The packing, not the injected probes, is the object of study in inverse gas chromatography (IGC). Known volatile probes are pulsed or flowed through a column filled with the solid, polymer, or powder of interest; their retention volumes, peak shapes, and temperature dependence report surface energy, acid–base parameters, solubility parameters, and porosity of that packing ^[1]. In the infinite-dilution limit, retention is governed by probe–surface interactions at vanishing coverage; at finite concentration the same experiment yields adsorption isotherms ^[2]. Conceptual boundaries exclude ordinary gas chromatography, in which the sample is unknown and the column is a calibrated separating medium, and they exclude inverse liquid chromatography. Essential features are a well-characterized probe set, a defined temperature and carrier-gas flow, and a data reduction that converts retention into physicochemical descriptors of the stationary material ^[3]. Surface-energy measurements of powders and crystalline polymorphs confirm that an inverse-gas chromatogram is interpreted as a property of the solid rather than as identification of the probe vapor ^[4].