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refractive index will only match at one particular wavelength. As a result the fully transmitted light will be largely monochromatic. Light of different wavelengths will be proportionally dispersed depending on the wavelength at which the two media have the same optical dispersion. Thus, a change in mobile phase refractive will change both the intensity of the transmitted light and its wavelength. This device was made by GOW-MAC Inc., who claimed it had a sensitivity of 1 x 10-6 refractive index units (the maximum that cold be expected). This would be equivalent to a sensitivity of 9 x 10-6 g/ml of benzene (refractive index 1.501) eluted in n-heptane (refractive index 1.388). The cell volume was kept to 8 ml (a little large for modern sensors) which was small enough to work satisfactorily with 4.6 mm I.D. LC columns. Different cells packed with appropriate materials were necessary to cover the refractive index range of 1.31 to 1.60. A diagram of the Christiansen detector is shown

. The change in direction is called the refraction and the relationship between the angle of incidence and the angle of refraction is given by Snell's law, namely,   where (i) is the angle of incident light in medium (A), (r) is the angle of refractive light in medium (B), (nA) is the refractive index of medium (A), (nB) is the refractive index of medium (B), and (n'B) is the refractive index of medium (B) relative to that of medium (A). Refractive index is a dimensionless constant that normally decreases with increasing temperature. The reported values are usually taken at 20o or 25oC and are mean values measured for the two sodium lines. If the mobile phase is allowed to flow through a hollow prism and a ray of light passes through the prism it will be diverged from its original path and can be focused onto a photocell. If the refractive index of the

been claimed for the detector and a linear dynamic range of about three orders of magnitude. The thermal lens detector is, in fact, a special form of the refractive index detector and might, therefore, be considered a universal detector. Nevertheless, like other bulk property detectors, it can not be used with gradient elution or flow programming and has sensitivity that  is no better, if as good as, other refractive index detectors. The Dielectric Constant Detector The refractive index of a substance is a complementary property to the dielectric constant and in some circumstances is a direct function of it. For non-polar substances, the relationship between dielectric constant (e) and refractive index (n) is given by                                           &

detector uses a monochromator to select the excitation wavelength and a second monochromator to select the wavelength of the fluorescent light. This instrument gives the maximum versatility and allows the maximum sensitivity to be realized for any type of solute. The system can also provide a fluorescence spectra by arresting the flow of mobile phase when the solute resides in the detecting cell and scanning the fluorescent light. The Refractive Index Detector The refractive index detector is one of the least sensitive LC detectors. It is very sensitive to changes in ambient temperature, pressure changes, flow-rate changes and can not be used for gradient elution. Despite these many disadvantages, this detector is extremely useful for detecting those compounds that are nonionic, do not adsorb in the UV, and do not fluoresce. There are many optical systems used in refractive index detectors (9) but one of the most common is the differential refractive index detector

The refractive index detector is often a 'choice of last resort' and is selected for those applications where, for one reason or another, all other detectors are inappropriate or impractical. However, the detector has one particular and unique area of application and that is in the separation and analysis of polymers. For those polymers that contain more than ten monomer units, the refractive index is directly proportional to the concentration of the polymer and is practically independent of the molecular weight. A quantitative analysis of a polymer mixture can, therefore, be obtained by the simple normalization of the peak areas in the chromatogram (there being no need for the use of individual response factors). Some typical specifications for the refractive index detector are as follows:- Typical Specifications for a Refractive Index  Detector        Sensitivity (

Corporation.   Figure 51  Optical Diagram of a Low Angle Laser Light Scattering Detector   Between the forward detector lens and the rear detector lens is A filter holder and an analyzer/polarizer. Finally the light is focused through a sensor aperture to an opal diffuser that spreads the scattered light through a red filter  and onto the photo- multiplier. The device is frequently used with a refractive index detector in series to coincidentally measure the refractive index of the eluent. This is necessary to calculate (K) from the refractive index. As discussed, the molecular weight of a solute is determined from the intercept of the graph relating () to the solute concentration (c) as shown in figure 52. The concentration calculated from the refractive index detector employing the data from prior calibration