Sweeps/ejections

Throughout the final phases, the solar wind will sweep across the Earth, tearing up the surface. The atoms of our dead will be driven from below and returned to the Sun. The Earth will surrender her dead. Finally, lit up by the bright white glare of the central star, the ejected matter will blossom into a beautiful planetary nebula rather like the one that so delightfully ornaments the constellation of the Lyre. The atoms of all human beings will shine in the sky, mixed with those of the animals and the stones. 

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). 

An obvious solution is to eject selectively all of the most abundant Ions, leaving only the small peaks of interest. As argued from Figure 4, multiple frequency-sweeps are not sufficiently selective for this purpose. However, SWIFT tailored ejection of the most abundant ions (23 in this case) removes the broadening and overlap from the large peaks. The resultant spectrum of the less abundant ions (bottom of Figure 5) exhibits better mass resolution (because Coulomb broadening has been reduced) and flatter baseline... 

To exploit the advantages of FTMS fully we have implemented several predefined ejection, activation and acceleration options, like single shots, covers, sweeps over some mass window, sweeps around some unaccelerated mass window, tickling belts with phase inversion, parent ion selection chirps, activation shots for daughter ion production, etc. 

The distributed array of drag elements in vegetation canopies creates a mean wind profile that contains an elevated shear layer centred near the canopy top that more closely approximates a plane mixing layer than a wall layer. This velocity stmcture is responsible for turbulence characteristics that differ substantially from those over a smooth surface. Velocity spectra are sharply peaked, streamwise and vertical velocities have probability densities that are strongly skewed, streamwise and vertical velocities are correlated more strongly that would be expected over a smoother surface, and transport is dominated by coherent flow structures with sweeps more important than ejections. 

In the zone close to the wall the momentum exchange with the outer regions is mainly sustained by ejection and sweeps, the events which are thought to be responsible for the stretching. 

It is now well established that in a turbulent boundary layer flow the transport of momentum to the wall is intermittent. The events responsible for this locally large instantaneous momentum transport are called sweeps and ejections depending on whether the transport is towards or away from the wall. A variety of models describing the flow field in such events as well as their overall organisation in the main flow have been proposed. Visualisation of ejections show that they are associated with a jet-like and a vortex-like flow field. The relation between the strengthes of these fields are not well known since in order to measure the vorticity in such events,one would need probes which are not available to-day. 

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. 

As ion trapping devices, FT-ICR instruments belong to the tandem-in-time category of instruments. The stage of precursor ion selection (MSI) is accomplished by selectively storing the ions of interest, whereas all others are ejected by means of a suitably tailored excitation pulse. For this purpose, the SWIFT technique [124] or correlated sweep excitation (CHEF) [125,126] are used (Chap. 4.7.7). Both methods generate tailored waveforms that cause excitation of all but the selected ions. Like QITs and LITs, FT-ICR analyzers are also capable of MS". 

Turbulence is a state of fluid motion where the velocity fluctuates in time and in all three directions in space. These fluctuations reflect the complex layering and interactions of large and small structural elements, such as vortices, sheets, ejections, and sweeps of a variety of shapes and sizes. In turbulent flows, scalar fields are rapidly dispersed compared to their laminar counterparts. At the time of writing, there is no completely acceptable way to model complex turbulent flow. 

Turbulent motions in single-phase boundary layers, pipe flows, and jets are not completely random but have coherent or ordered structure [42-45], According to the four-quadrant classification method, the turbulent motions are grouped into four distinct categories, as illustrated in Fig. 2.24. In this figure, ejection denotes a higher momentum fluid motion directed outward, outward interaction denotes a lower momentum fluid motion directed outward, sweep denotes a lower momentum... 

The measured values of for each turbulent motion remain almost unchanged in the radial direction, but in a strict sense, those for ejection and sweep changed trend around r/b = 1.0. This implies that the Fle-Wood s metal bubbling jet has two large-scale coherent structures, the boundary being located around r/by = 1.0. This result is consistent with the above-mentioned findings on the appearance frequency and the contributions of each turbulent motion to the turbulence kinetic energies and the Reynolds shear stress. 

Over the years, several methods of isolating ions in a QIT have been implemented. Methods include RF/DC isolation, forward and reverse RF resonance ejection isolation, " and various forms of TWF isolation. " The RF/DC isolation methods position the ion of interest near the boundaries of the stability diagram for isolation. In this case, the parameter is set to a nonzero value (see RF/DC isolation point in Figure 9.3b). The RF resonance ejection method sweeps the main RF amplitude and/or the resonance ejection frequency in both the forward and/or reverse directions to eject all but the ion of interest. This technique has been shown to yield high-resolution isolations and can be used to analyze multiply charged ions. ... 

---