Ion mobility spectra

Three ion peaks are seen in this mobility spectrum. The first, at a drift time of about 6 ms, is called the reactant ion peak (RIP) and is present in the spectrum even when a sample is not. The next two ion peaks are ion peaks of 4,6-dinitro-o-cresol, a common contaminant from smoke, with a reduced mobility of 1.59 V cm s and 2,4,6-trinitrotoluene (TNT) with a reduced mobility of 1.54 V cm s. For 

FIGURE 8.2 Drift tube ion mobility spectrum of 2,4,6-trinitrotoluene (TNT) and 4,6-dinitro-o-cresol (4,6DNOC). (From Wu et al.. Construction and characterization of a high-flow, high-resolution ion mobility spectrometer for detection of explosives after personnel portal sampling, Talanta 2002,57,123-134. With permission.) 

Ion mobility spectra for the TW-IMS look similar, but because the electric field is applied as successive waves, sweeping the ion toward the detector, mobilities and collision cross sections cannot be calculated directly from TW-IMS but must be calibrated to DTIMS instruments. 

FIGURE 8.3 Ion mobility spectrum from an aspiration-type ion mobility spectrometer. Spectrum is an array of Faraday plates producing a histogram-type display.  

FIGURE 8.4 DMA spectrum of 9.2-kDa polystyrene. The sharp peak to the left is from small background ions due to the electrospray process. The next peak to the right beginning at a 1/Z value of about 5 is the monomer, followed by the dimer, trimer, and so on. The resolving power of this spectrum is approximately 2. (From Ku et al.. Mass distribution measurement of water-insoluble polymers by charge-reduced electrospray mobility analysis. Anal. Chem. 2004, 76, 814-822. With permission.) 

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Figure 11. Ion mobility spectra obtained by the production of Cl" by electron capture to CCI4 in the ion source with the following partial pressures of CHCI3 added to the drift gas (a) none, (b) 1.61x10", (c) 3.2 x 10", (d) 6.5 x 10", (e) 1.29 x 10" , (f) 2.6...

Fig. 6a-c. Ion mobility spectra of bradykinin ions at m/z=1061 a the two features seen at 441 K are the singly protonated monomer (M+H)+ and the doubly protonated dimer (2M+2H)+ b at a cell temperature of 463 K the dimer dissociates into two monomer units accounting for the fill-in between the two peaks c at 510 K all of the dimer ions have disappeared upon exiting the cell... 

Ion homeostasis 971 Ion mobility spectra 1348 Ion mobility spectrometer, see IMS lonomycin 272 Iontophoresis 1382... 

Bell, S.E. Nazarov, E.G. Wang, Y.F. Eiceman, G.A., Classification of ion mobility spectra by chemical moiety using neural networks with whole spectra at various concentrations, A a/. Chim. Acta 1999, 394, 121-133. 

Eiceman, G.A. Nazarov, E.G. Rodriguez, J.E., Chemical class information in ion mobility spectra at low and elevated temperatures. Anal. Chim. Acta, 2001,433,53-70. 

IMS matches well as a detector for GC and can be interfaced directly to the exit of a gas chromatograph as a selective detector. Because ion mobility spectra are obtained at a frequency of 20 to 50 Hz, many IMS spectra can be obtained for... 

IMS has been compared with the UV detector after separation with SFC of benzoates and esters, demonstrating the ability of IMS for the detection of compounds that do not have sensitive chromophores for UV detection Often, polymers do not have sufficient UV-visible (Vis) absorbance for detection after LC or SFC, while SFC-IMS can be used for both efficient separation and detection of a variety of polymeric materials. IMS detection of a variety of drugs, such as various steroids, opiates, and benzodiazepines, after SFC separation demonstrated the ease and utility of acquiring ion mobility spectra at ambient pressure. While SFC has only captured a small portion of the separation maiket, the potential of IMS for detecting compounds that cannot be easily seen with a standard UV-Vis approach has led to its use as a stand-alone detector for LC. 

Cao, L. Harrington, P.B. Harden, C.S. McHugh, V.M. Thomas, M.A., Nonlinear wavelet compression of ion mobility spectra from ion mobility spectrometers mounted in an unmanned aerial vehicle. Anal. Chem. 2004, 76, 1069-1077. 

Ewing, R.E. Eiceman, G.A. Harden, C.S. Stone, J.A., The kinetics of the decompositions of the proton bound dimers of 1,4-dimethylpyridine and dimethyl methylphosphonate fiom atmospheric pressure ion mobility spectra, Int. J. Mass Spectrom. 2006, 76-85, 255-256. 

