80 UADIATION BIOLOGY 



method has soomed even more attractive for pharmaceutical liquids such 

 as vaccines, serums, l)lood, and blood plasma, to which pasteurizing tem- 

 peratures may he destructive. Such licjuids are much more al)sorpti\'e 

 of ultraviolet than is water. 



Kjjvdivv Depth of Penetration. Absorption of ultraviolet by these 

 li(iuids is expressed in Fig. 2-lG, in practical terms of the depth or film 

 thickness through which there is a 90 per cent absorption of 2537 A ultra- 

 violet. This "effective depth of penetration" can, however, be made 

 fully effective only in film-spreading devices which ensure enough tur- 

 l)ulence in the film to expose all particles to the full range of ultraviolet 

 intensities through the depth of the film during the exposure time. 



Film Spreaders. Liciuids with effective depths of penetration less than 

 0.1 in. must be irradiated in special ecjuipment which will expose those 

 liquids in moving layers of less than this depth. With the more absorp- 

 tive or more viscous liquids, films of controlled thickness cannot be pro- 

 duced by gravity, and it is necessary to resort to centrifugal methods or 

 to the wetting of moving surfaces from which the liquids are removed 

 after irradiation. Some of the many possible thin-film irradiators are 

 suggested schematically in Fig. 2-18. The film thickness and rate of 

 flow (exposure time) is by gravity and limited to it. In type C, both 

 thickness and flow are by centrifugal force and thus are subject to con- 

 siderable control. In type D, the film flow is by gravity and the slope 

 of the cylinder, \vhile the film is spread by centrifugal force, the latter 

 to some extent decreasing the gravity flow. Type E is almost entirely 

 dependent on adhesion and viscosity for the film formation and is inher- 

 ently limited as to the thinness of film formed. Type C provides for 

 short-time irradiation of thinner films, but with difficulty in providing the 

 required high ultraviolet intensit.y. Types D and E provide for longer 

 time, lower intensity irradiation, but with diflRculty in providing suffi- 

 ciently thin films of the more absorptive liquids. In types B, C, and D, 

 the irradiated liquid may be protected from contact wdth air and ozone by 

 filling the irradiators with a neutral gas such as nitrogen or carbon diox- 

 ide. In types C and E, the thickness of the irradiated film can be con- 

 trolled, regardless of viscosity, by the use of mechanical film spreaders. 

 Type F defines the film thickness between flat plates of fused-quartz 

 glass and the exposure time by the rate of flow. All but type F have the 

 time-proved feature of irradiation of and through the free-flowing and 

 continually renewed surface of the irradiated liquid. Type F has the 

 important feature of being inherently a closed system adaptable to con- 

 tinuous pressure operation. The slower moving liquid surface in con- 

 tact with the device is exposed for a longer time to compensate partially 

 for the reduction of the ultraviolet intensity by the absorption of the 

 liquid. Deposits of the nature of polymerization products, which usually 

 form on the film contact surfaces of these devices, are less objectionable 



