, CBr₂F₂ (the ratio of ¹³C atoms was 30 %), which was produced in the first stage infrared multiple photon decomposition and separated, was given into a reaction cell and irradiated with pulsed laser rays at the 9P(28) line, that is, 1039.37 cm⁻¹ emitted from a CO₂ laser apparatus. The fluence of the laser rays was 3.3 J cm⁻² and the number of the irradiated pulses was 100. CBr₂F₂ was under a pressure of about 1.3 kPa (10 Torr). The product was C₂Br₂F₄. The ratios of ¹²C and 13 C atoms in the C₂Br₂F₄ molecules, which were measured by means of gas chromatography(GC) and mass spectrometry(MS), are shown in Table III. In the gas chromatography, a column of 3 mm X 6 m billed with Gaskuropack® 55 was used at 150°C. The ion, which was measured by the mass spectrometer, was CF⁺. It was found that the ratio of ¹³C had increased to 95%. On the other hand, O₂ at 1.3 kPa (10 Torr) was added to CBr₂F₂ (the ratio of ¹³C atoms was 30%) at 1.3 kPa (10 Torr). The mixed gas was irradiated with 300 laser pulses under the same irradiation conditions, namely that the wavenumber of the laser rays was 1039.37 cm⁻¹ and the fluence was 3.3 J cm⁻². When O₂ was present, the product was COF₂. When H₂O reacted with this product, it rapidly changed to CO₂. The ratios of ¹³C and ¹²C atoms in the CO₂ molecules, which were measured by GC and MS, are shown in Table IV. From this result it was found that the ratio of ¹³C had increased to 91 %.

Claims: 1. A method for the enrichment of carbon 13 by means of laser irradiation, including the steps of irradiating a mixture of CHClF₂ and Br₂ with laser rays emitted from a carbon dioxide gas laser to produce CBr₂F₂, and irradiating the produced CBr₂F₂ with laser rays emitted from a carbon dioxide gas laser. 2. The method according to claim 1, wherein a mixture of the CBr₂F₂ produced in the first step and O₂ is irradiated with laser rays emitted from a carbon dioxide gas laser.

Inventors: ARAI, SHIGEYOSHI, ISOMURA, SYOHEI, KAETSU, HAYATO, SUGITA, KYOKO
Applicants: RIKAGAKU KENKYUSHO
Application year: 1989
Main classification: B01D59
IPC codes: B01D59