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J Biomol Tech. 2007 February; 18(1): 39.
PMCID: PMC2291832

P113-M Voltage/Current Divider Effects on Nanoelectrospray Threshold Voltage and Electrospray Stability

Abstract

Most commercially available nanoelectrospray ionization (NSI) sources possess well-defined electrical ground points in the mobile phase flow path. Inclusion of a ground point eliminates possible safety hazards involving source high voltage. Most home-built sources, however, lack a specifically defined ground point; the ground point often resides in injection valves, auto-samplers, or chromatographic pumps. Poorly placed ground-points affect analytical performance because high-voltage (HV) source current flows to this point. HV for on-line NSI is commonly provided through an electrode contacting the mobile phase upstream from the emitter. Current flow between HV supply, ground point, and nanoelectrospray plume (e.g., the current that flows from emitter to inlet) engenders the functional equivalent of a voltage-divider circuit. Relative impedance between the ground point and HV supply, and the impedance between the HV supply and NSI plume, determines effective spray voltage at the emitter tip. If relative impedance to the ground point is too low, effective voltage at the emitter sinks below the HV supply, affecting plume stability or even preventing spray formation; this obstacle is amplified in gradient elution because mobile phase surface tension changes during the analysis. Periodic or chaotic spray drop-out is a typical result. Plume formation and stability with gradient elution will be investigated using zero-dead-volume, optically clear, platinum electrode assemblies for HV and ground connections. Figure 11 shows a 150 μm platinum wire for HV contact at the junction between 75 and 50 μm ID fused-silica tubing. HV contact is made through a 150 μm platinum wire. Minimum requirements of tubing dimensions (length, inside diameter) between HV and ground points, while minimizing NSI threshold voltage disturbance, will be determined.


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