A detector converts light into a proportional electrical signal which in turn provides the response of the spectrophotometer.
The human eye serves as a sensitive detector for colour changes and was used effectively in colour matching colorimetric instruments. Though the human eye served the desired purpose but the analysis was prone to individual bias and judgement .This necessitated the requirement for other sensitive detection options. In the years that followed detectors based on electrical charge transfer properties were developed. Modern-day spectroscopic detectors can be classified into four basic categories:
- Photomultiplier tube
- Diode array detector
- Charge coupled devices
The operation of these categories is discussed in this article.
A phototube comprises of a light-sensitive cathode and an anode inside an evacuated quartz envelope. A potential difference of approximately 100 V is applied between the two electrodes. A photon entering the tube strikes the cathode and results in ejection of an electron which strikes the anode and results in flow of current. The current is generally of low intensity and needs to be amplified. The response of the phototube is dependent on wavelength of incident light.
The photomultiplier tube is the most popular detector used in UV- Visible spectroscopy. It comprises of a photosensitive cathode, anode and several dynodes.
Photons entering the tube strike the cathode resulting in emission of electrons. The electrons are accelerated towards the first dynode which is 90 V more positive than the cathode.The electrons striking the first dynode resulted in several electrons for each incident electron. The process repeats itself from one dynode to next and after about 10 dynodes each photon results in production of 106 to 107 electrons. The resulting current often needs to be amplified.
Photomultipliers have high sensitivity for UV and visible radiation and have fast response times. However, they are susceptible to damage when exposed to high intensity light. Photomultiplier tube is inherently more sensitive than the photo tube.
Diode array detector
The diode array detector is a multichannel detector capable of simultaneous measurement of all wavelengths of dispersed radiation. It comprises of an array of silicon photodiodes on a single silicon chip (generally 1024). The individual diodes are subsequently scanned for response.
The diode array detector is less sensitive than the photomultiplier tube but offers the advantage of simultaneous measurement of different wavelengths
A diode array detector is more rugged than the photomultiplier tube as alignment problems are non-existent. Further there are no optical performance variations with wavelength change as in case of scanning monochromator instruments.
Charge coupled device detectors
Charge coupled device (CCD) detectors are highly susceptible detectors which are used mainly for detection of extremely low intensity light signals. These are similar to diode array detectors but instead of diodes they consist of an array of photo capacitors arranged in one or two-dimensional arrays. The photo capacitors comprise of thousands or even millions of detector elements also called pixels. Simultaneous detection of emitted light from lowest to highest wavelength is possible. CCD detectors have low noise, high sensitivity in comparison to diode array detectors.