Refractive index monitors

Figure 14.17 Schematic diagram of the on-line coupled LC-GC system VI, valve foi switcliing the LC column outlet to the GC injector V2, valve for switching the LC column to back-flush mode V3, LC injection valve RI, refractive index monitor detector UV, ulti avio-let monitor detector FID, flame-ionization detector.

Fig. 15.10 The sensitivity of the MCLW device for refractive index monitoring of various concentrations of glycerol in aqueous solution. Reprinted from Ref. 19 with permission. 2008 Elsevier...

Methylation analysis was run by the method of Hakomori ( ), followed by hydrolysis with trifluoracetic acid, sodium borohydride reduction, and acetylation. 6LC was performed on a Hewlett-Packard 5970, used as an inlet for a mass spectrometer. Molecular weight was determined on a Sephacryl S-500 column (2.6 x 70 cm), using deionized water as solvent, upward flow, 2.75 ml/min, and detection by refractive index monitor. Model R-401 (Waters Associates). 

There are excellent HPLC systems available on the market today, yet there is one area of concern with this instrumentation, and this rests with the detection units. Certainly the most widely used detector system employs a low dead-volume micro-ultraviolet detector. This latter unit operates near 200 nm and detects mainly unsaturated linkages in phospholipids (or lipid) samples. Some contribution by carbonyl functions can be expected. This approach is an advantage when the sample under study contains olefinic groups, but will not detect those with saturated side (hydrocarbon) chains. An alternative detector is the refractive index monitor which is often called a universal detector, since it is based on the concept that the refractive index of the solvent changes when a solute is present. The drawback of the latter unit lies in its sensitivity, which is approximately 15- to 20-fold less than that of the ultraviolet monitor. 

Our ThFFF channel is similar to the model TlOO Thermal Fractionator (FFFractionation, Inc., Salt Lake City, UT), with a channel thickness of 0.10 mm. When the carrier liquid was tetrahydrofuran (THF) or cyclohexane, a UV monitor set at 254 nm was used for sample detection when toluene was the carrier liquid, a refractive index monitor was used instead. The temperature difference was 60.0 K and the cold wall temperature was 298.2 K. Intrinsic viscosities were measured with a CannonFenske ASTM-25 viscometer obtained from Fisher Scientific (Santa Clara, CA). Viscosities were measured in a thermostated temperature bath set at T g. All solvents were high-performance liquid chromatography grade. 

Refractive index monitors are used in food analysis, for detecting carbohydrates, alcohols, and other substances with weak or no UV absorption. 

An example of the second type of detector is the refractive index monitor which functions by recording the refractive changes in the eluant as the solute passes through the detector cell. Bulk property detectors, though more versatile, are generally several orders of magnitude less sensitive than specific property detectors and the choice for a particular application is often dictated by solute characteristics. 

Apart from the above two, refractive index monitors and fluorescence detectors are also available with HPLC systems. 

Since the original model of Tiselius there have been many papers published that describe different methods of refractive index monitoring. From these publications, four general methods emerge for measuring refractive index the angle of deviation method, the critical angle method, the Fresnel method and the Christiansen method. The theory behind each of these methods will now be discussed. 

Unlike a single wavelength UV monitor, or a refractive index monitor, a NIR analyser is not a single parameter measurement device. In a process situation, NIR analysis is carried out in a manner similar... 

You may assume that the purity of this stream can be evaluated by an on-line refractive index monitor placed on Stream 15. 

When necessary, the liquid flow can be split between the analytical colunm and the reference colunm. A valve placed before the reference column permits the flow rate through that column to be completely shut-off or varied depending on the requirements of the system. This is particularly useful when using the refractive index monitor at higher flow rates. For column flow rates up to 2 cm min good stability could be obtained by careful balance of flow. The reference column was either filled with the same liquid-liquid support employed in the analytical column or merely filled with uncoated glass beads. With the ultraviolet detector the reference column is not used. Column head pressures are read on 1 - 3,000 psi or 0 - 5,000 psi standard gauges. 

A comparative study is presented for different types of optical fibre sensor developed to monitor the cure of an epoxy resin system. The sensors were based on transmission spectroscopy, evanescent wave spectroscopy and refractive index monitoring. 19 refs. 

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