A method and apparatus for sampling for the chemical warfare agent VX, where it is advantageous for the VX to be converted to its G-analog to facilitate aspirator induced movement of the sample through a sample line, to a sampling tube or to sample detection apparatus. A conversion tube assembly has a conversion tube member defining inlet and outlet openings and having a conversion filter therein, which is saturated with an A to G reagent for a chemical warfare agent of interest. The conversion filter assembly includes non-impregnated filter elements and the ends thereof and is retained in position intermediate to the ends of the conversion tube member by retainer elements, which may be tubes or rings. The conversion filter assembly is adapted to be received by a compression fitting or may be directly assembled to a sampling tube or sampling line.