Subject index detector

When entering the field of liquid chromatography, the scientist is always faced with the problem of detector selection. The subject of detector choice will be dealt with later in this book but at this point, it should again be emphasized, that there is no ideal LC detector. Consequently, the practicing liquid chromatographer needs to have at least two, if not more, different types of detector available, or the full versatility of the technique will not be realized. It is therefore recommended that one of the detectors available should be a bulk property detector, which should probably be the refractive index detector. This would be a particularly appropriate detector if preparative or semi-preparative chromatography is likely to be required. If the separation and quantitative analysis of ionic materials are contemplated, then the refractive index detector might be replaced... 

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. 

UV detectors can be subject to baseline shifts due to changes in the refractive index of the carrier solvent. This effect can be bothersome when a gradient is carried out. 

A source of error in the HPLC method is the very short retention time of PEG in reversed-phase systems. This makes quantification by differential refractive index subject to interference by peaks from water, solvent impurities, and other unretained substances. This source of error may be eliminated by coupling the HPLC column to a GPC column, so that PEG is resolved from both the surfactant and from other impurities (36). Even with this modification, if phenol impurity were present in the initial alkylphenol, the resulting ethoxylated phenol could interfere with PEG determination by this method. Its presence is detected by using a UV detector in series with the RI detector. Another source of interference is the anion, such as acetate or lactate, added during the neutralization of the catalyst. For careful work, this should be removed by ion exchange prior to HPLC analysis (36). 

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