Ionization and transmission efficiency in an electrospray ionization—mass spectrometry interface

Abstract

The ionization and transmission efficiencies of an electrospray ionization (ESI) interface were investigated to advance the understanding of how these factors affect mass spectrometry (MS) sensitivity. In addition, the effects of the ES emitter distance to the inlet, solution flow rate, and inlet temperature were characterized. Quantitative measurements of ES current loss throughout the ESI interface were accomplished by electrically isolating the front surface of the interface from the inner wall of the heated inlet capillary, enabling losses on the two surfaces to be distinguished. In addition, the ES current lost to the front surface of the ESI interface was spatially profiled with a linear array of 340-m-diameter electrodes placed adjacent to the inlet capillary entrance. Current transmitted as gas-phase ions was differentiated from charged droplets and solvent clusters by measuring sensitivity with a single quadrupole mass spectrometer. The study revealed a large sampling efficiency into the inlet capillary (Ͼ90% at an emitter distance of 1 mm), a global rather than a local gas dynamic effect on the shape of the ES plume resulting from the gas flow conductance limit of the inlet capillary, a large (Ͼ80%) loss of analyte ions after transmission through the inlet arising from incomplete desolvation at a solution flow rate of 1.0 L/min, and a decrease in analyte ions peak intensity at lower temperatures, despite a large increase in ES current transmission efficiency. -6]. The sensitivity of ESI-MS is largely determined by the effectiveness of producing gas-phase ions from analyte molecules in solution (ionization efficiency) and the ability to transfer the charged species from atmospheric pressure to the low-pressure region of the mass analyzer (transmission efficiency) [7][8][9][10].Ionization efficiency is affected by a number of factors, such as flow rate, interface design, solvent composition, and analyte properties. In general, ionization efficiency increases as the liquid flow to the ES emitter decreases [11][12][13]. The primary reason for this increase is the production of smaller charged droplets at the lower flow rates [11,14]. The smaller droplets enable more efficient solvent evaporation and fewer coulombic fission events are required to create gasphase ions [11,14]. Also, the ES current in cone-jet mode increases approximately as the square root of the volumetric flow rate [14], increasing the number of available charges per analyte molecule as the flow rate decreases. Finally, smaller initial droplets and increased amount of charge available per analyte molecule improve the ionization of analytes with lower surface activity, improving quantitation and reducing matrix suppression effects [15,16]. In addition to its importance for transmission efficiency, the ESI interface on the mass spectrometer also plays a key role in ionization efficiency [17]. Adding energy to the charged droplets-such as using heated nitrogen as a background gas-enhances desolvation and liberates more analyte ions [17][18][...

Transmission mode desorption electrospray ionization

“…With the solvent flow rate set to 10 L/min and the nebulizing gas pressure set to 100 psi, D SC and D TC were varied incrementally throughout their possible range (i.e., 5 mm Ͻ D TC Ͻ 21 mm and 1 mm Ͻ D SC Ͻ 20 mm) and an average peak area for the protonated species at each position was used to construct a contour plot of the response (see Supplementary material, Figure S1). Under these electrospray conditions, the largest responses were observed when the sample mesh was placed between 8 and 10 mm from the capillary inlet and the electrospray tip was held 2 to 3 mm from the mesh (i.e., D TC between 10 and 12 mm) These findings correlate with what is already known about the optimal distance between an electrospray tip and a capillary inlet in a standard ESI experiment [47] and the distance between the ESI tip and a sample surface in a DESI experiment [5]. Furthermore, additional experiments conducted at lower flow rates (e.g., 3 L/min) favored D SC values of less than 8 mm, thereby indicating that a more compact geometry that is similar to traditional DESI may also be appropriate for TM-DESI analysis of dried residues.…”

Transferability of the Electrospray Ionization Efficiency Scale between Different Instruments

“…This condition is not easy to fulfill because a number of instrumental factors are known to influence analyte response in ESI-MS, in particular spray voltage, ion transfer capillary temperature, ion transfer capillary voltage, and tube lens voltage [17,18].…”

The influence of material and mesh characteristics on transmission mode desorption electrospray ionization

“…Numerous studies have investigated fundamental electrospray characteristics at various geometries, flow rates, gas pressures and applied voltages [41][42][43]. However, the transmission of an electrospray through a material is not typically addressed.…”