The purpose of this program has been to develop and characterize a high-purity filter medium for measuring particulates in stack gases at 230 to 540C. Sufficient purity to allow optimum use of the detection limits of atomic absorption (AAS) and flame emission spectrometry (FES), and reproducible collection efficiency of 99% for dense particles as small as 0.05 um were primary objectives. The work has included analysis and purification of filter materials, filter handsheet preparation, and filter characterization. Johns-Manville Co. 99.2% silica fibers have been made into handsheet filters with satisfactory efficiency, temperature resistance, cost (about $1/square foot), strength (about 1 lb/in.), and flexibility. Almost all purity requirements for optimum AAS and FES analysis have been achieved. The strengthened filters are insensitive to humidity, insoluble in most acids and organic solvents, and slightly alkaline.