Differential, detector viscometer

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. 

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. 

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... 

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. 

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)... 

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 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. 

In operation, the viscometer of Figure 2 will generate two separate signal detector traces for recording. The differential log-anplifier will generate a viscosity (In hi) trace vdiile the concentration detector will generate a concentration (c) trace. 

DIFFERENTIAL PRESSURE CAPILLARY VISCOMETER AS AN IN-LINE VIsensITV DETECTOR... 

The size exclusion chromatography for this study was done in the routine manner execept for the inclusion of an online viscosity detector called a Differential Viscometer <3> (Viscotek Corp., Porter, Texas, USAl. This instrument together with an RI concentration detector permits the calculation of intrinsic viscosities across the chromatogram. An IBM PC data system with software is also provided (5). The software acquires data from both detectors, and performs calculations of intrinsic viscosity and molecular weight distributions using the Universal Calibration Method. 

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... 

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. 

Size exclusion chromatography has been greatly enriched recently by the advent of two commercial detectors, the real-time differential viscometer (DV) and the low angle laser light scattering (LALLS) photometer. The only DV detector currently available is offered by Viscotek in Porter, TX, as the model 100. 

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

The combination of the differential refractive index (RI) detector and on-line viscometer allows the direct use of the universal calibration and thus true molecular weight determination. The RI detector is concentration-sensitive, and the viscometer records specific viscosity. The ratio of the specihc viscosity to the concentration is equivalent to intrinsic viscosity (as discussed in Section 6.1), and the continuous dependence of this ratio versus the retention volume could be related to the universal calibration curve, thus allowing the correlation of each point on the chromatogram with the true molecular weight. 

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.

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