Differential Ion Mobility Spectrometers

Advances in continuous ion flow with alMS methods have occurred as dimensions of the inlet aperture have become identified with limitations in resolving power of alMS instruments. Two approaches, one pneumatic and one electrical, were developed to enter a thin stream of ions into the cross section of an alMS. These methods have improved the performance of alMS instruments and brought additional capabilities into the technical options for IMS. 

Tyndall, A.M., The Mobility of Positive Ions in Gases, Cambridge Physical Tracts, Editors OUphant, M.L.E. RatcUffe, J.A., Cambridge University Press, Cambridge, UK, 1938. 

Bradbury, N.E. Nielsen, R.A., Absolute values of the electron mohUity in hydrogen. Physical Review 1936,49(5), 388-393. 

Author s note The ceramic two-part spring tensioned ion shutter is unique to PCP, Incorporated, drift tubes at least from the mid-1980s. No open reference to this design is known. 

Rajapakse, R.M.M.Y. Eiceman, G.A., Preparation of Bradbury Neilson shutter with temperature conditioning, 2012, in preparation. 

---

While time-dispersive ion mobility devices of the type used for drift tube IMS require aperture grids prior to the Faraday plate to preserve the resolving power of the instrument, ion filters and scanning mobility spectrometry such as differential mobility spectrometry (DMS), field asymmetric IMS (FAIMS), differential mobility analysis (DMA), and aspiration IMS (alMS) do not require an aperture grid and can efficiently detect ions with a simple Faraday plate. In these devices, ions do not travel as a discrete swarm, and the exact arrival time of the ions is not critical. Figure 7.3 shows a schematic of a typical differential ion mobility spectrometer (DIMS) in... 

FIGURE 7.3 Differential ion mobility spectrometer (DIMS) showing both positive and negative Faraday plates used for detecting the ions that pass through the tunable ion filter. Because ions are detected continuously, no aperture is required. (From Sionex Corporation.)... 

Schematic view of a differential ion mobility spectrometer. For ions of positive a, the effective ion mobility increases with increase in electric field, while for ions with a < 0, the mobility decreases with increase in...

The first part of this chapter is a brief presentation of the interactions encountered by an ion that is moving through a neutral gas under the influence of a weak electric field. This simplified treatment pertains mainly to the classic form of linear ion mobility spectrometers (IMSs) and aspiration IMS devices. The motion of ions in other ion mobility devices, like the differential mobility spectrometer (DMS) and traveling wave (TW) IMS is also discussed. In the second part of the chapter, the implications on ion behavior in these embodiments of IMSs are discussed. The effects of the experimental parameters temperature, drift gas composition, moisture level of the supporting atmosphere, and concentration of the analytes are described in Chapter 11. 

Differential ion mobility spectrometry Field asymmetric ion mobility spectrometry Ion mobility spectrometer Plasma chromatography Time-of-flight ion mobility spectrometry... 

Fig. 6. Schematic of a differential mobility spectrometer showing the principles of ion separation in a differential mobility spectrometry (DMS) drift tube. Ion paths are governed by both the asymmetric electric field and field dependence of mobility for an ion. The inset displays the asymmetric waveform of separation electric field used in the DMS drift tube. The waveforms shown are theoretical (top part) and actual or experimental (bottom part) used in these experiments.

The first description of a differential mobility spectrometer is shown in Fig. 9 with a schematic from the 1993 article by Buryakov et al. [8-10], Subsequently, the technology from this team was migrated to the USA [39] and then Canada [40] as field asymmetric ion mobility spectrometry (FAIMS) with a cylindrical design for the analyzer. The FAIMS analyzer was attached to a mass spectrometer [41], and a line of study on large instrumentation was begun where the FAIMS was an ion filter for the mass spectrometer in environmental and biological studies [42 14], Refinements were made and a commercial inlet for mass spectrometers was introduced [45], but no determinations with... 

Differential mobility spectrometers are attractive through the mechanical simplicity of an analyzer assembly without ion shutters, an aperture grid, or multiple components to establish a voltage gradient to move ions as found in conventional IMS drift... 

An important tool in the study of protein conformation and noncovalent protein complexes is the on-line combination of ion-mobility spectrometry (IMS) and MS. The IMS-MS instruments consists of an ESI source with related ion optics, a drift tube, and a mass spectrometer [75-76]. Quadrapole and TOF-MS instruments have been applied most frequently. In an IMS instrument, ions drift through a buffer gas under the iirfluence of a weak uniform electric field. The IMS separation of ions is based on differential mobility of ions related to shape and charge state. Within a particular charge state, compact ions show a higher mobility than more extended structures, because they experience fewer collisions. In this way, conformation differences between ions can be discovered. Compact ions have a smaller collision cross section. 

DIFFERENTIAL MOBILITY SPECTROMETER AND THE DEPENDENCE OF ION MOBILITY ON THE ELECTRIC FIELD STRENGTH... 

An, X. Stone, J.A. Eiceman, G.A., A determination of the effective temperatures for the dissociation of the proton bound dimer of dimethyl methylphosphonate in a planar differential mobility spectrometer, Int. J. Ion Mobil. Spectrom. 2010, 13, 25-36... 

In companion publications, pesticides were sampled with a laser desorbed vapors were passed into a differential mobility spectrometer (DMS). Fast detection of pesticides was made using apples, grapes, tomatoes, and peppers, with detection limits in the nanogram range. The detection of pesticides was improved for DMS with an atmospheric pressure photoionization ion source modified with dopants such as benzene, anisole, and chlorobenzene. Improvements of detection limits up to two orders of magnitude were observed, and peaks were displaced on the compensation voltage (CV), axis as expected with modified gas atmospheres in DMS. 

Karpas, Z., Eiceman, G.A., Krylov, E.V., Krylova, N., Models of ion heating and mobility in linear field drift tubes and in differential mobility spectrometers. Int. J. Ion... 

After presenting an overview chapter and the fundamentals, the book focuses on instrumentation and ionization sources. It describes an ion-mobility-capable quadrupole time-of-flight mass spectrometer, the differential mobility analyzer, a cryogenic-temperature ion mobility mass spectrometer, the atmospheric solids analysis probe method, and laserspray ionization. In the final applications-oriented chapters, the contributors explore how homebuilt and commercial instruments using electrospray ionization and matrix-assisted laser desorption/ionization (MALDl) methods are employed to solve biological and synthetic issues. 

Fig. 4.69. Basic design of an ambient-pressure IMS-TOF mass spectrometer. Ions from an electrospray source are entering a desolvation chamber from where packets are pulsed into the ion mobility tube (uneven ion path to indicate diffusion of ions) by means of an ion gate. After ion mobility separation the ions are transferred into the W-type double reflector oaTOF analyzer via a differentially pumped interface. Courtesy of TofWerk AG, Thun, Switzerland. 

---