Refractive index detectors refraction-type

Hplc techniques are used to routinely separate and quantify less volatile compounds. The hplc columns used to affect this separation are selected based on the constituents of interest. They are typically reverse phase or anion exchange in nature. The constituents routinely assayed in this type of analysis are those high in molecular weight or low in volatility. Specific compounds of interest include wood sugars, vanillin, and tannin complexes. The most common types of hplc detectors employed in the analysis of distilled spirits are the refractive index detector and the ultraviolet detector. Additionally, the recent introduction of the photodiode array detector is making a significant impact in the analysis of distilled spirits. 

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. 

Detection of the these types of compounds are sometimes difficult as many components of cosmetic products are aliphatic, do not possess a UV chromophore and are not easily reacted to give fluorescent derivatives. Providing the concentration of the component of interest is sufficiently high, then a refractive index detector is often used. If... 

The normalization method is the easiest and most straightforward to use but, unfortunately, it is also the least likely to be appropriate for most LC analyses. To be applicable, the detector must have the same response to all the components of the sample. An exceptional example, where the normalization procedure is frequently used, is in the analysis of polymers by exclusion chromatography using the refractive index detector. The refractive index of a specific polymer is a constant for all polymers of that type having more than 6 monomer units. Under these conditions normalization is the obvious quantitative method to use. 

Differential refractive index detectors measure the difference in refractive index between the solvent and the eluting polymer solution. The DRI can be used with almost any polymer-solvent combination, as long as the incremental increase in refractive index of the polymer solution with increase in polymer concentration is sufficient. Although a large number of differential refractometers are available in the market,167168 they usually conform to three main types, namely... 

The refractive index detector operates by comparing the refractive index of the mobile phase prior to the column with the refractive index of the column eluate. This detector responds to nearly all solutes but it is highly temperature-sensitive (Skoog et al., 1998). This type ofdetector can be used for sugars and fatty acids. 

Figure 3.18—Example of a differential refractive index detector. Opticalpath through the cell. Control of the position of the refracted beam is obtained with a dual stage photodiode. Chromatogram of a mixture of sugars obtained with this type of detector.

In the recent review on column LC by Majors et al. (21), a survey on the use of detector types was carried out in the same manner as that for the use of the various separation modes already mentioned. The results, shown in Table VI, were tabulated for the periods 1982-83 and 1980-81. The increased use of electrochemical and refractive index detectors is significant in these data. The authors speculated that the increased use of refractive index detectors resulted from the increased number of publications on the separation of carbohydrates. The increased use of electrochemical detection is probably a function of many different factors cell designs that are easier to use, expanding sales... 

Figure 25-21 Deflection-type refractive index detector. 

Whether eluted from columns or from thin-layer plates, the quantitative determination of sugars was traditionally based on colorimetric reactions involving the use of chemical reagents, e.g., anthrone. These detection methods have been largely replaced in modem HPLC by the refractive index detector, although ultraviolet detectors are also employed. Recently we have also seen the introduction of other types of detector (e.g., the mass detector), as will be discussed later. 

Since, as has already been said, the separation of alcohols is performed very often together with other analytes, such as organic acids and sugars, the choice of the type of detector also takes into account their chemical-physical properties. Usually, the choice is the ultraviolet detector (UV), the refractive index detector (RI), or the electrochemical detector (EC). Of the last, various types exist, which we shall describe briefly. 

A refractive index detector can see both of these types of compounds. A variable UV detector at 206 nm or 195 nm will work for both at reasonable concentrations. I would recommend either a CAD (see Chapter 5) or a mass spectrometer (see Chapter 15) if you need to do gradients or high-sensitivity detection. Both are expensive but give good results. The MS will give you molecular weight data for your separated peaks as well. 

Figure 9.28. Optical diagrams for refractive index detectors for LC (a) Fresnel type by permission of LDC/Milton Roy (h) deflection type by permission of Millipore Corporation, Waters Chromatography Division.

Four detectors have found widespread application. These are the ultraviolet-visible detector, the fluorescoice detector, the refiactive index d ector, and the electrochemical detector. Only the refractive index detector can be considered as a imiversal detector as virtually all compounds cause a (diange in refiactive index whoi solved in [Pg.202]

A non-specific detector responds to all solutes present in the mobile phase and its catholic performance makes it a very useful and popular type of detector. Unfortunately, non-specific detectors in LC tend to be relatively insensitive. The FID is a nonspecific detector in that (with very few exceptions) it responds to all solutes that contain carbon. As an added advantage the FID also has a very high sensitivity. In LC the refractive index detector is probably the most non-specific detector but as already mentioned it also has the least sensitivity of the commonly used detectors. 

Since the original model of Tiselius there have been many types of refractive index detectors introduced and a number of different optical systems utilized. However, only those in common use or having particular interest will be described here. 

Refractive index detectors. There are three types of commercially available RI detectors, namely, deflection, Fresnal and interference. Each measures the change in the refractive index of the base eluant due to the presence of analytes and hence in principle should provide the basis for a universal detector. Difficulties in RI detection arise due to the sensitivity of the solvent RI to fluctuations in temperature, pressure, the presence of dissolved gases and eluant composition. The RI of a solvent changes by 4 X 10 " per °C and by 4 x 10 per atmosphere thus baseline noise is 1 X 10 with a mixed solvent giving corresponding sensitivities of 5 pg ml . ... 

---