TSKgel High Temperature GPC Column Durability

Introduction

In gel permeation chromatography (GPC), just like all chromatography modes, the heart of the separation lies in the quality, applicability and selectivity, or resolution, of the column. The selectivity of a GPC column is based on the ability of the pores in the column packing material to differentiate between species of varying hydrodynamic volume. The pores of the packing material within a GPC column are sampled by the analytes as they travel through the column in a size dependent manner. Due to their size, the larger components of the analyte sample either a smaller number of pores or, within a given pore, a smaller pore volume than the smaller components of the analyte, thus the larger components elute from the column prior to the smaller components.

GPC is considered a low resolution technique and does not provide infinite resolution of species with different hydrodynamic volume. As a result of the low resolution of the separation technique, each slice eluting from the GPC column has some residual polydispersity. This residual polydispersity, combined with extra dead volume in detectors and instrument tubing, leads to the overestimation of sample polydispersity because the peak eluting from the GPC column is broadened and appears to cover a wide molar mass range.1 The superficial broadening of the molar mass range due to the resolution ofa GPC column has a direct impact on accuracy of the molar mass averages. The accuracy of the number and z-average molar masses, M_n and M_z respectively, can decreases by more than 10% as the resolution of a GPC column set decreases, while the accuracy of the weight-average molar mass, M_w, remains virtually unaffected by resolution.^{1, 2}The ability to obtain accurate molar mass averages for polymers by GPC, without superficially broadening the distribution, is essential as the molarmass averages and distributions affect the processing and end-use properties of materials. To obtain the best separation, thus most accurate molar mass averages possible, a GPC column must provide a linear calibration in the molar mass range of interest, narrow particle size distribution of the packing material, a large number of theoretical plates or high resolving power and durability.² Column manufactures, such as Tosoh, focus their innovations in GPC column technology on these characteristics as they directly affect column quality, applicability and selectivity, or resolution. Column characteristics such as column durability become even more important in high temperature GPC analysis as these columns are not only exposed to harsh organic solvents but are also continuously exposed to extreme temperatures and repetitive temperature cycling. Here we have studied the durability and stability of Tosoh’s new TSKgel high temperature GPC columns compared to other commercially available columns for polymer analysis at temperatures above 80°C.

Experimental Conditions

Sample analysis was performed on a system consisting of an EcoSEC^® High Temperature GPC System (HLC-8321 GPC/HT) equipped with RI detector. Separation of unfiltered 200 μL injections occurred over a column bank consisting of one 7.8 mm ID x 30 cm, 13 μm particle size TSKgel GMHHR-H(S) HT column (exclusion limit 4 x 10⁸ g/mol) (PN 18393) (Tosoh Bioscience) or one 7.8 mm ID x 30 cm, 13 μm particle size, commercially available high temperature GPC column. The mobile phase and solvent were 1-chloronaphthalene (Fisher) at a flow rate of 1.0 mL/min. Detetor, pump oven, and column oven were maintained at 220°C. The polymer samples were dissolved in 1-chloronaphthalene at 250°C for one hour using the Tosoh sample prep system (PN 23801). The final sample concentrations were approximately 2.0 g/L. Data was processed with the EcoSEC GPC Workstation software. Temperature cycling was performed by flowing 1-chloronaphthalene through the EcoSEC High Temperature GPC System at a flow rate of 1.0 mL/min and slowl raising the column oven to 220°C over nine hours. Samples were injected and molar mass averages were determined after the system reached equilibration at 220°C. The EcoSEC High Temperature GPC System requires three hours to equilibrate. Temperature cycling times were extended beyond the equilibration time to test the column durability when exposed to extreme temperatures for a prolonged period. Following sample analysis 1-chloronaphthalene remained flowing through the EcoSEC HighTemperature GPC System at a flow rate o 1.0 mL/min and the column oven temperature was slowly lowered from 220 °C to room temperature over nine hours. After the EcoSEC High Temperature GPC System was lowered to room temperature the flow was stopped.The entire temperature cycling process was performed multiple times to test column durability. Molar mass averages were determined for each polymer sample using a calibration curve. A calibration curve for each column set was created for the RI detector at 220°C using Tosoh polystyrene standards A-500, F-1, F-4, F-10, and F-40. Polystyrene standards were prepared for a final concentraion of 10 g/L.

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