An optical digital external modulator having a cascaded arrangement of interferometers is provided. In one embodiment, an emissions treatment system for an exhaust stream is provided having a catalyst comprising silver on a particulate alumina support, the silver having a diameter of less than about 20 nm. More specifically, the invention provides a method for use with digitized mass spectra that facilitates the distinction between low level signals and noise using the correlation of signals therein based on their mass differences.