Mass selective accumulation

DE Goeringer, KG Asamo, SA McLuckey, D Hoekman, SW Stiller. Filtered noise field signals for mass-selective accumulation of externally-formed ions in a quadrupole ion trap. Anal Chem 66 313—318, 1994. 

ScHULTZ,K.N. HAKANSSON,K.RapidElectronCapture dissociation of mass-selectively accumulated oligodeoxynucleotide dications. Int. J. Mass Spectrom. 2004,234, 123-130. 

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. 

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

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.)...

Figure 14 Inductively coupled plasma mass spectra of a mixture of lanthanides, Y, and Th (a) without stored waveform inverse Fourier transform ion trap (SWIFT) excitation and (b) with the selective accumulation of Ce+ via SWIFT. (From Ref. 58.)...

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. 

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. 

Ion trap operates in a pulsed mode so that ions are accumulated mass selectively over time. Collision-induced dissociation in the ion trap is produced by several hundred collisions of a mass-selected ion with helium buffer gas atoms. An advantage of ion-trap instruments is the ability to perform MS . 

Since 2000, the field has moved increasingly toward hybrid FT-ICR instruments in which the FT-ICR is interfaced with a front-end mass analyzer. The groups of Marshall [46,47] and Smith [48,49] introduced the quadrupole-FT-ICR. That configuration is available commercially. The hybrid linear ion trap FT-ICR [87] was introduced commercially in 2003. Hybrid instruments offer greater versatility in terms of mass-selective external accumulation with the associated increase in sensitivity and dynamic range. 

Further benefits can be realized by the implementation of mass-selective external accumulation. The dynamic range and sensitivity of the instrument are improved. Mass-selective external accumulation can be achieved by interfacing a quadrupole mass filter with the FT-ICR mass spectrometer. The quadrupole can be operated either in RF/DC mass filtering mode, in which one m/z region traverses the quadrupole, or in RF-only resonant dipolar excitation mode. The latter allows selective removal of multiple m/z peaks. For example. Smith and co-workers showed that this mode could be applied to remove the [M -i- 16H] +and [M + 14H] +ions of myoglobin from the charge-state envelope + 13 through -i-18 [49]. 

Hendrickson, C.L. Quinn, J.P. Emmett, M.R. Marshall, A.G. Mass-selective external ion accumulation for Fourier transform ion cyclotron resonance mass spectrometry. 49th ASMS Conference on Mass Spectrometry and Allied Topics. May 27-31, 2001. Chicago, IL. Patrie, S.M. Charlebois, J.P. Whipple, D. Kelleher, N.L. Hendrickson, C.L. Quinn, J.P. Marshall, A.G. Mukhopadhyay, B. Construction of a hybrid quadrupole/Fourier transform ion cyclotron resonance mass spectrometer for versatile MS/MS above 10 kDa J. Am. Soc. Mass Spectrom. 2004, 75(7), 1099-1108. 

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. 

Data dependent acquisition (DDA) is switched on automaticaUy when an MS data threshold is reached. MS/MS data is coUected until a preselected signal is accumulated, after which the instrument returns to MS operation. The mass selection (MS) fragmentation (MS/MS) cycle is continued as long as the chromatographic peak intensity exceeds a predetermined threshold. 

The mass spectrometric analysis of the raw material showed that at least two more polyhedral oligosilsesquioxanes had been formed. By means of MALDI TOP spectrometry they were identified as hexakis[4-(trimethylsilylethynyl)phenyl]hexasilsesquioxane (4) and tetrakis[4-(tri-methylsilylethynyljtetrasilsesquioxane (5). However, neither a separation by crystallization nor a selective accumulation of one of these species has succeeded so far. 

Quadrupole-Orthogonal Acceleration TOF Instrument As of today, quadrupole (Q)-orthogonal acceleration (oa) TOF (oa-TOF) instrument is the most popular hybrid instrument [58,59] a simphstic pictorial representation is shown in Figure 4.11. The quadrupole section consists of a normal massresolving quadrupole and an rf-only quadrupole. The latter serves as a collision cell and as an ion-accumulation device. For precursor-ion scan, the ions desired are mass-selected by the main quadrupole, accumulated in the collision cell, and a packet of the CID product ions is pushed into the TOF analyzer. The precursor-ion selection by the quadrupole is at a medium resolution, but the product-ion analysis by the TOF section is at a reasonably high resolution. 

Linear Ion Trap-oa-TOF Instrument A useful alternative design combines a quadrupole linear ion trap (LIT) with an oa-TOF mass analyzer [62]. This instrument consists of a short rf quadrupole ion guide, a quadrupole mass filter that operates in the rf-only mode, and an oa-TOF mass analyzer. The quadrupole mass filter is operated as an LIT to serve as an ion-accumulation and precursor-ion selection device. The mass-selected precursor is activated via a resonant excitation mode in the LIT, while the TOF instrument provides high-speed mass analysis of the resulting product ions. MS experiments have also been demonstrated with this hybrid system. A combination of QIT with an oa-TOF mass analyzer has also proven to be an effective tandem MS system [63,64]. The QIT performs tasks similar to those performed by an LIT in the LIT-oa-TOF design (i.e., its functions are ion accumulation, mass separation, and ion activation. 

In an alternative arrangement, ions fall through a potential difference as they pass from Qj to Qc so that CID occurs in Qc. The nascent product ions are accumulated in the linear ion trap Qc and then ejected mass selectively to the detector. 

In yet a further arrangement made possible by the serendipitous selection of the original experimental platform, ions fall through a potential difference as they pass from Qj to Qc so that, again, CID occurs in Qc. The nascent product ions pass through Qc and enter into and are accumulated within the linear ion trap Qi that is operated at rf only. Ions confined in Qz are excited radially and, in the fringing field at the exit of Qz, are ejected axially and mass-selectively. In this mode, since Qz is not pressurized directly, it is identified as a low-pressure linear ion trap. 

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