Ion mobility spectrometry as an immunoassay detection technique

Conventional immunoassays employ a variety of techniques which usually have a spectrophotometric or fluorescence spectral method of detection to signal the presence of the targeted antigen in a suspect sample. As the concentration of the antigen is changed, a sigmoid-shape curve results from the response of the assay. A hand-held ion mobility spectrometer, which detects analyte ions at atmospheric pressure, was used to probe the ortho-nitrophenol (ONP) product vapor of an immunoassay reaction. Bacillus cereus organisms were employed in an enzyme-linked immunosorbent assay (ELISA) with the beta-galactosidase: ortho-nitrophenyl-beta- d-galactoside (ONPG) enzyme-substrate biochemical couple. ONP has an intense yellow color, and the ion mobility vapor responses were compared to that of the conventional spectrophotometric assay. Both detection techniques produced a sigmoid-shape curve characteristic of immunoassay experiments, and the bacterial detection limit with the ion mobility spectrometry technique was estimated at below 1000 cells for an 8 min assay time. Thus, a commercial, hand-held chemical vapor detector is shown to have the potential of detecting the presence of microorganisms using an immunoassay format.