Low Flow Voltage Free Interface for Capillary Electrophoresis and Mass Spectrometry Driven by Vibrating Sharp-Edge Spray Ionization

Abstract

Capillary electrophoresis-mass spectrometry is a powerful technique for high-throughput and high efficiency separations combined with structural identification. Electrospray ionization is the primary interface used to couple capillary electrophoresis to mass analyzers; however, improved designs continue to be reported. A new interfacing method based on vibrating sharp-edge spray ionization is presented in this work to overcome the challenges of decoupling applied voltages and to enhance the compatibility with separations performed at near-neutral pH. The versatility and ease of use of this ionization source is demonstrated using β-blockers, peptides, and proteins. The cationic β-blocker pindolol was injected electrokinetically, and detected at concentrations ranging from 10 nM to 5 μM, with an estimated detection limit of 2 nM. The vibrating sharp-edge spray ionization functions with flow rates from 70 to 200 nL/min and did not perturb the capillary electrophoresis separation electroosmotic flow as evidenced by the observation that most migration times differed less than 7% (n = 3) across a lab-built system interfaced to mass spectrometry and a commercial system that utilizes absorbance detection. For cationic beta-blockers the theoretical plates achieved in the capillary electrophoresis-mass spectrometry setup were 80%-95% of that observed with a commercial capillary electrophoresis-UV absorbance detection system.

Figure 1.

Concept diagram and image of the spray probe, capillary and electrode alignment in the (A) z-direction and the (B) xy-direction. (C) Image of the CE-VSSI interface in front of the linear trap mass spectrometer.

Figure 2.

(A) CE-VSSI-MS separation of 1 μM pindolol and acebutolol. The extracted ion electropherograms were created using masses of 249.1554 and 337.2061 for pindolol and acebutolol, respectively, with a mass tolerance of 10 ppm. Separation was achieved with a 40 cm (total and effective length), 50 μm i.d. capillary at an applied voltage of +10 kV with a current of 12 μA. (B) CE-UV separation of 50 μM pindolol and 100 μM acebutolol. Separation was achieved with a 50 cm (total length), 40 cm (effective length), 50 μm i.d. capillary at an applied voltage of +12.5 kV with a current of 12 μA. All separations were achieved with a background electrolyte of pH 6.5, 25 mM ammonium acetate.

Figure 3.

(A) CE-VSSI-MS separation of 50 μM somatostatin and oxytocin. The extracted ion electropherograms were created using masses of 820.1 and 504.3 for somatostatin and oxytocin, respectively, with a mass tolerance of 500 mmu. (B) CE-UV separation of 50 μM somatostatin and oxytocin. The peak marked with the asterisk in 3B is an electroosmotic flow marker (dimethylformamide). Separation conditions are listed in Figure 2.

Figure 4.

(A) CE-VSSI-MS separation of ubiquitin and trypsin inhibitor with an extracted ion electropherogram of m/z 1428.2765 and 1817.0015 for ubiquitin (black trace) and trypsin inhibitors (blue trace), respectively, with a mass tolerance of 10 ppm. (B) CE-UV separation. Mass spectra of ubiquitin (C) and trypsin inhibitor (D) display the most abundant charge states for the width of the peaks in (A) at half the maximum intensity. Separation conditions are as in Fig 2, except the background electrolyte is 50 mM ammonium acetate at pH 6.5 (i = 22 μA).