Fourier transformation liquid chromatography: increasing sensitivity by a factor of 50.

Methods that increase sensitivity are a constant topic in research. To increase the sensitivity for high-performance liquid chromatography (HPLC) a continuous injection method with a sinusoidal analyte concentration profile was developed. The sinusoidal analyte concentration profile is obtained by a sinusoidal variation of the relative volume contents between two solvent reservoirs, one containing a pure solvent and the other an analyte stock solution prepared with the same solvent. Discrete Fourier transformation enables the analyte-specific phase angle shift to be calculated from the chromatograms. The proposed method is established and evaluated for size exclusion chromatography (SEC). Accordingly, the phase angle shift is used for molecular weight determination by establishing a molecular weight calibration using different narrowly distributed polystyrene (PS) calibration standards. In a comparison with conventional SEC, the Signal-to-Noise ratio (S/N), normalized to the square root of the time of the evaluated data set, increases by a factor of approximately 50.

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

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.

High performance liquid chromatography with mid-infrared detection based on a broadly tunable quantum cascade laser.

This work introduces a tunable mid-infrared (mid-IR) external cavity quantum cascade laser (EC-QCL) as a new molecular specific detector in liquid chromatography. An EC-QCL with a maximum tunability of 200 cm(-1) (1030-1230 cm(-1)) was coupled to isocratic high performance liquid chromatography (HPLC) for the separation of sugars with a cation exchange column (counter ion: Ca(2+)) and distilled water as the mobile phase. Transmission measurements in a 165 µm thick flow cell allowed for on-line coupling and independent quantification of glucose, fructose and sucrose in the concentration range from 5 mg mL(-1) to 100 mg mL(-1) in several beverages. The results obtained with the EC-QCL detector were found to be in good agreement with those obtained using a differential refractive index detector as a reference. The standard deviation of the method for the linear calibration was better than 5 mg mL(-1) for all sugars and reached a minimum of 1.9 mg mL(-1), while the DRI detector reached a minimum of 1 mg mL(-1). Besides the quantification of sugars for which a calibration was performed, also chromatographic peaks of other components could be identified on the basis of their IR absorption spectra. This includes taurine, ethanol, and sorbitol.

Online coupling of size-exclusion chromatography and IR spectroscopy to correlate molecular weight with chemical composition.

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.