Drift tube cells

A new development, the selected-ion-drift tube or SIFT technique (Smith and Adams, 1979), enables the isolation of reactions due to a given set of ions, of a selected mfe value, with a neutral. Ions are produced outside the flow cell under high vacuum, and injected into the flow tube through a mass filter. This filter eliminates all ions except the ones selected (according to a specific m/e value), and removes the neutral precursor of the ion in question by differential pumping. 

Fig. 19 Detector block for the light-scattering detector. (1) nebulizer, (2) drift tube, (3) heated copper block, (4) light-scattering cell, (5) glass rod, (6) glass window, (7) diaphragm. (Reproduced from A. Stoly-hwo, H. Colin, and G. Guiochon, J. Chromatogr. 265 1 (1983) with permission.)...

Ions exiting the drift tube are mass analyzed in mass spectrometer MS2, an important feature if reactions are occurring in the drift cell. Ions are generally detected after MS2 by ion counting techniques. The mass spectrometers MSI and MS2 are typically quadrupole mass filters, and either one or the other can be run in RF-only mode for better signal but without mass selection, if desired. 

The unique detection principle of evaporative lightscattering detectors involves nebulization of the column effluent to form an aerosol, followed by solvent vaporization in the drift tube to produce a cloud of solute droplets (or particles), and then detection of the solute droplets (or particles) in the light-scattering cell. 

Evaporative light scattering detection involves three successive and interrelated processes nebulization of the chromatographic eluent, evaporation of the volatile solvent (mobile phase), and scattering of light by residual analyte particles. The three major parts of the system are the nebulizer, drift tube, and light-scattering cell. 

The Fourier transform ion cyclotron resonance (FT-ICR) trapping of mass-selected cluster ions is an important emerging technique for the study of ion cluster reactivity. " This technique offers very high resolution and sensitivity. An alternative approach has been used by Brucat et al. who demonstrated that the reactivity of cluster ions can be studied in an experimental configuration identical to that used for the study of neutrals, except that ions are detected directly by pulsed extraction in the time-of-flight mass spectrometer. Other experiments " are exploring the reactions of mass-selected cluster ions in beam-gas-cell or drift-tube type configurations. This approach avoids the problems of mass overlap and offers a direct probe of cluster and cluster-adduct stabilities. For further experimental details, the reader is referred to the references. 

DCIM-MS has been coupled to liquid chromatography (LC) for analyzing complex peptide samples [36], Peptides eluting from the LC column were analyzed on an IMS-Q-TOF mass spectrometer. The ions generated from the source were accumulated in an ion trap and injected periodically into the drift tube. After mass analysis by the quadrupole, ions were subjected to collision-induced dissociation (CID) within an octopole collision cell and the product ions were analyzed by a TOF analyzer. Using this instrumental configuration, the urinary proteome [37], the Drosophila melano-gaster head proteome [38], and the human plasma proteome [39] have been analyzed. While many additional measurements, compared to standard mass spectrometry-based proteomics experiments, were obtained (for example, collision cross-sections), these were not used to improve upon protein identification results. 

For more accurate mass analysis, an IMS is coupled to a quadrupole or TOF mass analyzer [75-79], Similar to LC-MS systems, the IMS serves as a separation device and the quadrupole or TOF mass analyzer as a detection device, but has the added advantage that separation times are in milliseconds. ESI and MALDI ion sources have both been coupled to IM-MS instruments [75-78], Such systems can be employed for the analysis of mixtures of proteins and tryptic peptides [75,77,78], An instrument that depicts the coupling of IM with TOF mass spectrometry is shown in Figure 3.26. It consists of a MALDI source, an ion mobility cell, a CID cell, and an oa-TOF mass analyzer. Ions exit the drift tube when the axial field strength of the ion mobility cell is ramped up, and enter the source region of an oa-TOF instrument, where the ions are detected intact in the nsnal manner (see Section 3.5.4). Alternatively, ions can be fragmented in the CID prior to their detection by the TOF-MS. 

A modified version of the detector is shown in Fig. 2.6. In this version, the earlier used He-Ne laser is substituted for a diode laser, MDL-200-670-5 (Laser Max Inc., Rochester, NY). In order to allow a major part of the analytes to pass through the light beam, the drift tube ends as closely as possible to the beam. The diode laser produces a faint halo around the light beam, and if this halo hits the drift tube, an increased photodiode background current results. Therefore, the halo must be removed, and for this purpose a slit is included. The position of the slit is 45 mm from the end of the photodiode. In addition, a mirror has been inserted opposite the photodiode in the detector cell. With this detector, the limit of detection for... 

Often, the metastable ion fragmentation will not suffice to extract all of the structural information of interest, and consequently the internal vibrational energy of the molecules must be increased to generate more fragments. The most common method to accomplish this is by collisions with neutral gas molecules, typically in a specially designed collision cell. A typical tandem TOF instrument (often referred to as a TOF/TOF instrument) is shown in Figure 2.6 [52-55]. The first linear drift tube separates the ions into packets of different m/z. An ion gate in front of the collision cell is then switched open at a correctly chosen delay time for a short period, such that only the precursor ions of interest are passed. 

Drift tube In the drift tube, volatile components of the aerosol are evaporated. The non-volatile particles in the mobile phase are not evaporated and continue down the drift tube to the light-scattering cell to be detected. Non-volatile impurities in the mobile phase or nebulizing gas will produce noise. Using the highest-quahty gas. 

Five lipids (ceramide IV, sphingosine, sphingosine phosphate, and dihydro- and phytosphingosine) were isolated from cell cultures and analyzed on a silica column (ELSD, drift tube T = 70°, N2 nebulizer gas at 1.6 bar) using a 90/01/1/1 chloroform/ethanol (200 proof)/triethylamine/formic acid mobile phase [782]. Good resolution and peak shapes were achieved and elution was complete in 16 min. A linear response from 1.7 to 17 ig injected was generated and a detection limit of 100 ng injected were reported. 

The initial setup was developed into a hybrid quadrapole-ion-mobility-TOF instrument [148], The collision cell region of this instrument features three traveling-wave stacked ring ion guides, of which the middle one is used as ion-mobility drift tube and the other two may be used as collision cell, when applicable. The 185 imn long IMS part is operated at pressures up to 1 mbar with up to 200 ml/min Ar whereas the collision cells are 100 mm long and operated at 10 mbar with up to 10 ml/min gas [148]. The system can be used for a wide variety of applications. 

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