A method for analyzing formation fluid in a subterranean formation is disclosed, wherein the method includes the steps of: adding a scavenger compound to an analytical reagent to form a reagent solution; collecting an amount of formation fluid into a formation tester, wherein the formation tester includes at least one fluids analyzer comprising at least one probe, at least one flow line, at least one reagent container, and at least one spectral analyzer, wherein the fluids analyzer is configured such that the collected formation fluid is fed through the at least one flow line to the at least one spectral analyzer; mixing an amount of the collected formation fluid with an amount of the reagent solution to form a mixture; and analyzing the mixture downhole.