Vacuum System Filtration

The aspiration of tissue culture media from monolayer cultures and of supernatants from centrifuged samples into primary collection flasks is a common laboratory procedure. Protection should be provided against pulling biohazardous aerosols or overflow fluid into the vacuum system. This protection is provided by the use of an air filter in the line immediately leading into the house vacuum line and an overflow flask for liquids between the collection flask and the air filter.

Two techniques for protecting the vacuum system are shown. A cartridge-type filter provides an effective barrier to passage of aerosols into the house vacuum system. The filter has a capacity to remove airborne particles 450 nm (0.45μ) or larger.

For assembling either apparatus, flexible tubing is used of appropriate inside diameter for the flask and filter fittings and of sufficient wall thickness for the applied vacuum. Filter flasks of capacities from 250 to 4000 ml may be used for the overflow flask, depending on available space and amount of fluid that could be accidentally aspirated out of the collection flask.

The overflow flasks contain a disinfectant solution appropriate for the biohazardous material under study. It is essential that an antifoam, such as Dow Corning Antifoam A, be added to the overflow flask, since bubbling of air through the disinfectant probably will cause considerable foam which, if allowed to reach the filter, will shut off the vacuum.

If the filter becomes contaminated or requires changing, the filter and flask can be safely removed by clamping the line between filter and vacuum source. The filter and flask should be autoclaved before the filter is discarded. A new filter can then be installed and the assembly replaced.

The apparatus shown is composed of two suction flasks, a filter, rubber stoppers, flexible vacuum tubing, glass tubing and a small glass sparger. Various small fritted glass or ceramic spargers or gas dispersion tubas are commercially available. The coarse or medium porosity sparger assures that any aerosol passing through the collection flask is dispersed in small bubbles so that adequate contact is made with the disinfectant solution. 

The apparatus depicted in B has the feature of automatically shutting off the vacuum when the storage flask is full. It consists of a 1 liter filter flask with a small glass Buchner funnel (15 ml capacity, 29 mm filter disc) inserted upside down in a No. 8 rubber stopper in the mouth of the flask. A hole, 2 cm in diameter, is cut into the bottom of the stopper with a cork borer and of sufficient depth that the filter disc is level with the bottom of the stopper. 

A 14 gr (1/2 oz) rubber bulb measuring 6 cm (2 3/8 inches) in length and 3.2 cm (1 1/4 inches) in diameter, with the end plugged with a solid glass rod measuring 0.6 cm (1/4 inch) in diameter and approximately 5.7 cm (2 1/2 inches) in length, is placed inside the flask.

If liquids enter the overflow flask the rubber bulb rises until it presses against the mouth of the Buchner funnel and shuts off the vacuum. The entire unit is autoclavable, but the filter assembly should be thoroughly dried before reuse.