Narrow standard calibration

Commercial narrow standards [such as poly(ethylene glycol) (pEG), polystyrene sulfonate, pAA, poly w-vinyl pyrrolidinone, dextrans] are available from American Polymer Standards Corporation, Polymer Laboratories, Polymer Standards Service USA, Toyo Soda, and others. While these standards are often not as narrow as pSty or pMMA that has been anionically polymerized, they are acceptable for narrow standard calibrations. 

Extrapolations can also be applied to log Tg-retention volume plots (dashed line in Figure 5) to provide a better estimate of the number-average radius. There is reasonable superposition of the values of the broad standard on the narrow standard calibration data, but unlike the log M plot in Figure 4, there is considerable scatter in the data at long retention volumes (small sizes). In this region the 15° and 90° signals are nearly identical and the ratio of the two signals depends heavily on... 

This conventional narrow standard calibration procedure yields relative molecular-weight values because the averages obtained are relative to the cahbration polymer. For example, if polyethylene molecular weights are determined with a cahbration curve produced with polystyrene narrow standards, the results would be incorrect for polyethylene. For many purposes relative molecular-weight values are adequate, since the results obtained with an unknown sample may be compared to preestablished acceptable relative values. If narrow standards of the same polymer as the unknown sample are available, then it is possible to calculate absolute values. 

Absolute system calibration with a set of narrow distributed nb glucans (pullulans) by means of dual detection of mass and scattering intensity applying a mixed peak position/ broad standard calibration... 

Establishing a calibration function with one single broad distributed sample is an alternative to traditional peak postion calibration of SEC systems with a set of narrow distributed standards. An obvious advantage of this technique is time for peak position calibration elution profiles for the set of standards need to be determined for broad standard calibration the elution profile of one sample needs to be determined only. Establishing a linear calibration function with a broad distributed standard includes startup information [M (true), Mn(true)] and an iterative (repeat.. . until) algorithm ... 

Commercial grades of PVP, K-15, K-30, K-90, and K-120 and the quaternized copolymer of vinylpyrrolidone and dimthylaminoethylmethacrylate (poly-VP/ DMAEMA) made by International Specialty Products (ISP) were used in this study. PEO standard calibration kits were purchased from Polymer Laboratories Ltd. (PL), American Polymer Standards Corporation (APSC), Polymer Standards Service (PSS), and Tosoh Corporation (TSK). In addition, two narrow NIST standards, 1923 and 1924, were used to evaluate commercial PEO standards. Deionized, filtered water, and high-performance liquid chromatography grade methanol purchased from Aldrich or Fischer Scientific were used in this study. Lithium nitrate (LiN03) from Aldrich was the salt added to the mobile phases to control for polyelectrolyte effects. 

Acrylic esters can be polymerized by a number of routes. Anionic polymerization gives the narrow standards used primarily for calibration, but is not used on an industrial/commercial scale. Free-radical polymerization is the dominant mode of polymerization for making these polymers on an industrial scale. Significant volumes of polymer are made by both solution polymerization... 

Molecular weight calibration from a monomer to several million daltons can be carried out by a variety of techniques. Because narrow standards of p(methyl methacrylate) (pMMA) are available, these are often used. Narrow standards of p(styrene) (pSty) are also available and can be used. Using the Mark-Houwink-Sakurada equation and the parameters for pSty and pMMA, a system calibrated with pSty can give pMMA-equivalent values, and vice versa. 

Figure 19. Universal calibration curves for GPC 1 and GPC 2 based on hydrodynamic volume in GPC 1 (Vthf) (J) 100% THF (B3, Table I) obtained by sampling narrow standards infected into GPC 1 (A3, Table I) (2) 57% n-heptane in THF (B4, Table 1) (obtained as in 1 above) (3) 57% n-heptane in THF (B4, Table 1), obtained by direct injection into GPC 2 (4) 62% n-heptane in THF (B5, Table 1) (obtained as in 3 above) (5) 100% THF (A3, Table I), obtained by sampling GPC 1 (NBS706, 0.375 and 0.750 mg into GPC 1 and...

