The ion collectors

The currents carried by the major and minor ion beams are of the order of 10 and 10 A respectively. These pass to the major and minor head amplifiers that have high impedence, low current noise characteristics and are normally of d.c. vibrating reed design. High value feedback resistors in parallel with the head amplifiers ensure maximum gain. Treatment of the output voltage from the head amplifiers is considered in the following section. 

Measurement of the isotopic ratio. This is normally achieved using either a bridge—amplifier balance system or the infinite bridge null system. The former method provides that the output of the major amplifier supplies a resistor divider network whose output is fed back to the input of the minor amplifier. 

Adjustment of the divider to attain a given pen position on a potentiometric recorder provides the ratio from the sum of the divider and recorder readings. 

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Stop must be used to prevent photons from the plasma flame reaching the ion collector, which would produce a spurious high background signal. 

After the skimmer, the ions must be prepared for mass analysis, and electronic lenses in front of the analyzer are used to adjust ion velocities and flight paths. The skimmer can be considered to be the end of the interface region stretching from the end of the plasma flame. Some sort of light stop must be used to prevent emitted light from the plasma reaching the ion collector in the mass analyzer (Figure 14.2). 

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) by separating the individual m/z values and then recording the numbers (abundance) of ions at each m/z value to give a mass spectrum. Quadrupoles allow ions of different m/z values to pass sequentially e.g., ions at m/z 100, 101, 102 will pass one after the other through the quadrupole assembly so that first m/z 100 is passed, then 101, then 102 (or vice versa), and so on. Therefore, the ion collector (or detector) at the end of the quadrupole assembly needs only to cover one point or focus for a whole spectrum to be scanned over a period of time (Figure 28.1a). This type of point detector records ion arrivals in a time domain, not a spatial one. 

In an EW- of a B/E-linked scan using an electric/magnetic-sector instrument, a precursor ion is selected. In this case it is m, which might be a molecular ion but equally could be any fragment ion. All product ions (mj, m3, m4) from decomposition of m, in the first field-free region between the ion source and the ion collector are found, thereby giving connections mpm, m -m3, m -m4. 

The total trajectory of the ions is approximately V-shaped, the top of one leg of the V being the position of the pusher electrode and the top of the other being the position of the ion collector (a microchannel plate detector). 

Ions that fragment in flight between the ion source and the ion collector are called metastable ions and can give information about connections between fragment ions. 

The mass spectrometer provides a mass spectrum that is actually an analog voltage varying in amplitude with time as ions of different m/z values arrive at the ion collector within a period of a few seconds. An important exception to this generalization occurs with ion collectors, called time-to-digital converters because their output is already digitized. 

The discovery of the x-ray effect spurred the development of a new generation of hot-cathode gauges designed to minimise this effect. One of the earhest, and commercially the most successful, the Bayard-Alpert gauge shown in Eigure 14b, was developed in 1950 (13). A fine wire, the ion collector, is... 

The magnetic scan is synchronised with the x-axis of a recorder and calibrated to appear as mass number (strictly m/e). The amplified current from the ion collector gives the relative abundance of ions on the y-axis. The signals are usually pre-processed by a computer that assigns a relative abundance of 100% to the strongest peak (base peak). 

The electrons emitted from the cathode impinge on the anode, releasing photons (soft X-rays). These photons, in turn, trigger photoelectrons from surfaces they strike. The photoelectrons released from the ion collector flow to the anode, i.e. the ion collector emits an electron current, which is indicated in the same manner as a positive ion current flowing to the ion collector. This photocurrent simulates a pressure. This effect is called the positive X-ray effect, and it depends on the anode voltage as well as on the size of the surface of the ion collector. 

Adsorbed gases can be desorbed from a surface by electron impact. For an ionization gauge this means that, if there is a layer of adsorbed gas on the anode, these gases are partly desorbed as ions by the impinging electrons. The ions reach the ion collector and lead to a pressure indication that is initially independent of the pressure but rises as the electron current increases. If such a small electron current is used so that the number of electrons incident at the surface is small compared to the number of adsorbed gas particles, every electron will be able to desorb positive ions. If the electron current is then increased, desorption " inaease because more electrons impinge on the surfece. This finally leads to a... 

Fig. 3.15 Explanation of the X-ray effect in a conventional ionization gauge. The electrons e emitted by the cathode C cottide with anode A and trigger a soft X-ray radiation (photons) there. This radiation strikes, in part the ion collector and generates...

The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by Redhead, offers pressure measurement in which errors due to X-ray and ion desorption effects can be quantitatively taken into account. In this arrangement there is a second thin wire, the modulator, near the anode in addition to the ion collector inside the anode. If this modulator is set at the anode potential, it does not influence the measurement. If, on the other hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current that flows to the ion collector becomes smaller. The indicated pressure p, of the ionization gauge with modulator set to the anode potential consists of the portion due to the gas pressure pg and that due to the X-ray effect pg ... 

After switching the modulator from the anode potential over to the ion collector potential, the modulated pressure reading p is lower than the p, reading because a portion of the ions now reaches the modulator. Hence ... 

In the gauge developed by Bayard and Alpert, Ix was decreased by reducing the surface area of the ion collector. In Bayard-Alpert gauges (BAGs), the collector is a thin wire surrounded by a cylindrical, coaxial open anode. With BAGs, pressures down to 10 9 mbar can be measured. 

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