Direct separation and detection of biogenic amines by ion-pair liquid chromatography with chemiluminescent nitrogen detector

Analysis of biogenic amines is critical to pharmaceutical and food industry due to their biological importance. For many years, the determination of biogenic amines has relied on high performance liquid chromatography (HPLC) coupling with pre-, on-, or post-column derivatization procedures to enable UV or fluorescent detections. In this study, 14 biogenic amines were separated on a Phenomenex Luna Phenyl-Hexyl column by an ion-pair liquid chromatography method using perfluorocarboxylic acids as ion-pair reagents and detected by a chemiluminescent nitrogen detector (CLND). This direct separation and detection HPLC method eliminated the time consuming and cumbersome derivatization procedures. Compared with HPLC-UV (post-column derivatization with ninhydrin) and HPLC-charged aerosol detector (CAD) methods, this HPLC-CLND technique provided narrower peaks, better baselines, and improved separations and detections. Excellent linearity was acquired by CLND for each of the 14 biogenic amines ranging from less than 1 ng to about 1000 ng (on-column weights). The relative response factors determined by this LC-CLND method were proportional to the numbers of nitrogen atoms in each compound, which has been the characteristic of the equimolar determinations by CLND. In addition, a number of samples including beer, dairy beverage, herb tea, and vinegar were analyzed by the LC-CLND method with satisfactory precision and accuracy.     

Thermodynamic and kinetic factors in the hydrothermal synthesis of hybrid frameworks: zinc 4-cyclohexene-1,2-dicarboxylates

Experimental and computational studies indicate that the formation of a series of zinc 4-cyclohexene-1,2-dicarboxylates takes place under thermodynamic rather than kinetic control.     

Pressure‐Induced Bond Rearrangement and Reversible Phase Transformation in a Metal–Organic Framework

Pressure‐induced phase transformations (PIPTs) occur in a wide range of materials. In general, the bonding characteristics, before and after the PIPT, remain invariant in most materials, and the bond rearrangement is usually irreversible due to the strain induced under pressure. A reversible PIPT associated with a substantial bond rearrangement has been found in a metal–organic framework material, namely [tmenH₂][Er(HCOO)₄]₂ (tmenH₂²⁺=N,N,N′,N′‐tetramethylethylenediammonium). The transition is first‐order and is accompanied by a unit cell volume change of about 10 %. High‐pressure single‐crystal X‐ray diffraction studies reveal the complex bond rearrangement through the transition. The reversible nature of the transition is confirmed by means of independent nanoindentation measurements on single crystals.     

In Situ Observation of Successive Crystallizations and Metastable Intermediates in the Formation of Metal–Organic Frameworks

Understanding the driving forces controlling crystallization is essential for the efficient synthesis and design of new materials, particularly metal–organic frameworks (MOFs), where mild solvothermal synthesis often allows access to various phases from the same reagents. Using high‐energy in situ synchrotron X‐ray powder diffraction, we monitor the crystallization of lithium tartrate MOFs, observing the successive crystallization and dissolution of three competing phases in one reaction. By determining rate constants and activation energies, we fully quantify the reaction energy landscape, gaining important predictive power for the choice of reaction conditions. Different reaction rates are explained by the structural relationships between the products and the reactants; larger changes in conformation result in higher activation energies. The methods we demonstrate can easily be applied to other materials, opening the door to a greater understanding of crystallization in general.     

Synchrotron X-ray and Neutron Diffraction Studies in Solid-State Chemistry

Neutron diffraction studies, especially with powders, play an important role in structural solid-state chemistry, making possible the precise determination of the location of light atoms, particularly hydrogen, and enabling a distinction to be made between certain neighboring elements in the periodic table that are difficult to distinguish in experiments with X-rays. Neutron diffraction investigations also make a unique contribution in the area of magnetic structure determination. The availability of intense synchrotron X-rays sources, however, is opening up new opportunities to the structural chemist, many of them complementary to the “traditional” strengths of neutron methods. The key features of synchrotron radiation in relation to structural studies are the wavelength tunability, which facilitates the use of resonant diffraction methods, and the high brightness and excellent vertical collimation of the source, which make possible the construction of diffractometers with unparalleled angular and spatial resolution. The following types of experiments are now possible with synchrotron X-ray diffraction: (1) The ab initio determination of structures from powder diffraction data. (2) The differentiation between different oxidation states of an element (valence contrast experiments) based upon the sensitivity of an absorption edge to the valence of the element in question. (3) The differentiation of elements adjacent to each other in the periodic table, which is now feasible with synchrotron X-rays for all elements beyond chromium. (4) Site-selective X-ray absorption spectroscopy. (5) The study of cation occupancies in materials where more than one element occupies a site that is, or may be, partially occupied. (Such problems are important in zeolite chemistry and high-temperature superconductors.) (6) The determination of crystal structures from microcrystals. (7) In situ and rapid, time-resolved diffraction studies. This review examines the roles played by X-ray and neutron diffraction studies in modern solid-state chemistry, and describes some recent examples in which the use of neutron radiation or synchrotron X-rays has been advantageous.