The unresponsive locus is an intersection of the level sets of the adsorption functions for each MOF in the sensor array, B = ∩ M i=1 {x : a i (x) = m i,op }. By definition, the sensor array cannot distinguish between gas compositions in the unresponsive locus. A change in the gas composition from x op ∈ B to x ∈ B is undetectable despite x = x op . Fundamentally, the response of a sensor array composed of cross-sensitive MOFs remains constant under the composition change from x op to x because the adsorption of one set of species balances [in mass] the desorption of another set of species (see Fig. ). A trivial example of a non-trivial unresponsive locus is when all MOFs do not adsorb a particular component of the gas.