Selective ionization of melamine in powdered milk by using argon direct analysis in real time (DART) mass spectrometry

5-Hydroxymethylfurfural (5-HMF) is a compound with the elemental composition C6H6O3 that is present in powdered milk. Protonated 5-HMF (calculated m/z 127.0395) has the same nominal m/z as protonated melamine (calculated m/z 127.0732) and can interfere with direct analysis of melamine in powdered milk. Tandem mass spectrometry and high-resolution mass spectrometry have been previously used to distinguish melamine from 5-HMF. An alternative approach is presented here that uses the direct analysis in real time (DART (TM)) ion source operated with argon gas in combination with acetylacetone and pyridine reagent gases to selectively ionize melamine and eliminate the interference from 5-HMF. High-resolution/accurate mass data were used to verify the elimination of the 5-HMF interference and confirm the melamine elemental composition. With further refinement, this technique could lead to a rapid analysis method for screening large numbers of samples.