Filtering the Ion Beam

The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode, in which it allows all ions in a beam to pass through, whatever their m/z values. In doing so, the ion beam is constrained, so it leaves the hexapole as a narrow beam. This constraint is important because the ion beam from the inlet system tends to spread due to mutual ion repulsion and collision with residual air and solvent molecules. By injecting this divergent beam into a hexapole unit, it can be refocused. At the same time, vacuum pumps reduce the background pressure to about 10 mbar (Figure 22.1). The pressure needed in the TOF analyzer is about 10 ... 

Two further important parameters have to be defined for the energy filter layout the channel length and the width of the outlet aperture. The channel length is important because the ion beam enters the channel with a certain width. Since ions entering the channel close to the inner or outer radius are deflected differently, ions of the desired energy are focused at 63 °. 

Extraction and acceleration of electrons and ions, focusing, and energy filtering of the ion beam rely on stable but tunable electric fields. Different potentials have to be applied to the electrode structures to generate these fields. 

Analysis Conditions. For the ISS analyses, the ion beam incidence is normal to the sample surface and the scattered ions are energy analyzed with a CMA in a full annular ring at 138° with respect to the incident beam direction. For the SIMS analyses, the sample is tilted until the ion beam incidence reaches 75° with respect to the surface normal. The secondary ions are first energy filtered before being mass discriminated in the quadrupole mass spectrometer. More details about both ISS and SIMS analyses are given in Table II. 

The ions from the ion beam are separated by their mass to charge ratio (m/z) (where commonlyz= 1), providing the series mi/zi, m2/z2, m3/z3,...mn/Zn that will give the mass spectrum. The ion separation can be done using special ion optics that differentiate the mass spectrometers as follows magnetic sector instruments, quadrupole, time-of-flight, ion trap, Wein filter, ion cyclotron resonance, etc. 

SIMS requires an ultra-high vacuum environment, similar to AES and XPS. The ultra-high vacuum environment ensures that trajectories of ions remain undisturbed during SIMS surface analysis. The SIMS vacuum chamber and pumping system is not much different from that for AES and XPS. Figure 8.5 illustrates common SIMS structure in a vacuum chamber in which there are two main components a primary ion system and a mass analyzer system. The primary ion system includes a primary ion source, a Wien filter and ion beam deflector. The mass analysis system includes a secondary ion extractor filter, mass analyzer and ion detector. 

Brooker (1997) measured the Raman spectra using a Laser Raman Microprobe Renishaw and a conventional spectrometer Coderg PHO. A super-notch filter served as a monochromator in front of the entrance slit of a single grating, which in turn disperses the Raman beam onto a 400 x 600 CCD detector. The Laser Raman Microprobe was equipped with a 632.8 nm helium-neon laser of 10 mW power and a 514.5nm argon ion laser of 50 mW power with the appropriate super-notch filters. The laser beam was focused into the sample by a lens with an Olympus microscope and the back-scattered Raman light was collected by the same lens. Samples of molten salts were sealed in capillary tubes under dry nitrogen or vacuum. 

In an alternative approach to the use of DMS as a filter for ESI before a high-end MS, a low-resolution QMS was fitted with a DMS for real-time chemical analysis in the field. The instrument had a mass resolution of 140 with two stages of differential pumping and an electrodynamic ion funnel to transport the ion beam from ambient pressure to the MS. This prototype DMS-MS detected approximately 1 ppb of dimethyl methyl phosphonate (DMMP) as a simulent for chemical warfare agents. 

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