Flow detectors, viscometers

It is often useful to monitor simultaneously not only concentration and molar mass but also composition of eluting mactomolecules. Detectors that for example combine a differential refiactometer with the ultraviolet photometer, and the flowthrough light scattering monitor or the flow-through viscometer are comercially available. 

The combined detectors, which simultaneously monitor not only concentration and molar mass but also chemical composition of eluting macromolecules became quite popular in modem SEC. Most of combined detectors include a differential refractometer, an ultraviolet photometer, a flow-through light scattering monitor and sometimes also a flow-through viscometer. 

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. 

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. 

Mod i f 1 ed Mjymbrajne Viscometer Foi- the pulsed system a coil of tubing (the injection loop) was placed after the prefilter and liefore the membrane holder as shown in Figure P. Directional valves at each end of the loop controlled the flow path. Solvent or solution could be pumped directly to the UV to establish baseline absorbance or for calibration. To make P measurements the flow was directed through the membrane and then into the differential UV spectrophotometer. The flow could also be brought to the upstream portion of tlie membrane holder and then to the UV detector in an effort to measure the concentration at the membrane surface. 

Two specialized detectors have been developed with specific applications for SEC. The first is the laser light-scattering detector, which has been around for almost 20 years but now has new electronics and computer data acquisition capabilities. Substitution of bulky He-Ne gas lasers with small, inexpensive diode lasers has greatly reduced the size and cost of laser light-scattering detectors, and the development of reference flow viscometers has provided similar size and cost advantages for viscometer detectors. 

It is possible to calculate the offsets between detectors from the independent calibration curves. This was done and the results are shown in Figure 6. The calculated offset between the DRI and UV detectors is constant within experimental uncertainty. In contrast, the calculated offset between the viscometer and DRI detectors shows deviations at its extremes. The cause of these deviations is unclear, but they may be a manifestation of flow effects such as those observed in systems with single capillary viscometers (7). 

A Waters GPC 150 CV, equipped with the DRI prototype 4 and a single capillary viscometer, was used for this study. A low-angle laser lightscattering (LALLS) detector (Chromatix CMX 100) was inserted between the column set and the GPC 150 CV detectors. Tetrahydrofuran (THF) was used at 40 °C and at a flow rate of 1 mL/mn. THF was filtered on a Millipore membrane-type FH and stabilized by lonol at a concentration of 0.04%. The columns used were a set of Waters Ultrastyragel (103—106 A). The narrow standards used for calibration were a set of polystyrene standards... 

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. 

The viscosity data are a quantitative gauge for monitoring the loss of high-molecular-weight chains within the polymer. Because the viscometer detector operates by measuring the pressure drop across a capillary tube, the intrinsic viscosity at a constant flow rate can be measured as follows [11,16] ... 

The last differential viscometer design is the Waters Corporation detector [9], which is in the Alliance GPCV2000 high-temperature instrument. It is composed of three capillaries, two differential pressure transducers, and two holdup reservoirs it is represented in Fig. Id. The pressure transducers are connected flow-through this eliminates the need for frequent purges. This detector provides, at the same time, relative viscosity information and relative flow information. This design does not require a perfect matching of the capillaries. 

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... 

An alternative molecular weight-sensitive detector is the on-line viscometer. All current instrument designs depend upon the relationship between pressure drop across a capillary through which the polymer sample solution must flow and the viscosity of the solution. This relationship is based upon Poiseuille s law for laminar flow of incompressible fluids through capillaries ... 

The only universal detector sensitive enough to detect asphalt (because of its relatively low molecular weight) is the Viscotek differential viscometer (108,109). It utilizes a Wheatstone bridge flow resistance scheme that measures intrinsic viscosity differences between the column eluant and the carrier solvent. Other viscosity detectors measure absolute intrinsic viscosity of the eluant and are not as precise. In Figure 19, several supercritically refined asphalt fractions having a variety of molecular weights (MJ are seen to have similar RI and IV... 

Application of a molar mass sensitive detector eliminates the column calibration. In principle, there are two kinds of the molar mass sensitive detectors. They are the light scattering photometers and viscometers. Actually the viscometers are not truly molar mass detectors since the measured quantity is the intrinsic viscosity and not the molar mass. The molar mass is determined from the experimental intrinsic viscosity and the so-called universal calibration, i.e., the relation log(M[j ]) versus elution volume that is independent of polymer composition and structure. Likewise, in conventional SEC the obtained results are affected by the flow rate and temperature... 

SEC experiments were carried out on a Watters 150CV instrument (Waters Associates, Milford, Massachusetts, U.S.A.) equipped with both differential refractive index single-capUlary viscometer detectors. The solvent/mobile phase was H20/0.02% NaNs, at the flow rate of 1.0 ml/ min. Pump, solvent, and detector compartments were maintained at 50°C. Separation occurred over a column bank consisting of three analytical columns preceded by a guard column Shodex KB-G, KS-802, KS-803, and KB-804 (Phenomenex, Torrance, California, U.S.A.). Universal calibration was performed using a series of oligosaccharides (Sigma, St. Louis, Missouri, U.S.A.), and PuUuIan Standards (American Polymer Standards, Mentor, Ohio, U.S.A., and Polymer Laboratories, Amherst, Massachusetts, U.S.A.). 

---