Ion Detection and Spectra Acquisition

After the mass analyser has dispersed the ions in space or in time according to their various m/z values, they may be collected by a detector. In modern mass spectrometry, a detector consists of a planar assembly of small electron multipliers, called an array in one case (spatial separation) and a microchaimel plate in the other (temporal separation). These collectors can either detect the arrival of all ions sequentially at a point (a point ion collector) or detect the arrival of all ions simultaneously (an array or multipoint collector). 

Quadrupole mass spectrometers (mass filters) allow ions at each m/z value to pass through the analyser sequentially. Therefore, the ion collector at the end of the quadrupole unit needs only to cover one point or focus in space and can be placed immediately behind the analyser. A complete mass spectrum is recorded over a period of time (temporarily), which is set by the voltages on the quadrupole analyser. The resolution of m/z values is dependent solely on the analyser and not on the detector. 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled side by side over a plane. A multipoint collector may be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a time-of-flight analyser, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. 

Other types of mass spectrometer may use point or array or both types of ion detection. Ion trap mass spectrometers may detect ions sequentially or simultaneously and, in some cases, may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight, according to their m/z values. 

High Resolution Fourier Transform Ion Cyclotron Resonance (ICR) Mass Spectrometry 

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