Biochemical Techniques and Instrumentation

2.7.2.5. Detectors

Detector detects the sample components and subsequently signals a peak on the chromatogram. There are many types of detectors.

1. Refractive Index detectors:

They measure the ability of sample molecules to bend or refract light, i.e. based on the refractive index of a particular compound. They respond to all the compounds and changes the refractive index either positive or negative. They have modest sensitivity (10^-7gcm^-3) and commonly used in the analysis of carbohydrates.

2. Ultra violet (UV) detectors:

They are based upon ultraviolet-visible spectrophotometry. They are capable of measuring absorbance down to 190 nm. They use continuous flow cells with the optical path length of 10 mm, to continuously monitor the components. They have high sensitivity of 5x10^-10gcm^-3.

3. Diode Array detectors:

They use diode array techniques that can scan the complete spectrum of compound within seconds and display as a 3D plot on a screen in real time.

4. Fluorescent detectors:

They have very high sensitivity (10^-12gcm^-3) than UV detector but reduced linearity. They have limited applications as relatively few compounds fluoresce. Pre-derivatization of the sample can broaden their applications.

5. Radiochemical detectors:

They involve the use of radiolabelled material usually tritium (³H) or carbon-14 (¹⁴C). They detect the fluorescence associated with beta-particle ionization. They are most popular in metabolic research and has got high sensitivity limit of 10^-9 to 10^-10 g cm^-3.

6. Electrochemical detectors:

They measure compounds that undergo oxidation or reduction reactions accomplished by measuring gain or loss of electrons from samples as they pass between electrodes at a given difference in electrical potential. It has got a high sensitivity of 10^-12 to 10^-13g cm^-3.

7. Mass spectroscopy detectors:

The molecule is ionized and passed through a mass analyzer, where the ion current is detected. It has got detection limit of 10^-8 to 10^-10g cm^-3.

8. Nuclear magnetic resonance detectors:

They give the structural information of the compounds.

9. Light- scattering detectors:

They rely on the vapourization of the compounds, evaporation of the mobile solvent and then quantification of the compounds by light scattering detected by a photodiode. The intensity of the scattered light gives the quantity of the compound present and its particle size.

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