Types of Detectors Used in UV-Vis Spectroscopy System

What are Detectors?

Detectors are instruments that measure the amount of ultraviolet or visible light absorbed by the sample molecules. In chromatography, they measure light absorbed by components of a mixture while eluted out of the chromatography column. They are used for the identification and detection of analytes in the sample as different molecules have absorption peaks at different wavelengths.

Detectors serve as a valuable tool in quality control, quality assurance, teaching, and research laboratories. Meanwhile, they are indispensable in the pharmaceutical, biotech, and biopharma industries. Hence, it is essential to know about types of detectors to select one, which is compatible with our purpose.

In provision to this regard, this write-up sheds light on different types of detectors used in UV-Visible spectroscopy Besides, advantages and limitations of each type have been discussed.

Types of Detectors

A detector is an important instrument in the UV-Vis spectrophotometer, used in the conversion of light into proportional electrical signals, which provides the response of a spectrophotometer. 

Nowadays, detectors used in UV-Visible spectroscopy are classified into four types, namely

• Photomultiplier tube
• Phototube
• Diode array detector
• Charge coupled device

Phototube

Phototube is also known as a photoelectric cell. It is filled with gas under low pressure. It contains a light-sensitive cathode and anode inside an evacuated quartz envelope. Between the electrodes, a potential difference of 100 V is applied. A photon entering the tube strikes the cathode and leads to the ejection of an electron, which strikes the anode and results in the flow of current. The resulting current is of low intensity and requires amplification. The response in the phototube depends on the wavelength of incident light.

Phototubes utilize the photoelectric effect for the generation of current from absorbed light. Light is absorbed by a metallic surface having a low work function.

In order to avoid an excessive current density on a part of the cathode, it is necessary to illuminate a larger spot on the photocathode. Quantum efficiency is dependent on the used tube glass, used photocathode, angle of incidence, and optical wavelength.

Due to thermionic emission at longer wavelengths, a dark current is generated. In that case, tubes are cooled with liquid nitrogen.

Photomultiplier Tube

Photomultiplier tube is the popular detector nowadays used in UV-Vis spectrophotometers. It has an anode, cathode, and many dynodes.

Photon when entering the tube, strikes the cathode, resulting in the emission of electrons. The emitted electrons are accelerated towards the first dynode, which is 90 V more positive than the cathode. An electron striking the first dynode will result in the production of several electrons. The process repeats itself continuously from one dynode to another and after passing to 10 dynodes, there is the production of 100 electrons from a single electron. The resulting current requires it to be amplified.

Pros are

• Detection of very low levels of light
• High wavelength resolution due to the use of narrow slit widths
• More sensitive than phototube
• Faster response time and higher sensitivity for ultraviolet and visible radiations

Con is 

When exposed to higher intensity light, photomultiplier tube is vulnerable to damage

Diode Array Detector

A diode array detector is a multichannel photon detector, which is capable of measuring all wavelengths of dispersed radiation simultaneously. On a single silicon chip, there is an array of silicon photodiodes. Subsequently, Individual diodes are scanned for a response. 

A diode array detector is less sensitive than a photomultiplier tube but offers simultaneous measurement of various wavelengths. It is more rugged than a photomultiplier tube as there are no alignment problems. Also, there are no optical performance variations with the change in wavelength.  It is useful for recording the UV-Vis spectrum in HPLC instruments.

There are many silicon photodiodes formed on a single silicon chip. The number of photodiodes can be from 64 to 4096, but 1024 photodiodes are the most commonly used. Storage capacitors and diodes are present in each diode. Individual diode circuits are sequentially scanned. It is placed in the focal plant of the monochromator, so that spectrum falls on the diode array.

Each diode in the array is reverse biased. Through the transistor switch, each diode in the array is completely charged. When light reaches, charge carriers are generated and they neutralize the stored charges having opposite polarity. The lost charge is directly proportional to light received by the detector and is measured during the recharge process of each diode, thereby measuring the amount of current needed for recharging.

Pros are

• Recording of complete spectrum in a short period of time as the movement of diffractor does not affect scan time
• Capacity to monitor at all wavelengths
• Detection of transient signals like in flow analysis and HPLC
• Quantifies larger photon fluxes
• High quantum efficiency
• High height-to-weight aspect ratio

Cons are

• Presence of relevant dark current
• High read noise

Charge Coupled Device

Charge coupled device (CCD) is a highly susceptible detector. Hence, it is used for the detection of extremely lower light intensity signals. It is similar to diode array detectors except that it has photo capacitors instead of diodes. They contain an array of photo capacitors arranged in single or two dimensions. Each photocapacitor consists of millions of detector elements called pixels. CCD can detect emitted light from lowest to highest wavelengths simultaneously. CCD has lower noise and higher sensitivity than diode array detectors. 

There is a silicon chip in the CCD which converts light into an electric signal. The chip absorbs a photon and releases one electron. It is covered by electrodes, which hold electrons in pixels or an array of electrons. Building up of charges corresponds to the pattern of incident light.

Some pros of CCD are 

• Low dark count rate
• High UV-Vis quantum efficiency
• Low read noise

Conclusion

Different detectors are used in the instrumentation of UV-Vis spectroscopy. The analysis should be of lower cost and simple. Each of the detectors can be used in different situations and samples depending on the need. Available sample volume, the concentration of analyte, and reaction mixture play a role in the selection of detectors. Detection should be done at a very lower concentration of samples in ultraviolet-visible spectroscopy. In addition, they should be of higher quality and miniaturized.