• the partial pressure of the interferent, p 0 i . Even in the absence of measurement noise, the inverse problem has infinite solutions because: (Perspective 1) The response of the GS places only one constraint (through eqn. 4) on the two unknowns p 0 a , p 0 i . (Perspective 2) The mapping from the 2D composition space (p 0 a , p 0 i ) into the 1D sensor response space m by eqn. 4 is many-to-one .