Solvent systems refractive index

Another classification of detector is the bulk-property detector, one that measures a change in some overall property of the system of mobile phase plus sample. The most commonly used bulk-property detector is the refractive-index (RI) detector. The RI detector, the closest thing to a universal detector in lc, monitors the difference between the refractive index of the effluent from the column and pure solvent. These detectors are not very good for detection of materials at low concentrations. Moreover, they are sensitive to fluctuations in temperature. 

For the determination of the molecular weight (Mw) by light scattering, the number of solvent systems is limited. The refractive index difference should be at least 0.1 and the solvent should not have an electrolytic effect. Useful solvents include formic acid containing KC1 salt and fluorinated alcohols. 

The association of methanol and water was examined by Katz, Lochmiiller and Scott (11) using volume change on mixing and refractive index data and established that the methanol/water solvent system was indeed a complex ternary system. 

The SEC mechanism demands only an isocratic (constant composition) solvent system with normally a single solvent. The most frequently used organic solvents are THF, chloroform, toluene, esters, ketones, DMF, etc. The key solvent parameters of interest in SEC are (i) solubility parameter (ii) refractive index (iii) UV/IR absorbance (iv) viscosity and (v) boiling point. Sample solutions are typically prepared at concentrations in the region of 0.5-5 mg mL-1. In general an injection volume of 25-100p,L per 300 x 7.5 mm column should be employed. For SEC operation with polyolefins chlorinated solvents (for detector sensitivity and increased boiling point) and elevated temperatures (110 to 150 °C) are required to dissolve olefin polymer. HFIP is the preferred solvent for SEC analysis of polyesters and polyamides. 

Fig. 10. A SPR Detection realized in a BIAcore system. A fan of polarized light passes a prism and is focused at the interface to an aqueous phase under conditions of total reflection. An evanescent wave enters the solvent phase. If the prism is coated with a thin gold layer at the interface the free electrons in the metal absorb energy from the evanescent wave for a distinct angle, depending on the refractive index of the solvent near the interface. B The gold layer can be modified with, e.g., a carboxydextrane matrix, where catcher molecules can be immobilized by standard chemistry. If a ligand is applied with the aqueous phase it may interact with the catcher and accumulate in the matrix, causing a shift in the resonance angle. If no specific binding occurs the refractive index in proximity of the sensor is less affected...

For HPLC, some fairly broad generalizations can be made about the selection of certain preferred solvents from the large number available. A suitable solvent will preferably have low viscosity, be compatible with the detection system, be readily available in pure form, and if possible have low flammability and toxicity. In selecting organic solvents for use in mobile phases, several physical and chemical properties of the solvent should be considered. From the standpoint of detection, the refractive index or UV cutoff values are also important. 

For accuracy in light-scattering measurement the proper choice of solvent is necessary. The difference in refractive index between polymer and solvent should be as large as possible. Moreover, the solvent should itself have relatively low scattering and the polymer-solvent system must not have too high a second virial coefficient as the extrapolation to zero polymer concentration becomes less certain for high A2. Mixed solvent should be avoided unless both components have the same refractive index. 

Model RR/066 351 and 352 pumps models 750/16 variable-wavelength UV monitor detector 750/11 variable filter UV detector, MPD 880S multiwave plasma detector, 750/14 mass detector 750/350/06 electrochemical detector refractive index detector HPLC columns column heaters, autosamplers, pre-columns derivatization systems, solvent degassers, preparative HPLC systems... 

As will be seen later (Section V.l), meaningful molecular weights in multicomponent systems can be determined, if the specific refractive index increment appertains to conditions of constant chemical potential of low molecular weight solvents (instead of at constant composition). Practically, this can be realised by dialysing the solution against the mixed solvent and then measuring the specific refractive index increment of the dialysed solution. The theory and practice have been reviewed4-14-1S> 72>. 

In Eq. (88), dn0/dl expresses how the refractive index % of the binary solvent alone varies with its composition expressed as volume fraction 4>y of liquid-1. Clearly, if liquids 1 and 3 are iso-refractive or nearly so, then M = M2, that is, a LS experiment will yield the true molecular weight irrespective of the composition of the mixed solvent. This situation is exemplified133) by the system polystyrene -ethyl-acetate (l)-ethanol (3) for which the molecular weight in mixed solvents of different 0i is the same as that obtained in pure ethylacetate (Fig. 40). The values of dn /d0j for the mixed solvents are only of the very small order of ca. 0.01, whilst the values of dn/dc for the polymer solutions are large (ca. 0.22 ml/g). 

Clearly, if one is using only a single solvent, these three requisites are mutually exclusive the criteria may well be satisfied with different solvent in each case. Expression (104) means that poly-A and poly-B are iso-refractive and that they have a very different refractive index from that of the solvent. Such systems are comparatively... 

In any chromatographic analysis the method of detection is determined by the nature of the analyte and the mobile phase used must not interfere with this system. The use of ultraviolet absorption detection systems is very common but the solvents used must not absorb significantly at the wavelength used. For instance, absorption at 280 nm is frequently used to detect protein but some solvents, e.g. acetone, absorb at this wavelength. Similarly the use of concentration gradients in the mobile phase may present problems with refractive index and electrochemical detection systems. 

Research has been done showing that rapid pressnre-driven LC analysis can be done with little solvent consumption, demonstrating this as a viable process analytical tool. Using electrokinetic nanoflow pumps LC can be miniaturized to the point of being a sensor system. Developments in terms of sampling to enable sampling directly from a process stream, to the separation channel on a chip are critical for the application of miniaturized process LC. The components (valves and pumps) required for hydrodynamic flow systems appear to be a current limitation to the fnll miniatnrization of LC separations. Detection systems have also evolved with electrochemical detection and refractive index detection systems providing increased sensitivity in miniaturized systems when compared to standard UV-vis detection or fluorescence, which may require precolumn derivatization. 

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