Cyclotron domain

Other types of mass spectrometer may use point, array, or both types of collector. The time-of-flight (TOF) instrument uses a special multichannel plate collector an ion trap can record ion arrivals either sequentially in time or all at once a Fourier-transform ion cyclotron resonance (FTICR) instrument can record ion arrivals in either time or frequency domains which are interconvertible (by the Fourier-transform technique). 

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. 

In 1929 Lawrence invented the cyclotron, which instrument played (and still plays) an important role in nuclear physics. That work led directly to the award of the Nobel Prize in Physics for 1939, just one of his many honors. During World War II E. O. Lawrence made vital contributions to the development of the atomic bomb holding several high-level appointments in the Manhattan Project. He played an influential role in the decision to develop and later employ electromagnetic methods for uranium isotope separation (Calutrons) during the early 1940s. (Photo credit http //wikipedia.org, public domain)... 

Wang, F. Li, W. Emmett, M.R. Marshall, A.G. Corson, D. Sykes, B.D. Fourier transform ion cyclotron resonance mass spectrometric detection of small Ca +-induced conformational changes in the regulatory domain of human cardiac troponin C. J. Am. Soc. Mass Spectrom. 1999, 10, 703—710. 

FT/ICR experiments have conventionally been carried out with pulsed or frequency-sweep excitation. Because the cyclotron experiment connects mass to frequency, one can construct ("tailor") any desired frequency-domain excitation pattern by computing its inverse Fourier transform for use as a time-domain waveform. Even better results are obtained when phase-modulation and time-domain apodization are used. Applications include dynamic range extension via multiple-ion ejection, high-resolution MS/MS, multiple-ion simultaneous monitoring, and flatter excitation power (for isotope-ratio measurements). 

Fig. 4.4. (a) Excitation-ionization spectrum of the H atom Balmer series around the ionization limit in a static homogenous magnetic field, (b) Fourier-transformed time domain spectrum of the spectrum shown in (a). The square of the absolute value is plotted. The time scale is given in units of the cyclotron period Tc = 2 k/u c. Reprinted from Main, Holle, Wiebusch, and Welge (1987). 

Wigger, M. Eyler, J.R. Benner, S.A. Li, W. Marshall, A.G. Fourier Transform-Ion Cyclotron Resonance Mass Spectrometric Resolution, Identification, and Screening of Non-Covalent Complexes of Hck Src Homology 2 Domain Receptor and Ligands from a 324-Member Peptide Combinatorial Library, J. Am. Soc. Mass Spectrom. 13, 1162-1169 (2002). 

Application of an oscillating electric field of the same frequency, to, leads to cyclotron resonance in which the ions absorb energy from the electric field and move increasingly faster, with increased orbital radius, whilst keeping v/r constant. Ions of particular mfz are therefore selected on the basis of the frequency of the applied field. This makes this method particularly appropriate for frequency-domain Fourier transform (FT) analysis, and gives potentially very high resolution. 

Fig. 3.7. Data (time) domain signal produced by 9.4-Tesla Fourier-transform ion cyclotron resonance mass spectrometer (left), and frequency domain signal (i.e. mass spectrum) after fast Fourier transformation and frequency-to-mass calculation (right). The sample was a mixture of poly(ethylene) glycol polymers used for internal calibration.

If several different masses are present, then one must apply an excitation pulse that contains components at all of the cyclotron frequencies. This is done by using a rapid frequency sweep ( chirp ), an impulse excitation, or a tailored waveform. The image currents induced in the receiver plates will contain frequency components from all of the mass-to-charge ratios. The various frequencies and their relative abundances can be extracted mathematically by using a Fourier transform which converts a time-domain signal (the image currents) to a frequency-domain spectrum (the mass spectrum). 

To obtain a mass spectrum over the desired m/z interval, all ions within this interval are excited simultaneously by a rapid frequency sweep of the voltage on the transmitter plates. The excitation pulse increases the orbital radii of all ions and puts ions of the same m/z ratio in phase. The orbiting ions create a complex wave signal in the circuit connecting the receiver plates, which is monitored over time as the coherent motion of the ions is destroyed by collisions (Figure 1.25). Fourier-transformation of this time-domain signal furnishes the individual cyclotron frequencies and, hence, the m/z values (Eq. 1.17) of the ions (Figure 1.25). 

Figure 2. Ion motion during the detection period of a Fourier transform ion cyclotron resonance (FT-ICR) experiment. The cyclotron motions of the ions in the sample are excited along spiral paths as shown in Figure 1 by an oscillator which is then turned off. The excited ions then proceed on the circular paths shown and induce a time domain signal in the plates of the capacitor which is amplified and detected. Fourier transformation of this time domain signal yields the frequency domain spectrum...

FTMS instruments can operate at very high resolving power, and if the residual pressure is low and the density of ions within the cell is small, the measurement of the cyclotron frequency can be very precise. This allows very accurate measurement of mIz and very precise separation of adjacent peaks. In a 4.7T magnetic field at 10 Pa, the time domain signal measured for miz 18 over 51s has been recorded to give a resolving power of M/AM = 100000000. 

FTICR-MS instruments operate on the principle of ion cyclotron resonance. As ions have resonant frequencies, these frequencies can be used to isolate the ions prior to further fragmentation or manipulation. For example, a resonant frequency pulse on the excite plates (E+/— in Figure 2.8b) will eject the ions at, or near, that frequency. Furthermore, frequency sweeps - carefully defined to not excite the ion of interest - can be used to eject unwanted ions. However, the most elegant method for ion isolation is that of Stored Waveform Inverse Fourier Transform (SWIFT) [86] in which an ion-exdtation pattern of interest is chosen, inverse Fourier-transformed, and the resulting time domain signal stored in memory. This stored signal is then clocked-out, amplified, and sent to the excite plates when needed. The typical isolation waveform in SWIFT uses a simple excitation box with a notch at the frequencies of the ion of interest, a few kHz. 

Fourier transform (FT) instruments, whether used for infrared, NMR, ion cyclotron resonance, etc., all operate in the time domain. The intensity of the analytical signal measured by the detector is related to the time at which the measurement was made, rather than the frequency, or energy, of the signal itself. Time-domain spectra are meaningless to the spectroscopist, and must be converted into the frequency domain to appear intelligible. This is done using a mathematical, computationally intense, procedure known as Fourier transformation. The Fourier transformation for a continuous function is given in equation (10.3). 

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