Collector slit

Curvature and rotation lenses correct for any imperfections (aberrations) in the cross-sectional shape of the beam before it reaches the collector slit. The curvature lens provides a means of changing any banana-shaped beam cross-section into a rectangular shape (Figure 24.8). The rotation lens rotates the beam such that the sides of the beam become parallel with the long axis of the collector slit (Figure 24.8). 

Through the use of sequential electric (electrostatic) and magnetic fields (sectors) and various correcting lenses, the ion beam leaving the ion source can be adjusted so that it arrives at the collector in focus and with a rectangular cross-section aligned with the collector slits. For the use of crossed electromagnetic fields. Chapter 25 ( Quadrupole Ion Optics ) should be consulted. 

There are three main reasons for this choice. Firstly, it becomes more and more difficult to obtain recordable, molecular-ion signals from un-derivatized carbohydrates as their M, increases significantly above 3000. Secondly, the mass spectrometers that have been used in all high-mass-carbofiydrate studies published at the time of writing this article are not capable of very sensitive analysis above —3800 mass units (see later). Thirdly, at masses >4000, it is usually not practicable to work at the resolution necessary for adjacent peaks to appear as separate signals in the spectrum. To do so would require that the source and collector slits be narrowed to such a degree that there would be an unacceptable loss in sensitivity. Thus, spectra acquired at mass >4000 are usually composed of unresolved clusters. 

So how does the IRMS get its stability Collector slits are several times the width of the ion beams. This gives a flat-topped peak shape (Fig 6) which makes the ion current intensive to drift. The main source of drift is temperature variation which both affects the electronic components used for mass selection and caused expansion and contraction of mechanical parts. Simultaneous measurement of ion beams using a double or triple collector is more precise than sequential measurement by mass scanning with a single detector. Finally, frequent comparison of sample gas under identical conditions also contributes to stability. Ion beam stability is more important than resolution for isotopic measurements. 

The gas stream from the inlet system (Fig. 2.2) enters the ionization chamber (operated at a pressure of about 10-6 — 10"5 torr) in which it is bombarded at right angles by an electron beam emitted from a hot filament. Positive ions produced by interaction with the electron beam are forced through the first accelerating slit by a weak electrostatic field. A strong electrostatic field then accelerates the ions to their final velocities. To obtain a spectrum, the applied magnetic field is increased, bringing successively heavier ions into the collector slit. A scan from mass (strictly m/z see above) 12 — 500 may be performed in seconds. 

Figure 9 Magnetic sector pulse-counting mass spectrometer (a) ion source, (b) ion lens, (c) source defining slit, (d) collector slit, (e) electron multiplier. (From Ref. 47.) (Courtesy of R H. Hemberger, Los Alamos National Laboratory.)...

The instrument was operated in the electron ionization (El) mode with 70-eV electrons, a source temperature of 200 °C, the conversion dynode at -5000 V and the secondary electron multiplier at 2400 V. The source and the collector slit widths were adjusted to obtain trapezoidal peaks with flat tops. The GC-MS interface was at 280 °C and high-purity He was used as a carrier gas. Data were acquired in the selected-ion monitoring (SIM) mode using voltage peak switching and the quantitation was based on peak areas. 

Fig. 39 c. Magnetic sector mass spectrometer 1) = Ion source 2) = FFR 1 collision cell 3) = Source slit = Magnetic sector 3) = Focusing element 6) = Intermediate slit 7) = FFR 2 collision cell 8) = Focusing element 9) = Electric sector 10) = Collector slit 11) = Conversion dynode 12) = Secondary electron multiplyer (SEM)... 

The mass spectra are generally obtained by magnetic scanning, that is, increasing B, at constant V. Thus, the ions of progressively higher m/z reach the required value of radius to pass through the collector slit sequentially. 

---