In a cloud chamber detector, ions at atmospheric pressure are electrically focused into a cloud chamber filled with cold water or octane vapors. The presence of the ion serves as the nucleus for the formation of small droplets that can scatter light from a laser beam passing through the cloud chamber. When mobility-separated ions entered the cloud chamber, perturbation in the laser light due to the formation of ion-nucleated particles was detected by a PMT. When the chamber was supersaturated with water, the scattered light intensity increased in the presence of ions, but when the chamber was supersaturated with octane, the intensity decreased in the presence of ions. Mobility spectra of difluorodibromomethane have been reported using cloud chamber detection." ... 

Low-pressure ion detectors for IMS are typical of those used in mass spectrometry. In fact, whenever low-pressure ion detection methods are used with IMS, the IMS is interfaced to a mass spectrometer. Mass spectrometers after IMSs serve as mass filters or mass dispersive devices prior to low-pressure ion detection. Mass discrimination is not necessary, however, as in the case of a quadrupole mass spectrometer operating in the radio frequency (RF)-only regime, where complete ion mobility spectra can be obtained. A more detailed discussion of the advantages... 

As with chromatography, the position of the peak in IMS provides qualitative information. The location of the ion swarm as it exits a drift region is dependent on the type of instrumentation used For drift tube instruments, it is the arrival time of the ion swarm at the Faraday plate or mass spectrometer orifice for DMS, it is the compensation voltage required to create a stable path through the instrument, and for aspiration-type instruments, it is the location of the Faraday plates as a function of the strength of the electric field. All of these qualitative measurements can be related to the mobility of the ion swarm, although in some cases this relation is complex and not well understood. Nevertheless, the relationships of K, K, and fl to ion mobility spectra have been described elsewhere in this book and serve as the qualitative basis of IMS. Until the fundamental relation of ion-molecule interactions can be understood sufficiently to model ion behavior in IMS instruments, IMS standards will serve to calibrate the various IMS platforms. 

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. 

FIGURE14.6 Ion mobility spectra of heroin and tetrahydrocannabinol (THC) in different drift gases air, nitrogen, carbon dioxide, and nitrous oxide. The spectra illustrate the two compounds can be resolved with high-polarizability drift gases carbon dioxide and nitrous oxide. 

FIGURE 15.4 Ion mobility spectra of 50 pg of Cymbalta drug product thermally desorbed into a Ni IMS. (a) The background swab showing on the reactant ions, (b) The swab after swiping it over a contaminated surface. (From Strege, Total residue analysis of swabs by ion mobility spectrometry, Anal. Chem. 2009, 81, 4576-4580. With permission.)... 

FIGURE 13.3 Ion mobility spectra as functions of the concentration of DMNB at 443 K. The peak at 0 ppm DMNB is the chloride ion alone and peaks with DMNB are dne to the Cr(DMNB) adduct where drift time is dependent upon vapor concentration of DMNB in the drift tube. (Reproduced from Lawrence, A.H., et al, Int. J. Mass Spectrom. 2001, 209, 185-195. With permission from Elsevier.)... 

The chloride ion was formed by dissociative electron capture by CCI4 in a Ni ionization source n-alkyl bromide, at a known concentration, was present in the drift region. The traces in Figure 13.4 are ion mobility spectra obtained at three concentrations of methyl bromide. Figure 13.4a shows the CF peak in addition to two small peaks, each marked with an asterisk, that arise from impurities the small... 

Rate constants for the first-order dissociation of symmetrical proton-bound dimers, M H + —> MH + + M, have been determined for organophosphorus compounds (M = 2,4-dimethylpyridine (DMP) and dimethyl methylphosphonate (DMMP)), where the shapes of the mobility spectra are of the form shown in Figure 13.2d [44]. Some proton-bound dimers decompose in the time taken for the ions to travel between the shutter and the detector plate, and this residence time was varied by changing the electrostatic drift field strength. Typical ion mobility spectra obtained at different field strengths are shown in Figure 13.5 and peaks were mass identified as first peak, H + (DMP), the protonated monomer and second peak H (DMP)2, the proton-bound dimer. The raised baseline between the peaks was due entirely to (DMP)H +, from the decomposition of the proton-bonnd dimer as in Equation 13.25... 

Finally, we briefly mention several other ANNs that have seen limited chemical applications. A connectionist hyperprism ANN has been used in the analysis of ion mobility spectra. This network shares characteristics of Kohonen and backpropagation networks. The DYSTAL network has been successfully used to classify orange juice as either adulterated or unadulte-rated.200 A learning vector quantizer (LVQ) network has been used to identify multiple analytes from optical sensor array data. A wavelet ANN has been applied to the inclusion of P-cyclodextrin with benzene derivatives, anj a... 

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