This report describes the relation between peak capacity and mass spectrometric detectivity on peptides analysis with low-density octadecylsilylated monolithic silica capillary columns for several acidic additives in mobile phase using a nano-flow LC/MS. As the acidic additives, trifluoroacetic acid (TFA), 3,3,3-trifluoropropionic acid (TriFPA), formic acid (FA), acetic acid (AA), and cyanoacetic acid (CAA) were evaluated. Peak capacity and mass spectrometric detectivity were evaluated by using the peptides mixture in the gradient elution of water/acetonitrile with 0.1% (v/v) additives at 1-50% acetonitrile composition for 35 minutes on low-density octadecylsilylated monolithic silica capillary columns. Peak capacity is usually estimated by using the ratio of gradient time to average peak width of peptides. However, we estimated the peak capacity by using the ratio of t₀ to the time of final peak to average peak width of peptides because the retention time of peptides was varied by difference of additives. As a result, peak capacity was increased by utilizing higher acidity additives, while the mass spectrometric detectivity was decreased. This result suggested that both high peak capacity and high detectivity for peptides analysis is irreconcilable. To overcome this relation, the impact of CAA was investigated because of its thermolysis behavior. In the condition of mobile phase with CAA at high temperature condition of mass spectrometry capillary inlet, higher peak capacity than those with TriFPA and even detectivity for those with FA were observed.