Detection of 1 zmol injection of angiotensin using capillary zone electrophoresis coupled to a Q-Exactive HF mass spectrometer with an electrokinetically pumped sheath-flow electrospray interface

An electrokinetically pumped sheath-flow nanoelectrospray interface provides an efficient means of transferring ions from a capillary electrophoresis system into a mass spectrometer. To characterize its performance, we analyzed angiotensin solutions prepared in a background of 0.25 mg/mL of a BSA tryptic digest. Calibration curves were prepared from 10 zmol (10(-2)1 mol) to 10 fmol (10(-1)4 mol) of angiotensin injected into the capillary. A parallel reaction monitoring approach was used; MS1 was set to m/z = 523.8 +/- 2, and fragment ion signals at 263.1389 (y(2+)) and 784.4095 (b(6+)) were used to generate selected ion electropherograms. Calibration curves based on peak areas were linear (log-log slope of 0.94 for the y(2+) fragment and 0.98 for the b(6+) fragment). We then injected 1-zmol (600 copies) of angiotensin in the BSA background using a 10- mu m ID separation capillary. Triplicate analyses consistently produced co-migrating peaks for the two fragment ions. The b(6+) electropherogram showed no background signal, whereas the y(2+) electropherogram showed a few noise spikes that were smaller than the peak maximum. Bienayme-Tchebycheff inequality was used to estimate detection limits of 230 ymol (140 ions) injected into the separation capillary. To the best of our knowledge, these electropherograms present the smallest number of molecules detected using mass spectrometry coupled with a separation.