U.S. Department of Energy

Pacific Northwest National Laboratory

Characterizing an ESI-MS Interface Based on the Ion Utilization Efficiency

Introduction 
The achievable sensitivity of electrospray ionization mass spectrometry (ESI-MS) is largely determined by the ionization efficiency of the ESI source and the ion transmission efficiency through the ESI-MS interface.  These performance characteristics are difficult to evaluate and compare among different ESI-MS interface designs. We present here an effective method to evaluate the overall ion utilization efficiency of an ESI-MS interface by measuring the gas phase ion current transmitted through the interface and correlating it to the observed ion abundance in the corresponding mass spectrum. This method allows us to systematically compare the ion transmission characteristics and the ion utilization efficiencies under different ESI-MS interface configurations, including a heated capillary interface and a subambient pressure ionization with nanoelectrospray (SPIN) MS interface.

Methods
The evaluation of ESI-MS interfaces was performed on an Agilent TOF-MS with its standard interface replaced by a tandem ion funnel interface. Four different interface configurations, including single emitter/single capillary, single emitter/multi-capillary, single emitter/SPIN, and emitter array/SPIN, were compared for their ESI and ion transmission efficiencies. The overall ion utilization efficiency, defined as the percentage of analyte molecules in a sample solution being converted into gas phase ions and transmitted through the interface, under different interface configurations was systematically evaluated by measuring the ion current transmitted through the high pressure ion funnel at different RF amplitudes, using the low pressure ion funnel as a charge collector, and correlating it to the observed ion abundance measured in the corresponding mass spectra. 

Preliminary Data 
To probe both the ionization and ion transmission efficiencies under different ESI-MS interface configurations, the nature of the ion cloud in the high pressure ion funnel was systematically characterized.  This was accomplished by measuring both the total transmitted electric current through the high pressure ion funnel, and the total ion current (TIC) measured at MS detector for various interface configurations at different ion funnel RF peak to peak voltages. In general, the charged particles inside of the high pressure ion funnel contain both fully desolvated gas phase ions and the residue/not fully desolvated charged analyte/solvent clusters/particles. The portion of fully desolvated gas phase ions determines the final intensity of the ion current detected by MS, and the best sensitivity is achieved when this is maximized for any ESI-MS interface configuration. Over an order of magnitude increase in transmitted analyte ion current was observed by using the 10 emitter/SPIN-MS interface as compared to using the standard single emitter/single heated capillary ESI-MS interface, which consistently correlated to an observed similar analyte MS peak intensity improvement. 
By further calculating the theoretical maximum analyte ion current, corresponding to all the analyte molecules in solution being completely converted to the gas phase ions, and using the measured gas phase ion current transmitted through the interface and the TIC and analyte extracted ion current (EIC) measured at the mass detector, the ion utilization efficiency can be determined for any given ESI-MS interface configurations.  This established a universal performance metric to evaluate the overall efficiency of any ESI-MS interface design. Of the four interfaces tested in this study, an emitter array/SPIN-MS interface demonstrated the greatest ion current, highest MS-signal intensity and subsequently the best ion utilization efficiency at a given total ESI flow rate.      

Novel Aspect

A universal method to evaluate the overall ion utilization efficiency of any ESI-MS interface design

 

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