Nephelometry

Nephelometry is an analytical procedure that is based on the dispersion of radiation that particles of matter pass through. When light passes through a transparent medium in which there is a suspension of solid particles, it is dispersed in all directions and as a consequence it appears cloudy. Dispersion does not involve the net loss of radiant power, only the direction of propagation is affected, because the intensity of the radiation is the same at any angle. The intensity depends on: the number of suspended particles, their size, their shape, the refractive indices of the particle and the dispersing medium, and the wavelength of the scattered radiation. The relationship between variables and a theoretical treatment is more feasible, but due to its complexity it is rarely applied to specific analytical problems. The procedure is generally empirical and only 3 factors are considered:

1. Concentration: The higher the number of particles, the greater the scatter.
2. Particle size: factors such as pH, the speed and order of the mixture, concentration of the reagents, duration of the rest state and the ionic force.
3. Wave length: Usually the samples are illuminated with white light, but if they are coloured, a portion of the electromagnetic spectrum should be chosen in which the absorption of the medium is reduced to a minimum.

Surfactant agents (such as gelatin) are added to the samples to prevent coagulation of the colloid. Reliable data is only obtained if the variables that affect particle size are scrupulously controlled.

Application

Generally, nephelometry and turbidimetry are used in the analysis of the chemical quality of water to determine clarity and to control treatment processes. These methods may also be appropriate for quantitative assays using antigen-antibody complexes or for measuring the amount of proteins in fluids. Turbidimetric and nephelometric measurements occupy a prominent position in clinical laboratories. They have been applied for the determination of immunoglobulins, complement proteins, acute phase proteins, coagulation proteins, among others.

Difference with turbidimetry

The choice between one of the two methods lies in the light dispersion, if it is extensive, it is appropriate to apply turbidimetry, on the other hand, if it is minimal, nephelometry is appropriate. Better results are obtained from nephelometry, because it determines concentrations of a few parts per million (ppm), with a precision of 1 to 5%.

Nephelometry is based on the measurement of scattered radiation, while turbidimetry is based on the measurement of the intensity of a diminished beam.

The instrument used in nephelometry, the nephelometer, resembles the fluorimeter. On the other hand, in turbidimetry, the turbidimeter is used, which is a filter photometer.