The first category are ratios close to zero (i.e. < 0.1 ratio), which account for ≈10 % of the samples. These measurements report negligible or no uptake of nitrogen or argon. Figure shows that most of the materials close to the x-axis (near-zero measured pore volume) also have a below-average geometric pore volume in the range of 0.25-0.5 cm 3 g -1 . With such a 0.00 0.25 0.50 0.75 1.00 1. 25 small pore volume computed from the crystal structure, one could suspect small interstices where the nitrogen (or argon) may not fit or permeate.