Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Refractive Index Detector > Page 20

 

Therefore, to detect a concentration of one part per million of carbon tetrachloride (at a signal to noise ratio of two), then the temperature variation must be maintained below 1.8 x 10-4 oC. Such temperature stability is extremely difficult to maintain and, thus, temperature control will limit the sensitivity obtainable from the detector. Even the heat of adsorption and desorption of the solute on the stationary phase can produce temperature changes of this order of magnitude.

Similarly, the density of the contents of the cell will change with pressure and, if there is a significant 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.