Improved Separation Capability Research Articles

Enhanced Visible-Light Response of Ultrathin Er-Doped MoS2 Nanosheets through a Wet-Chemical Method

Doping is one of the important technologies for modifying the properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). Herein, ultrathin Er-doped MoS2 nanosheets with an enhanced visible-light response have been synthesized by a wet-chemical method. The few-layer characteristic of the MoS2:Er nanosheets is beneficial from the H2S generated during the preparation process. Near-infrared emission at ∼1530 nm from MoS2:Er nanosheets has been observed under 980 nm laser excitation, which corresponds to the energy transition from 4I13/2 to 4I15/2 of Er3+. Meanwhile, the enhanced visible-light response of MoS2:Er has been demonstrated by the photocatalytic performance (removing rhodamine B) and photocurrent response compared with undoped MoS2. Based on the experiments and density functional theory calculation, the enhanced visible-light response is attributed to the improved separation capability of photogenerated electron–hole pairs of the ultrathin MoS2:Er nanosheets. Er-doping induces an increase of S vacancies and the energy transfer between Er3+ ions and the host. The work demonstrates a developed universal and recyclable synthesis strategy for various metal-doped MoS2 nanosheets and their applications in the fields of photocatalysis and photodetection.

Using multiple heart-cutting valves and stop-flow operation to enable variable second dimension gradient times in online comprehensive two-dimensional liquid chromatography

Online comprehensive two-dimensional liquid chromatography (LC×LC) is the preferred method currently for the separation of highly complex mixtures of non-volatile compounds. With fully automated commercial instrumentation and software making the method appealing to both researchers and industry, the demand for systems with improved separation capabilities for highly complex mixtures such as those found in natural products or proteomics has increased. In this study we report an approach that enables variable second dimension analysis times based on the use of multiple heart-cutting valves and stop-flow operation to circumvent the requirement for very fast second dimension (2D) analyses in online LC×LC. As application, the HILIC×RP-LC analysis of condensed tannins (proanthocyanidins) in cocoa, grape seed and quebracho extracts is used to demonstrate the performance on the proposed methodology. The method offers increased flexibility compared to conventional online LC×LC separations in that longer 2D gradients can be used to accommodate more complex portions of the chromatogram, while shorter 2D gradients can be used in sections containing fewer peaks, while largely maintaining the benefits of comprehensive separation. We present an evaluation of the performance of the variable gradient time stop-flow HILIC×RP-LC method compared to a comparable, conventional online HILIC×RP-LC system in terms of practical peak capacities using established 2D-LC theory. The improved separation of especially low-level intermediate molecular weight proanthocyanidin oligomers by the former method demonstrates the benefits of the developed approach.

Heart-cut nano-LC-CZE-MS for the characterization of proteins on the intact level.

Multidimensional separation techniques play an increasingly important role in separation science, especially for the analysis of complex samples such as proteins. The combination of reversed-phase liquid chromatography in the nanoscale and CZE is especially beneficial due to their nearly orthogonal separation mechanism and well-suited geometries/dimensions. Here, a heart-cut nano-LC-CZE-MS setup was developed utilizing for the first time a mechanical 4-port valve as LC-CE interface. A model protein mixture containing four different protein species was first separated by nano LC followed by a heart-cut transfer of individual LC peaks and subsequent CZE-MS analysis. In the CZE dimension, various glycoforms of one protein species were separated. Improved separation capabilities were achieved compared to the 1D methods, which was exemplarily shown for ribonuclease B and its different glycosylated forms. LODs in the lower μg/mL range were determined, which are considerably lower compared to traditional CZE-MS. In addition, this study represents the first application of an LC-CE-MS system for intact protein analysis. The nano-LC-CZE-MS system is expected to be applicable to various other analytical challenges.

