Ion Mobility Analyzers

Mass spectrometrists have always been concerned with the measurement of the mass and intensity of analyte ions. Investigation/utilization of the shapes of molecules is now possible with ion mobility techniques that utilize differences in the cross sections of ions as they move through a gas. Think in terms of two pieces of paper, one crumpled and the other flat. If dropped at the same time, the crumpled one will hit the floor first because it will encounter less air resistance than the flat piece. A similar situation applies to ions with different shapes as they travel through a gas. Although ion mobility has been examined with home-built instruments for years, only recently has this type of analyzer become available commercially. There are two significantly different types, the high-field asymmetric waveform ion mobility spectrometer (FAIMS) and the ion mobility separator (IMS). The FAIMS separator is placed between the ion source and the analyzer, while the IMS cell is located between the analyzers of an MS/MS instrument. 

1 High-Field Asymmetric Waveform Ion Mobility Spectrometer (FAIMS) 

The asymetric voltage wave (above) is applied to the parallel analyzer electrodes. 

Ions will drift to a plate because of an asymmetric voltage wave applied to the electrodes. 

Ions with the same miz but having different shapes are separated as only one structure is rendered stable when a specific, empirically determined voltage is placed on one of the electrodes. 

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Eiceman, G.A. Nazarov, E.G. Tadjikov, B. Miller, R.A., Monitoring volatile organic compounds in ambient air inside and ontside bnildings with the use of a radio-frequency-based ion-mobility analyzer with a micromachined drift tnbe. Field Anal. Chem. Tech. 2000,4, 297-308. 

Meng, Q. Karpas, Z. Eiceman, G.A., Monitoring indoor ambient atmospheres for VOCs using an ion mobility analyzer array with selective chemical ionization, Int. J. Environ. Anal. Chem. 1995, 61, 81-94. 

Finally, the long-standing limitation of ion mobility instruments as point sensors with all the limitations may be solvable with multiple instruments either distributed through some area or region or on airborne platforms in motion. This has been demonstrated with the unmanned airborne vehicle (UAV) work of C. S. Harden and colleagues and could develop with reduced size and cost of ion mobility analyzers and UAVs. 

The main advantage of ion mobility analyzers is the addition of another dimension of separation to mass spectrometry. FAIMS improves signal-to-noise ratios by removing isobaric ions that have three-dimensional shapes that are different from those of the analyte. IMS enables the measurement of the cross-sectional areas of ions in addition to the dimensions of retention time, mass and intensity provided by the chromatograph and mass spectrometer, respectively. IMS can also improve the quality of spectra by separating species that overlap chromatographically and would otherwise give mixed spectra. 

FAIMS has been referred to as an ion mobility filter instead of an ion mobility analyzer. ( ) Both FAIMS and T-wave IMS systems are relatively low-resolution ion mobility separation devices. Thus the primary focus of this chapter is on the DTIMS, which can have sufficient resolution to separate ions in complex mixtures based on ion mobility alone. 

The determination of the activity size distribution of the ultrafine ions is of particular interest due to their influence on the movement and deposition of Po-218. These ultrafine ions are the result of radiolysis and their rate of formation is a function of radon concentration, the energy associated with the recoil path of Po-218, and the presence of H O vapor and trace gases such as S02 a joint series of experiments utilizing a mobility analyzer, the separate single screen method, and the stacked screen method were conducted to examine the activity size distribution of the ultrafine mode. 

From the data obtained by the utilization of the mobility analyzer it was observed that H O decreased the amount of Po-218 ions and the addition of SC>2 increased particle formation. From Figure 3, it can be seen that with an increase in the relative humidities and absence of SC>2, there is a corresponding decrease in the number of counts. This decrease in the number of counts recorded is a result of an increase in neutralization of the Po-218 ions by water vapor. Since the mobility analyzer is only capable of detecting ions, the mobility spectra obtained in the presence of water are of a different type than those spectra obtained in the absence of water. 

