We emphasize that the sensitive and unresponsive directions are (i) depend on the operation gas composition x op and (ii) pertain to small changes in the gas composition since they rely on the linear approximation of the generally non-linear function a(x) in eqn. 2. We remark on the uniqueness of the SVD in eqn. . The singular values are unique. The left and right singular vectors associated with the distinct, non-zero singular values are unique, up to both corresponding vectors being multiplied by -1. The right singular vectors spanning the null space are much more arbitrary in the sense that they can be any orthonormal basis for the null space.