Image current detection

Ion detection is carried out using image current detection with subsequent Fourier transform of the time-domain signal in the same way as for the Fourier transform ion cyclotron resonance (FTICR) analyzer (see Section 2.2.6). Because frequency can be measured very precisely, high m/z separation can be attained. Here, the axial frequency is measured, since it is independent to the first order on energy and spatial spread of the ions. Since the orbitrap, contrary to the other mass analyzers described, is a recent invention, not many variations of the instrument exist. Apart from Thermo Fischer Scientific s commercial instrument, there is the earlier setup described in References 245 to 247. 

Examples are given in References 249 and 250 of about 100 ions detected in a single scan. This is about the practical detection limit for image current detection due to thermal noise in the detection system. Bradykinin has been detected from a sample concentration of 3 nM [249] and detection of sub-femtomole levels on a column is readily obtained [251]. 

Since a minimum of about 100 ions is needed to generate a detectable signal under normal circumstances (ion counting is inherently more sensitive than image current detection) and space-charge effects become influential with more than 106 to 107 ions, the dynamic range is relatively poor, about 104. The same applies to the FTICR as to the QIT and orbitrap. The signal depends on other species present in the trap at the same time, which limits quantification quality. 

Image current detection is (currently) the only nondestructive detection method in MS. The two mass analyzers that employ image current detection are the FTICR and the orbi-trap. In the FTICR ions are trapped in a magnetic field and move in a circular motion with a frequency that depends on their m/z. Correspondingly, in the orbitrap ions move in harmonic oscillations in the z-direction with a frequency that is m/z dependent but independent of the energy and spatial spread of the ions. For detection ions are made... 

Because the number of ions leaving the mass analyser at a particular instant is generally quite small, significant amplification is often necessary to obtain a usable signal. Indeed, 10 incident ions per second at the detector corresponds to an electric current of 1.6 x 10 18 A. In consequence, subsequent amplification by a conventional electronic amplifier is required. Furthermore, with the exception of Faraday cup and image current detection, the other detectors multiply the intensity of the signal by a cascade effect. 

In particular, the ion motion in the z (axial) direction may be described as an harmonic oscillation and Eq. 2.18 showed the relationship between the axial frequency and the mJz (m/q) value of the trapped ion. By the same approach used for FT-ICR, in the case of Orbitrap ion detection is obtained by image current detection on the two outside electrodes, and by a FT algorithm the complex signal due to the copresence of ions of different m/z values (and hence exhibiting different coz values) is separated into its single m/z components. The typical mass resolution obtained by this analyzer is up to 105. 

Figure 3.23. Shape of an orbitrap, and ion motion and modes of mass analysis (a) image-current detection mode and (b) mass-selective instability detection. (Reproduced from ref. 66 by permission of the American Chemical Society, Washington, DC, copyright 2000.)...

The same lack of apphcation to trace quantitative analysis appears to be true of a more recent innovation, the Orbitrap analyzer (Makarov 2000) indeed a recent extensive review of this device (Hu 2005) also does not mention quantitation at all. Like the FTICR analyzers, the Orbitrap operates under very high vacuum ( 10 torr) to achieve its ultimate performance it also uses image current detection. At this time neither of these analyzers appears to be suitable for trace level quantitation experiments of the kind discussed in this book. 

Online intact protein separation was the same as for the Q-TOF LC-MS (above) for consistent protein retention times across platforms. For LC-MS/MS the eluent flow was split to a flow rate of 350 nL/min via the TriVersa NanoMate (Advion BioSci-ences, Ithaca, NY) chip-based nanospray source and analyzed with a LTQ-Oibitrap XL (Thermo Fisher, San Jose, CA) mass spectrometer. The instrument was operated in a top-three data dependent mode, with both MS spectra and collision-induced dissociation (CID) MS/MS spectra acquired at 60,000 resolving power in the Oibi-trap. CID collision energy was operated at 15 %. Each MS spectrum was composed of three microscans, and each MS/MS spectrum was the average of 10 microscans. To facilitate the analysis of intact proteins, the instrument was operated with the HCD gas off and the delay before image current detection shortened to 5 ms. 

The ICR voltage signal strength at the detector plates is inversely proportional to ion mass if the monitoring circuit is predominantly resistive, and is independent of ion mass if the circuit is predominantly capacitive [200]. Image current detection at room temperature is typically less sensitive than ion counting techniques in... 

The orbitrap analyzer incorporates a completely new concept of m/z analysis [15,205]. Commercialized by Thermo Fisher Scientific in 2005, the Orbitrap delivers high resolving power and accurate mass measurement at a level rivaling FT-ICR to a certain extent [15,206-210]. It is the special charm of the orbitrap to operate without a magnetic field, and therefore, to be available at a much lower price and lower installation room requirements than FT-ICR instruments. Nonetheless, it has one important feature with FT-ICR in common the orbitrap also employs image current detection of ion oscillations and Fourier transformation for the conversion of the transient into the frequency domain. 

The Kingdon trap, an ion trapping device that consists of an outer barrel-like electrode and a coaxial inner spindle-like electrode that form an electrostatic field with quadro-logarithmic potential distribution. The frequency of harmonic oscillations of the orbitally trapped ions along the axis of the electrostatic field is independent of the ion velocity and is inversely proportional to the square root of m/z so that the trap can be operated as a mass analyser using image current detection and Fourier transformation of the time domain signal. 

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