Friday, August 10, 2012

Focus On: Turbidity Measurements in Life Science

The measurement of turbidity, which is the haziness or cloudiness of a fluid that sometimes cannot been seen by the naked eye, can be used to determine the amount of insoluble particles in a solution. Amongst the life science applications that can use turbidity measurements include: drug compound solubility, bacterial or fungal growth studies, immunoprecipitations, protein aggregation, antibody-antigen interaction, and polymerization monitoring.

Turbidity measurements can be performed with an absorbance instrument that measures light transmission through a sample. The more turbid a sample is, the less amount of light will pass through the sample. This relationship is directly proportional in a linear fashion and can be used to determine the degree of turbidity of a sample, i.e. the amount of insoluble particles in solution. However, this linear relationship only occurs to a certain degree of turbidity. For most absorbance instruments, the turbidity measurements deviate from a direct linear relationship usually above 3 O.D., that is when only 1 photon of light in 1,000 is transmitted through the sample. BMG LABTECH microplate readers that can perform turbidity absorbance measurements are: SPECTROstar, FLUOstar, and POLARstar Omegas; FLUOstar and POLARstar OPTIMAS; SPECTROstar Nano; as well as the PHERAstar Plus and PHERAstar FS.

turbidity measurements
Fig 1. Light-scattering can be used to measure the turbidity of a sample.
Turbidity measurements can also be performed with a nephelometric instrument that measures light-scattering (see figure 1), such as the NEPHELOstar Plus from BMG LABTECH. This instrument uses a diode laser that passes light through each sample. If there are insoluble particles in the solution, the laser light will be scattered by the particles. Similar to absorbance measurements, the amount of scattered light is directly proportional to the amount of particles in solution.

Fig. 2: Solubility diagram of CD-Econazole-nitrate complex
using light-scattering
However, light-scattering measurements do not suffer from the same upper limitation as absorbance measurements, thereby providing a wider dynamic range for turbidity measurements. An example turbidity application is shown in Figure 2, where a drug compound precipitates out of solution at its insolubility point.

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