Capillary-based multi nanoelectrospray emitters: improvements in ion transmission efficiency and implementation with capillary reversed-phase LC-ESI-MS

Abstract

We describe the coupling of liquid chromatography (LC) separations with mass spectrometry (MS) using nanoelectrospray ionization (nano-ESI) multiemitters. The array of 19 emitters reduced the flow rate delivered to each emitter, allowing the enhanced sensitivity that is characteristic of nano-ESI to be extended to higher flow rate separations. The signal for tryptic fragments from proteins spiked into a human plasma sample increased 11-fold on average when the multiemitters were employed, due to increased ionization efficiency and improved ion transfer efficiency through a newly designed heated multicapillary MS inlet. Additionally, the LC peak signal-to-noise ratio increased approximately 7-fold when the multiemitter 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 microL/min.

Figure 1

Experimental setup. (A) Schematic depiction of the TOF instrument modified with an interchangeable heated capillary inlet and differentially pumped tandem ion funnels. The entrance and exit diameters of both the high-pressure and standard ion funnels were 25.4 mm and 2.5 mm, respectively. (B) Front view of the heated capillary inlets used for instrument Configurations 2-4 (Table 1). Capillary spacing (center-to-center) was 500 μm in Configuration 3 and 1.0 mm in Configuration 4. (C) Photograph of an array of 19 emitters positioned in front of a heated multi-capillary inlet (Configuration 4).

Figure 2

Sensitivity comparison of a mixture of standard peptides for the four different emitter/inlet configurations in Table 1. The solution flow rate was 2.0 μL/min. Additional description is in the text.

Figure 3

Mass spectra of Angiotensin I [M+2H]²⁺ (inside of dashed lines) from (A) configuration 1 and (B) configuration 4. The shaded baseline to the left of the analyte signal was magnified 10-fold for clarity.

Figure 4

Total ion chromatograms of depleted human plasma (1 μg total sample) using a multi-emitter (configuration 4) and a single emitter (configuration 2).

Figure 5

Signal enhancement for 10 peptides spiked into depleted human plasma tryptic digest. Peptide I.D.s are listed in Table 2. Additional description is in the text.

Figure 6

Extracted ion chromatograms of Peptide 10 (Table 2) from analyses employing a single emitter and an array of 19 emitters. The peaks are overlaid in the inset, showing identical peak shapes and widths.

Figure 7

Extracted ion chromatograms of Peptide 7 (Table 2) from 3 replicate separations using (A) configuration 4 and (B) configuration 2. Scale bar in left panel of (A) equals 1 min and applies to all panels.