Miniature 3D Gas Chromatography Columns with Integrated Fluidic Connectors Using High-resolution Stereolithography Fabrication

In this manuscript we report on design, fabrication, and characterization of miniature gas chromatography columns with arbitrary 3D geometry and integrated connectors as separation elements for a mobile ethylene sensor system. In contrast to planar microfabrication, these columns have been realized in a single step additive manufacturing process using high-resolution stereolithography printing. This maskless technology enables the combination of freeform 3D microstructures with packaging aspects in one bulk element. For applications in food monitoring, the columns were packed with Carbosieve®-SII particles as adsorbents. With the printed columns a minimal gas concentration of 35 ppb (3σ) ethylene can be detected. Retention times of 400 s for ethylene and 1035 s for water and no signal peak overlap show an improved separation capability.

Retention behaviors of natural products in reversed‐phase liquid chromatography using mobile phase comprising methanol, acetonitrile and water

The three-component mobile phase, methanol-acetonitrile-water, has been frequently used for the separation of natural products in complicated herbal extracts by reversed-phase liquid chromatography, especially when two-component solvent systems do not work. However, the rational for optimization of this three-solvent system is not clear, so far. In this study, the retention behavior of different types of natural products in RP-LC with a methanol-acetonitrile-water elution system was studied. A total of 27 compounds from four classes, including steroids, flavonoids, phenolic acids, and triterpene saponins were analyzed by high-performance liquid chromatography coupled with mass spectrometry or ultraviolet detectors. It was interesting to find that prolonged retention times and improved separation capabilities were obtained when the acetonitrile-to-methanol ratio was 3-16%. This rule applied to all four classes of natural products under different instrumental conditions, and could be valuable for the separation of natural products in complicated samples.

Theory for Inverse Pulsing of The Shutter Grid in Ion Mobility Spectrometry

The fundamental transport theory for ion mobility spectrometry is modified to include effects of space charge. The new theory is then applied to describing the performance of "inverse ion mobility spectrometry" recently reported in Tabrizchi, M.; Jazan, E. Anal. Chem. 2010, 82, 746-750 using a discharge ionization source. The improved separation capabilities arise from space charge repulsion of the greater number of ions that are introduced into the drift tube by the technique. A larger effective diffusion coefficient and additional displacement velocities for the leading and trailing edges of the ion mobility peak account for the results. Performance is compared to conventional linear ion mobility spectrometry, with and without a radioactive source for ionization.

Comparison of CZE, open‐tubular CEC and non‐aqueous CE coupled to electrospray MS for impurity profiling of drugs

Open-tubular CEC and non-aqueous CE (NACE) methods were developed for the analysis of six pharmaceutical compounds and their respective process-related impurities, comprising 22 analytes in total with a range of functional groups and lipophilicities. These methods were assessed for orthogonality of analyte separation with respect to existing CZE-ESI-MS and HPLC-ESI-MS methods, in order to complement a generic analytical strategy for impurity profiling of pharmaceutical compounds. Open-tubular CEC, using etched and chemically modified capillaries, induced weak reversed-phase-type interactions between some of the analytes and the bonded phases (0.811<k(app)<0.996). However, the separations were primarily influenced by electrophoretic mobility rather than chromatographic retention, and hence no significant change in selectivity compared with CZE was observed. NACE optimum separating conditions were 10 mM ammonium acetate-100 mM acetic acid in methanol far UV acetonitrile (1/1 v/v ratio). The ion-pair reagents triethylamine or dimethylhexylamine did not induce further changes in selectivity, but tridecafluoroheptanoic acid significantly modified the electrophoretic mobility of bases. The results indicate that one of three generic CZE methods previously reported should be replaced by NACE, due to its improved separation capabilities. The NACE-ESI-MS method complements the two CZE-ESI-MS and the four HPLC-ESI-MS methods recommended in a previous publication; these together form the basis of a generic approach to impurity profiling of pharmaceutical compounds.

An analytic technique to separate cochannel FM signals

A new technique is presented to separate two cochannel frequency modulation signals. Two candidate solutions for the phases are analytically derived, and a sequence of phase solutions is chosen to match the expected power spectral density of each constituent signal. This is accomplished with a one-step linear predictor and a simple two-state Viterbi algorithm. Simulations on recorded radio frequency data indicate improved separation capability over other techniques such as the joint Viterbi algorithm and cross-coupled phase-locked loop.