Broad-standard linear calibration

An added benefit of the direct SEC-[n] calibration approach is that a new independent way of determining K and a values, using only broad MW standards, has also resulted. As few as three standards (or four, if all are narrow MWD) are needed to obtain both MW and [x]] calibration curves for a particular polymer-solvent system by using the broad-standard, linear calibration approach. From the experimental calibration constants of the two calibration curves, one can calculate K and a directly as described later. 

The broad-standard linear calibration curves for polystyrene in THF in Figure 1 and 4 are used to illustrate the K and a calculations as follows. The particular calibration curves are found to correspond to the following calibration equations ... 

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 ... 

FIGURE 16.5 Broad standard calibration (linear mode) of a semipreparative Sephacryl S-IOOO system (95 x 1.6 cm) with an aqueous mixture of Blue Dextran, Dextran T-SOO, and glucose eluent 0.005 M NaOH V, i = 75 ml. = 162 ml. 

C. Linear SEC-[nl Calibration Using Broad Standards. It is to be noted that the viscosity calibration curve in Figure 3 is essentially linear. This linearity is derived from bimodal SEC column sets. 

Figure 4. Molecular weight calibration curves for polydextran obtaimd u g linear calibration and two broad M WD standards and by plotting Mrm ( / Mif Mw) vs, peak retention volume ((%) Mrms, (---------) M(v) = 0,37 X exp(—OJ v))...

Table V. Results of Case Study 2 - Linear Calibration with Two Broad MWD Polydextran Standards with Known Mj and...

Dow 1683 polystyrene standard was utilized as a broad MWD standard in the linear calibration method due to its distribution symmetry and particular lack of significant tail at the low end of its MWD. 

Those which employ one or more broad MWD standards with known M or [n] and assume a linear molecular weight calibration curve and finally those which employ one broad MWD... 

Methods based on universal calibration will be illustrated using nonaqueous SEC and broad MWD PVC standards. The and of these standards is known. Methods based on the linear molecular weight calibration will be illustrated using aqueous SEC and broad MTO polydextran standards for which M and My are known. 

Several assumptions were made in using the broad MWD standard approach for calibration. With some justification a two parameter equation was used for calibration however the method did not correct or necessarily account for peak speading and viscosity effects. Also, a uniform chain structure was assumed whereas in reality the polymer may be a mixture of branched and linear chains. To accurately evaluate the MWD the polymer chain structure should be defined and hydrolysis effects must be totally eliminated. Work is currently underway in our laboratory to fractionate a low conversion polydlchlorophosphazene to obtain linear polymer standards. The standards will be used in polymer solution and structure studies and for SEC calibration. Finally, the universal calibration theory will be tested and then applied to estimate the extent of branching in other polydlchlorophosphazenes. 

Nonlinear regression enables broad molecular weight distribution standards to be used for SBC calibration and permits simultaneous resolution correction euid calibration. Furthermore, it greatly increases the variety of equations 4iich can be fit to calibration curves, detector linearity data and chromatogreun shapes. 

More recent flame photometric methods rely on direct measurement of the phosphorus emission. If organophosphorus compounds are injected into a hydrogen flame, a continuous emission is obtained in the 490-650 nm region. A broad band system, with an intensity maximum at 526 nm, is superimposed on this background139 it is attributed to the HPO species formed in the flame. An early determination of phosphorus at 0.01-0.04 m concentrations was based on examination of the continuous emission standard and sample solutions were injected into the burner and the intensities were measured at 540 nm the calibration graph was linear down to the detection limit of 10 4 M phosphorus sodium or calcium, if present in the sample, interfered with the results140. 

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