Microspheres are used as an aerosol challenge to test filter integrity at the filter manufacturer and in situ in cleanroom installation. Microspheres are specified where oils or oil vapor / outgassing are considered Airborne Molecular Contaminants ( AMC ). The Milholland Microsphere Stock Solution is convenient and easy to use. Simply add 50ml to 3.8 liters (one US gallon) of DI water and add to generator. 500ml of stock solution makes 10 gallons (38 liters) of test solution.