Viscometer Detector

Detectors must be sensitive and must have a wide linear range in order to respond to both trace amounts and large quantities of material, if necessary. They must be nondestructive to the eluting components if they are to be collected for further analysis. There are different types of detectors for GPC, the most common ones being the refractive index (RI) detector, the UV detector, viscometer detector, as well as light scattering and infrared detectors. 

While RI detectors are used for isocratic HPLC, the other detectors, viscometer and light scattering, are not generally suitable for HPLC. As noted in See. II.D. absorbance detectors, either UV/visible or photodiode array (PDA), are much more useful. Table 5 and Fig. 13 show the HPLC of a polymer additive mixture. The reproducibility of 12 consecutive injections shown in Fig. 13 demonstrates exceptional reproducibility of the analysis, espeeially considering that both the solvent eomposition and solvent flow rates were programmed for this work. 

Refractive index detectors are used to measure concentration profiles and (if used alone) molar masses based on a calibration. In addition, they are used to measure the slice concentration when working with online light scattering detectors, viscometers, or online mass spectrometry. They are also used to measure the refractive index increment, dnidc, in a single point experiment when light scattering or triple detection is used. In combination with other concentration detec-... 

The latest trend is to smaller beads in smaller columns, as this saves eluent and shortens the time for a chromatographic analysis. This argument can be correct if only one suitable detector is used. However, these modern small columns are not optimal for a combination of detectors. So-called multiple detection is a combination of some detectors with different measurement principles (differential refractometer, spectral photometer, light-scattering detector, on-line viscometer) behind the last column, mostly in series, seldom in a branched ( parallel ) order. In this way, the tedious preparative fractionation of a polymer sample can often be avoided. 

These combined HDF and GPC separations require the use of detectors such as static light scattering or viscometers to help sort out the convoluted elution profiles seen in those type of experiments. It should also be remembered in these situations that the typical refractive index or ultraviolet detector responses may not be representative of the actual mass fraction of insolubles eluting from the column because of the significant light scattering that can occur with those large particles in the detector cell. 

Detectors Model 410 differential refractive index (Waters Corporation) Model 100 differential viscometer (Viscotek Corporation) Dawn-F multiangle laser light-scattering photometer (Wyatt Technology)... 

At the moment, one recommends to determine the molecular characteristics of pectins using SEC chromatography equipped with a differential refractometer, a multiangle laser light scattering detector and a viscometer as previously described [25]. This technique needs no calibration with the usual molecular weight standards such dextrans and pullulans... 

Lesec, J. and Volet, G., Data treatment in aqueous GPC with on-line viscometer and light-scattering detectors, /. Liq. Chromatogr.,13, 831, 1990. 

Haney, M. A., The differential viscometer. II. On-line viscosity detector for size-exclusion chromatography, /. Appl. Polym. Sci., 30, 3037, 1985. 

Novolac molecular weights were measured in THF at 35°C by high pressure size exclusion chromatography using a Waters Model 510 pump (flow rate=1.0 ml/min), 401 differential viscometer detector and a set of Dupont PSM 60 silanized columns. A universal calibration curve was obtained with a kit of 10 narrow molecular weight distribution, linear polystyrene standards from Toya Soda Company. Data acquisition and analysis were performed on an AT T 6312 computer using ASYST Unical 3.02 software supplied with the Viscotek instrument. 

A Waters Model 150C ALC/GPC was interfaced to a minicomputer system by means of a microcomputer for automated data collection and analysis. Programs were developed for conventional molecular weight distribution analysis of the data and for liquid chromatographic quantitative composition analysis of oligomeric materials. Capability has been provided to utilize non-standard detectors such as a continuous viscometer detector and spectroscopic detectors for compositional analysis. The automation of the instrument has resulted in greater manpower efficiency and improved record keeping. 

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. 

A continuous capillary viscosity detector has been developed for use in High Performance Gel Permeation Chromatography (HPGPC). This detector has been used in conjunction with a concentration detector (DRI) to provide information on the absolute molecular weight, Mark-Houwink parameters and bulk intrinsic viscosity of polymers down to a molecular weight of about 4000. The detector was tested and used with a Waters Associates Model 150 C ALC/GPC. The combined GPC/Viscometer instrumentation was automated by means of a micro/mini-computer system which permits data acquisition/reduction for each analysis. 

The automation of the HPGPC/Viscometer system is achieved by interfacing the differential refractometer (DRI) and viscosity detector to a microcomputer for data acquisition. The raw data subsequently, are transferred to a minicomputer (DEC PDP-ll/HiI) for storage and data analysis. Details of the instrument automation are given elsewhere.(6)... 

Details of the data analysis for the GPC/Viscometer system have been reviewed by Ouano.(T ) The data reduction scheme is summarized in Figure 2 and briefly will be discussed here. The intrinsic viscosity of the effluent at a given retention volume [n](v) is determined from the DRI and continuous viscosity detector responses according to the following equation... 

The high sensitivity of the viscosity detector to the high molecular weight fractions is demonstrated in the analysis of a sample of very high molecular weight polyCmethyl methacrylate) shown in Figure 8. A shoulder at 3,000,000 molecular weight detected by the DRI becomes a peak when detected by the viscometer detector. 

Determination of the Dead Volume Between the Viscometer and the Concentration Detector... 

Another requirement for accurate GPC/Viscometer data analysis is accounting for the dead volume (aV) between the viscometer and the concentration detector. 

Figure 9. Slope versus AV plot for the determination of the dead volume between DRI and viscometer detectors.

Table I indicates good agreement between the molecular weight distribution statistics obtained by coupled GPC/Viscometer method and the nominal values for t BS 706. The discrepancy between the Mark-Houwink parameters obtained here and the reported values for polystyrene standard ( ) in THF at 25°C (i.e., a = 0,706 and k = 1.60 x 10 ) may in part be due to the uncertainty involved in the determination of the dead volume between DRI and viscometer detectors. Our simulation studies over a range of dead volume values (0 to 120u)l) showed that a and k are quite sensitive to the dead volume between the detectors. Larger dead volume results in smaller o and larger k values. This is a direct result of a clockwise rotation of log [q] vs. log M(v) curve (Figure 12) which occurs when the dead volume correction is applied in quantitative analysis. The effect on the molecular weight statistics, however, appeared to...

The concentration of the polymer molecules eluting from SEC columns is continuously monitored by a detector. The most widely used detector in SEC is the differential reftactometer (DRI), which measures the difference in refractive index between solvent and solute. Other detectors commonly used for SEC are functional group detectors ultraviolet (UV) and infrared (IR), and absolute molecular weight detectors low angle laser light scattering (LALLS) and in-line continuous viscometers. Applications of these detectors to SEC analysis will be discussed later in the Multiple Detectors Section. Other detectors also being used are the densimeter (11-19) and the mass detector (20-23). 

A New Stand-Alone Capillary Viscometer Used as a Continuous Size Exclusion Chromatographic Detector... 

The sample fluid could be any neat liquid or a sample of polymer solution. Under favorable conditions, a single viscosity determination on a polymer solution at high dilution can provide a direct measure of the polymer intrinsic viscosity, without the need of polymer concentration extrapolation. With this viscometer used as a continuous viscosity detector for SEC, it is possible to achieve SEC molecluar weight calibration by way of the universal SEC calibration methodology without the need of molecular weight standards for the unknown polymers. 

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