For some polymers, like polystyrene or poly(methyl methacrylate), narrow standards of known molar mass and small polydispersity are commercially available, which can be used for calibration. Unfortunately, such standards are not available for all polymers and then the obtained true molar masses of a specific polymer might differ by a factor of two from the value obtained by calibration with, e.g., polystyrene [30] (see Section 9.1). This problem can be resolved by the so-called universal calibration, which is based on the finding that the retention volume of a polymer is a single-valued function of the hydrodynamic volume of the polymer, irrespective of its chemical nature and... 

Analysis Is carried out on a cream stated to contain 2% w/w of both miconazole and hydrocortisone. An ODS column is used with a mobile phase consisting of acetonitrile/ acetate buffer pH 4.0 (70 30) and the eluent is monitored at 220 nm. A narrow range calibration curve, within 20% of the expected concentration of each analyte in the sample extract was prepared for each analyte by plotting the ratio of the areas of the analyte peaks against fixed amounts of the internal standards for both analytes. The internal standards used were econazole and hydrocortisone 21-acetate for miconazole and hydrocortisone, respectively. 

Table I illustrates the molecular weight averages found from universal calibration (narrow standards) for four aspen lignins and two quinonemethide-derived polymers using the Unical software. The poly-dispersities were the same within the experimental errors for all lignin samples. In contrast, the polydispersities found for the quinonemethide-derived polymers by universal calibration were near 1.1. The molecular weight averages found for the four acetylated lignins studied by universal calibration were substantially larger than those determined from previous work using conventional GPC (e.g., AESE and BM lignins from refs. 7 and 12 had approximately one-third those values found by HPSEC-DV in the present study).

Table I. MWD of Acetylated Aspen Lignins and Model Compounds from Universal Calibration with Narrow Standards 1...

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... 

Multiple detection applied to the SEC characterization of copolymers is attractive because it yields both CCD and MWD information. A dual detection system based on two concentration detectors, for example, RI and UV, is useful where narrow standards of the homopolymers are available and where both homopolymers obey universal calibration. However, in other copolymer systems the addition of a third detector, LALLS, can offer the advantage of on-line determination of molecular weight for each eluting species. The triple detection approach gave similar values to the dual detector approach for a model copolymer system (PS-PMMA) studied. It was also applicable to a more difficult copolymer system (PS-PEO), although it appeared that where one homopolymer was present in very small quantities, an average p value gave more consistent results than correction for pi across the distribution. 

Fresh solutions of the standards are prepared in the solvent used as chromatographic eluent. Calibration solutions should be as dilute as possible, in order to avoid any concentration dependence of sample retention volumes. The concentration effect causes an increase of retention volumes with increased sample concentration. As a rule of thumb, when high efficiency microparticulate packings are used, the concentration of narrow standards should be < 0.025% (w/v) for MW over 10 , < 0.05% for MW between 10 and 2 x 10 , and < 0.1% for MW down to lOl With a lower MW and in the oligomer range, the sample concentration can be higher than the previously suggested values. 

The goodness of different equations fitted to the experimental data points is assessed by the results of statistical analysis or by simply considering the standard error of the estimate. It should be also considered that the adequacy of the calibration function for the determination of correct MW values is also dependent on the quality of the narrow MWD standards. Their nominal MWs are determined with independent absolute methods and are affected by experimental errors which may be different between samples with different MWs, or coming from different producers. A check of the quality of the narrow standards may be obtained by calculating the percent MW deviation of each standard from the calibration curve ... 

The experimental dependence M = /(V), i.e. the classical SEC calibration curve usually obtained by using narrow standards, in such a case can be obtained directly without calibration from the on-line LS detector. By combining the experimental function M = /(V) and the concentration profile (from DRI), one can construct the complete MMD of the HA sample. The differential and cumulative MMD of a high molar mass HA sample (Mw = 652 kDa, D = 2.1) are shown in Fig. (10). Starting from the initial MMD, the molecular weight averages and dispersity index (Mn, Mw, Mz, and D) could be easily calculated using the appropriate definitions. 

Analyse the product using gel permeation chromatography (GPC) calibrated with a range of PMMA narrow standards to determine M, and... 

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