Refractive index detectors weight

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. 

Select the detector. To acquire molecular weight distribution data, use a general detector such as a refractive index detector. To acquire structural or compositional information, employ a more selective detector such as an ultraviolet (UV) or infrared (IR) detector. Viscometric and light-scattering detectors facilitate more accurate molecular weight measurement when appropriate standards are not available. 

FIGURE 16.24 Inulin isolated from mature Topinambur tubers sparated on Bio-Gel P-6 (140 X I. Scm) flow rate 0.33 ml/min eluent H20(dest) + 0.002% NaNs detection Waters 403 R differential refractive index detector, sensitivity 8X applied sample I ml of a 20-mg/ml aqueous solution weight average degree of polymerization dp = 9 number average degree op polymerization dp = 2 polydisper-sity dp /dp = 4.5. 

A common error is to confuse the GPC distribution with the weight distribution. The response of a refractive index detector is proportional to the mass of polymer. The GPC elution volume (V) typically scales according to the logarithm of the degree of polymerization (or the logarithm of the molecular... 

The refractive index detector, in general, is a choice of last resort and is used for those applications where, for one reason or another, all other detectors are inappropriate or impractical. However, the detector has one particular area of application for which it is unique and that is in the separation and analysis of polymers. In general, for those polymers that contain more than six monomer units, the refractive index is directly proportional to the concentration of the polymer and is practically independent of the molecular weight. Thus, a quantitative analysis of a polymer mixture can be obtained by the simple normalization of the peak areas in the chromatogram, there being no need for the use of individual response factors. Some typical specifications for the refractive index detector are as follows ... 

The molecular weight distribution of cell wall polysaccharides was estimated by gel filtration with a TOSOH TSK gel G4000 PWXL (7.8 x 300 mm) column equilibrated and eluted with 0.05 M sodium acetate, 0.01 M EDTA, 0.05 M NaCl (pH 5.0) in polyuronide and 0.05 M sodium citrate, 0.1 M NaCl (pH 5.5) in the hemicellulose fraction. Samples (1 mg/ml) of 100 ml were injected. The eluate was monitored by a refractive index detector (Shimadzu R1D-6A, Kyoto, Japan) and collected at the fraction size of 0.4 ml. 

The determination of the molecular weight of nanoparticles is performed by gel permeation chromatography (GPC). The experimental setup consists of a high performance liquid chromatography system with a size exclusion column and a refractive index detector. The nanoparticles are usually freeze-dried and dissolved in tetrahydrofuran for analysis on the system. Poly(styrene) or poly(methylmethacrylate) standards are used to calibrate the column, to enable the determination of number average molecular weight (Mn), as in... 

Multiangle light-scattering detectors are increasingly used to obtain on-line information on protein molecular weight. However, they must be used in combination with refractive index detectors, and so this technique is not compatible with reversed-phase gradient elution. 

Other analysis methods dependent on multiple detectors can be implemented using this automated system. Two methods under development are the use of a continuous viscometer detector with a refractive index detector to yield absolute molecular weight and branching, utilizing the universal calibration curve concept (4), and the use of a UV or IR detector with the refractive index detector to measure compositional distribution as a function of molecular weight. 

Gel permeation chromatograms were generated from a Waters Associates, Inc. GPC equipped with a refractive index detector. The following operating conditions were employed mobile phase, THF flow rate 1 ml/min., columns ICP, 10, 500, 100 A . Sample concentrations were prepared at 0.2% (w/w) a 100 microliter aliquot was used for molecular weight analysis. Standard polystyrene samples (Polymer Laboratories, Inc.) were used to create a calibration curve. 

Figure 8. Gel filtration of ethylated (/ -0-4)-(/ -/ )-DHP 16. Solid line Ethylated (/ -0-4)-(/ -/ )-DHP 16 after removal of low molecular weight fractions. The column was calibrated with (/ -0-4)-(/ -/ ) lignin substructure model trimer 6 (molecular weight 642) /3-0-4 lignin model dimer 1 (molecular weight 348) and polystyrenes of molecular weight 9000, 4000 (void), 2200 (indicated by A). Column Sephadex LH-20, 1.1 x 48 cm. Eluent DMF, 13.5-14.4 ml/hr. Detector Refractive index detector RI-2 (Japan Analytical Industry Co., Ltd.).

