Current detection

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 

Goldstein. Uber eine noch nicht untersuchte Strahlungsform an der Kathode inducirter Entladungen. Sitzungsber. Konigl. Preuss. Akad. Wiss. Berlin, 2(1886) 691-699. 

Untersuchungen fiber die electrische Entladung in verdfinnten Gasen. Ann. Physik, 301(1898) 440-452. 

Thomson. Rays of Positive Electricity. Proc. Roy. Soc. London, A89(1913) 1-20. 

Inghram and W. A. Chupka. Surface Ionization Source Using Multiple Filaments. Rev. Sci. Instrum., 24(1953) 518-520. 

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A directional G/F relay basically is a power-measuring device, and is operated by the residual voltage of the system in conjunction with the residual current detected by the three CTs used for non-directional protection, as shown in Figure 21.19. To provide directional protection, therefore, a residual VT is also essential, in addition to the three residual CTs. The voltage phasor is used as a reference to establish the relative displacement of the fault current. In healthy conditions, i.e. when the current flows in the right direction. = 0. (refer to. Section 15.4.3 for details), and the relay remains inoperative. The relay operates only when the current flows in the reverse direction. 

This relay may be used only under unrestricted fault conditions, with three CTs as shown, tf the scheme is used under a restricted fault condition, with the fourth CT in the neutral, the directional relay will remain immune to any fault occurring outside the zone of the three CTs, as the fault current through the fourth CT will offset the residual current, detected by the three CTs (.Section 2l.6.,f), rendering the whole scheme non-functional. 

The build-up is less than the current detectable level of 50 ppb. 

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Coordination of NO to the divalent tetrasulfonated phthalocyanine complex [Co(TSPc)]4 results in a complex formally represented as [(NO )Coin(TSPc)]4 kf= 142M-1s-1, KA 3.0 x 105 M-1). When adsorbed to a glassy carbon electrode, [Co(TSPc)]4- catalyzes the oxidation and reduction of NO with catalytic currents detectable even at nanomolar concentrations. Electrochemistry of the same complex in surfactant films has also been studied.905 Bent nitrosyl complexes of the paramagnetic trivalent tropocoronand complex Co(NO)(TC) ((189), R = NO) have also been reported.849... 

Peaks and their number are instead currently detected and measured by an integrator or scanning devices and thus are instead observable quantities. The main drawback is that they are evaluated on the basis of established threshold and resolution criteria that very often are not precise. Nonetheless, the response values, even if poorly defined, can be used to estimate m—a hidden quantity, see above—in a random retention pattern, as described below. 

GBA (Orchid) Midlevel High with current detection scheme High High Low... 

Figure 3.67 Typical disc potential and ring current behaviour during galvanoslatic reduction of the passive him on pure iron (area 0.5 cm2) at a rotation speed of 25 Hz. The Pt ring potential is maintained at 0.2 V to oxidise all Fc(II) to Fellll), and the collection efficiency is 0.28. Note that the residual current detected on the ring beyond 70s corresponds to re-oxidation of hydrogen generated galvanostatically on the disc. Reprinted from Corrosion Science, 28, P. Southworth, A. Hamnett. A.M. Riley and J.M. Sykes, An Ellipsometric and RRDE Study of iron Passivation and Depassivation in Carbonate Buffer , pp. 1139-1161 (1988), with kind permission from Pergamon Press Ltd.. Headington Hill Hall. Oxford 0X3 OBW. UK.

The gain process is noisy and introduces a factor into current-detection measurements. 

The dark current in current detection always exceeds the dark-current equivalent of dark counts in photon counting because of the leakage. Also, no component of the dark current can be eliminated by discrimination as in photon counting. 

Total Ion Current Chromatogram Chromatogram obtained by plotting the total ion current detected in each of a series of mass spectra recorded as a function of retention time. See related entry on extracted ion chromatogram. 

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. 

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Current detection limits are < 1 ng for full scan mass spectra and < 1 pg for multiple ion monitoring mass spectrometry. Compound identifications are based on the comparisons with authentic standards, GC retention time, literature mass spectra and the interpretation of mass spectrometric fragmentation patterns. The MS methods used for the various markers and studies are listed in Table 1. 

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