NOVELTY - The method comprises passing a reactant sample through a fuel cell (12) to form reaction products that exit the fuel cell in an output stream (28), adding a derivatizing reagent to the output stream to form a derivatized output stream, directing a charged solvent towards the derivatized output stream for ionizing the derivatized output stream, and directing the ionized, derivatized output stream to a mass spectrometer. The mass spectrometer is configured to detect the derivatized reaction product. The derivatizing reagent reacts with a potential reaction product.    USE - The mass spectrometry-based method is useful for directly detecting fuel cell reaction products such as formaldehyde and carbon dioxide, where the fuel cell includes a proton-exchange membrane fuel cell and a direct methanol fuel cell (all claimed) that is useful as an excellent energy source for portable electronics, land vehicles, airplanes and submarines.    ADVANTAGE - The mass spectrometry-based method is capable of directly detecting the fuel cell reaction products with high selectivity and sensitivity.    DETAILED DESCRIPTION - The method comprises passing a reactant sample through a fuel cell (12) to form reaction products that exit the fuel cell in an output stream (28), adding a derivatizing reagent to the output stream to form a derivatized output stream, directing a charged solvent towards the derivatized output stream for ionizing the derivatized output stream, and directing the ionized, derivatized output stream to a mass spectrometer. The mass spectrometer is configured to detect the derivatized reaction product. The derivatizing reagent reacts with a potential reaction product to form a derivatized reaction product if the potential reaction product is present. The charged solvent is generated by desorption electrospray ionization. The derivatized output stream is ionized by one of photo, ion beam, high energy particle or plasma-based ionization. The derivatizing reagent is directly doped with the charged solvent. An INDEPENDENT CLAIM is included for an apparatus for detecting fuel cell reaction products.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross sectional view of an apparatus for directly detecting the fuel cell reaction products.    Fuel cell (12)    Reaction chamber (20)    Electrode (22)    Output stream (28)    Ambient ionizer. (40)