Nephelometry and Turbidimetry – Instrumental Methods of Analysis B. Pharma 7th Semester

Nephelometry and Turbidimetry

Nephelometry-and-Turbidimetry

Nephelometry and Turbidimetry

Objectives

At the end of the session the student will be able to

       Identify Nephelometry and Turbidimetry as  important methods of determination of solid  pharmaceutical substances

       Distinguish and compare Nephelometry and turbidimetry

       Compare and differentiate between Nephelometry and Fluorimetry

       Interpret the  data obtained in Nephelometric determinations

Nephelometry:

       Nephelometry is concerned with measurement of scattered light from a cuvette containing suspended particles.

        The components of a nephelometer are the same as a light spectrophotometer except that the detector is placed at a specific angle from the incident light.

       The detector is a photomultiplier tube placed at a position to detect scattered light.

       Detectors may be placed at 90o, 70o or 37o depending on the angle at which most scattered light are found.

Turbidimetry

       Turbidimetry (the name being derived from turbidity) is the process of measuring the loss of intensity of transmitted light due to the scattering effect of particles suspended in it.

CHOICE OF THE METHOD:

Depends upon the amount of light scattered by suspended particles present in solution.

       TURBIDIMETRY: high conc.Suspensions

       NEPHELOMETRY: low conc. Suspensions more accurate results

TURBIDIMETRY-NEPHELOMETRY

Difference between Nephlometry and Turbidimetry

  Nephlometry Turbidimetry
Definition The measurement of the intensity of scattered light at right angles to the
direction of the incident light as a function of the concentration of the
dispersed phase, It is most sensitive for very dilute suspensions (100 mg
/L).
Light passing through a medium with dispersed particles, so the intensity of light transmitted is measured.
Instrument used Nephelometer spectrophotometer
Type of light measured Scattered light Transmitted light
Arrangement of photometer Measure the light scattered at right angle to the direction of the propagation of light from the source.

It could be movable detectors which allow operator to vary the angle of detection

Made in the same direction as the propagation of the light from the source.

 

Clinical uses Ag-Ab reaction, immuno complex reaction, ppts, lipoproteins Ag-Ab reaction, immuno complex reaction, ppts, liver
dysfunction, protein in urine or CSF

INSTRUMENTATION:

Nephelometer
Nephelometer
Turbidimeters

Turbidimeters

1- Light source:

       Tungsten: Its relatively low intensity makes it less useful for samples with low light scattering.

       Alternatives are: quartz halogen lamp, xenon lamp and laser which have higher intensities than tungsten lamp.

2- Lens assembly:

       Light enters the sample holder through lens assembly.

3- Monochromator:

       there is provision for the insertion of filter between the sample and source of light

 4- Detector (photo –cell):

       It is shielded to minimize interference from stray light.

5-Read out device:

       Light intensity is converted to an electrical signal by the detector.

Considerations in turbidimetry and nephelometry: 

• The reaction in turbidimetry & nephelometry does not follow Beer’s Law 

• Therefore, standard curves must be plotted and the concentration of the unknown is determined from the standard curve. 

• The standard solution which is used for the standard curve must have similar size in suspension as unknown.

• Mix the sample well prior to placing the cuvette in the instrument, and, b) keep the same time for measurement of every sample throughout the measurement. 

• Kinetic reactions (measurement of the progress of reaction with time) provide higher degree of accuracy, sensitivity, precision and less time than end-point reactions (measuring the reaction at the start and finish of the reaction)

Selection of a wavelength

• If both solution and suspended particles are colorless, then use any wave length in the visible range

• If the solution is coloured but the particles are not coloured, then use a wave length that gives minimum absorption for the solution

• If the particles are coloured and the solution is colorless then use a wavelength that gives maximum absorption with the particles

• If both solution and particles are coloured then use two wavelengths; one that gives minimum absorbance for the solution and the other one maximum absorbance for the particles. Subtract the solution absorbance from the particles absorbance.

Clinical Applications of turbidimetry

       Determination of the concentration of total protein in biological fluids such as urine and CSF which contain small quantities of protein (mg/L quantities) using trichloroacetic acid

       Determination of amylase activity using starch as substrate. The decrease in turbidity is directly proportional to amylase activity.

       Determination of lipase activity using triglycerides as substrate. The decrease in turbidity is directly proportional to lipase activity.

Clinical applications of nephelometry.

       Widely used to determine concentrations of unknowns where there is antigen-antibody reactions such as

       Determination of immuno globulins (total, IgG, IgE, IgM, IgA) in serum and other biological fluids

       Determination of the concentrations of individual serum proteins; hemoglobin, heptoglobin, transferring c-reactive protein, !1-antitrypsin, albumin (using antibodies specific for each protein)

       Determination of the size and number of particles (laser-nephelometer}

Summary

       Nephelometry and Turbidimetry are  based on scattering  of light

       They are used determine the solid dispersions

       Concentration ,shape and size of the particles determine the choice of the method

       In Nephelometry ,the intensity of light scattered is measured

       In Turbidimetry the transmitted light after scattering is measured.

       The stability of many dispersions is uncertain and hence a reference standard dispersion of formazine is used.

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