We have explored the utility of gas-phase fractionation by mass spectrometry (MS) in the mass-to-charge (m/z) dimension (GPF(m/z)) for increasing the effective number of protein identifications in cases where sample quantity limits the use of multi-dimensional chromatographic fractionation. A peptide digestate from proteins isolated from the membrane fraction of natural killer (NK) cells was analyzed by microcapillary reversed-phase liquid chromatography coupled online to an ion-trap (IT) mass spectrometer. Performing GPF(m/z) using eight narrow precursor ion scan m/z ranges enabled the identification of 340 NK cell proteins from 12 microg of digestate, representing more than a fivefold increase in the number of proteins identified as compared to the same experiment employing a standard precursor ion survey scan m/z range (i.e., m/z 400-2000). The results show that GPF(m/z) represents an effective technique for increasing protein identifications in global proteomic investigations especially when sample quantity is limited.