Size calibration, linear

Straight chain alkanes from Applied Science, aromatics from Fisher Scientific Company and polystyrene standards from Waters Associates were used without purification for the linear molecular size calibration of the GPC. Since the solubility of the larger alkanes in THF is very low, appproximately 0.2 -1 mg of each standard was dissolved in 50 microliters of the THF for the molecular size calibrations. 

Monodisperse spheres are not only uniquely easy to characterize, but also very rarely encountered. Polymerization under carefully controlled conditions allows the preparation of the polystyrene latex shown in Figure 1.8. Latexes of this sort are used as standards for the size calibration of optical and electron micrographs (also see Section 1.5a.3). However, in the majority of colloidal systems, the particles are neither spherical nor monodisperse, but it is often useful to define convenient effective linear dimensions that are representative of the sizes and shapes of the particles. There are many ways of doing this, and whether they are appropriate or not depends on the use of such dimensions in practice. There are excellent books devoted to this topic (see, for example, Allen 1990) and, therefore, we consider only a few examples here for the purpose of illustration. 

The mercury vapor produced by bubbling the sulfur dioxide through a mercurous nitrate solution was detected by the gold-coated crystal due to the formation of a mercury amalgam. The optimum conditions were temperature = 27°C flow rate = 55 ml/min [Hg2 ] = 4 x 10 M purge time = 10 min. The sensitivity of the detector depended on the sample size, and linear calibration curves in the concentration... 

Atomic force microscopes may use sophisticated algorithms to modify the voltages applied to actuating elements (piezoelectric ceramics) that incorporate the effects of scan rate, scan size,non-linearities,andcross-talk.Thisapproach is referred to as an open loop method. Closed loop approaches are also available. These provide an independent method of dimensional measurement (strain gauges, capacitance sensors, or optical interferometers) in order to apply realtime correction to actuator motion. Closed loop is more readily applied in x and y dimensions recent advances include z motion. Closed loop calibration can introduce additional electric noise, and care is required to make sure that ultimate resolution is not compromised. 

Polysaccharides TSK-GEL GMPWxl TSK-GEL GSOOOPWxl -I- TSK-GEL G3000PWxi Large pore size, linear calibration curve, small particles, high resolving power... 

Synthetic polymers TSK-GEL GMPWxi TSK-GEL GSOOOPWxl G3000PWxl Large pore size, low adsorption, linear calibration curve... 

The linear column (PSS SDV 5 /mm linear) has a wider molar mass fractionation range while keeping the analysis time roughly the same. Therefore the slope of the calibration curve is much steeper and the resolution will be poorer in this case. The second column with a single pore size (PSS SDV 5 /mm 1000 A) separates only below 50,000 Da, but does this very efficiently in the same time. 

Most size exclusion chromatography (SEC) practitioners select their columns primarily to cover the molar mass area of interest and to ensure compatibility with the mobile phase(s) applied. A further parameter to judge is the column efficiency expressed, e.g., by the theoretical plate count or related values, which are measured by appropriate low molar mass probes. It follows the apparent linearity of the calibration dependence and the attainable selectivity of separation the latter parameter is in turn connected with the width of the molar mass range covered by the column and depends on both the pore size distribution and the pore volume of the packing material. Other important column parameters are the column production repeatability, availability, and price. Unfortunately, the interactive properties of SEC columns are often overlooked. 

Two linear columns from Showa Denko, Shodex SB-806M and Shodex SB-806MHQ, and two linear columns from TosoHaas, TSK GM-PWxl and TSK GM-PW, were evaluated. Prior to the evaluation, the number of theoretical plates for Shodex SB-806MHQ, SB-806M, PWxl, and PW was determined to be 15,100, 15,700, 11,390, and 4710, respectively, as per manufacturer inspection. The lower plate count of the TSK PW column is due to the larger particle size of this column. Two mobile phases, water with 0.1 M LiNOi and 50 50 methanol/water (v/v) with 0.1 M LiNOi, were used for each of the four columns. These four columns were new and only PEO and PVP were analyzed with these columns in this study. Waters Ultrahydrogel columns have also been used in this laboratory. However, Ultrahydrogel columns are exactly the same as the TSK GM-PWxl columns based on the calibrations curves supplied by the manufacturers and by the pyrolysis GC data discussed later. 

Calibration Each of the solutions is injected once and a linear regression is calculated for the five equidistant points, yielding, for example, Y = -0.00064 + 1.004 X, = 0.9999. Under the assumption that the software did not truncate the result, an r of this size implies a residual standard deviation of better than 0.0001 (-0.5% CV in the middle of the LO range use program SIMCAL to confirm this statement ) the calibration results are not shown in Fig. 4.39. 

The linear MW calibration curve of the bimodal columns, as shown in Figure 1, is the result, of proper matching of the column packing pore size -, and such a matching of pore size is relatively easy to accomplish in practice. li) Th linear MW calibration can be expressed by equation 4 ... 

The calibration technique used in conventional SEC does not always give the correct MWD, however. The molecular size of a dissolved polymer depends on its molecular weight, chemical composition, molecular structure, and experimental parameters such as solvent, temperature, and pressure ( ). If the polymer sample and calibration standards differ in chemical composition, the two materials probably will feature unequal molecular size/weight relationships. Such differences also will persist between branched and linear polymers of identical chemical composition. Consequently, assumption of the same molecular weight/V relation for dissimilar calibrant and sample leads to transformation of the sample chromatogram to an apparent MWD. 

Compare molecular size/weight characteristics of branched and linear species eluting at V in each chromatogram. Under the universal calibration formalism branched and linear components have the same hydrodynamic volume at V ... 

Figures 6, 7 and 9 show calibration curves using two multi-column combinations and illustrate the degree of "optimization obtained in this system. The mobile phases for Figures 6 and 7 contained 0.025 g polyethylene oxide and ion exclusion and adsorption effects should therefore be largely eliminated. Figure 6 shows that reasonably good resolution can be obtained with a combination of five columns but does exhibit some loss of peak separation at the low cuid high MW ends. In Figure 7 the effect of adding a sixth column of small pore size is illustrated and it is seen that resolution at the low MW end is thereby somewhat improved. This calibration curve is effectively linear with a change of slope at 500,000 MW. It should provide a useful aqueous GPC system for MW and MWD determination of nonionic polyacrylamides.

---