Different types of detectors used in Spectrophotometer

Detectors:

Three different types of detectors are commonly used in Spectrophotometer which are:

I. Photovoltaic cell

II. Phototubes

III. Photo Amplifier s

I. Photovoltaic cell:

There are different designs of photovoltaic cells. It operates without the use of a battery. A typical photovoltaic cell consists of a metal base plate (irons or aluminium) acts as one electrode. On the surface of the base plate, semiconductor thin metal layer of selenium is coated. Further it is covered by a thin layer of silver or gold on the outer most surface. It acts as a second collector electrode. When incident radiation strikes at surface it generates electrons in Se-Ag interface. Then the electrons are collected by silver and created electric voltage. It is limited to visible region (450-650 nm) and sensitive to the whole visible range. However output depends upon the wavelength of the incident light. Current can’t be amplified readily in this design and so there are fatigue effects after long time operation.

Figure 1: Photovoltaic cells

II. Phototubes:

It is also known as photo-emissive cells. It consists of an evacuated glass bulb, inside which a light sensitive cathode in form of a half cylinder of metal is fitted. The cathode is coated with light sensitive layer, cesium, potassium oxide or silver oxide. A metal ring is inserted near the center acts as an anode for capturing of electrons. The incident beam when falls on cathode it emits photoelectrons which are attracted by an anode. The electrons return via the external circuit which is amplified to read out readily. It measures the amount of the light striking the photo-sensitive surface and expressing it in absorbance or transmittance or concentration forms. For the wavelength of 350-450 nm the cathode should be coated with sodium.

Figure 2: Phototubes

III. Photo amplifiers:

It is one of the highly sensitive devices used today.  It consists of an electrode covered with a photoemissive material. A large number of plates known as dynodes are used. A dynode is cover with a material which emits several electrons. Once electrons are ejected from the cathode and accelerated to the sensitive surface of the dynode, secondary electrons are emitted in greater amount than the striking the plate by 4 x to 5 x factor Each dynode is maintained at 75-100 V more positive than the preceding dynode. Over all amplification by about 10 dynodes will be 10⁶. So it can be used to measure the intensities about 200 x weaker than those by conventional methods. It also prevents from stray light.

Figure 3: Photo amplifier

Figure 4: Photograph of photo amplifier