1.4 SAMPILN RATES. The sampling rate must correlate with the size of the filter and the impingers used. Ut is criticak that the combination of the filter size and sampling rate of 11-12 cm/s (9). At lower face velocities, H2SO4 may condense on the filter. The most convenient and the recommended combination is a 47-mm filter, a sampling rate of 12L/min and two 250 mL of 3% H2O2. An alternate smaller-scale arrangement involves a 25-mm filter, a sampling rate of 1 l/min and two 25 mL midget impingers in series, each containing 15 mL of 3% H2O2.