Technique in which a mobile phase additive causes a positive detector signal output. When a solute elutes from the column, it dilutes the signal and gives a negative peak (a vacancy). The technique has been most recently applied to single-column ion chromatography, in which mobile phases that absorb in the UV region, such as citrate and phthalate buffers, are used. When a none sorbing anion elutes, it dilutes the UV-absorbing background and causes a negative peak the detector output leads are usually reversed so that the chromatogram looks normal.