An HPLC system, equipped with a Waters solvent delivery system (M-45), two PLgel 20 p,m Mixed-A columns (300 x 7.5 mm) with 20 p,m guard column (50 x 7.5 mm) (Polymer Laboratories, Amherst, MA) and a refractive index detector (model 2410) (Waters, Milford, MA), can be used to study the molecular size and size distribution (e.g. molecular weight and weight distribution) of starch. 

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. 

H and Si NMR spectra were recorded on a Brnker Avance 300 spectrometer respectively at 300.13 and 75.6MHz at room temperatnre. CDClj and toluene were used as solvents. For Si NMR, Cr(acac)3 (0.03 M) was added in the tube and a delay between pulses of 20 s was set. IR spectra were recorded on a Perkin-Ehner IR ET 1760-X. The average molecnlar weight of the linear polymers was determined by size exclusion chromatography in TFIF (flow rate l.OmL.min" ) on an apparatus equipped with a Waters refractive index detector, a Waters column pack (Ultrastyra-gel 10, 10 100 A) and a Minidawn Wyatt light scattering detector. 

Molecular weight and polydispersity of the acetone extracted fractions from thermally degraded samples were determined by SEC with a Jasco PU-1580 HPLC pump equipped with two Pigel mixed D columns (Polymer Laboratories UK) connected in series, and a Jasco 830RI refractive index detector. Sample elution was with THF at 1 ml/minute flow rate. The instrument was calibrated with standard polystyrene samples. 

Molecular weights were obtained by GPC in dichloromethane (1 mL/min) using a Waters pump model 6000, an injector (Rheodyne) and a refractive index detector (RID-6A Shimadzu), equipped with a PL gel 5)iim mixed-C linear column. The system was calibrated using polystyrene standards with low polydispersity. 

The original soybean oil (SBO), the processed soybean oil under similar conditions without catalyst (SBO control), and polymers of soybean oil (PSBO) were dissolved in THF. Molecular weights and molecular weight distribution were measured by GPC with a differential refractive index detector using THF as an eluent. The flow rate was 1.00 mL/min at 40 C. The injection volume was 100 pL. Linear polystyrene standards (Polymer Laboratories (PL), Mn = 580-lOOK, Mw/Mn = 1) were used for calibration of molecular weights of all polymers of PSBO. 2 PL gel 3 pm mixed E columns (300 mm x 7.5 mm) in series were used to resolve the samples. 

Gel permeation chromatography (GPC) was used to determine molecular weights and molecular weight distributions, Mw/Mn, of polymer samples. The GPC consisted of an Alliance 2690 pump equipped with a Wyatt Rex Differential Refractive Index Detector and utilized three Polymer Labs PL Mixed B GPC Columns at 70°C (DMF) or 40°C (THF) at a flow rate of 1.0 mL/min. The molecular weights were calculated relative to the retention times of polystyrene and polyethylene oxide standards for DMF or polystyrene and polytetrahydrofuran standards for THF using Empower software. 

The device is often operated with a refractive index detector in series in order to coincidentally measure the refractive index of the eluent. This is necessary to calculate (K) from the refractive index as given in equation (5). A common refractive index detector used for this purpose is that manufactured by the Wyatt Technology Corporation and it is described as the interferometer detector in chapter 11. As discussed above the molecular weight of a solute is determined from the intercept of the graph relating... 

The overall sensitivity of the detector appears to be very similar to that of the refractive index detector with about the same linearity. However, the most important characteristic of this detector is not its propensity for accurate quantitative analysis but its proficiency in providing molecular weight data for extremely large molecules. 

The differential refractive index detector response on the ordinate of the SEC chromatogram in Fig. 3-8 can be transformed into a weight fraction of total polymer while suitable calibration permits the translation of the elution volume axis into a logarithmic molecular weight scale. 

Figure 5. Light-scattering and refractive index detector tracings for a hypothetical MWD containing a small amount of high-molecular-weight material. (Reproduced with permission from reference 3. Copyright 1995 Marcel Dekker)...

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