The Study On Transmission Efficiency Of Atmospheric Nanoelectrospray Ionization Source

The electrospray ionization mass spectrometry (ESI-MS) has become one of the most powerful and broadly applicable tools in the analytical chemistry. It allows for detection of many compounds in a wide range of masses with low detection limits, high resolving power and mass accuracy. However, the ion transmission efficiency from the atmospheric pressure region into the high vacuum region of the MS is low, mainly because of by the MS sampling interface that acts as a conductance limit through which only a small fraction of all analytically significant ions can pass through as fully liberated gas-phase ions.To improve the ion transmission efficiency between the electrospray (ESI) emitter and the mass spectrometer (MS) at atmospheric pressure, a new ion-focusing electrode based on SU-8polymer was developed in a nano-electrospray ionization (nano-ESI) system. The novel SU-8structure with a three-stepped microhole was fabricated by utilizing the MEMS technology including three-step UV photolithography and a single step development. The SU-8electrode was characterized in the nano-ESI system and a10-fold increase in the ES ion current was observed for certain operation conditions.It has been demonstrated that industrial air amplifiers can be used to enhance the ionization efficiency and to focus gas-phase ions into the MS. However, these devices are usually large, which may prohibit their broader utilization. For this reason, the advantages of computer simulations and the MEMS processing were employed for design and fabrication of a micromachined air amplifier based on the polydimethylsiloxane (PDMS) material. The air amplifier is based on Bernoulli’s principle and the Coanda effect to focus electrosprayed droplets and liberated gas-phase ions. Computer simulations were used to optimize the structure and and their results provided a design guideline for the device’s fabrication. Experimental results show a30-fold improvement in the ES current for certain operation conditions when the air amplifier was incorporated in the nano-electrospray ionization (nano-ESI) process. Compared with traditional air amplifiers, the MEMS-based air amplifier provides good performance while keeping the fabrication process simple and cost effective.We also propose and demonstrate a fabrication process for picoliter electrospray ionization (pico-ESI) systems. The pico-ESI can be effectively realized using out-of-plane silicon emitters or with in-plane emitters imprinted into polymer substrates, such as PET, PMMA or SU-8. The proposed pico-ESI chips could be used as sources for ESI in a low flow rate (pL/min) regime in future applications for mass spectrometry, and for electrohydrodynamic (EHD) printing and controlled deposition of various inks, suspensions of nanoparticles and nanowires onto substrates in microelectronics, biotechnology and other areas. An improved air amplifier design that takes advantage of combined effects of aerodynamic and electrodynamic focusing was further developed to improve the MS sensitivity by coupling a nano-ESI source with the heated MS inlet. The new design comprises an electrodynamic ion funnel integrated into the main air pathway of the air amplifier to more effectively focus and transmit the gas-phase ions. Numerical computational fluid dynamics simulations were carried out using commercial software package ANSYS Fluent to provide more detailed information about the device’s performance. The gas flow field as well as the electric field patterns and the Lagrangian ion motion were conveniently simulated using this single package and custom written user defined functions. Experiment results show a nearly5-fold improvement in reserpine ion intensity with the air amplifier operated at a nitrogen gauge pressure of40kPa and no DC or RF potentials applied to the ion funnel as compared to direct sample infusion from the same distance without the air amplifier. More importantly, a nearly3-fold additional gain in ion intensity was measured when both DC and RF potentials were co-applied, resulting in more than13-fold overall ion intensity gain which could be attributed to the combined air amplifier aerodynamic and ion funnel electrodynamic focusing effect.A commercial CFD package ANSYS Fluent was employed to simulate ion transport at atmospheric pressure between a nano-electrospray emitter and the mass spectrometer sampling tube inside a second iteration of an air amplifier device incorporating a radiofrequency ion funnel. The flow field, electric field and the ion trajectory calculations were carried out in separate steps. User-defined C/C++functions were written to accommodate the electric field and the Monte Carlo ion-gas collisions. The ion transmission efficiency was evaluated for different operating conditions by tracking250sample reserpine ions. It was found that the high velocity gas stream and the external electric fields can reduce space-charge effect and the beam divergence. Profiles of the total (stagnation) pressure were measured in the horizontal and the vertical direction to evaluate the radial flow symmetry of the Coanda gap and the outlet aperture. Measurements of the static pressure along the centerline of the air amplifier showed good correlation with the computational fluid dynamics (CFD) simulations.