Wiley-McLaren

Figure Bl.7.17. (a) Schematic diagram of a single acceleration zone time-of-flight mass spectrometer, (b) Schematic diagram showing the time focusing of ions with different initial velocities (and hence initial kinetic energies) onto the detector by the use of a reflecting ion mirror, (c) Wiley-McLaren type two stage acceleration zone time-of-flight mass spectrometer.

The velocity mapping-ion counting technique increases the resolution of imaging data. This is clearly seen when it is compared with the data from the conventional Wiley-McLaren imaging apparatus2 or even the... 

The femtosecond laser pulses shaped by the AOPDF are amplified by the CPA up to 0.5mJ/pulse. Ethanol vapor is continuously flow into the vacuum chamber through a micro-syringe (70 pm) with stagnation pressure of 7 Torr at room temperature. The laser pulses are focused on a skimmed molecular beam of the ethanol vapor with an achromatic lens (/ = 145 mm). The focal spot size of the laser beam is 20 pm(j>. The peak intensity of the transform-limited laser pulse is calculated to 4 x 1015 W/cm2. The fragment ions are mass-separated with Wiley-McLaren type time-of-flight (TOF) mass spectrometer, and are detected with a microchannel plate (MCP) detector. 

The relationship between the initial velocity of the incoming neutral species and the TOF for the ionized species to reach the MCP detector is well known for the set-up shown in Figure 9.1, which is known as the Wiley-McLaren-type TOF mass spectrometer [21]. The right and left ordinates of the lower panel of Figure 9.6 show the relationship between the initial velocity and the flight time under our experimental set-up. Detailed discussion on the relation will be made elsewhere [18]. Instead, we will focus on... 

Today, the modern version of time-lag focusing, or delayed extraction,14 16 is used in almost all MALDI-TOF mass spectrometers. It is in fact a special case of the Wiley and McLaren method, in which the initial spatial distribution (usually a very thin sample and matrix mixture dried on a stainless steel plate) is assumed to be zero. Therefore, after the delay time the velocity and spatial distributions are correlated, i.e. ions of highest velocity have moved the greatest distance. This space velocity correlated focusing has been described by Colby and Riley,17 and provides considerably better focusing than can be obtained from two independent and uncorrelated initial distributions in space and velocity/energy that comprise the general Wiley-McLaren case. 

In the Wiley McLaren instruments produced for many years by the Bendix Corporation (Rochester, NY, USA) the mass-dependence problem was addressed by scanning the delay time, bringing a different mass into focus in... 

When reaction products are formed in a finite region, rather than a point source, it is best to extract the ions with a combination of acceleration and drift regions which are consistent with the Wiley-McLaren space focusing condition (Wiley and McLaren, 1955). Specific ratios of acceleration and drift distances for given electric fields are imposed so that the ion TOP is independent of the precise initial position of ionization. This is easily explained as follows. Suppose that two ions (with initially zero kinetic energy) are formed at positions Uj + 8 and tij — 8 as measured from the second acceleration, or drift region. Clearly the ion closer to the drift region will reach this... 

Figure 5.25 Ion TOP distributions for isotropic angular distribution of dissociation products. The mass of the fragment ion is 30 amu, the electric fields in the first two acceleration regions are 10 and 100 V/cm and the acceleration distances are 2 and 1 cm, respectively. The drift distances are indicated in the figure. The d = 55 cm corresponds to the Wiley-McLaren space focusing condition. For each drift distance, the ion TOF was calculated for three product translational energies of 0, 0.5, and 1.0 eV.

The TOE under Wiley-McLaren conditions and the Doppler techniques are similar in that the initial velocity produces a linear response in the measured quantity (T — T and Vobs o> respectively). Likewise the energy analysis of product ions produces a signal... 

Time of flight mass spectrometer Wiley McLaren, 7... 

An example of a Fourier transform TOF mass spectrum of toluene obtained from this instrument is shown in Figure 7.3. Their overall duty cycle of 25% is considerably greater than that of the Wiley-McLaren design for electron impact ionization. 

Passive Ion Storage. The IRLD instrument was constructed from a modified CVC Products (Rochester, NY) model 2000 time-of-flight mass spectrometer, a Wiley-McLaren design that included a pulsed electron beam. When used in the electron impact (El) ionization mode it was possible to monitor the NJ ion signal as a fimction of the electron-beam pulse width from 0.1 ps (where the signal was proportional to the pulse width) to 3 (is (where losses due to ion drift from the source equal the rate of ion production). From this, one could determine the rate of ion loss from the extraction volume due to drift at thermal energies. For the N2 ion (wj/z = 28) the half-life in the extraction volume was about 1 ps. Since the drift velocity is ... 

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