Gradient elution refractive index effects

Refractive index detectors. These bulk property detectors are based on the change of refractive index of the eluant from the column with respect to pure mobile phase. Although they are widely used, the refractive index detectors suffer from several disadvantages — lack of high sensitivity, lack of suitability for gradient elution, and the need for strict temperature control ( + 0.001 °C) to operate at their highest sensitivity. A pulseless pump, or a reciprocating pump equipped with a pulse dampener, must also be employed. The effect of these limitations may to some extent be overcome by the use of differential systems in which the column eluant is compared with a reference flow of pure mobile phase. The two chief types of RI detector are as follows. 

The major advantage of this detector is that it is almost universal. All substances have their own characteristic refractive index (it is a physical property of the substance). Thus, the only time that a mixture component would not give a peak is when it has a refractive index equal to that of the mobile phase, a rare occurrence. The disadvantages are that it is not very sensitive and the output to the recorder is subject to temperature effects. Also, it is difficult to use this detector with the gradient elution method because it is sensitive to changes in the mobile phase composition. 

Fluorescence detectors can also be used and while their sensitivity may be greater, they are less widely applicable owing to the smaller number of fluorescent compounds. Differential refractometers will detect changes in the refractive index of the solvent due to the presence of solutes and, while they are less sensitive than the other detectors and often cannot be used effectively with gradient elution techniques, they are capable of detecting the presence of any solute. 

New stationary phases were synthesized by binding substituted aromatic groups to silica. Their separation capacity was determined by using various native CDs and CD dal-vatives. The mobile phase for the gradient elution consisted of a mixture of MeCN, H2O, and formic acid. CDs were detected with a refractive index detector and ELSD. The measurements indicated that the N-(4-nitrophenyl)-carbamide group-bonded silica was the most effective. ... 

---