To measure signals, it is necessary to convert photons in electric current. This is the role of the detection system. The most common systems used are photomultiplier tubes and solid-state detectors. Both have advantages and drawbacks.
A photomultiplier tube (PMT) is made of a cathode followed by several dynodes to amplify the signal. Photons reach the cathode through a window made of a material suitable for the wavelength range that has to be covered. Electrons are created by the cathode and amplified by dynodes.
Solid-state detectors are based on CCD (Charge Coupled Device) or CID (Charge Injection Device) technology. According to the optical mounting, linear or 2-dimensional detectors may be found. These detectors use the photonssilicon interactions to allow measuring the signal. The photons reaching the detector are converted into electrons through an electron-hole pair that can be transported to the reading device and measured. With a CCD, reading the signal is destructive for the acquisition, while a CID allows a non-destructive reading of the pixels.
PMTs have many advantages:
The single drawback of PMTs is:
PMTs have many advantages for ICP-OES spectrometry.
The only drawback is the fact that the full spectrum cannot be covered at the same time and then, the analysis time depends on the number of wavelengths measured. Nevertheless, integration time used with PMTs is less important than with solid-state detectors and for up to 10 elements, analysis time with PMT-based ICP-OES spectrometers can be less or equal to analysis time obtained with solid-state detection instruments.
For solid state detectors, we have to consider both CCDs and CIDs.
Despite the advantage on the speed of analysis that is directly linked to the full wavelength coverage, solid-state detectors still exhibit many drawbacks:
Solid-state detection instruments have many drawbacks. Despite all these drawbacks, many instruments use such devices because of the speed of analysis.