Ion Mobility Separator IMS

A series of voltage waves passes along a set of plates contained within a gas cell (mTorr of nitrogen). 

Isomeric ions with different cross sections (i.e., different shapes) will he separated those ions with the larger cross sections are retarded hy their interaction with the gas molecrdes while compact species (smaller cross section) encounter gas molecules less frequently and pass through the analyzer more rapidly than ions with larger cross sections. 

The time scale for mobility separations is in milliseconds thus, IMS can be combined conveniently with LC-TOF instruments where the LC peaks are seconds wide and the TOF analyzers operate on a microsecond scale. Compounds eluting from the LC column are ionized, exit the source, and are stored in an ion trap prior to their release into the IMS. During the milliseconds that the first set of ions is traversing the IMS, a second packet of ions (from the source) is accumulated in the trapping region. At the other end of the instrument, the TOF uses orthogonal ion injection, as do other ESI-TOF systems. The microsecond timescale of the TOF enables the collection of spectra for the separated compounds emerging from the IMS. 

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. 

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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. 

In ion mobility separation (IMS), ions move in a medium-pressure chamber (about 1 mmHg). Ions with... 

Ion mobility spectrometry (IMS) [3,12] is the most widely used instrument for drug detection. The sample is heated to vaporize the analyte, which is then ionized by atmospheric (ambient) pressure chemical ionization (APCI) [3]. The resulting gas-phase ions travel through a drift tube and are separated by their distinct velocities (mobilities) in a weak electrostatic field. IMS instruments use ambient air or nitrogen as the carrier gas, making it particularly adaptable to field applications. 

Ion mobility spectrometry (IMS), which has the ability to separate ionic species at atmospheric pressure, is another technique that is useful for detect and characterising organic vapours in air [97]. This involves the ionisation of molecules and their subsequent drift through an electric field. Analysis is based on analyte separations resulting from ionic mobilities rather than ionic masses. A major advantage of operation at atmospheric pressure is that it is possible to have smaller analytical units, lower power requirements, lighter weight and easier use. 

IM-TOF MALDI positive ion mode Full-scan MS Ion mobility separation (29)... 

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. 

Ion mobility spectrometry (IMS), in which ions are separated on the basis of differences in the cross-sectional areas, can be used to determine the conformation and folding—unfolding kinetics of proteins.151,152 The basic idea behind these measurements is that because of their distinct shapes, different conformers will travel at... 

Baumbach et al. were the first to show that IMS can be used to quantify MTBE with ion mobility spectrometry (IMS) both in gasoline and in aqueous samples [67]. They coupled the IMS with a 25-cm multi-capillary column (MCC) in order to separate BTEX compounds and MTBE. Sensitivity depended very much on the ionization source used (see Table 7). Aqueous samples were introduced into the MCC by a membrane inlet but no further enrichment step has been applied. 

Ion-mobility spectrometry (IMS) is also being applied to vapor-sensing devices. This has the ability to separate ionic species at atmospheric pressure although research... 

Ion mobility can add an extra dimension of separation when coupled to a mass analyzer. Ion mobility spectrometry (IMS) separates ions according to their interactions with a buffer gas, in addition to differences in their m/z ratios. This can provide separation of ions (i.e., isobaric or conformational isomers), which cannot be accomplished using traditional mass analyzers. It can also be used to reduce interfering chemical noise. Ion mobility separates ions based on how long they take to migrate... 

Because IMS is a method that separates gas phase ions through collisions with a buffer gas, all analytes must be transported from the sample matrix and converted to a gas phase ion before ion mobility separation and detection can be performed. Thus, the type of introduction method largely depends on the physical characteristics of the analyte. The remainder of this chapter is divided into four sections based on the characteristics of the sample vapor, semivolatile, aqueous, and solid. While these categories are somewhat arbitrary with significant sample overlap, it is useful to think of volatile samples as those compounds that exist or partially exist as vapors under ambient temperature and pressure semivolatile samples as those compounds that can be volatilized but have vapor pressures too low to detect by IMS under ambient temperature and pressure aqueous samples as those compounds that are not volatile but can be dissolved in water and solid samples as compounds not in a solution. Table 3.1 lists a number of example analytes according to the categories discussed in this chapter. 

The process of converting a swarm of gas phase ions into an electrical signal that provides both arrival time information and amplitude information is known as ion detection. The ion detector is typically located after the ion mobility separation cell and has a number of ideal requisites to accomplish the transduction of mobility-separated ions in a manner that minimizes loss of sensitivity and IMS resolving power. 

