Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > Page 18
LC Detectors Based on Refractive Index Measurement
LC detectors range from those 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.
Refractive index 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, such as carbon tetrachloride (specific gravity 1.595), at a level of 1 mg/ml in n-heptane (specific gravity 0.684).