Image current detection is unique among ion detection methods, in that it is inherently non-destructive. Ion all other detection schemes, ions are driven into pieces of metal in order to detect them. Upon striking the metal, the ion is neutralized and can no longer be detected. Image current detection does not require the ions to strike any objects in order to detect them but rather relies on the ion's motion inducing a current on pieces of metal.
As an ion approaches a piece of metal, it will induce an increasing "image charge" on said metal. For example, as positive ions approach a piece of metal, negative charges build up on the surface due to electrostatic attraction. As the ions get closer and closer to the metal, more charges build up on the surface. This increase in image charge is referred to an an "image current". Conversely, as positive ions move away from the piece of metal, the amount of negative charge on the surface will decrease, causing an image current in the opposite direction. By monitoring the current flowing to and from the piece of metal, it is possible to detect the motion of ions.
Image current is most commonly used in mass spectrometry with two different mass analyzers: FT-ICR and the Orbitrap. The principle of operation in both cases is essentially the same. At a very basic level, ions oscillate back and forth with periodic motion between two pieces of metal, with the frequency of oscillation depending on each ion's mass-to-charge ratio. This motion is generated by magnetic fields in the case of FT-ICR and electrostatic fields in the case of the Orbitrap:
As the ions oscillate back and forth, they induce positive and negative image currents on the two pieces of metal. The more ions there are, the stronger the induced image current will be. Also, the closer the ions come to the metal plates, the stronger the image current will be. The instrument's circuitry measures this induced current, which then runs through a somewhat complex signal processing chain. At a very base level, the signal processing performs a Fourier transform. In a real world scenario, there will be many ions of varying m/z values that are oscillating between the metal plates simultaneously. Ions with different m/z values will be oscillating back and forth with different frequencies and therefore inducing currents with different frequencies. As such, the total image current picked up by the circuitry is a complicated mixture of numerous sine waves with various frequencies and intensities. The Fourier transforms takes this complex mixture and breaks it into its respective sine waves, providing the various frequencies and amplitudes of ion motion that were picked up. Since frequency is related to m/z and intensity is related to ion counts, a mass spectrum can be generated based on this information.