|Chemically Sensitive Online Detectors for SEC – current advances for SEC-MR-NMR and SEC-QCL-IR
|Comprehensive Chromatography - The State of the Art
|Dr Johannes Höpfner
|Prof Manfred Wilhelm
|Karlsruhe Institute of Technology (KIT)
Abstract Information :
Polymers have three important molecular characteristics: the molecular weight distribution (MWD), the chemical composition and the topology. The MWD is usually determined using size exclusion chromatography (SEC). SEC detectors commonly in use, such as refractive index detectors, light scattering or viscometers, do not provide information about the chemistry or topology. This information is normally gained in separate experiments. Especially current spectroscopic methods such as IR and NMR are very powerful in in obtaining detailed insights. However, when it comes to analyzing complex materials like copolymers, blends or unknown samples, the correlated measurement of size and chemical properties is of special interest but tedious with dedicated but separate instruments.
Online coupling of IR or NMR spectroscopy with SEC is a promising approach to gain this correlated information. The inherent challenges of this approach are the low signal-to-noise ratios due to low concentrations after separation on a column as well as the strong solvent signals overlapping regions of interest in the analytes spectrum. Therefore, carefully optimization of the sensitivity and solvent signal reduction are most important.
The first approach is the coupling of a table-top medium resolution (MR) NMR spectrometer to a SEC system. The commercial spectrometer consists of a permanent magnet with a magnetic field strength of 1 T (62 MHz for 1H-NMR). The sensitivity is optimized via different flow probes, chromatographic conditions and signal treatment. The setup is run on non-deuterated solvent and thus the solvent suppression relies on selective pulse sequences and mathematical solvent signal reduction. The best resulting LOD is in the order of 100 g/ml for PMMA and is sufficient for first applications.
In a second approach, our group previously reported on a SEC-FTIR coupling with a limit of detection (LOD) for the carbonyl group in PMMA as low as 30µg. To gain an even higher sensitivity, different infrared light sources are needed. We present results from a SEC coupled with an IR spectrometer using a tunable Quantum Cascade Laser (QCL) light source, which has a higher light intensity, but a limited bandwidth. In this application, the SEC-QCL-IR has the best sensitivity when operated in single wavelength mode. This makes SEC-QCL-IR ideal for investigating specific features of interest, such as an end-groups, functional groups or branching points. Therefore, SEC-QCL-IR measurements complement, but do not replace SEC-FTIR results. The LOD for PMMA could consequently be reduced by a factor of 3.8 to 8µg.
The method development for SEC-QCL-IR and MR-NMR-SEC including the general setup, the sensitivity enhancement, the solvent suppression and the influence of chromatographic conditions will be presented. Examples illustrating the benefits of such hyphenated techniques will be shown as well.