Evaluating the Temperature Shift in Analytical Temperature Rising Elution Fractionation

May 22, 2014

This article presents a new method to evaluate the temperature shift observed in analytical temperature rising elution fractionation (ATREF). The evaluation is based on the dependence of the measured peak temperature as a function of heating rates. Application of the proposed method does not require any knowledge of the fluid circuit characteristics geometry and avoids the use of narrow preparative TREF standards. The results are found to be more accurate than the method that is usually applied.

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Since the publication of the Wild article in 1982,1 analytical temperature rising elution fractionation (ATREF) has been a method of choice for polyolefin characterization. In 1991 Exxon introduced the first patents in which polyethylenes were described by their compositional distribution breadth index (CDBI), an ATREF parameter equivalent to the polydispersity index of the molecular weight distribution. Exxon was followed by other material producers like Dow and Cryovac, and the number of patents incorporating claims covered by ATREF analysis steadily increased to about 30 patents per year. At the beginning, the ATREF systems were built by modifying gel permeation chromatography (GPC) systems. The increasing demand for ATREF measurements fostered the introduction of the first automated commercial ATREF system by Polymer Char in 1996. No standardized method was published for this technique, and most of the patents make reference to Wild's publication.1

To calculate the CDBI, several standards of polyethylene with narrow compositions and known short-chain branching characteristics are injected into the ATREF unit and chromatograms are recorded as a function of time while monitoring the column temperature. These data are used to construct thermograms, in which the detector signal is plotted as a function of the column temperature. The recorded column temperature is usually higher than the real melting temperature of the eluted fraction because of the delay between melting in the column and detection. This delay depends on several experimental parameters, including column dimensions, tubing characteristics between the column and detector, and heating and elution rates. These equipment-specific parameters prevent the use of any short chain branches (SCB)-elution temperature calibration curves available in the literature.1–3 Actually, as recommended by the commercial ATREF manufacturer,4 the use of an identical procedure and identical equipment is mandatory to be able to compare samples.

The procedure would be greatly simplified with a robust method to calculate the temperature shift, allowing for the correlation between results obtained with different instruments and methods.

To circumvent that problem, the temperature shift can be calculated based on the tubing volume between the column and the detector. As the heating rate and the flow rate during the heating step are usually constants, the tubing volume can be converted into temperature shift using the following relationship:

As this method assumes a detailed knowledge of the tubing and does not take into consideration the heat transfer delay in the column heating system, it is expected that the calculated temperature shift will be lower than the actual one observed.

An alternative method to evaluate the temperature shift is to analyze narrow preparative TREF standards.5,6 In this case, the temperature shift is given by the difference between the measured ATREF peak temperature and the preparative TREF temperature of the standard; the difficulty of the method is the fact that narrow TREF standards, isothermally eluted in a temperature interval of about 3 °C, are not easily available. In addition, the high preparative column volume introduces measurement errors close to the temperature interval used to elute the preparative fraction (3 °C). Another potential issue with this method is the fact that usually the analytical TREF solvent (trichlorobenzene) is different from the solvent used in preparative TREF (xylene).

In this article, we present a new method to evaluate the temperature shift for ATREF that does not require either the detailed knowledge of the tubing between the column and the detector or the availability of narrow standards obtained by preparative TREF. To apply the method, it is possible to use any unimodal sample such as a commercial metallocene polyethylene or high-density polyethylene. The method is based on the idea that the temperature shift is zero for isothermal elution steps. Because finite heating rates are used with ATREF, we propose to find the real melting temperature by analyzing the same sample with different heating rates and to extrapolate the measured elution temperatures to isothermal conditions. After finding the real melting temperature by extrapolation, we can then use a general rule to calculate the temperature shift for a given heating rate.