SEC-viscometer system

In a previous paper (16) the hardware design of the SEC/Viscometer system used in this work has been described. The effects of operational variables, e.g. pump pulsations, flow rate and flow irregularities on the performance of the viscometer and the enhancement of the signal-to-noise ratio by using mechanical dampers also has been described. Our current work has focused on the development, implementation and application of automated data... 

Data Reduction Procedures. Details of the data analysis for the SEC/Viscometer system have been described previously.(16) The data reduction scheme is summarized in Figure 1 and briefly will be reviewed here. The intrinsic viscosity [n](V) of the effluent at a given retention volume V is determined from the DRI and continuous viscosity detector responses according to the following equation... 

Details of the SEC/Viscometer detector system have been described previously.(16) The key component of the viscometer detector is a differential pressure transducer (CELESCO Model P-7D, Canoga Park, CA) with a jf25 psi pressure range. The transducer monitors the pressure drop across a section of stainless steel capillary tubing (length 2 ft., I.D. = 0.007 in.). Pump pressure fluctuations... 

The automation of the SEC/Viscometer detector system is achieved by interfacing the DRI and viscosity detectors to a microcomputer for real time data acquisition. The raw data subsequently are transferred to a minicomputer (DEC PDP-11/44) for storage and data reduction. The automated data acquisition and analysis system for the size exclusion chromatograph with multiple detectors used in this study have been described previously.(23)... 

A more sensitive viscometer than the drop-time glass capillary method is also needed in size exclusion chromatography (SEC) such as the gel permeation chromatographic (GPC) analysis of polymer molecular weight distribution (MWD). In an SEC system, a concentration detector is commonly used for providing the weight concentration profile of the polymer elution curve. 

The cone-and-plate viscometer is an in vitro flow model used to investigate the effects of bulk fluid shear stress on suspended cells. Anticoagulated whole blood specimens (or isolated cell suspensions) are placed between the two platens (both of stainless steel) of the viscometer. Rotation of the upper conical platen causes a well-defined and uniform shearing stress to be applied to the entire fluid medium as described by Konstantopolous et al. (1998). The shear rate (y) in this system can be readily calculated from the cone angle and the speed of the cone using the formula i/ = where y is the shear rate in sec-1, mis the... 

Due to the problems encountered with SEC-LALLS and SEC-viscometry, a triple-detector SEC technology has been developed, where three on-line detectors are used together in a single SEC system. In addition to the concentration detector, an on-line viscometer and a LALLS instrument are coupled to the SEC... 

Chromatography. Filtered polysaccharide solutions were analyzed using an SEC system consisting of an automatic sampler (Waters WISP, Waters, Milford, MA) with a high-performance liquid chromatography pump (Waters model 590), pulse dampener (Viscotek, Houston, TX), viscometer... 

The opportunity to measure the dilute polymer solution viscosity in GPC came with the continuous capillary-type viscometers (single capillary or differential multicapillary detectors) coupled to the traditional chromatographic system before or after a concentration detector in series (see the entry Viscometric Detection in GPC-SEC). Because liquid continuously flows through the capillary tube, the detected pressure drop across the capillary provides the measure for the fluid viscosity according to the Poiseuille s equation for laminar flow of incompressible liquids [1], Most commercial on-line viscometers provide either relative or specific viscosities measured continuously across the entire polymer peak. These measurements produce a viscometry elution profile (chromatogram). Combined with a concentration-detector chromatogram (the concentration versus retention volume elution curve), this profile allows one to calculate the instantaneous intrinsic viscosity [17] of a polymer solution at each data point i (time slice) of a polymer distribution. Thus, if the differential refractometer is used as a concentration detector, then for each sample slice i. 

