Mass-Selective Stability Mode

Mass-Selective Stability Mode This mode is analogous to the operation of... 

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. 

Figure 3.24. Stability diagram and scan line of an orbitrap in the mass-selective instability mode. (Reproduced from ref. 66 by permission of the American Chemical Society, Washington, DC, cop)night 2000.)...

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 QIT is about 10 -10 , but it reduces to about 10 -10 if unit mass resolution in an RF scan is desired. Axial modulation, a subtype of resonant ejection, allows to increase the number of ions stored in the QIT by one order of magnitude while maintaining unit mass resolution [152,153]. 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., Pz = 0.98, we have Qz = (0 + 0.98/2) x Q = 0.49 x O.. 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. 

Figure 7 Stability diagram in space for the region of simultaneous stability in both rand z directions near the origin for the three-dimensional quadrupole ion trap the iso-yS, and iso-/S lines are shown. The axis intersects the = 1 boundary at q = 0.908, which corresponds to in mass-selective instability mode.

If a compound has low volatility or if the parent mass cannot be determined, it may be possible to prepare a suitable derivative. The derivative selected should provide enhanced volatility, a predictable mode of cleavage, a simplified fragmentation pattern, or an increased stability of the molecular ion. 

Nuclear reactions producing exotic nuclei at the limits of stability are usually very non-specific. For the fast and efficient removal of typically several tens of interfering elements with several hundreds of isotopes from the nuclides selected for study mainly mass separation [Han 79, Rav 79] and rapid chemical procedures [Her 82] are applied. The use of conventional mass separators is limited to elements for which suitable ion sources are available. There exists a number of elements, such as niobium, the noble metals etc., which create problems in mass separation due to restrictions in the diffusion-, evaporation- or ionization process. Such limitations do not exist for chemical methods. Although rapid off-line chemical methods are still valuable for some applications, continuously operated chemical procedures have been advanced recently since they deliver a steady source of activity needed for measurements with low counting efficiencies and for studies of rare decay modes. The present paper presents several examples for such techniques and reports briefly actual applications of these methods for the study of exotic nuclei. 

The electrospray stability of this device was studied with two different samples 1 pg mL 1 imipramine- 3 directly dissolved in 75% methanol, 25% water and 0.1% formic acid and extracted urine sample containing only methylphenidate-d3, which corresponds to the blank calibration standards. Figure 6.13A shows the total ion current of full mass range scan (m/z 250 to 350) of the imipramine-J3 sample over 15 min with a scan speed of 1.16 s per scan while Figure 6.13B shows the extracted ion current of the base peak (m/z 284.2) from Figure 6.13A. The RSD for the total ion current over the 15 min periods was 10.7% and the protonated molecule ion was even more stable with an RSD of 5.7%. Figure 6.13C shows the total selected ion current for the SRM data (m/z 234.2 > m/z 84.1, m/z 234.2 > m/z 84.1) from 150ngmL 1 methylphe-nidate-fi 3 sample. The SRM mode was also very stable with an RSD of 3.05% over a period of 5 min. 

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