Online Coupling of Two-Dimensional Liquid Chromatography and NMR for the Analysis of Complex Polymers

Online coupling of liquid chromatography and two-dimensional diffusion ordered spectroscopy for the analysis of oligostyrenes

The aim of this study was to introduce a powerful coupling of Liquid Adsorption Chromatography (LAC) and Diffusion-Ordered Spectroscopy (DOSY) for comprehensive structure analysis. This new hyphenation approach facilitated the simultaneous separation of a polymer mixture and the determination of molar masses within a single 3D experiment. The online coupling of High-Performance Liquid Chromatography (HPLC) and two-dimensional DOSY-NMR will be called 3D-LAC-NMR-DOSY experiment. Our methodology enabled the chromatographic separation of analytes based on their chemical heterogeneity, and provided accurate molar masses of the analytes through 2D-DOSY. This new method was demonstrated on a polystyrene oligomer mixture. In this case, the oligostyrenes could be separated with LAC according to their tacticity and chain length in protonated acetonitrile as eluent and DOSY measurements provided the molar masses of each oligomer. In order to show the power of the 3D-LAC-NMR-DOSY method, the comparison to 2D-DOSY, 3D-DOSY and LAC-NMR was separately evaluated. Furthermore, the recently published solvent-independent molar mass calibration of diffusion coefficients was also successfully applied in our LAC-DOSY studies for molar mass predictions of the oligomers in acetonitrile. The predicted molar masses were in good agreement with the LAC-DOSY measurements and were verified by calibrations of diffusion coefficients and mass spectrometry. Finally, this pioneering 3D technique offers a powerful new tool for advancing structure analysis and enhancing our understanding of complex systems such as oligostyrenes.

Critical conditions for liquid chromatography of statistical polyolefins: Evaluation of diene distribution in EPDM terpolymers

Liquid chromatography at critical conditions is of interest as it may unravel molecular information on macromolecular structures not accessible by any other analytical techniques. Yet so far, such conditions have never been experimentally established for copolymers, where a particular need for such information exists. Toward this goal, critical conditions for statistical ethylene propylene copolymers were identified. In the first approach the composition of the binary mobile phase was varied at a constant temperature, and secondly by modulating the adsorption-desorption temperature at constant mobile phase composition. Solvents for both methods were identified by using a novel approach that combines structure retention relationships with Hansen Solubility Parameters. As a result, for the first time, the heterogeneity of an ethylene propylene diene terpolymer sample with regard to the pendant double bond of the diene could be determined. The novel chromatographic approach was validated by measuring the composition of fractions taken over the chromatographic run offline by nuclear magnetic resonance. In summary, this work gave the first experimental evidence for the existence of critical conditions for polyolefin random copolymers, as postulated by Brun. This novel chromatographic approach holds immense potential to engineer complex polymers towards future applications by making use of the now-accessible molecular information.

Deuterated Modifiers in Sub/Supercritical Fluid Chromatography for Streamlined NMR Structure Elucidation

Isolation and chemical characterization of target components in fast-paced pharmaceutical laboratories can often be challenging, especially when dealing with mixtures of closely related, possibly unstable species. Traditionally, this process involves intense labor and manual intervention including chromatographic method development and optimization, fraction collection, and drying processes prior to NMR analyses for unambiguous structure elucidation. To circumvent these challenges, a foundational framework for the proper utilization of supercritical carbon dioxide (scCO₂) and deuterated modifiers (CD₃OD) in sub/supercritical fluid chromatography (SFC) is herein introduced. This facilitates a streamlined multicomponent isolation with minimized protic residues, further enabling immediate NMR analysis. In addition to bypassing tedious drying processes and minimizing analyte degradation, this approach (complementary to traditional reversed-phase liquid chromatography, RPLC) delivers highly efficient separations and automated fraction collection using readily available analytical/midscale SFC instrumentation. A series of diverse analytes across a wide spectrum of chemical properties (acid, basic, and neutral), combined with different stationary-phase columns in SFC are investigated using both a protic organic modifier (CH₃OH) and its deuterated counterpart (CD₃OD). The power of this framework is demonstrated with pharmaceutically relevant applications in the context of target characterization and analysis of complex multicomponent reaction mixtures from modern synthetic chemistry, demonstrating high isolation yields while reducing both the environmental footprint and manual intervention. This workflow enables unambiguous fast-paced structure elucidation on the analytical scale, providing results that are comparable to traditional, but time-consuming, RPLC purification approaches.

