Dr. Lhoëst G.J.J.
The detector is a device where ions separated by the mass analyzer are converted into a measurable electric current (signal). The analogue current is the further converted to a digital signal stored by the data system. All the mass analyzers require an ion-detection device except for the FTICR instrument which is a mass analyzer and a detector at the same time. the most important characteristics for an ion-detection device are speed, dynamic range and sensitivity (gain)
3.1 Single Point Ion Collectors
3.1.1. Faraday Cup Detectors
This type of detector consists of an open-ended metal cage having a metal collector plate that is grounded through a high-ohmic resistor (1010 to 1012 Ohm).
Positive ions arriving at the collector plate are neutralized by accepting electrons from the grounded metal plate and negative ions donate electrons. The acquisition or depletion of electrons creates a potential drop across the resistor that is proportional to the abundance ot the arriving ions. The flow of electrons resulting from the potential drop results in a small electric current that can be amplified and converted to a voltage for subsequent display and/or recording. Reflected ions and ejected secondary electrons are prevented by the shape of the detector.
3.1.2 Secondary Electron Multipliers (SEM)
Positive ions leaving the analyzer are acelerated into the multiplier by a -2 to 5 kV potential applied to the front of the multiplier. The output of the multiplier is grounded. The reverse situation is true for negative ions. The accelerated ions impinge on a conversion dynode plate made of a Cu-Be alloy from which
two secondary electrons are ejected for every ion arriving. The released electrons are then accelerated and strike another dynode from which about 2 electrons are ejected for every arriving electron. This cascading continues through a series of stages. The avalanche of electrons produced over 12-18 dynode stages causes an electric current large enough to be detected by a sensitive preamplifier. Such a detector is called secondary electron multiplier (SEM) Depending on the applied voltage, SEMs provide a gain of 106-108 and the resulting current at the electron trap is the input of a low-noise preamplifier providing another 106-107 gain. Its output voltage is converted to a voltage signal which can be tranlated to an intensity value by means of an analog-to-digital converter (ADC). The gain of all types of secondary electron amplifiers decreases with age as the impinging ions burn into the surface of the conversion dynode.
3.1.3 Channel Electron Multiplier (CEM)
The cascade of secondary electrons can be produced in a continuous tube. In the channel type multipliers, the cascading of the secondary electrons takes
place along the inside wall of a curved leaded glass tube coated with a resistive (109 Ohm) material (silicon dioxide) in order to withstand high voltage of about 2 kV. This system acts as a continuous dynode when a potential difference is placed between the ends of the tube. The cascading of the secondary electrons is repeated approximately 20 times and the gain is a function of the length-to-diameter ratio. These channel electron multipliers (CEM) are often used with quadrupole instruments where the kinetic energy of the ions in the analyzer is only a few electron volts.
3.2 Multipoint Array Detectors
In the various array detectors, a portion of the mass scale being scanned is detected simultaneously. The spatial array detectors are electro-optical ion
detectors based on a number of very small microchannel electron multipliers called channeltrons arranged in a line and/or above the other in a very small space. The number of array elements if often limited to 100 for space and cost considerations and consequently a limited section of the mass range can be covered at a given time. The term microchannel plate is used to describe an assemblage of small single point detectors all of which are connected so as to act as a single large monitoring element. The most important application of array collectors is with nanosecond scale, pulsed ion configurations , such as MALDI, where the abundance of a range of ions produced at the same instant must be detected in a short interval of time. TOF analyzers use microchannel plate detectors almost exclusively and often with the oa-TOF configuration because they are capable of generating instantaneous (30 µs) spectra and also the oa-TOF-microchannel plate combination can sum approximately 30.000 spectra per second giving very high signal/noise ratios.