Axial modulation

During the ejection phase corresponding to the detection of ions, the value V of the trapping potential of the ions is progressively increased while a radiofrequency of 

The most recent ion traps on the market are equipped with a system referred to as triple resonance, which operates differently from axial modulation. Triple resonance allows ejection of all the ions toward the detector (not the 50% with axial modulation), thus doubling the transmission factor. 

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Ion trapping devices are sensitive to overload because of the detrimental effects of coulombic repulsion on ion trajectories. The maximum number of ions that can be stored in a QTT is about 10 -10, but it reduces to about 10 -10 if unit mass resolution in an RF scan is desired. Axial modulation, a sub-type of resonant ejection, allows to increase the number of ions stored in the QIT by one order of magnitude while maintaining unit mass resolution. [160,161] During the RF scan, the modulation voltage with a fixed amplitude and frequency is applied between the end caps. Its frequency is chosen slightly below V2 of the fundamental RF frequency, because for Pz < 1, e.g., = 0.98, we have z = (0 + 0.98/2) = 0.49 x... 

At the stability boundary, ion motion is in resonance with this modulation voltage, and thus ion ejection is facilitated. Axial modulation basically improves the mass-selective instability mode of operation. 

Kaiser, R.E., Jr. Louris, J.N. Amy, J.W. Cooks, R.G. Extending the Mass Range of the Quadrupole Ion Trap Using Axial Modulation. Rapid Commun. Mass Spectrom. 1989, 5, 225-229. 

Fig. 23 a, b. a An amplitude micrograph of PMMA brushes adsorbed on mica from a dilute solution in chloroform measured by tapping mode SFM. b Three-dimensional SFM micrograph of single brush-molecules indicated by the arrow in (a). The axial modulations have been explained by phase segregation of the side chains into nodules (insert) [82]... 

For MS work, the electron impact (El) mode with automatic gain control (AGC) was used. The electron multiplier voltage for MS/MS was 1450 V, AGC target was 10,000 counts, and filament emission current was 60 pA with the axial modulation amplitude at 4.0 V. The ion trap was held at 200°C and the transfer line at 250°C. The manifold temperature was set at 60°C and the mass spectral scan time across 50-450 m/z was 1.0 s (using 3 microscans). Nonresonant, collision-induced dissociation (CID) was used for MS/MS. The associated parameters for this method were optimized for each individual compound (Table 7.3). The method was divided into ten acquisition time segments so that different ion preparation files could be used to optimize the conditions for the TMS derivatives of the chemically distinct internal standard, phenolic acids, and DIMBOA. Standard samples of both p-coumaric and ferulic acids consisted of trans and cis isomers so that four segments were required to characterize these two acids. The first time segment was a 9 min solvent delay used to protect the electron multiplier from the solvent peak. 

In another conception of nanowire heterostructures with axially modulated composition, Lieber and coworkers generated nanowires with lithographically defined segments of alternating metallic NiSi and semiconducting Ni was thermally evaporated on... 

Figure 9,13. Schematic diagram of an ion trap mass analyzer with scan function for electron impact ionization and axial modulation for mass analysis. (From ref. [121] John Wiley Sons).

A supplementary RE voltage, the axial modulation, is applied during the scan program to improve the resolution by increasing the efficiency with which the ions are resonantly ejected from the ion trap during the RE analytical ramp. 

Mass-Selective Instability Scan with Collisions and Axial Modulation... 

Wells proposed an ion isolation method [33], which consisted of two steps for the isolation of precursor ions. The method employed a modified mass-selective instability technology (axial modulation technology) to ejea sequentially ions with m/z less than that of the molecular precursor ion, M+, for example. Combined with an empirical calibration procedure, ions with mass/charge ratio > M Th are ejected resonantly with a broadband waveform. Figure 15.17 shows the scan function of this isolation method. 

Jackson et al. [42] have proposed another interesting CID method in which the CID process is accomplished during a mass acquisition scan. The method is very similar to that of axial modulation except that the amplitude of the AC dipole field is tuned to fragment precursor ions rather than ejecting precursor ions. They have shown this method to be fast, efficient, and highly energetic CID process. 

The development of resonant ion ejection scanning, commonly called axial modulation, increased the mass resolution to unit resolution over the entire mass range. The ITS-40 was introduced in 1989 with axial modulation scanning and a high temperature vacuum chamber. The increased power of microprocessors available at that time allowed more sophisticated mass centroiding algorithms to be applied. 

AGC [12] was a big leap forward, which allowed variable ionization times as a function of ion population. This feature controlled the space-charge effect allowing the user to generate quadrupole-like mass spectra for good matches against known databases. Axial modulation was the next big step in allowing the chemist to tune the instrument with standard compounds that were readily available, namely bromofluorobenzene (BFB) and decafluorotriphenylphosphine (DFTPP). The use of... 

The nodalizations for phases B,C,D are similar. The standard modules of CATHARE (axial module, volume module, boundary condition module) were used. There are several common features in all nodalizations ... 

Secondary sides of heat exchangers (PCC pools) are modelled as one volume module connected with one axial module. 

Yang C, Zhong Z, Lieber CM (2005) Encoding electronic properties by synthesis of axial modulation-doped sdicon nanowires. Science 310 1304-1307... 

Enhanced mass resolution Mass spectra with enhanced mass resolution are obtained with linear ion traps by reduction of the mass scanning rate and axial modulation amplitude. At a scanning rate of 5 Th s and with an auxiliary frequency of 450 kHz applied to Qc of Figure 2, a mass resolution (ml m) of 6000 was observed for the protonated reserpine molecule. 

Figure 6.22 Scan function used to obtain an El mass spectrum (one microscan) using a Paul trap with mass selective instability scanning with concurrent axial modulation. The scan function shows the ionization period A (this could be replaced by a period of ion injection from an external source), followed immediately by activation of the axial modulation waveform and the analytical ramp of Vg. Note that the pre-scan for the automatic gain control function is not shown. Reproduced from March, J. Mass Spectrom. 32, 351 (1997), with permission of John Wiley Sons, Ltd.

In addition, use of axial modulation scanning was also found to increase resolving power this effect is believed to arise as follows (March 1998). Immediately prior to ion ejection, as Vg is scanned upwards the axial secular motions of ions of a particular m/z value become resonant with the supplementary potential applied between the end caps, so that the axial excursions of the resonantly excited ions (and only these ions) increase in magnitude. As a result these ions escape from the space charge effects of the cloud of ions of higher m/z while the latter are stiU coUisionally cooled at the center of the trap. In this manner the ions that are resonantly excited become tightly... 

Some ion trap mass spectrometers use variants of the original axial modulation principle. For example, some instruments use an auxiliary AC potential angular frequency (o/3, apphed to the end cap nearer to the detector. In this case, when Vq is ramped, ion ejection occurs at a fixed working point well removed from the 3 = 1 boundary of the stability diagram. 

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