A formation tester comprises a body having an outlet; a probe extendable from the body and having a sealing pad; and a flow line within the body, wherein the flow line has an entry end connectable to the probe and has an exit end connectable to the outlet in the body; and a reactive filter material in the flow line downstream of the entry end of the flow line, wherein the reactive filter material sorbs and entraps an analyte in a wellbore fluid.