If you want to do quantitative comparisons with TPD, it is always recommendable to keep something constant or at least well-referenced. Basic parameters to keep constant would be either the surface concentration (so molecules/cm2) or the absolute coverage (so number of covered adsorption sites / number of all adsorption sites). Unless you know the absolute coverage calculation from other experiments, keeping the surface concentration constant is probably the best ides for the first series.

Dear Jürgen Weippert, thanks very much for your insightful response.