Selective ion accumulation

Figure 9 Glow discharge mass spectra of NIST SRM 1103 Free Cutting Brass, demonstrating selective ion accumulation by using mass selective instability (a) low-mass cutoff, m/z 15 (b) low-mass cutoff, m/z 45. (From Ref. 36.)...

Dynamic range extension in GD quadrupole/ion-trap MS based on selective ion-accumulation (e.g. by mass-selective instability, single-frequency resonance ejection, combined rf-dc and entrance end-cap dc methods) allows the selective accumulation of the analyte ions and enables the dynamic range to be increased by a factor of 105 [233]. The linearities and relative trapping efficiencies of the previous methods were assessed with respect to the injection time and the methods were used for the GD ion-trap MS determination of major and minor constituents in NIST SRM 1103 Free Cutting Brass. 

Bruce, J.E. VanOrden, S.L. Anderson,G.A. Hofstadler, S.A. Sherman,M.G. Rockwood, A.L. Smith, R.D. Selected Ion Accumulation of Noncovalent Complexes in a Eourier Transform Ion Cyclotron Resonance Mass Spectrometer, , 7. Mass Spectrom. 30, 124-133 (1995). 

Wang, Y. Shi, S.D-H. Hendrickson, C.L. Marshall, A.G. Mass-selective ion accumulation and fragmentation in a linear octopole ion trap external to a Fourier transform ion cyclotron resonance mass spectrometer. Int. J. Mass Spectrom. 2000,198, 113-120. 

In addition, ions can be accumulated in Q2 prior to dumping them into the TOF. This selected ion accumulation can improve detection sensitivity by as much as 100-fold, depending on the duty cycle of the instrument The LINAC arrangement from Sdex [111] assists with these experiments by allowing an improved ejection of these stored ions from Q2. 

Eiden, G. C., Barinaga, C. J., and Koppenaal, D. W. (1995) Selective ion accumulation in an ICP/ITMS using a filtered noise field. In Proceedings of the 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, GA. 

LITs capable of scanning, axial or radial excitation of ions, and precursor ion selection for MS/MS experiments [118,134-136] have lately been incorporated in commercial mass spectrometers (Fig. 4.39). The replacement of Q3 in a QqQ instrument with a scanning LIT, for example, enhances its sensitivity and offers new modes of operation (Applied Biosystems Q-Trap). Introduction of a scanning LIT [118,135] as MSI in front of an FT-ICR instrument (Thermo Electron LTQ-FT) shields the ultrahigh vacuum of the FT-ICR from collision gas and decomposition products in order to operate under optimum conditions. In addition, the LIT accumulates and eventually mass-selects ions for the next cycle while the ICR cell is still busy with the previous ion package. 

The amount of literature which has accumulated over the last few years concerning the subject of ion-molecule reactions of silicon cations with neutral molecules is tremendous. In a review of the various reactions of Si+ with neutral molecules published in 1990, Bohme44 lists almost 100 reactions studied by selected-ion flow tube (SIFT) techniques alone84,85. In the meantime, this list will have grown to even larger dimensions and we can therefore only confine ourselves to the description of a few exemplary cases45 50. 

The IT is used to accumulate ions and to perform ion selection and activation in MS/MS experiments before analysis in the TOF analyser. All the ions accumulated in the trap are then ejected in the RTOF analyser. Therefore, the TOF analyser is used for mass analysis instead of the classical ion ejection methods used with ITs, namely mass selective ion ejection at the stability limit or resonant ejection. In comparison with TOF instruments, higher sensitivity is achieved by ion accumulation in the IT. In comparison with IT instruments, the analysis by TOF reduces the time as the TOF analyser allows faster mass analysis, extends the mass range, and gives a better resolution and much better accuracy. 

Understanding how ions are transported through membranes is fundamental to biology9. No attempt will be made here to review the vast literature on this subject. Instead, we will concentrate on distinguishing the separate mechanisms by which cells tend to select and accumulate metal ions. 

Because ion accumulation in the MDE element-collector is due to diffusion and migration of ions in the field of the double electrical layer across the membrane, concentration reaches a maximum over time (Fig. 2-19, curve 3). The maximum time (Xmax) and maximum concentration (C, ax) depend on many factors and therefore measurement time (imes) is selected on the basis Tn,es > Xmax- As a rule x,nes is 20-24 hours. 

Uranium selectivity of hydrous titanium oxide can be derived from the concentration factors of several metal ions accumulated from sea water103) (Table 5). 

DeKalb XL 72) root segments was followed by multiple selected ion monitoring. The BS quickly accumulated in root tissues by non-energy-dependent processes possibly involving adsorption to cell membranes. However, a comparison of the uptake and release patterns of epiBR in living and frozen tissues revealed that in the former there is a greater element of irreversible binding of BS. How this is achieved still awaits clarification. 

The membrane is a conducting solid. Both single crystal and peUet-pressed crystalhne substance mixtures can be used in membrane construction. Table 2, accumulated from data available from several electrode manufactmers data sheets, shows information relative to crystal membrane electrodes and then-application capabilities. As can be seen, crystal membrane electrodes, with the exception of that selective to F, involve Ag2S or crystal mixtures where one component is Ag2S and the other the sulphide of the selective ion of interest. The membranes are generally produced by pressing the polycrystalline substance in a pellet press. 

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