Isotope Separation by Selective Photodissociation of Glyoxal

It has been found that glyoxal fulfills all of the above-cited requirements for an ideal photodissociation molecule.

The photodissociation of glyoxal is represented by the following reactions:

reaction (1), which occurs about 60% of the time, has a quantum efficiency of 0.6; the quantum efficiency of reaction (2) is much lower, of the order of about 1%. Thus, the major dissociation mode fulfills the requirement of high quantum efficiency.

Both formaldehyde and carbon monoxide are stable molecules. No radicals are produced in the glyoxal dissociation; hence, there is no problem of isotope mixing by radical reaction.

A significant advantage in using glyoxal as the photodissociation molecule is that a major absorption band is centered in the visible blue (4500A), a spectral region which matches existing highly efficient coumarin dye lasers. Therefore, frequency doubling is not required, and the energy efficiency of the overall enrichment process is correspondingly increased, generally by a factor of about ten.

In addition, glyoxal has a high vapor pressure (284 torr at 26.degree.C) and is stable in the vapor phase. Finally, glyoxal is easily prepared by passing ethylene over selenium dioxide at 150.degree. - 200.degree.C, thereby fulfilling requirement of ready availability.

The selective photodissociation of glyoxal can be used to obtain isotopic enrichment of deuterium, tritium, carbon-13, carbon-14, oxygen-17, and oxygen-18. Any given sample of glyoxal molecules consists mainly of the isotopic species .sup.16 O.sup.1 H.sup.12 C.sup.12 C.sup.1 H.sup.16 O since .sup.1 H, .sup.12 C, and .sup.16 O are the most abundant isotopes of their respective elements. A much fewer number of glyoxal molecules has a deuterium atom substituted for one of the hydrogen atoms, the isotopic composition being .sup.16 O.sup.1 H.sup.12 C.sup.12 CD.sup.16 O, where D represents deuterium (.sup.2 H). Extending this idea, the glyoxal sample contains numerous isotopic species representing all possible ways that the isotopes of H, C, and O can combine to form OHC--CHO. The fraction of each isotopic species present depends on the natural abundance of the particular isotopes comprising that species. Thus, for example the isotopic species OHC--CTO where T represents tritium (.sup.3 H) would be present only in trace amounts because tritium is only a trace component of naturally occurring hydrogen. Each of the isotopic species comprising the glyoxal sample is a distinct entity having its own unique absorption spectrum. By choosing a single strong absorption line which does not overlap absorption lines of other species and tuning the emission of a monochromatic spectral source, such as a laser, to precisely that wavelength, one can induce a highly selective photodissociation of the chosen isotopic species.

As a specific example of isotopic enrichment according to the present invention, consider monodeuterated glyoxal (OHC--CDO), a compound whose absorption spectrum has been extensively studied. One particular line in the spectrum of OHC--CDO, at 4545A, matches the emission wavelength of an argon laser. Glyoxal, containing a mixture of naturally occurring isotopic species, is allowed to vaporize at room temperature to a total pressure of about 10 Torr in a meter-long, quartz absorption cell. Argon laser light at a wavelength of 4545A is passed through the vapor in the cell. Only the OHC--CDO molecules absorb radiation of that wavelength. The selectively excited OHC--CDO molecules dissociate to produce deuterated formaldehyde. The reaction is represented as follows:

the resulting vaporous mixture is passed through a cold trap at a temperature of about -70.degree.C to about -80.degree.C to freeze out undissociated glyoxal, thereby separating it from the gaseous deuterated formaldehyde. Because of the highly selective nature of the photodissociation, the resulting deuterium enrichment is correspondingly high.

Any source of monochromatic radiation which is capable of being tuned to the narrow bandwidth and exact wavelength requirements can be used in the present process. Lasers are by far the most desirable radiation sources by virtue of their high monochromaticity.

Although the invention has been described with reference to preferred embodiments, various modifications and changes will be apparent to those skilled in the art, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims.