Universal calibration

Quality control, however, needs more attention. For example, it has been reported that much of the chemical data produced by the International Indian Ocean Expedition is unusable because of doubts about its accuracy. Such reports are a perennial source of confusion in marine chemistry. Better calibration, universal standards, and interlaboratory comparison are essential if we are to continue our present field methods, in which independent investigators make measurements that are presumably comparable (NRC, 1971, pp. 54-55). 

SAYBOLT UNIVERSAL VISCOSITY. The efflux time in seconds (SUS) of 60 mL of sample flowing through a calibrated Universal onfice in a Saybolt viscometer under specified conditions. 

An interesting outgrowth of these considerations is the idea that In r versus K or Vj should describe a universal calibration curve in a particular column for random coil polymers. This conclusion is justified by examining Eq. (9.55), in which the product [i ]M is seen to be proportional to (rg ), with r = a(rg 0 ) - This suggests that In rg in the theoretical calibration curve can be replaced by ln[r ]M. The product [r ]M is called the hydrodynamic volume, and Fig. 9.17 shows that the calibration curves for a variety of polymer types merge into a single curve when the product [r ]M, rather than M alone, is used as the basis for the cafibration. 

H. A. Martens, Multivariate Calibration Ph.D. dissertation. Technical University of Norway, Trondheim, Norway, 1985. 

A. Melikov, ed. Calibration and requirements for accuracy of Thermal Anemometers for Indoor Velocity Measurements. Report ET-1E9701. Technical University of Denmatk, Laboratory of Indoor Environment and Energy, 1997. 

PMMA, on the unmodified porous glass and silica gel, and the universal calibration curves for polystyrenes and poly(methyl methacrylates) did not coincide (10,12,19). 

It is evident from these results that the interactive properties of the investigated SEC PS/DVB or DVB gels are very different. Because polar electroneutral macromolecules of PMMA were more retained from a nonpolar solvent (toluene) than from polar ones (THF, chloroform), we conclude that the dipol-dipol interactions were operative. Columns No. 1 and No. 2 were very interactive and can be applied successfully to LC techniques that combine exclusion and interaction (adsorption) mechanisms. These emerging techniques are LC at the critical adsorption point (18), the already mentioned LC under limiting conditions of adsorption (15,18), and LC under limiting conditions of desorption (16). In these cases, the adsorptivity of the SEC columns may even be advantageous. In most conventional SEC applications, however, the interactive properties of columns may cause important problems. In any case, interactive properties of SEC columns should be considered when applying the universal calibration, especially for medium polar and polar polymers. It is therefore advisable to check the elution properties of SEC columns before use with the... 

Long-time reproducibility of elution profiles broad standard calibration with dextran T-500 transformation of a scb-type calibration function into nb/Icb-type via universal calibration dp of synthetic glucans in the presence of significant amounts of monomer mass and molar degree of polymerization of Triticale (hybride) starch... 

Universal SEC calibration reflects differences in the excluded volume of polymer molecules with identical molecular weight caused by varying coil conformation, coil geometry, and interactive propenies. Intrinsic viscosity, in the notation of Staudinger/ Mark/Houwink power law ([77]=fC.M ), summarizes these phenom-... 

FIGURE 16.10 Intial dextran calibration ( ) and resultingnb/Icbglucan calibration ( )forthe Sepha-cryl S-SOO/S-1000 (60 + 9S X 1.6 cm) system achieved from broad standard calibration with Dextran T-SOO and universal calibration, respectively Staudinger/Mark/Houwink constants (dextran Ksu = 0.0978 ml Mg . Osw = O.SO nb/lcb amylose = pullulan K pi. = 0.0268 ml M g . = 0.6S). 

Electrostatic and adsorption effects conspire to make aqueous GPC more likely to be nonideal than organic solvent GPC. Thus, universal calibration is often not obeyed in aqueous systems. Elence, it is much more critical that the standard chosen for calibration share with the polymer being analyzed chemical characteristics that affect these interactions. Because standards that meet this criterion are often not available, it is prudent to include in each analysis set a sample of a secondary standard of the same composition and molecular weight as the sample. Thus, changes in the chromatography of the analyte relative to the standards will be detected. 

In SEC, universal calibration is often utilized to characterize a molecular weight distribution. For a universal calibration curve, one must determine the product of log(intrinsic viscosity molecular weight), or log([7j] M). The universal calibration method originally described by Benoit et al. (9) employs the hydro-dynamic radius or volume, the product of [tj] M as the separation parameter. The calibration curves for a variety of polymers will converge toward a single curve when plotted as log([7j] M) versus elution volume (VJ, rather than plotted the conventional way as log(M) versus V, (5). Universal calibration behavior is highly dependent on the absence of any secondary separation effects. Most failures of universal calibration are normally due to the absence of a pure size exclusion mechanism. 

To use universal calibration, intrinsic viscosity must be measured. An online, DV detector can measure specific viscosity, 7j,p, which is related to intrinsic viscosity by the expression... 

Mark-Houwink constants for PEG, PEO, and PSC are summarized in Table 20.4. These were measured in either an aqueous mobile phase of 0.10 N NaNOi or a mobile phase of 20% acetonitrile in aqueous 0.10 N NaNOi (which also exhibits universal calibration behavior). The values for a fall within... 

FIGURE 20.3 Universal calibration curve for TSK-PW columns with an aqueous mobile phase of 0.10 N sodium nitrate. 

In addition to polymer standards, a number of broad distribution, water-soluble polymers can be characterized on TSK-PW columns using universal calibration. These include both fully and partially hydrolyzed PVA, PAAM, PEE, and dextran. PVA, the world s largest-volume, synthetic, water-soluble polymer, was first successfully separated on TSK-PW columns by Hashimota et al. (10). In the 1980s, the use of low-angle, laser light-scattering detection... 

FIGURE 20.1 I Universal calibration curve of PVP using CATSEC columns in an aqueous mobile phase of 0.20 N sodium nitrate/0.1 % trifluoroacetic acid. 

The steel housing rarely exceeds 0.5 in. (12 mm) and a calibration is done in terms of API units, arbitrary units defined in a standard calibration pit located at the University of Houston. 

For scientific work the fundamental standard of mass is the international prototype kilogram, which is a mass of platinum-iridium alloy made in 1887 and deposited in the International Bureau of Weights and Measures near Paris. Authentic copies of the standard are kept by the appropriate responsible authorities in the various countries of the world these copies are employed for the comparison of secondary standards, which are used in the calibration of weights for scientific work. The unit of mass that is almost universally employed in laboratory work, however, is the gram, which may be defined as the one-thousandth part of the mass of the international prototype kilogram. 

The most widely used molecular weight characterization method has been GPC, which separates compounds based on hydrodynamic volume. State-of-the-art GPC instruments are equipped with a concentration detector (e.g., differential refractometer, UV, and/or IR) in combination with viscosity or light scattering. A viscosity detector provides in-line solution viscosity data at each elution volume, which in combination with a concentration measurement can be converted to specific viscosity. Since the polymer concentration at each elution volume is quite dilute, the specific viscosity is considered a reasonable approximation for the dilute solution s intrinsic viscosity. The plot of log[r]]M versus elution volume (where [) ] is the intrinsic viscosity) provides a universal calibration curve from which absolute molecular weights of a variety of polymers can be obtained. Unfortunately, many reported analyses for phenolic oligomers and resins are simply based on polystyrene standards and only provide relative molecular weights instead of absolute numbers. 

We can determine an approximate value of the pH of an aqueous solution very quickly with a strip of universal indicator paper, which turns different colors at different pH values. More precise measurements are made with a pH meter (Fig. 10.11). This instrument consists of a voltmeter connected to two electrodes that dip into the solution. The difference in electrical potential between the electrodes is proportional to the hydronium ion activity (as will be explained in Section 12.10) so, once the scale on the meter has been calibrated, the pH can be read directly. 

From this it follows that plotting a graph of log M[7]] against elution volume should give a straight line for all polymer-solvent combinations. This was shown by Benoit et al to be true for a large number of systems and is referred to as the universal calibration curve. It is illustrated in Figure 6.6. 

In reality, the universal calibration curve is not quite universal. Certain exceptions exist, such as polymers with very rigid backbones. However, the... 

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