Electrostatic mass filters

This consists of four rod-like electrodes arranged parallel to each other about a central axis. Potentials with a radio-frequency component are applied to these rods in order to create an electrostatic mass filter. Only masses within a specified range can pass down the axis of the filter and be detected. Ions outside this mass range exhibit unstable oscillations and eventually hit one of the electrodes. By varying the potentials applied to the electrodes mass scanning can be achieved. 

FIG. 35. Vertical cross section of the reaction chamber equipped with the mass spectrometer system. Indicated are QMF. the quadmpole mass filter ESA. the electrostatic analyzer CD, the channeltron detector DE, the detector electronics DT, the drift tube lO, the ion optics TMP, the turbomolecular pump PR, the plasma reactor and MN. the matching network. 

Figure 6. Diagram of our 1-atm ion mobility spectrometer (IMS) apparatus (a) stainless steel source gas dilution volume, (b) septum inlet, (c) needle valve, (d) Nj source gas supply, (e) source and drift gas exhaust, (f) flow meter, (g) pressure transducer, (h) insulated box, (i) drift tube, (j) ion source, (k) Bradbury-Nielson gate, (I) Faraday plate/MS aperture, (m) drift gas inlet, (n) universal joint, (o) electrostatic lens element, (p) quadrupole mass filter, (q) 6"-diffusion pump, (r) first vacuum envelope, (s) channeltron electron multiplier, (t) second vacuum envelope, (u) 3"-dif-fusion pump, (v) Nj drift gas, (w) leak valve, (x) on/off valves, (y) fused silica capillary, (z) 4-liter stainless steel dilution volume, (aa) Nj gas supply.

Once the ions have been focussed through the orifice and into the analyser portion they are further focussed by electrostatic lenses into the first of three axially aligned quadrupole arrays. The first and third arrays can be operated as mass filters while the central quad (Q2 in Figure 1) functions only as an ion guide, incapable of resolving one mass from another. 

The tandem MS includes a quadrupole mass filter, an octopole ion guide," a second quadrupole mass filter, and an ion detector. The ions from the flow tube are focused through electrostatic lenses into the first quadrupole, where a particular reactant ion is selected. These ions are then focused into the octopole, which passes through a cell that contains the collision gas. From the octopole, the dissociated and unreacted ions are focused into a second quadrupole for mass analysis. The detector is an electron multiplier operating in pulse-counting mode. 

A pioneering work on the simultaneous measurement of the angular and velocity distributions was carried out by Champion et al. [98—101] following the velocity work of Vance and Bailey [94] described above. Their apparatus, a tandem mass spectrometer system, consists essentially of three sections a primary ion gun, a collision chamber, and a product-ion analyser and detector. A mass-analysed, velocity-selected ion beam is directed into the collision chamber containing target gas at low pressure. The product ions are velocity-analysed with a 127° electrostatic velocity selector and mass-analysed in a quadrupole mass filter. The angular distributions of the product ions are obtained by rotating the analyser-detector system about the centre of the collision chamber. 

Fig. 3.2. High-resolution double-focusing mass analyser with Nier-Johnson geometry. The electrostatic sector filters ions according to kinetic energy before magnetic sorting according to m/z. The combination of electric and magnet fields produces narrow peaks and high spectral resolution.

Most process analyzers utilize either a Faraday cup or a secondary electron multiplier (SEM) for detection. The Faraday cup is the simpler and more mgged and stable of the two, but is generally useful for detection of species at higher concentrations (100 ppm to 100%). The SEM is much more sensitive, capable of measurements in the ppb range. It is quite common to configure a process MS with both detectors, along with a set of electrostatic lenses to switch the mass-filtered ion beam between the two detectors. This results in a single process analyzer that is capable of quantitation from 1 ppb to 100 % ... 

These ions are then accelerated by electrostatic fields into a mass filter to be separated and detected. The entire process takes place under high vacuum to minimize collisions between ions. 

A mass filter that separates ions according to their trajectories in a combined field consisting of perpendicular magnetic and electrostatic fields. It can also be used as a velocity filter. 

A set of DC-RF voltage on the rods will steer the analyte ions of interest electrostatically through the center of the quadrupole mass filter until the exit and they will be converted to an electrical pulse by the detector while other ions of different mass-to-charge ratios will stop in the quadrupole. In a multielemental analysis, the mass scan process is repeated one after another for all analyte ions of different mass-to-charge ratio until all the analytes in a multielement analysis have been measured. Quadrupole scan rates are typically on the order of 2,500 atomic mass units (amu) per second and can cover the entire mass range of 0-300 amu in about 0.1 s. However, real-world analysis speeds are much slower than the above. 

A mass filter that separates ions according to their trajectories in a two-dimensional hyperbolic electrostatic field with DC and radiofrequency AC components. Ions with stable trajectories transit the filter, while other ions discharge on the electrodes. Unlike a quadrupole ion trap, the Chan Hee Chon, Dongqing Li Department of Mechanical Engineering, Vanderbilt University, Nashville, TN, USA Chanhee.chon vanderbilt.edu... 

Fig. 3.13. Construction details of the combination of a temperature variable 22-pole ion trap and a pulsed supersonic beam. Primary ions are produced in a storage ion source and mass selected in a quadrupole mass filter (from the bottom, not shown). After deflection in an electrostatic quadrupole bender the ions are injected into the trap (see Fig. 3.12). There the ions are confined in radial direction by the rf field and in axial direction with dc voltages applied to the entrance and exit electrodes (less than 100 mV). Buffer gas is used, usually He, for thermalizing the ions. From the left a skimmed molecular beam traverses the trap without colliding with the cold surfaces. For detection, the stored ions are extracted to the right, mass selected and counted via a Daly detector. A laser beam can be injected via the detector tract for state selective excitation of the trapped ions.

Energy filters, covered in Section 4.23.2, also act to ensure that no line of sight exists between the sample and the detector. This is required to remove the secondary neutral emission also produced during the sputtering process. Note As secondary neutrals in the secondary ion beam cannot be mass filtered (these are unaffected by electrostatic and magnetic fields), they will introduce a background signal if not removed. 

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