Viscometer-differential refractometer

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. 

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... 

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)... 

A concentration detector such as the differential refractometer is shown here connected in series with the capillaries. Uie following conponents are typically used in the viscometer stainless steel capillaries of 1/16-in. o.d. and 0.016 in. i.d. X 8 in. long, 2 ml. sanple loop, Celesco pressure tramsducers of 1 psi rating, Valeo 6 port sanple valve. Burr Brown Log 100 OP. type differential log-anplifier, VWR-1145 circulation tenperature bath (-15 to 150 C). Several liquid chromatographic pumps have been used. A Du Pont 860 pump was used to obtain the data reported in this work. 

Fig. 3-8. Typical GPC raw data. The unils of the vcitical axis depend on the detectors used, while those on the horizontal axis are elapsed time. In this case tlie lower curve is that of the differential refractometer, while the upper curve is the trace produced by a continuous viscometer (which is described briefly in Section 3.4.4). The citrve proceeds from left to right.

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 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. 

SEC chromatogram of a mixture of three polystyrene standards showing the outputs of both a differential refractometer (top) and a viscometer (bottom). 

Differential refractometer (DRI) and viscometer outputs for a 1 1 mixture of 845,000 g/mol of poly methyl methacrylate and 170,000 g/mol of polystyrene. With this method [Equation (15)], the determined was 265,000 g/mol, compared with an expected value of 283,000 g/mol. (Adapted from Reference 21 and used with permission from John Wiley and Sons, Publishers.)... 

Chromatograms of polybistrifluoroethoxyphosphazene using different detectors (a) differential refractometer, (b) differential viscometer, and (c) intrinsic viscosity. Column PLgel mixed bed. Mobile phase acetone, cyclohexanone. Temperature 30°C, 40°C. [From T. H. Mourey et al. (1989)... 

The column compartment is reasonably large and can hold up to ten 60-cm X 1-cm analytical columns as well as the differential refractometer—the only standard detector supplied. Other detectors can be fitted (IR, UV, viscometers and LALLS), but these are normally fitted externally and require heated extension tubes with additional plumbing to the equipment. The refractometer has a sensitivity of 1 x 10 RI units full-scale deflection, with an average noise level of 5 x 10 RI units. A quartz halogen bulb light source is used for increased sensitivity. Very good temperature control is required when using a RI detector. 

A series of random, block and graft copolymers of VC with styrene (S), butadiene (B), MMA, VAc, and VDC was characterized with the help of three consecutive detectors a differential refractometer (RI), an ultraviolet absorption detector (UV) and an automatic viscometer [33]. Figure 5.2 summarizes the relation between solution viscosity and molecular mass for these copolymers. 

Fig.12A,B. LCCC chromatograms of blends of PMMA and PnBMA under the critical condition of PMMA. Column LiChrospher 300 A +1000 A, mobEe phase methyl ethyl ketone/cy-clohexane (72/28, v/v), detector A differential refractometer B capElary viscometer. The large injection solvent peak is suppressed in the chromatograms recorded by the viscosity detector.

Precision on molecular weight, molecular size, and intrinsic viscosity is typically less than 0.3% RSD, whieh permits excellent feedback for control of the polymerization process. The integrated detector array consists of three primary detectors a light scattering detector that measures molecular weight, a four-eapillary differential viscometer that determines molecular density and size, and a differential refractometer that measures concentration. 

There are a number of detection options, some used primarily for GPC and others that have use for GPC as well as other modes of HPLC. The differential refractometer, viscometer, and light-scattering detectors are associated mostly with GPC, while absorbance detectors such as the UV/ visible or photodiode array (PDA) are widely used in all HPLC modes, including GPC. The UV/visible and PDA are especially useful for characterizing polymers and oligomers with chromophoric groups and for HPLC analyses of additives. Mass spectrometry is also used for some analyses. This is described in Sec. ILF. 

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