RF ramping

Several methods for ion isolation exploit relatively straightforward combinations of DC voltages, RF ramps and tickle frequencies to eject unwanted ions. An example is the so-called DC isolation method , which is based upon an earlier method used for selective ion storage (Bonner 1977 FuRord 1978). This isolation method uses... 

Fig. 3.4.2 Schematic description of the three-dimensional SPRITE imaging technique. Gz, Gx and Gy are the phase encode magnetic field gradients and are amplitude cycled. A single data point is acquired at a fixed encoding time tp after the rf excitation pulse from the free induction decay (FID). TR is the time between rf pulses. Notice that Gx is ramped (+GZ max to -GXt max) and one /c-space point is acquired for each value of the magnetic field gradient. Gy and Gz are on during the Gx magnetic field gradient ramp and turned off at the end.

Fig. 3.4.4 Schematic description of the one-dimensional double half k (DHK) SPRITE technique. The phase encode magnetic field gradient, Gz, ramped through half of /(-space beginning at the center and a single data point is acquired at a fixed time (tp) after the rf excitation pulse. The second half of /(-space is acquired after a 5T time delay. The time between rf pulses is TR.

Ramping of both DC and RF voltages in a simultaneous fashion produces resonant or stable trajectories (Fig. 11.9) for ions of sequential... 

The final implant annealing process schedule developed during this research is shown in Figure 4.19. A 6-slm UHP Ar flow is first established in the reactor. When the RF generator is turned on, the susceptor is heated to the annealing temperature (typically 1,600°C) using a controlled thermal ramp. To avoid the formation of Si droplets, silane is not introduced into the reactor until a substrate temperature of 1,490°C is reached. At that time the premixed silane in Ar gas is introduced into the Ar carrier flow at a flow rate of 20 seem. All flows are controlled using calibrated... 

Shaped pulses are created from text files that have a line-by-line description of the amplitude and phase of each of the component rectangular pulses. These files are created by software that calculates from a mathematical shape and a frequency shift (to create the phase ramp). There are hundreds of shapes available, with names like Wurst , Sneeze , Iburp , and so on, specialized for all sorts of applications (inversion, excitation, broadband, selective, decoupling, peak suppression, band selective, etc.). The software sets the maximum RF power level of the shape at the top of the curve, so that the area under the curve will correspond to the approximately correct pulse rotation desired (90°, 180°, etc.). When an experiment is started, this list is loaded into the memory of the waveform generator (Varian) or amplitude setting unit (Bruker), and when a shaped pulse is called for in the pulse sequence, the amplitudes and phases are set in real time as the individual rectangular pulses are executed. 

At this stage, different MS experiments can be performed. In the full-scan mode, ions of different m/z are consecutively ejected from the trap towards the external detector by ramping the RF voltage at the ring electrode. Resonant ion ejection may be supported by additional waveforms applied to the end-cap electrodes. Ion ejection can be achieved with unit-mass resolution, or at enhanced resolution by slowing down the scan rate [42]. 

An example of the application of optimal control theory for experiment design is described in Fig. 8, which in panel (a) shows the transfer efficiency achievable by optimal control sequences by the solid line, while corresponding efficiencies for the DCP experiment under ideal and inhomogeneous rf conditions as well as the performance of ramped versions of DCP are illustrated by the various dashed/dotted lines. We should... 

Lorentzian rf inhomogeneity, (—) ramped DCP with ideal rf, and (— ramped DCP with 5% Lorentzian rf inhomogeneity, (b) x- (solid line) and y-phase (dotted line) rf amplitudes for the 2.4 ms optimal control sequence marked by an arrow in (a). Upper and lower panels correspond to the N and C rf channels, respectively, (c) Experimental DCP (left) and DCP (the sequence in b) - spectra of glycine. Reproduced from Ref. 70 with permission. 

Fig. 4. Pulse sequences for 2D PISEMA experiments on static or slow-spinning samples. It has been shown that the use of ramped rf pulses for S-spin-lock during the ti period makes the pulse more tolerant to experimental errors. The original PISEMA sequence (A) does not suppress the effects of phase transients. Incorporation of the modified SEMA sequences, (B) and (C), in the ti period of the PISEMA sequence (A) have been shown to suppress the effects of phase transients.

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