Refractive index optical detection systems

The refractive index of a substance is a dimensionless constant that normally decreases with increasing temperature values are taken at 20°C or 25°C using the mean value taken for the two sodium lines of the spectrum. The optical systems that are used to exploit the refractive index for detection purposes are many and varied. One procedure is to construct a cell in the form of a hollow prism through which the mobile phase can flow. A ray of light is passed through the prism, which will be deviated from its original path, and is then focused onto a photocell. As the refractive... 

Inserting Equations (23)-(26) into Equation (20) and using Equation (22), we obtain an equation for the diffusion coefficient, Z>app> determined using an optical detection system based on the use of the refractive index increment ... 

Several experimental techniques may be used, such as acid/base titration, electrical conductivity measurement, temperature measurement, or measurement of optical properties such as refractive index, light absorption, and so on. In each case, it is necessary to specify the manner of tracer addition, the position and number of recording stations, the sample volume of the detection system, and the criteria used in locating the end-point. Each of these factors will influence the measured value of mixing time, and therefore care must be exercised in comparing results from different investigations. 

Seo et al. (1999) used a planar optic biosensor that measures the phase shift variation in refractive index due to antigen binding to antibody. In this method, they were able to detect S. enterica serovar T) himurium with a detection limit of 1 x 10 cfu/ml. When chicken carcass fluid was inoculated with 20 cfu/ml, the sensor was able to detect this pathogen after 12 h of nonselective enrichment. A compact fiber optic sensor was also used for detection of S. T) himurium at a detection limit of 1 X 10" cfu/ml (Zhou et al., 1997, 1998) however, its efficacy with food samples is unproven. Later, Kramer and Lim (2004) used the fiber optic sensor, RAPTOR , to detect this pathogen from spent irrigation water for alfalfa sprouts. They showed that the system can be used to detect Salmonella spiked at 50 cfu/g seeds. An evanescent wave-based multianalyte array biosensor (MAAB) was also employed for successful testing of chicken excreta and various food samples (sausage, cantaloupe, egg, sprout, and chicken carcass) for S. T) himurium (Taitt et ah, 2004). While some samples exhibited interference with the assay, overall, the detection limit for this system was reported to be 8 x 10 cfu/g. 

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