Used for non-UV absorbing or nonfluorescing analytes. A UV-absorbing or fluorescent compound added to the mobile phase maintains a high background signal; when a nonabsorbing or nonfluorescing analyte is eluted, the background is diluted and a negative peak is observed for that analyte. When an analyte acts to increase the concentration of the indicating species, it produces a positive peak. When a negative signal is detected, the detector signals are reversed to the output device.