If all distributions of an HA sample (MMD, RGD, IVD) are required, use of absolute on-line detectors as MALS and Vise is needed. A multidetector SEC-LS-Visc system is composed of an LS detector (LALS or MALS), a viscometer (differential viscometer DV or SeV), and a concentration detector (DRI or UV). As a result, from the record of the on-line MALS detector, values of M and Rg can be obtained, while from the on-line Vise detector one can calculate [r)]. Fig. (9) shows the experimental relationships M = /(V), Rg = /(V), and [rj] = /(V) obtained using a SEC-MALS-SCV system for an HA sample (Mw =1314 kDa, D = 1.6). The use of absolute on-line detectors, as MALS and Vise, for the characterization of HA samples was described previously [270,279]. 

Measurement of viscosity [r ] is more complicated than estimation of the molar mass, due to the non-Newtonian behaviour of HA solutions during the flow. It is well known that the HA viscosity strongly depends on the shear rate (7) even for very dilute solutions. Unfortunately, the shear rate range of the usual viscometers used for the [rj] measurements, both off-line (i.e. Ubbelohde viscometer 1200-1500 s ) and on-line to a SEC system (i.e. DV 2500-3000 s ) are too high for measuring the HA... 

There is big theoretical and practical interest for the scaling relationship [q] = /(M) for HA. Theoretically, the Mark-Houwink-Sakurada (MHS) plot for HA could be obtained by using an on-line viscometer combined with an SEC system applied to an appropriate number of broad MMD HA samples. Unfortunately, the 7 range of on-line DV detectors is too high for HA even at very low flow rates. To obtain a reliable MHS plot for HA for an extended range of molar masses, Mendichi et. al. [275] used a modified on-line SCV... 

Table III shows the viscosity characteristics of these systems as obtained from the Epprecht rotational viscometer. With this instrument, shear rate can be obtained in units of reciprocal seconds (sec-1). These data are plotted in Figure 9. Under these shearing rates (19.2 to 125 sec"1), all three formulations are pseudoplastic.

Hyphenated methods use SEC apparatus equipped with two or three detectors, mounted in parallel or in series. In the SEC/IntrinsicViscosity method, one places a viscometer after the SEC apparatus and records two traces, namely, the RI response and the viscometer response. This technique encounters some problems. For instance, the pistons of the solvent delivery system cause systematic pressure drops that disturb the measurement. Furthermore, it cannot be applied to samples with broad MMD (say with a polydispersity index MwIMn larger than 1.8-2.0). In fact, it has been shown by computer simulation that for broad MMD samples, the time-lag between the two detectors is overestimated, which implies that there is a discrepancy, AC, given by... 

Size Exclusion Chromatography (SEC) is widely used technique polymer characterization. The standard configuration of a SEC apparatus consists of three parts (/-5), viz., a solvent-delivery system (equipped widi an injection port), the columns, and a differential refractive index (RI) detector. This basic configuration can be modified by adding an additional detector, either in series or in parallel to the RI detector (J-5). The second detector can be a viscometer (2), a concentration detector (3), an UV detector (4) or a Light Scattering (LS) device (5),... 

This situation also results in the imcertainty of characterization of stmcture of porous bodies with help of SEC designated inverse SEC . In other words, SEC is a relative, non-absolute method. Each column (or column system) must be calibrated, or molar mass of species leaving column must be monitored by special absolute detectors (light scattering measuring devices, viscometers, mass spectrometers, and so on (see sections 11.6.1.4and 11.7.3.2). 

An important application of SEC -viscometiy in conjunction with universal calibration is to determine the Mark-Houwink coefficients for a given polymer system. The coefficients can provide information about solvent quality and molecular conformation. In addition, once the coefficients for a polymer-solvent system are known, that polymer can then be characterized using conventional universal calibration without an on-line viscometer. All references listed in Table 1 report the Mark-Houwink coefficients for the systems studied. 

The intrinsic viscosity and Mark-Houwink constants of standards can be determined from a static capillary viscometer or an on-line viscometer detector in an SEC system. If the intrinsic viscosity is to be used for constructing a universal calibration curve, it is important to use the identical conditions in performing the SEC analysis and the intrinsic viscosity measurement. A Mark-Houwink plot for five PAM standards and one PAA standard is shown in Figure 4. The intrinsic viscosity of PAM may decrease with time and becomes constant after about one week. It is recommended that the PAM solution be analyzed while still fresh. 

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