Semi-quantitative determination of free cyclic and linear polytetrahydrofurane in coatings using critical chromatography

Liquid chromatography at critical conditions (LCCC) is being used more frequently, especially for the determination of cyclic structures in a matrix of the parent linear molecules, analyses that cannot be performed easily by other techniques. In practice, analyses are often more complex than separating the linear from the cyclic structures, e.g. in the determination of low concentrations of free (linear and cyclic) polyols like polytetrahydrofurane (pTHF) in functional polymers and coatings. To this end different chromatographic phase systems at the critical conditions for the polyols were investigated to obtain the maximum amount of information. The identification of different oligomeric series was performed by means of an on-line coupling of LCCC with atmospheric pressure chemical ionization - mass spectrometry (APCI-MS). Mass summated chromatograms were introduced to improve the signal to noise ratio of specific oligomeric series. MS cannot be used for quantitative purposes here; therefore the evaporative light scattering detector (ELSD) and the refractive index (RI) detectors were evaluated for routine analyses. Specific tailing problems observed with a drift tube in the ELSD are discussed. Two LCCC systems (reversed phase and normal phase) were compared for best selectivity.