Selection of on-site analytical techniques involves evaluation of many factors including the specific objectives of this work. Numerous instrumental techniques, GC, GC-MS, GC-MS-TEA, HPLC, HPLC-MS-MS, IR, FTIR, Raman, GC-FTIR, NMR, IMS, HPLC-UV-IMS, TOF, IC, CE, etc., have been employed for their laboratory-based determination. Most, however, do not meet on-site analysis criteria, (i.e., are not transportable or truly field portable, are incapable of analyzing the entire suite of analytes, cannot detect multiple analytes compounded with environmental constituents, or have low selectivity and sensitivity). Therefore, there exists no single technique that can detect all the compounds and there are only a few techniques exist that can be fielded. The most favored, portable, hand-held instrumental technique is ion mobility spectrometry (IMS), but limitations in that only a small subset of compounds, the inherent difficulty with numerous false positives (e.g., diesel fumes, etc.), and the length of time it takes to clear the IMS back to background are just two of its many drawbacks. 

Figure l Schematic of pre-concentrator designed to trap particulate matter including traces of explosives on a metal mesh screen. After a sample collection step, the apertures are closed, gas is passed at a low flow rate over the mesh which is resistively heated to 200°C or more, releasing vapors into the gas flow and passed to an analyzer, commonly an ion mobility spectrometer. 

Figure 9 An ion mobility spectrometer called the Quantum Sniffer has an inlet with laser or flash-lamp to warm a surface and a vortex sampler (left frame) to pull sample into the analyzer without contact between analyzer and surface (right frame).

The device resembles a cylindrical differential mobility analyzer (DMA) in that a sample flow is introduced around the periphery of the annulus between two concentric cylinders, and charged particles migrate inward towards the inner cylinder in the presence of a radial electric field. Instead of being transmitted to an outlet flow, the sample is collected onto a Nichrome filament located on the inner cylinder. The primary benefit of this mode of size-resolved sampling, as opposed to aerodynamic separation into a vacuum, is that chemical ionization of the vapor molecules is feasible. Because there is no outlet aerosol flow, the collection efficiency is determined by desorption of the particles from the filament, chemical ionization of the vapor, separation in a mobility drift cell, and continuous measurement of the current produced when the ions impinge on a Faraday plate. 

Spectrometry Spectroscopy4 is basically an experimental subject and is concerned with the adsorption, emission or scattering of electromagnetic radiation by atoms or molecules [15, p. 1]. A wide variety of applications of this concept have been applied in analyzing many substances. In the particular case of explosive molecules the most prominent are several forms of mass spectrometry and ion mobility spectrometry. Each has certain advantages and disadvantages. Each is discussed in detail in a later chapter. The former is most often used in fixed applications the latter, in both fixed and portable applications. 

Rosell-Llompart, J., I. G. Loscertales, D. Bingham, and J. F. de la Mora, Sizing Nanoparticles and Ions with a Short Differential Mobility Analyzer, J. Aerosol Sci, 27, 695-719 (1996). 

There are currently two vendors of ion mobility spectrometers Smith s detection offers the Ionscan and GE Sensing offers the Ion Trap Mobility Spectrometer . A description will be provided with an emphasis on their applicability to analyzing... 

Ion mobility spectrometry (IMS) is an instrumental method where sample vapors are ionized and gaseous ions derived from a sample are characterized for speed of movement as a swarm in an electric field [1], The steps for both ion formation and ion characterization occur in most analytical mobility spectrometers at ambient pressure in a purified air atmosphere, and one attraction of this method is the simplicity of instrumentation without vacuum systems as found in mass spectrometers. Another attraction with this method is the chemical information gleaned from an IMS measurement including quantitative information, often with low limits of detection [2 1], and structural information or classification by chemical family [5,6], Much of the value with a mobility spectrometer is the selectivity of response that is associated with gas-phase chemical reactions in air at ambient pressure where substance can be preferentially ionized and detected while matrix interferences can be eliminated or suppressed. In 2004, over 20000 IMS-based analyzers such as those shown in Fig. 1 are placed at airports and other sensitive locations worldwide as commercially available instruments for the determination of explosives at trace concentration [7],... 

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... 

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