Trapping-Enrichment Multi-dimensional Liquid Chromatography with On-Line Deuterated Solvent Exchange for Streamlined Structure Elucidation at the Microgram Scale

At the forefront of chemistry and biology research, development timelines are fast-paced and large quantities of pure targets are rarely available. Herein, we introduce a new framework, which is built upon an automated, online trapping-enrichment multi-dimensional liquid chromatography platform (TE-Dt-mDLC) that enables: 1) highly efficient separation of complex mixtures in a first dimension (¹ D-UV); 2) automated peak trapping-enrichment and buffer removal achieved through a sequence of H₂ O and D₂ O washes using an independent pump setup; and 3) a second dimension separation (² D-UV-MS) with fully deuterated mobile phases and fraction collection to minimize protic residues for immediate NMR analysis while bypassing tedious drying processes and minimizing analyte degradation. Diverse examples of target isolation and characterization from organic synthesis and natural product chemistry laboratories are illustrated, demonstrating recoveries above 90 % using as little as a few micrograms of material.

Development of a chemically sensitive online SEC detector based on FTIR spectroscopy

A new FTIR–SEC coupling method provides correlated information about the molecular weight distribution and the chemical composition and has the potential to be applied as a standard SEC detector.

Investigations of multiple detection of polymers

Theoretical and experimental aspects of multiple detection analysis of polymers are critically revised for size exclusion chromatography (SEC). In particular, different combinations of detectors, the importance of the injected mass and the influence of the tacticity of polymers are evaluated in respect to the accuracy of the weight fractions of the polymer components. It is also shown how overlapping detector responses of the chemical components will affect the accuracy of the weight fractions. The calculation of the weight fractions is performed with equations derived for n different chemical components and detectors by using the slopes and intercepts of the linear response equations. Several detector combinations of dual detection in SEC are evaluated with PS-b-PMMA diblock copolymers to determine the comonomer compositions and for the first time different combinations of triple detections are performed for the determination of the weight fractions of a blend of three homopolymers, respectively. The correct determination of the weight fractions of minor and main components of polymers is strongly affected by the chosen combination of detectors, the injected mass and the intercept of the response calibrations. It is shown how these conditions have to be changed to obtain correct quantifications of the weight fractions according to the experimental setups. The experimental results are approved with online SEC-(1)H/NMR where no response factors are required.

Toward absolute chemical composition distribution measurement of polyolefins by high-temperature liquid chromatography hyphenated with infrared absorbance and light scattering detectors

Chemical composition distribution (CCD) is a fundamental metric for representing molecular structures of copolymers in addition to molecular weight distribution (MWD). Solvent gradient interaction chromatography (SGIC) is commonly used to separate copolymers by chemical composition in order to obtain CCD. The separation of polymer in SGIC is, however, not only affected by chemical composition but also by molecular weight and architecture. The ability to measure composition and MW simultaneously after separation would be beneficial for understanding the impact of different factors and deriving true CCD. In this study, comprehensive two-dimensional chromatography (2D) was coupled with infrared absorbance (IR5) and light scattering (LS) detectors for characterization of ethylene-propylene copolymers. Polymers were first separated by SGIC as the first dimension chromatography (D1). The separated fractions were then characterized by the second dimension (D2) size exclusion chromatography (SEC) with IR5 and LS detectors. The concentrations and compositions of the separated fractions were measured online using the IR5 detector. The MWs of the fractions were measured by the ratio of LS to IR5 signals. A metric was derived from online concentration and composition data to represent CCD breadth. The metric was shown to be independent of separation gradients for an "absolute" measurement of CCD breadth. By combining online composition and MW data, the relationship of MW as a function of chemical composition was obtained. This relationship was qualitatively consistent with the results by SEC coupled to IR5, which measures chemical composition as a function of logMW. The simultaneous measurements of composition and MW give the opportunity to study the SGIC separation mechanism and derive chain architectural characteristics of polymer chains.