Medium Resolution ¹H‐NMR at 62 MHz as a New Chemically Sensitive Online Detector for Size‐Exclusion Chromatography (SEC–NMR)

Abstract

A state-of-the-art, medium-resolution H-NMR spectrometer (62 MHz) is used as a chemically sensitive online detector for size-exclusion chromatography of polymers such as polymethylmethacrylate (PMMA) and polystyrene (PS). The method uses protonated eluents and works at typical chromatographic conditions with trace amounts of analytes (<0.5 g L after separation). Strong solvent suppression, e.g., by a factor of 500, is achieved by means of T -filtering and mathematical subtraction methods. Substantial improvements are made with respect to previous work in terms of the sensitivity (signal-to-noise ratio up to 130:1, PMMA OCH ) and selectivity (peak width, full width half maximum (FWHM) 4 Hz on-flow). Typical homopolymers and a blend are investigated to deformulate their composition along the dimensions of molecular weight and NMR chemical shift. These results validate this new hyphenated chromatography method, which can greatly facilitate analysis and is much more effective than previously published results.

A New Quantum Cascade IR‐Laser Online Detector: Chemical‐Sensitive Size‐Exclusion Chromatography Measurement at Unprecedented Low Levels

“…These are the first results from this unique IR‐spectrometer illustrating its potential to measure, for example, single end groups in a polymer chain, as this rare functionalization is of high importance in case surface absorbance is intended. The strength of this result can be further put into perspective by comparing the measured LODs for SEC‐FTIR and SEC‐MR‐NMR (measured on polystyrene in CHCl 3 ), which were reported to be 13.4 µg and 110 µg, respectively, and are reproduced in Figure . Therefore, each of the coupled SEC‐IR and SEC‐NMR techniques offers unique advantages where SEC‐FTIR and SEC‐MR‐NMR measure full spectra as a function of the polymer size at lower sensitivities and SEC‐EC‐QCL measures only over limited wavelengths at a substantially higher sensitivity.…”

“…Comparable method development work was done by Lendl and co‐workers where they coupled liquid adsoption chromatography, LAC, to a QCL setup to measure sugars in beverages and later stand‐alone QCL setups were used to measure biological applications such as proteins in aqueous solutions, glucose levels, and multianalyte blood analysis . Analog correlation between chemical composition and molecular weight are currently conducted via SEC‐NMR indicating the needs for the acquisition of correlated information . Previous results showed that the LOD measured for the QCLS‐B was about a factor of four better than the Bruker Vertex 70 FTIR, even though a liquid nitrogen cooled MCT detector is installed on the FTIR Vertex spectrometer and a far less sensitive (balanced) LiTaO 3 detectors was used in the first prototype QCLS‐B.…”

“…A 30 s boxcar filter was applied to the data. For comparison, published LOD results on our coupled SEC‐FTIR and SEC‐MR‐NMR are shown to illustrate the sensitivity limits of each coupled technique. The LOD (SNR = 3) and the limit of quantification (LOQ; SNR = 10) are included in the graph to show the instrument capabilities.…”

“…The LOD (SNR = 3) and the limit of quantification (LOQ; SNR = 10) are included in the graph to show the instrument capabilities. Data for the SEC‐FTIR and SEC‐NMR are reproduced with permission. A typical trace of the blend for the LOD and LOQ determination is illustrated in Figure S2, Supporting Information.…”

“…A typical trace of the blend for the LOD and LOQ determination is illustrated in Figure S2, Supporting Information. Data for the SEC‐FTIR and SEC‐NMR, are reproduced with permission . Copyright 2018, Wiley‐VCH, respectively.…”