Waveform, notch

Notch — Disturbance of the normal power voltage waveform lasting less than a half cycle the disturbance is initially of opposite polarity than the waveform and, thus, subtracts from the waveform. Figure 1.7 shows notch and noise produced by the operation of a converter in a variable speed drive. 

Some manufacturers recommend using a filter at the output of the inverter to smooth the waveform applied to the motor and to reduce the sharp rise and fall in the notches that may be present, as in the case of current-fed motors. Steep sided notches cause a high dV/dt across the insulation of the motor, which can reduce the life expectation of the insulation. The filter may also be required to reduce electromagnetic interference (EMI). 

FIG U RE 15.17 The process of ion isolation is execnted seqnentiaUy. First, lower masses are ejected with a modified mass-selective axial instability scan that is, essentially, the same as normal analytical scanning. Second, higher masses are ejected resonantly with a broadband waveform. The ejection order may be reversed to avoid product ions of higher m/z-values. WFl is a notch waveform to eject unwanted ions during ionization and the post-ionization period WF2 is a broadband waveform to eject higher mass ions during higher mass isolation period. 

Several methods for constructing a notch waveform have been proposed [34-36]. Marshall developed originally the SWIFT technique [34] for Fourier transform ion cyclotron resonance, FT-ICR, spectrometry. However, because the trajectories of trapped ions in each of an ICR cell and a quadrupole ion trap are characterized by secular frequencies of ion motion, the SWIFT technique can be applied also to the quadrupole ion trap. The SWIFT technique is able to generate a near perfect notch... 

FIGURE 15.20 The basic concept of notch waveform isolation. In the isolation process, the working point of precursor ions is moved first to a j-value at which the secular frequency of precursor ions lies inside the window of the notch waveform. Subsequently, the notch waveform is turned on to eject all unwanted ions. Due to the limited duration of application of the notch waveform, the window is not a perfect box as is seen in the bottom picture of this figure. 

Noise Field (FNF) notch waveform [36] will have a performance somewhere between that of the quadratic SWIFT notch waveform and the random phase notch waveform. 

Buttrill has shown that in a notch waveform, the amplitude of each frequency component should be weighted [37], He suggested a formula, (A/A ) = in... 

FIGURE 15.21 (a) Notch waveform created with SWIFT technique, (b) The frequency... 

Schwartz and co-workers have proposed a modified notch isolation method that has somewhat higher resolution [39,40], The method combines a notch waveform with slow RF resonance ejection scanning. In the first step of the method, most of the unwanted ions, except the precursor and ions of mass/charge ratio similar to that of the precursor ions, are ejected by a notch waveform. The trapping RF is then scanned slowly down to eject ions with m/z-values higher than that of the precursor ions. Next, the trapping RF is ramped slowly up to eject ions with m/z-values lower than that of the precursor ions. An isolation resolution. Am, of ca 0.3 Th has been demonstrated with the technique. 

FIGURE 15.23 In the notch waveform, the two frequency components adjacent to the notch window have higher relative amplitudes compared to the other components. 

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. 

The specimens are either smooth or notched. They are subjected to either a cyclic load or a cyclic deformation. The cyclic parameter may follow a sinusoidal, triangular, or trapezoidal function or correspond to some other waveform. Figure 11.43 defines the three mechanical parameters of a corrosion fatigue test with a sinusoidal applied... 

Fig. 5.12a-f. Normal waveform changes in the cavernosal arteries during the onset of erection, a Phase 0. Monophasic flow with minimal or no diastolic flow occurring in the flaccid state, b Phase 1. Increased systolic and diastolic flow, c Phase 2. Dicrotic notch appearance at end systole and progressive decrease of the diastolic flow, d Phase 3. End diastolic flow disappearance, e Phase 4. Diastolic flow reversal, f Phase 5. Reduction of the systolic peak during rigid erection... 

High-Frequency ECG Surfece electrodes 100 Hz-1 kHz 100 pV-2 mV Notchs and slus waveforms superimposed on the ECG. 

Soni, M.H. and Cooks, R.G. (1994) Selective injection and isolation of ions in quadrupole ion trap mass spectrometry using notched waveforms created using the inverse Fourier transform. Anal. Chem.,... 

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