Online Coupling of Size‐Exclusion Chromatography and IR Spectroscopy to Correlate Molecular Weight with Chemical Composition

Abstract

The determination of molecular weight and correlated chemical composition is of major interest for the advanced analysis of copolymers, blends, or unknown samples. In this work, we present a new way of online coupling IR spectroscopy and SEC to achieve a chemically sensitive, universally applicable SEC detector. Our method overcomes the limitations of existing spectroscopy-SEC combinations. We solved the major problems, like huge intensity of solvent signals (polymer concentration in detector <1 g L(-1) ) and short measuring time (<30 s), by recording the IR spectra with fully optimized sensitivity and by following mathematical solvent suppression. The measuring time for a certain S/N was reduced in several optimization steps by a factor of more than 70 000. The resulting sensitivity allows online coupled IR-SEC measurements.

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

“…SEC–MS couplings were intensively investigated and recently reviewed . For NMR and IR couplings, far fewer literature exists and the reason is most likely twofold . In SEC, the analyte concentrations after separation are very low (<0.5 g L −1 ) and consequently sensitivity is a major issue.…”

“…For the coupling of SEC–FT‐IR, one successful application is in high‐temperature SEC of polyolefins using trichlorobenzene as a solvent, where a convenient spectroscopic window exists. [2, and references therein] In earlier work by our group on FT‐IR, careful optimization of the signal‐to‐noise ratio (SNR) and mathematical solvent signal subtraction allowed for online identification of typical polymers such as polystyrene (PS), polymethylmethacrylate (PMMA), and polyisoprene (PI) at typical chromatographic conditions . Recently, the light source for the on‐flow measurements in the mid‐IR range was changed from a globar to a quantum cascade laser (QCL), which produces a much higher photon flux .…”

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

“…We previously published work on an online setup combining size exclusion chromatography (SEC) and FTIR (SEC‐FTIR) and with a first prototype QCL spectrometer from Bruker Optik GmbH (Ettlingen, Germany, abbreviated as QCLS‐B) used as an online chemically sensitive detector for SEC. The measured limit of detection (LOD) was compared with results from a high‐end research IR spectrometer (Vertex 70 FTIR spectrometer from Bruker Optik GmbH) .…”

“…An auto sampler injected 500 µL of a 5 g L −1 solution (2.5 mg sample) onto a semipreparative 10 µm particle size Linear M SDV column (300 × 20 mm inner diameter) from Polymer Standards Service GmbH (Mainz, Germany). The reader is referred to our previous work for information on the QCLS‐B experimental setup and the online coupled SEC‐FTIR setup . The EC‐QCL spectrometer was designed to house up to eight EC‐QCL sources (four per box, two boxes) in a single instrument using an automated beam switch to access all the sources.…”

“…The flow cell volume depends on the position of the rods and has a minimum volume of ≈275 µL with an IR path length of 3.8 mm. In comparison, the SEC‐FTIR setup could only be realized with a 6‐reflection attenuated total reflection flow cell with an estimated IR path length of about 60 µm . Consequently, the EC‐QCL has a factor of 60 increase in path length.…”