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

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

that are exclusively non specific (i.e., bulk property detectors, e.g., the refractive index detector) through those that are partially specific (i.e. partial solute property detectors, e.g., the UV detectors) to the totally specific detectors (i.e., solute property detectors, e.g., the fluorescence detector). In general, the sensitivity increases progressively as the detector becomes more specific, the highest sensitivities being obtained from the specific detectors.    Refractiveindex is a bulk property of the column eluent and so detection depends on the solute modifying the overall refractive index of the mobile phase sufficiently to provide a signal twice that of the noise. Bulk property detectors have an inherently limited sensitivity irrespective of the instrumental technique that is used. Consider an hypothetical bulk property detector that monitors the density of the eluent leaving the column. Assume it is required to detect the concentration of a dense material

. 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

pressure drop across the cell, also with flow rate. These stability problems apply to all bulk property detectors and, thus, bulk property detectors in general will all have a limited sensitivity (on average for most compounds, this will be about 10-6 g/ml). In addition, even to achieve this sensitivity, the sensor must always be operated under very carefully controlled conditions.   The Refractive Index Detector One of the first on-line detectors to be developed was the refractive index detector originally described by Tiselius and Claesson (14) in 1942. Despite its limited sensitivity, this detector can be very useful for detecting those compounds that are nonionic, do not adsorb in the UV, and do not fluoresce. Since 1942, there have been many types of refractive index detectors introduced and a number of different optical systems utilized. Only those in common use or having particular interest will be described here

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                                           &