Analytical Instrument Documents

Analysis of complex mixtures with gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC-HRTOFMS) can produce a large amount of data. A new software program was recently reported that integrates all of the available mass spectrometric information from GC-HRTOFMS analysis into a concise report. New capabilities have now been added to the software to incorporate retention index data and to identify differences between two samples.

Solid-state NMR is a valuable tool for elucidating the structures and dynamics of materials at an atomic level. Proton multiple-quantum (MQ) /single-quantum (SQ) correlation NMR spectroscopy is widely used to probe spatial proximity among protons. In the triple-quantum (TQ)/SQ correlation experiment, the excitation of triple-quantum (TQ) coherences is traditionally achieved by a 90° pulse in conjugation with double-quantum (DQ) recoupling sequences. Nevertheless, such sequences often suffer from low TQ filtering efficiency and may lead to overlapping spinning sidebands in the indirect TQ dimension, especially at a slow MAS frequency. Herein, we design several supercycled symmetry-based RNnν γ-free TQ recoupling sequences and compare their performance via extensive numerical simulation and experiments. Experimental results further confirm that pulse sequence gives the highest TQ filtering efficiency of around 20% in the slow MAS regime (∼10 kHz). The 2D TQ/SQ spectrum at slow MAS is completely free of spinning sidebands in the TQ dimension due to its γ-free nature. We establish that such a γ-free pulse sequence is a superior candidate for TQ spectroscopy at slow MAS frequency.

Herein, a novel non-planar 2D COF with a stair-stepped structure was constructed from a Z-shaped building block for the first time. Compared with its similar planar COF, the unique stair-stepped non-planar COF possesses larger surface area and stronger fluorescence, which was further applied for specific explosive detection through a fluorescence quenching mechanism. This work not only extends the traditional planar 2D COF structures to unique non-planar structures based on the bottom-up design principle, but also expands the potential applications of COF materials.

Diffusion-ordered NMR spectroscopy (DOSY) constructs multidimensional spectra displaying signal strength as a function of Larmor frequency and of diffusion coefficient from experimental measurements using pulsed field gradient spin or stimulated echoes. Peak positions in the diffusion domain are determined by diffusion coefficients estimated by fitting experimental data to some variant of the Stejskal–Tanner equation, with the peak widths determined by the standard error estimated in the fitting process. The accuracy and reliability of the diffusion domain in DOSY spectra are therefore determined by the uncertainties in the experimental data and thus in part by the signal-to-noise ratio of the experimental spectra measured. Here the Cramér–Rao lower bound, Monte Carlo methods, and experimental data are used to investigate the relationship between signal-to-noise ratio, experimental parameters, and diffusion domain accuracy in 2D DOSY experiments. Experimental results confirm that sources of error other than noise put an upper limit on the improvement in diffusion domain accuracy obtainable by time averaging.

The conversion of CO2 into functional materials under ambient conditions is a major challenge to realize a carbon-neutral society. Metal–organic frameworks (MOFs) have been extensively studied as designable porous materials. Despite the fact that CO2 is an attractive renewable resource, the synthesis of MOFs from CO2 remains unexplored. Chemical inertness of CO2 has hampered its conversion into typical MOF linkers such as carboxylates without high energy reactants and/or harsh conditions. Here, we present a one-pot conversion of CO2 into highly porous crystalline MOFs at ambient temperature and pressure. Cubic [Zn4O(piperazine dicarbamate)3] is synthesized via in situ formation of bridging dicarbamate linkers from piperazines and CO2 and shows high surface areas (∼2366 m2 g–1) and CO2 contents (>30 wt %). Whereas the dicarbamate linkers are thermodynamically unstable by themselves and readily release CO2, the formation of an extended coordination network in the MOF lattices stabilizes the linker enough to demonstrate stable permanent porosity.

