MEMS-Based Nanospray-Ionization Mass Spectrometer

Abstract

An electrospray ionisation mass spectrometer (ESI-MS) whose main components are all fabricated using silicon microelectromechanical systems (MEMS) techniques is demonstrated for the first time. The ion source consists of a microengineered alignment bench containing a V-groove mounting for a nanospray capillary, an ion extraction electrode, and a pneumatic nebuliser. The vacuum interface consists of two plates, each carrying a 50 µm diameter capillary, that are selectively etched and bonded together to provide a differentially pumped internal cavity. The quadrupole filter consists of a microfabricated frame that provides mountings for stainless steel rods measuring 650 µm in diameter and 30 mm in length. Two different quadrupoles are compared: a firstgeneration bonded silicon device, and a second-generation silicon-on-glass device with a Brubaker prefilter. Differential pumping of a MEMS component is demonstrated for the first time, atmospheric pressure ionisation and ion transfer into vacuum are characterised, ESI-MS operation is demonstrated and spectra are presented for a variety of compounds. KEYWORDS:Mass spectrometry, Electrospray, Quadrupole filter, MEMS IntroductionOver 25 years have passed since an electrospray ionisation (ESI) source was first coupled to a quadrupole mass spectrometer (MS) by Yamashita and Fenn [1]. Since then, ESI-MS has become a workhorse of analytical chemistry, allowing analytes to be ionised at atmospheric pressure with little fragmentation and passed into a vacuum system for analysis.Conventional ESI sources employ spray tips with an internal diameter (ID) of ≈100 µm[2]. In order to establish a Taylor cone and maintain stable emission of charged droplets, minimum flow rates of 0.5 -5 µL min -1 and voltages of 2.5 -4 kV are used. However, it has been convincingly demonstrated that high sensitivity and stable emission can be achieved at much lower flow rates and voltages using spray tips with IDs of ≈ 5 µm, and silica nanospray capillaries with a range of coatings have become widely available [3][4][5][6][7][8].Unfortunately, while these are inexpensive, operator expertise and costly positioning apparatus are often required for stable and reproducible performance.Electrosprayed ions are passed into a low-pressure chamber via a vacuum interface. The simplest interface is a single orifice or capillary that allows gas and entrained ions to pass directly into the vacuum chamber. Clearly, the orifice must be small enough or the pumps large enough to ensure that the pressure is consistent with mass analysis. However, the susceptibility of very small orifices to clogging and the inconvenience of large pumps led to this approach being abandoned. Modern instruments employ one or more stages of 4 differential pumping. Larger orifices can be tolerated since only a fraction of the flow is transmitted to the vacuum chamber, while the majority of the gas load can be pumped away at higher pressure using more modest pumps.The effective transfer of ions and molecules from atmospheric pressu...

A miniature mass spectrometer for liquid chromatography applications

“…These features cannot be seen in Fig. , as the capillaries are only visible on a magnified scale, while the internal cavity and pumping channels are concealed within the assembled structure. This interface provides a simple means of partitioning the gas flow.…”

“…The interface shown in Fig. is conceptually similar to the structure described previously, which was the first example of a differentially pumped MEMS vacuum interface. The gold‐plated, microengineered component is seen mounted centrally on a stainless steel collar that engages with alignment features on a flange.…”

“…More recently, an advanced quadrupole mass analyzer, a vacuum interface, and a microspray ionization source have all been developed using MEMS technology. The designs, operating principles, methods of fabrication, and performance characteristics of these individual components have been reported in detail elsewhere . In the present paper, we demonstrate for the first time, a complete miniature ESI‐MS system (Microsaic 3500 MiD) in which all the components used for ion generation, transmission, and mass analysis are microengineered.…”

A Microelectromechanical Systems-Enabled, Miniature Triple Quadrupole Mass Spectrometer

“…Complex structures can be built-up by etching both sides of the wafer, growing insulating oxide layers, selective metalization, and anodic bonding to glass wafers. As the design, fabrication, and performance characteristics of the MEMS components used here have been described in detail elsewhere [32,33,40,41], only a brief overview is given.…”

MEMS ion source for mass spectrometer integrated on a chip

“…So far, different individual modules of MEMS mass spectrometer have been presented [1][2][3][4], but up to now only one on chip integrated MS has been reported [5].…”