Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods391ticulates from their gas, liquid, or solid matrix. A second method, which isuseful for gas particles, solutes, and solids, is an extraction.Fi l t r a t i o nTo separate solid particulates from their matrix we use gravity or applysuction from a vacuum pump or aspirator to pull the sample through afilter. The type of filter we use depends upon the size of the solid particlesand the sample’s matrix. Filters for liquid samples are constructed from avariety of materials, including cellulose fibers, glass fibers, cellulose nitrate,and polytetrafluoroethylene (PTFE). Particle retention depends on the sizeof the filter’s pores. Cellulose fiber filter papers range in pore size from 30mm to 2–3 mm. Glass fiber filters, manufactured using chemically inertborosilicate glass, are available with pore sizes between 2.5 mm and 0.3 mm.Membrane filters, which are made from a variety of materials, includingcellulose nitrate and PTFE, are available with pore sizes from 5.0 mm to0.1 mm.Solid aerosol particulates are collected using either a single-stage or amultiple-stage filter. In a single-stage system, we pull the gas through a singlefilter, retaining particles larger than the filter’s pore size. When collectingsamples from a gas line, we place the filter directly in the line. Atmosphericgases are sampled with a high volume sampler that uses a vacuum pumpto pull air through the filter at a rate of approximately 75 m 3 /h. In eithercase, the filtering media for liquid samples also can be used to collect aerosolparticulates. In a multiple-stage system, a series of filtering units separatesand the particles in two or more size ranges.The particulates in a solid matrix are separated by size using one or moresieves (Figure 8.12). Sieves are available in a variety of mesh sizes rangingfrom approximately 25 mm to 40 mm. By stacking sieves of different meshsize, we can isolate particulates into several narrow size ranges. Using thesieves in Figure 8.12, for example, we can separate a solid into particles withdiameters >1700 mm, with diameters between 1700 mm and 500 mm, withdiameters between 500 mm and 250mm, and those with a diameter