ASMS 2007 - Individual Abstract Info - Session: Novel MS Instrumentation
Utilizing an Electrospray Membrane Probe to Explore Fundamental Mechanisms Underlying Electrospray Ionization in Mass Spectrometry
Craig M. Whitehouse; Thomas P. White
Analytica of Branford, Inc., Branford, CT
Electrospray membrane probes are used in studies of Electrospray signal response with different electrolyte counter ion species in Electrospray ionization.
Electrospray (ES) Ionization includes oxidation and reduction reactions at conductive surfaces in the sample solution flow path. The type and concentration of electrolyte counter ion species effects ES ionization efficiency. ES signal is enhanced when specific organic acid species such as acetic and formic acids are added to organic and aqueous solvents. Conversely, ES signal is reduced when inorganic acids such as hydrochloric or triflouroacetic acid are added to ES sample solutions. Although mechanisms underlying variation in Electrospray ionization efficiency due to different electrolyte counter ion species have been proposed, explanations of these root modulators underlying Electrospray ionization processes remain speculative. A study has been conducted to shed light on the role of the electrolyte counter ion in Electrospray ionization.
An Electrospray membrane probe incorporating Nafion proton exchange membranes separating the sample solution and second solution flow channels was interfaced to an Analytica of Branford Corsair™ Time-Of-Flight mass spectrometer. Sample solutions included pure water, acetonitrile or methanol or incremental differences in percent organic solvent in aqueous solutions. ES signal response was recorded as a function of polar (methanol) and non polar (acetonitrile) organic solvent percent concentration while running Electrospray total current scans using acid concentration gradients in the Electrospray membrane probe second solution flow channel. Analyte signal was recorded during Electrospray membrane probe current scans for a range of acid electrolyte species. Results were compared with adding the same acid electrolyte species at different concentrations directly to the sample solutions.
Electrospray current and pH scanning was conducted using Electrospray membrane probes to provide a highly resolved map of Electrospray signal response for different Electrospray operating conditions. This study focused on the efficiency of positive and negative polarity Electrospray ionization when running different acid electrolytes added to a range of solvent compositions. Electrospray signal was compared for electrolyte species added directly to the sample solution flow and electrolyte concentrations scanned in the second solution flow of Electrospray Membrane Probes while running pure solvent sample solutions. Electrospray membrane probes were configured with proton exchange membranes isolating the sample solution flow from a second solution flow. Redox reactions occur at electrode surface in contact with the second solution flow. Acid or electrolyte concentrations were ramped using gradients in the second solution flow to control the Electrospray electrical current and pH while mapping the sample ion signal response. The electrolyte counterion permeability through the proton exchange membrane was species and concentration dependent but in general, the membrane probe provided considerable reduction of the electrolyte counter ion for a given ES electrical current compared with adding the electrolyte directly to the sample solution as is conducted in standard Electrospray. Signal maximum for a given analyte species occurred at different Electrospray currents for different acids run. For example, for a 1 uM Hexatyrosine sample Electrosprayed in 1:1 Methanol:Water at a flow rate of 10 ul/min, the maximum (M+H)+ signal achieved for Acetic Acid, Formic Acid and Nitric Acid occurred at 100 nA, 185 nA and 175 nA Electrospray total current values respectively. ES signal response versus ES current will be reported for a wide range of acid (electrolyte) types and solvent chemistries. Based on the data presented, an explanation of the role of the electrolyte counter ion in ES ionization processes will be proposed. Presentation Poster