330
10.
PARTICULATE SPECIMENS
It is the virologists who have found the nebulizing technique particularly important for their work. Naturally, some of their suspensions
are potentially infectious, and perhaps highly dangerous, so that one
must not spray into an open room. Home and Nagington (1959) have
described a simply constructed spraying box which attaches tightly to
the Vaponefrin nebulizer. The grids are fully enclosed while being
sprayed, and the exhaust air is passed first through a washing chamber,
and finally to the outside through a Bunsen burner flame.
The device described above can be used only with relatively low
viscosity media. High pressure spray guns mainly have industrial applications when highly viscous materials need to be sprayed. A relatively
simple gun was described by Backus and Williams (1950). A derivative
version of this is now commercially available from Ernest F. Fullam, Inc.
The latter uses a can of pressurized Freon gas as the driving force, and
introduces the material to be fragmented into the fast flow of this gas
as it escapes through a nozzle. This high velocity stream then can be
sprayed on prepared grids. Low viscosity media, however, are not broken
up at all as effectively by this instrument as by the Vaponefrin device.
Nixon and Fisher (1958) have described quite an elaborate spraying apparatus which purports to allow a superior control of the final drop size.
Its complications would hardly seem to warrant its construction except
for special purposes.
When there is an interest in knowing the average volume of sprayed
microdroplets, this can be determined by a method first described by
Backus and Williams (1949). This consists of adding indicator particles
in known concentration to the suspension. The droplet patterns will then
contain particles in proportion to their original volumes. Standardizing
the number of indicator particles in different suspensions, and then determining their ratios with the particulate material of interest, allows different suspensions to be quantitatively compared. Although any type
of dense, small, recognizable particle might be used as an indicator,
polystyrene latex particles* often can serve best.
The surface tension forces associated with drying tend to flatten particulate matter against the supporting film. (The degree of deformation
ordinarily can be determined quite accurately after heavy metal shadowing by measuring shadow length and knowing the angle of shadowing
* Available upon request from the Physical Research Laboratory, The Dow Chemical
Co., Midland, Michigan.
10.
PARTICULATE SPECIMENS
It is the virologists who have found the nebulizing technique particularly important for their work. Naturally, some of their suspensions
are potentially infectious, and perhaps highly dangerous, so that one
must not spray into an open room. Home and Nagington (1959) have
described a simply constructed spraying box which attaches tightly to
the Vaponefrin nebulizer. The grids are fully enclosed while being
sprayed, and the exhaust air is passed first through a washing chamber,
and finally to the outside through a Bunsen burner flame.
The device described above can be used only with relatively low
viscosity media. High pressure spray guns mainly have industrial applications when highly viscous materials need to be sprayed. A relatively
simple gun was described by Backus and Williams (1950). A derivative
version of this is now commercially available from Ernest F. Fullam, Inc.
The latter uses a can of pressurized Freon gas as the driving force, and
introduces the material to be fragmented into the fast flow of this gas
as it escapes through a nozzle. This high velocity stream then can be
sprayed on prepared grids. Low viscosity media, however, are not broken
up at all as effectively by this instrument as by the Vaponefrin device.
Nixon and Fisher (1958) have described quite an elaborate spraying apparatus which purports to allow a superior control of the final drop size.
Its complications would hardly seem to warrant its construction except
for special purposes.
When there is an interest in knowing the average volume of sprayed
microdroplets, this can be determined by a method first described by
Backus and Williams (1949). This consists of adding indicator particles
in known concentration to the suspension. The droplet patterns will then
contain particles in proportion to their original volumes. Standardizing
the number of indicator particles in different suspensions, and then determining their ratios with the particulate material of interest, allows different suspensions to be quantitatively compared. Although any type
of dense, small, recognizable particle might be used as an indicator,
polystyrene latex particles* often can serve best.
The surface tension forces associated with drying tend to flatten particulate matter against the supporting film. (The degree of deformation
ordinarily can be determined quite accurately after heavy metal shadowing by measuring shadow length and knowing the angle of shadowing
* Available upon request from the Physical Research Laboratory, The Dow Chemical
Co., Midland, Michigan.