Alternative sample-introduction technique to avoid breakthrough in gradient-elution liquid chromatography of polymers.

Gradient-elution liquid chromatography (GELC) is a powerful tool for the characterization of synthetic polymers. However, gradient-elution chromatograms often suffer from breakthrough phenomena. Breakthrough can be averted by using a sample solvent as weak as the mobile phase. However, this approach is only applicable to polymers for which a sufficiently strong solvent exists which is at the same time a weak eluent. Finding such a solvent for a given polymer can be laborious or may even be impossible. Besides, when working with comprehensive two-dimensional LC the effluent of the first dimension is the injection solvent of the second dimension. In this case, it is not possible to avoid breakthrough by adjusting the eluent strength of the second-dimension injection solvent. Therefore, another strategy to avert breakthrough has to be implemented. In this work, we successfully avoided breakthrough in GELC by mixing the mobile phase not before, but after the autosampler. This was demonstrated measuring a blend of poly(methyl methacrylate) standards with different molecular-weights as model mixture with comprehensive two-dimensional GELC×size-exclusion chromatography. The strategy is thought to be applicable to all substances with a sufficiently strong dependence of retention on mobile-phase composition. This typically applies to large molecules (synthetic and natural polymers) and allows efficient refocusing. Unretained and barely retained substances are not refocused and therefore suffer in the proposed setup from peak broadening.