The resolution of an ion mobility separation depends on both the resolving power Rp and the separation selectivity a of the IMS system. Resolution R determines how well two peaks, with drift times of tj and t2 and peak widths of Wj and W2 can be separated from one another and is defined as... 

An example of IMS-TrapMS analysis is shown in Figure 9.12. Spectrum 1 illustrates the ion mobility separation of three isomers (hesperidin, neohesperidin, and rutin) adducted with silver. Spectrum 2 shows the overlaid ion mobility spectra of the respective standards. Through the use of single-mobility monitoring, the ions contained in the drift time windows (a), (b), and (c) were fragmented to produce the mass spectra shown in 1(a), 1(b), and 1(c), respectively. Shown in bold text in 1(a) and 1(b), the ions 409 and 411 may be used to confirm the presence of either hesperidin or neohesperidin. However, the IMS separation prior to mass analysis is necessary to distinguish conclusively among the three isomers. 

Thus, IMS can serve as a stand-alone instrument in medical and biological applications for which only limited information from a small number of compounds is required. Ion mobility separation can be used to preseparate or filter interfering compounds prior to MS analysis or serve as a means of distinguishing between isomers based on structural differences. As mentioned, mobility data can also be used to determine the stereoscopic conformation of macromolecules and can thus serve as a means for assessing their biologic activity. 

A few review papers discussing the use of ion mobility and IM-MS measurements for studying macromolecules have been recently published. Combining MALDI with IM-MS was discussed in view of its applications in solvent-free structural studies, sequencing, and protein identification. The mobility measurements provided a new dimension in the analysis of biomolecules due to the fact that separation is based on the size and shape of the ion and not only on its mass. The role of multidimensional assemblies and aggregations in normal cellular processes and diseases, based on ESI-IM-MS experiments, was analyzed. ... 

Over the last decade, scientific and engineering interests have been shifting from canventional ion mobility spectrometry (IMS) to field asymmetric waveform ion mobility spectrometry (FAIMS). Differential Ion Mobility Spectrometry Nonlinear Ion Transport and Fundamentals of FAIMS explores this new analytical technology that separates and characterizes ions by the difference between their mobility in gases at high and low electric fields. It also covers the novel topics of higher-order differential IMS and IMS with alignment of dipole direction. 

This is the first book on differential ion mobility spectrometry (IMS), an analytical technique also called field asymmetric waveform ion mobility spectrometry (FAIMS) and, on occasion, several of the altemafive names mentioned in the Introduction. These terms refer to the evolving methods for separation and characterization of ions based on the nonlinearity of their motion in gases under the influence of a strong electric field. 

Ion mobility spectrometry (IMS) is now a well-established analytical technique that is employed throughout the world for the detection of explosives, drngs, and chemical warfare agents. The predominant approach is based on the nse of a drift cell in which ions migrate through a counter flowing bnffer gas in the presence of a low electric field. Separation of ions takes place as a resnlt of interactions between these ions and the buffer gas, and depends on the mass, charge, and shape of the ion. Because the drift cell was employed in the first ion mobility (IM) approach used (and is still the most common), the use of the drift cell in this manner is often referred to as IMS. In discussions of the development of the field of IM, it would be prudent to differentiate this experimental technique in which the drift cell is used from other... 

Ion mobility (IM) mass spectrometers are hybrid instruments that combine an IM separation system with conventional MS systems. An ion mobility spectrometer (IMS) can also serve as a stand-alone ion detection system [72]. An IMS uses gas-phase mobility rather than the m q ratio as a criterion to separate ions [73,74]. The mobility of ions is measured under the influence of an electrical field gradient and cross-flow of a buffer gas, and depends on ion s collision cross section and net charge. 

The typical time bin of an ion mobility separation is in the millisecond range. Because this time window is substantially larger than the duration of the TOF measuremenL many single mass spectra can be recorded during one IMS separation. Under optimal conditions the overall transmission efficiency is close to that achieved without IMS. Depending on the complexity of the sample, limits of detection in the subfemtomolar range are achieved in the MALDI-IMMS analysis of peptides. 

By connecting an ion mobility spectrometry (IMS) in front of a QTOF-MS, another dimension of separation is realized. Unseparated isobaric compounds, which have the same m/z value, can be separated after ionization by the structure-dependent drift time through the IMS. The combination of IMS with QTOF is also a powerful tool for nontarget analysis in complex samples, due to the fact that the chemical noise is drastically reduced by IMS. 

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