Structural analysis by NMR can provide not only a planar molecular structure but also three-dimensional structural information. In this Note, we describe a method for obtaining information on dihedral angles by using 1H-1H coupling constants (JHH values). For example, hydrogen atoms attached to a cyclohexane ring are either located in axial or equatorial positions in respect to the cyclohexane ring (Fig. 1). The dihedral angles between vicinal protons are known to be ∠Hax-C-C-Hax ≈ 180°, ∠Hax-C-Heq ≈ 60°, and ∠Heq-C-C-Heq ≈ 60°. If we look at the Karplus curve shown in Fig. 2, we can see that 3JHH of around 4 Hz can be expected in the case of the dihedral angle of 60°, while 3JHH of around 13 Hz corresponds to the dihedral angle of 180°. In reality, 3JHH values depend on substituents attached to the cyclohexane ring in substituted cyclohexanes, so the analysis is not straightforward, but the basic trend of having a larger J-value for a 180° dihedral angle compared to a 60° dihedral angle remains unchanged. Therefore, from the value of 3JHH of the methylene protons, it is possible to differentiate between the dihedral angle of 60° or 180°.

The new generation diffusion probe is specially designed for diffusion applications that requires a large magnetic field gradient. By improving the design around the coil, the recovery time after field gradient pulse has been significantly shortened compared to the conventional model. Using a newly developed 50A bipolar magnetic field gradient power supply, a magnetic field gradient of 20 T/m (2000 G/cm) can be applied, making it possible to measure diffusion coefficients on the order of 10-14 m2/s. This system is ideal for measuring the diffusion of ions in solid electrolytes.

Catalyzed light olefin oligomerization is widely used in petrochemical industries to produce fuels and chemicals. Light olefins such as propene and butenes are commonly selected as feedstocks. Solid phosphoric acid (SPA) and zeolite are representative acidic catalysts. Both the feedstocks and catalysts have an impact on the product composition. In this study, state-of-the-art instrumentation two-dimensional gas chromatography (GC × GC) coupled photoionization─time of flight mass spectrometry was employed to investigate the composition of dodecene products produced from olefin oligomerization. Information such as the olefin congener distribution, dodecene structural subgroup distribution, and individual dodecene isomers was obtained and utilized in the statistical analyses. By using specific data sets of the product composition, the distinguishment between SPA and zeolite catalysts as well as among the feedstocks was achieved by applying the unsupervised screening approaches (principal component analysis and hierarchical clustering analysis). The potential indicators of catalysts and feedstocks were selected by the feature selection methods (univariate analysis: analysis of variance and multivariate analysis: partial least squares-discriminant analysis).

Modification of the recently reported 19F-detected 1,1-ADEQUATE experiment that incorporates dual-optimization to selectively invert a wide range of 1JCC correlations in a 1,n-ADEQUATE experiment is reported. Parameters for the dual-optimization segment of the pulse sequence were modified to accommodate the increased size of 1JCC homonuclear coupling constants of poly- and perfluorinated molecules relative to protonated molecules to allow broadband inversion of the 1JCC correlations. The observation and utility of isotope shifts are reported for the first time for 1,1- and 1,n-ADEQUATE correlations.

Solid-state NMR is a useful tool to elucidate protein structures. Although fully 13C and 15N labelled samples are required, solid-state NMR provides complementary or unique information that is not accessible by other typical methods. For example, there are, in principle, no upper or lower limits to molecular weight in solid-state NMR, unlike solution NMR and cryo EM. In addition, solid-state NMR is a very sensitive probe of local dynamics. One of the stumbling block of solid-state NMR is sample amount. Indeed, 20 – 50 mg of fully labelled (expensive!) samples are required for 3.2 mm / 4 mm MAS rotors. However, the recent developments allowing MAS rates above 70 kHz change the situation completely. [1] Indeed, the 0.75 mm (1 mm) MAS rotors used for fast MAS experiments (up to 120 (80) kHz) only require 290 (800) nL of sample, enabling solid-state NMR measurements on sub-milligram amounts of sample. Another great advantage of fast MAS is the averaging out of most of the anisotropic interactions, allowing the use of low power schemes (decoupling, recoupling, cross-polarization etc.), which in turns avoids sample heating. High resolution 1H spectra become readily available in these conditions, especially for microcrystalline proteins. This is the biggest advantage of fast MAS, since 1H detection dramatically improves sensitivity compared to traditional 13C detection. As a consequence, fast MAS solid-state NMR is now a versatile tool to study the structure and dynamics of biomolecules, for which the first step is sequence-specific resonance assignment. To this end, requisite experiments for sequential backbone and sidechain assignments are well established. [2] Here, we introduce the tool box available for the Delta software together with examples on fully protonated Het-s (226-280) protein.