D Printed Annular Focusing Ambient Ion Mobility Spectrometer

Introduction

From its beginnings as “Plasma Chromatography™” for the analysis of trace organic molecules [1—4], to applications in security for the detection of explosives [5-7] and chemical warfare agents [8-10], and more recent applications in structural analysis of biomolecules [11-14], ion mobility spectrometry (IMS) has been demonstrated as a powerful analytical tool. The utility of IMS lies largely in its ability to distinguish structural and conformational differences between ion populations and the rapid manner in which ions may be separated from complex mixtures [15]. This utility is further enhanced through the coupling of IMS with MS, for enriched information output [16, 17]. One particular advantage of IMS is the wide range of pressure regimes under which it may be operated, ranging from <1 Torr to atmospheric pressure conditions [18].

There are several different embodiments of IMS, each of which separates gas-phase ions based on their electrophoretic mobility through a drift gas [19]. High-field asymmetric ion mobility spectrometry (FAIMS) [10, 20] and differential mobility analysis (DMA) [21, 22] are two techniques normally performed under atmospheric pressure that provide a continuously filtered stream of mobility-selected ions. In both FAIMS and DMA ions are carried by a laminar flow of drift gas between parallel plate electrodes, often constructed as a set of coaxial cylinders [23, 24]. The distinction between FAIMS and DMA lies in the electric field applied to the electrodes. Additionally, the separation between electrode plates is typically no larger than 2 mm in FAIMS instruments, whereas typical separation between plates within DMA instruments is much larger.

Separation in FAIMS is achieved by applying an asymmetric waveform (peak voltages over a single period must be opposite, but unequal in magnitude), to one electrode while a compensation voltage (CV) is applied to the opposite electrode [20]. The asymmetry of the waveform must be such that the high field portion is significantly greater than the low field and an integration of the waveform over one

© Springer International Publishing AG 2017 39

Z. Baird, Manipulation and Characterization of Electrosprayed Ions

Under Ambient Conditions, Springer Theses, DOI 10.1007/978-3-319-49869-0_4

period is equal to zero [25]. As ions are carried by the drift gas through the analysis region, they are separated based on differences in mobility under high- and low-field conditions. Through selection of an appropriate CV, a particular ion will achieve a balanced condition and exhibit a stable trajectory through the entirety of the device and will be detected.

DMA employs a similar configuration of parallel plate electrodes, but rather than a waveform, a DC voltage is applied to one electrode while the other is grounded. Ions are introduced into the analyzer region through a narrow opening in the grounded outer plate and are carried by a laminar flow of gas, between the parallel electrodes [26]. Throughout this transit ions migrate toward the inner electrode at different velocities, depending on the mobility of the ion in the carrier gas and the voltage applied to the electrode. Ions are sampled downstream of the gas flow through a slit (detector region) in the inner electrode. Only ions with a particular velocity will be sampled through this slit, their identities dependent on gas velocity and electric field strength as ions are effectively separated spatially within the DMA [24]. Because of this, the linear distance between the ion introduction region and the detector region is of critical importance in determining the collisional cross-section of a sampled ion, a property directly related to the ion’s size and conformation. Spectra are collected by scanning the applied potential on the central electrode.

Unlike DMA and FAIMS, drift tube ion mobility spectrometry (DT-IMS) separates a packet of ions temporally as they traverse a drift region [18]. The drift region is normally composed of a series of axially aligned, open-ring electrodes, to which a DC potential gradient is applied, thus creating a near-uniform electric field extending along the length of the drift tube. Drift tubes have also been constructed using monolithic resistive glass cylinders [27], rather than stacked rings. Upon injection into the drift region, ions are propelled along the length of the drift tube by the electric field. Ions traverse the length of the drift tube at different velocities, determined by their mobility in the drift gas [28], and are detected in a time resolved fashion, to construct an ion mobility spectrum.

More recently, travelling wave ion mobility spectrometry (TW-IMS) [29] which uses a stacked ring configuration similar to DT-IMS, has seen widespread adoption. In TW-IMS, a confining RF voltage is applied 180° out of phase to adjacent electrodes onto which voltage pulses are superimposed in a step-wise serial manner to generate potential “waves”, thus propelling ions along the drift tube where they are separated temporally [30]. Unlike other IMS approaches mentioned, TW-IMS is restricted to operation at pressures around 0.1-3 Torr [29, 31]. TW-IMS allows for high duty cycles and relative ease in coupling with MS instruments [29], but is electronically complex compared to DT-IMS.

As previously stated, one of the main advantages of IMS techniques is the ability to relate IMS spectra to conformation and size of ions. In the case of FAIMS, this relation is only known empirically due to the complex behavior of ions in a rapidly changing electric field. On the other hand, DMA is routinely used to size aerosol particles as the separation parameters can be directly correlated to size so long as the gas flow is well controlled and the electric field is well-defined [32]; however, due to Brownian motion, resolution is relatively poor for ions smaller than * 10 nm, even in specially designed analyzers [24].

Collisional cross section (CCS) measurements, a property directly correlated to an ion’s size and conformation, are often performed by DT-IMS as the velocity of an ion in a uniform electric field in the presence of a drift gas is well established [28, 33]. TW-IMS instruments may also be used to measure CCS, however the relationship of ion velocity to CCS is not as straightforward as in DT-IMS. Because of this, CCS standards of known mobility must be used to calibrate CCS measurement by TW-IMS instruments [34, 35].

The simplicity of DT-IMS instrumentation, combined with its wide operational pressure regime make it a good candidate in the analysis of size and conformation at a number of different conditions. In light of this, an effort was undertaken to construct a 3D printable, plastic DT-IMS for operation at ambient conditions that was easily interfaced with nanoESI. The drift tube employs a novel electrode geometry which serves to focus nanoelectrosprayed ions into an annulus, while simultaneously blocking the transmission of neutral droplets, without the use of supplementary gas flow or heating. SIMION-SDS simulations were used to explore electrode geometries and the results compared with experimental data. This approach is demonstrated as a viable rapid, low-cost alternative to traditional manufacture of IMS drift cells.

 
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