Capillary-Based Multi-Nanoelectrospray Emitters: Improvements in Ion Transmission Efficiency and Implementation with Capillary Reversed- Phase LC-ESI-MS Ryan T. Kelly, Jason S. Page, Rui Zhao, Wei-Jun Qian, Heather M. Mottaz, Keqi Tang, and Richard D. Smith * Biological Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352 Abstract We describe the coupling of liquid chromatography (LC) separations with mass spectrometry (MS) using nanoelectrospray ionization (nanoESI) multi-emitters. The array of 19 emitters reduced the flow rate delivered to each emitter, allowing the enhanced sensitivity that is characteristic of nanoESI to be extended to higher flow rate separations. The signal for peptides from spiked proteins in a human plasma tryptic digest increased 11-fold on average when the multi-emitters were employed, due to increased ionization efficiency and improved ion transfer efficiency through a newly designed heated multi-capillary MS inlet. Additionally, the LC peak signal-to-noise ratio increased ∼7-fold when the multi-emitter configuration was used. The low dead volume of the emitter arrays preserved peak shape and resolution for robust capillary LC separations using total flow rates of 2-μL/min. INTRODUCTION The rapid growth in the use of electrospray ionization mass spectrometry (ESI-MS) over the past 2 decades, particularly for biological applications, has been accompanied by ongoing efforts to increase its sensitivity. 1,2 These efforts have been motivated by the fact that only a small fraction of the analyte ions ever reach the detector. 3-6 While ion losses can occur in the mass analyzer, most of the losses presently encountered arise from incomplete droplet desolvation or during transport from atmospheric pressure to the high vacuum region of the mass spectrometer. Many innovations have been implemented to reduce ion losses at the atmospheric pressure interface, including the use of drying gases, 7 air amplifiers, 8 and electrostatic lenses. 9 In addition, interfaces that replace the standard skimmer with the electrodynamic ion funnel 10-16 have proven far more efficient for transmitting ions through the first vacuum stage. Besides hardware modifications, operation of ESI at low flow rates can also greatly improve efficiency. The smaller charged droplets produced by an electrospray in the so-called nanoESI regime facilitate desolvation and increase the amount of excess charge available per analyte molecule. 17-21 Quantitation is thus improved, 20,22,23 and the emitter can be positioned closer to the MS inlet, further reducing ion losses. 1,21 However, problems associated with nanoESI such as emitter clogging, and the much higher flow rates of most liquid chromatography (LC)- ESI-MS analyses, can preclude operation in the nanoESI regime with its associated benefits for sensitivity. Clogging issues 5,23,24 arise from the internal taper and the requisite narrow orifice of the pulled fused silica or glass emitters typically employed for nanoESI. Fortunately, the recent development of chemically etched fused silica emitters 25 in large part solves the * Corresponding Author. email: rds@pnl.gov NIH Public Access Author Manuscript Anal Chem. Author manuscript; available in PMC 2009 January 1. Published in final edited form as: Anal Chem. 2008 January 1; 80(1): 143–149. doi:10.1021/ac701647s. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript