BCEIA Conference 2007 - Individual Abstract Info - Session: Mass Spectrometry
Using an Electrospray Membrane Probe to Enhance Electrospray MS Performance
Craig M. Whitehouse; Thomas P. White and Shida Shen
Analytica of Branford, Inc., 29 Business Park Drive Branford, Connecticut 06405 USA
An Electrospray Membrane Probe has been developed as a tool to expand the analytical capability of Electrospray ion sources and to explore the fundamental mechanisms underlying Electrospray Ionization interfaced to mass spectrometry. Electrospray ionization includes five primary steps; (1) the production of monopolarity charged liquid droplets from a sample solution, (2) evaporation of the charged liquid droplets, (3) production of ions from the rapidly evaporating charged liquid droplets (4) gas phase charge exchange reactions and (5) ion transport into vacuum. All five steps must be taken into account when conducting a study to explain the mechanisms underlying overall Electrospray ionization efficiency. The generation of charged droplets in Electrospray ionization using a stable Taylor cone or with pneumatic nebulization assist requires the occurrence of oxidation or reduction reactions (redox) at conductive surfaces in the sample solution flow path. The redox or electrolysis reactions occurring in Electrospray ionization constitute half of an electrochemical cell. The complementary half of the electrochemical cell occurs in the gas phase in Electrospray ionization. The total Electrospray current supplied from these redox reactions depends on solution and electrical variables including the sample solution dielectric constant, conductivity, liquid flow rate, applied electric voltage, Electrospray needle and counter electrode geometries, conductive surface material and the location of the conductive surface along the sample solution flow path relative to the Electrospray needle tip. For a fixed applied voltage between the sample solution exiting the Electrospray tip and the endplate counter electrodes and for a constant liquid flow rate, the Electrospray total ion current is a function of the net solution conductivity to the conductive surface in the sample solution flow path. The effective sample solution conductivity can change with solvent composition and the electrolyte type and concentration added to the sample solution. Electrolytes such as acetic acid, formic acid or triflouroacetic acid (TFA) added to sample solutions aid in upstream separations and to increase the rate of electrolysis in Electrospray ionization increasing the Electrospray total ion current. Often an electrolyte, such as TFA is added to the sample solution to improve reverse phase gradient Liquid Chromatography separation efficiency but reduces the downstream Electrospray ionization efficiency.
The electrolyte added to the sample solution plays an electrochemical role in the redox or electrolysis reactions that occur on conductive surfaces in the sample solution flow path. Less understood mechanistically are the additional roles that added electrolytes play in the ion production from evaporating liquid droplets, gas phase charge exchange reactions and ion transport into vacuum. An Electrospray Membrane probe has been developed to provide fine control of Electrospray total ion current, electrolyte type and concentration, and to remove redox reactions from occurring on conductive surfaces in the sample solution flow path. The Electrospray (ES) Membrane probe incorporates a semipermeable membrane that separates the sample solution flow channel from a second solution flow channel. For the studies reported, Nafion™ proton exchange membranes were configured in the membrane probes. An electrode is configured in contact with the second solution and redox reactions occur at the surface of this electrode during Electrospray ionization. In the case of positive polarity Electrospray ionization, protons produced at the second solution electrode surface, are transferred through the proton exchange membrane into the sample solution, driven by the electric field, and pass through the Electrospray tip forming the Electrospray total ion current. An electrode is configured in contact with the second solution and redox reactions occur at the surface of this electrode during Electrospray ionization. Increasing the electrolyte concentration in the second solution increases the total Electrospray current by effectively increasing the conductivity of the second solution and the sample solution locally along the sample solution flow path between the membrane and the Electrospray tip. The second solution flow is supplied by a gradient syringe pump allowing the scanning or ramping of electrolyte concentration during Electrospray ionization. As the Electrospray current increases, in positive polarity ionization mode, the pH decreases proportionally in the sample solution flow path between the membrane and the Electrospray tip. Scanning Electrospray total ion current by ramping the electrolyte concentration in the second solution will effectively scan to lower pH in positive ion mode. This Electrospray membrane probe total ion current or pH scanning technique can be used to expand Electrospray analytical capability and to rapidly determine optimal Electrospray performance for different samples, solvent and electrolyte species, and concentrations. For example pH scanning can be used to rapidly fold and unfold proteins during Electrospray ionization.
A study was conducted using the Electrospray Membrane probe to quantify the Electrospray signal response for solvent and electrolyte compositions typically used in online Liquid Chromatography separations. Using the Electrospray Membrane probe to conduct conductivity and pH scans, Electrospray analyte signal response was measured for different organic solvent types and concentrations in aqueous solutions and for different electrolyte types and concentrations. The Electrospray Membrane probe was interfaced to an Analytica of Branford, Inc. Corsai® Time-Of-Flight Mass Spectrometer. Results obtained using the Electrospray Membrane probe were then compared to Electrospray MS measurements taken with electrolytes added directly to the sample solution with standard Electrospray probes. The concentrations of polar and non polar solvents in aqueous solutions were increased starting with 100% water. Electrospray total ion current and analyte signal was measured versus percent organic solvent. Concentration gradients of different electrolyte species were run through the membrane probe second solution flow channel for different sample solution organic solvent concentrations. Analyte Electrospray MS signal was mapped as a function of electrolyte species and concentration for different organic solvent types and concentrations. It was found that non polar solvents such as acetonitrile in aqueous solutions will reduce analyte Electrospray MS signal compared with a polar solvent such as methanol in aqueous solutions. The addition of inorganic acid electrolytes to polar aqueous solvents reduce the analyte Electrospray MS signal more than the case when inorganic acid electrolytes are added to non polar aqueous solvents. The results obtained in the study shed light on the roles played by the electrolyte species and solvent composition in Electrospray ionization efficiency. The study has shown that some electrolytes not conventionally used in Electrospray ionization can enhance Electrospray performance.