Online Detection of Functional Groups in SEC via Quantum Cascade Laser IR Spectroscopy

Abstract

The development of coupled techniques based on chemically sensitive detectors, such as FTIR or NMR spectrometers, for size exclusion chromatography (SEC) provides sophisticated methods for determining the molecular‐weight‐dependent chemical composition in polymers. However, the detection of rare functionalities such as end groups or branching points presents a challenge, especially for online coupled SEC detection, which is based on low‐concentration chromatography. To address this issue, for the first time, an external cavity quantum cascade laser (EC‐QCL) infrared spectrometer is coupled to an SEC. The system is evaluated using polystyrene/poly(methyl methacrylate) (PS/PMMA) blends. The current limit of detection for the carbonyl (PMMA) stretch vibration at 1730 cm−1 with this technique is 3.5 µg PMMA on a semipreparative column (typical load of 2.5 mg polymer in total). That equals 0.15 mol% of PMMA in the PS/PMMA blend and corresponds to one carbonyl group per 70 kg mol−1 polymer.

On-line SEC-MR-NMR hyphenation: optimization of sensitivity and selectivity on a 62 MHz benchtop NMR spectrometer

“…Among these hyphenated techniques, liquid chromatography coupled to Fourier transform infrared-(FT-IR), quantum cascade laser-(QCL) and NMR-spectroscopy (LC-IR/ QCL/NMR), are versatile and powerful combinations. 1,3,4 High performance liquid chromatography (HPLC) has become a powerful analytical tool for the separation of compounds since its introduction in the 1960s. 5 HPLC consists of three main modes of separation, namely (1) liquid adsorption chromatography (LAC), (2) liquid chromatography at critical conditions (LC-CC) and (3) size exclusion chromatography (SEC).…”

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

“…This allows the measurement of thicker samples. Thereby, the limit of detection (LOD) was reduced significantly for the SEC–QCL–IR coupling . However, such sources operate at a narrow band or can be tuned over a small range of wave numbers, typically 100–200 cm −1 .…”

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) . These measurements were performed in series and allowed for a direct comparison between the two instruments.…”

“…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. The QCLS‐B was based on a single external cavity QCL (EC‐QCL), which has the lowest system noise when operated in single wavelength mode .…”

“…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. The QCLS‐B was based on a single external cavity QCL (EC‐QCL), which has the lowest system noise when operated in single wavelength mode . This highlights the complementary nature of QCL spectrometers and standard FTIR instruments.…”