Large volume injection techniques in capillary gas chromatography

Large volume injection (LVI) is a prerequisite of modern gas chromatographic (GC) analysis, especially when trace sample components have to be determined at very low concentration levels. Injection of larger than usual sample volumes increases sensitivity and/or reduces (or even eliminates) the need for extract concentration steps. Also, an LVI technique can serve as an interface for on-line connection of GC with a sample preparation step or with liquid chromatography. This article reviews the currently available LVI techniques, including basic approaches to their optimization and important real-world applications. The most common LVI methods are on-column and programmed temperature vaporization (PTV) in solvent split mode. Newer techniques discussed in this article include direct sample introduction (DSI), splitless overflow, at-column, and "through oven transfer adsorption desorption" (TOTAD).     

GC/MS analysis of three Amaryllidaceae species and their cholinesterase activity

Alzheimer's disease (AD), characterized by the death of nerve cells in the cerebral cortex, is the most common subtype of dementia. Despite the exponential growth in the number of AD patients, acetylcholinesterase (AChE) inhibitors are currently used to treat AD. Plants of the Amaryllidaceae family are known to synthesize a particular type of bioactive compounds, named Amaryllidaceae alkaloids, which have shown AChE inhibitory activity. Alkaloid extracts of three species of Amaryllidaceae were studied with respect to their acetylcholinesterase and butyrylcholinesterase inhibitory activity and alkaloid patterns. Eleven alkaloids were identified by GC/MS. Significant cholinesterase inhibitory activity was demonstrated by the alkaloid extract of N. undulata (IC50,(HuAChE) = 14.3 +/- 1.2 microg/mL; IC50,(HuBuChE) = 33.9 +/- 1.9 microg/mL).     

Conformationally restricted calix[8]arenes substituted at all methylene bridges

Reaction under S(N)1 conditions of the octabromocalix[8]arene derivative 2d with alcohols proceeds in nonstereoselective fashion, but all-cis octaryl derivatives are the single major products in the reaction with arenes. The incorporation of aryl substituents at the bridges rigidifies the calix[8]arene skeleton.     

Ruggedness and other performance characteristics of low-pressure gas chromatography-mass spectrometry for the fast analysis of multiple pesticide residues in food crops

Low-pressure gas chromatography-mass spectrometry (LP-GC-MS) using a quadrupole MS instrument was further optimized and evaluated for the fast analysis of multiple pesticide residues in food crops. Performance of two different LP-GC-MS column configurations was compared in various experiments, including ruggedness tests with repeated injections of pesticides in matrix extracts. The tested column configurations employed the same 3 m x 0.15 mm i.d. restriction capillary at the inlet end, but different analytical columns attached to the vacuum: (A) a 10 m x 0.53 mm i.d., 1 microm film thickness RTX-5 Sil MS column; and (B) a 10 m x 0.25 mm i.d., 0.25 microm film thickness DB-5MS column. Under the optimized conditions (compromise between speed and sensitivity), the narrower analytical column with a thinner film provided slightly (<1.1-fold) faster analysis of <5.5 min separation times and somewhat greater separation efficiency. However, lower detection limits for most of the tested pesticides in real extracts were achieved using the mega-bore configuration, which also provided significantly greater ruggedness of the analysis (long-term repeatability of analyte peak intensities, shapes, and retention times). Additionally, the effect of the increasing injection volume (1-5 microl) on analyte signal-to-noise ratios was evaluated. For the majority of the tested analyte-matrix combinations, the increase in sensitivity caused by a larger injection did not translate in the same gain in analyte detectability. Considering the costs and benefits, the injection volume of 2-3 microl was optimal for detectability of the majority of 57 selected pesticides in apple, carrot, lettuce, and wheat extracts.     

Chromatographic methods for the separation of biocompatible iron chelators from their synthetic precursors and iron chelates

Chromatographic methods have been developed for the separation of the three novel biocompatible iron chelators pyridoxal isonicotinoyl hydrazone (PIH), salicylaldehyde isonicotinoyl hydrazone (SIH), and pyridoxal 2-chlorobenzoyl hydrazone (o-108) from their synthetic precursors and iron chelates. The chromatographic analyses were achieved using analytical columns packed with 5 microm Nucleosil 120-5 C18. For the evaluation of all chelators in the presence of the synthetic precursors, EDTA was added to the mobile phase at a concentration of 2 mM. The best separation of PIH and its synthetic precursors was achieved using a mixture of phosphate buffer (0.01 M NaH2PO4, 5 mM 1-heptanesulfonic acid sodium salt; pH 3.0) and methanol (55:45, v/v). For separation of SIH and its synthetic precursors, the mobile phase was composed of 0.01 M phosphate buffer (pH 6.0) and methanol (60:40, v/v). o-108 was analyzed employing a mixture of 0.01 M phosphate buffer (pH 7.0), methanol, and acetonitrile (60:20:20, v/v/v). These mobile phases were slightly modified to separate each chelator from its iron chelate. Furthermore, a RP-TLC method has also been developed for fast separation of all compounds. The chromatographic methods described herein could be applied in the evaluation of purity and stability of these drug candidates.     

Desilylation of t-butyldimethylsilyl ethers of hydroxyquinones

Desilylation of a series of hydrolytically stable hydroxyquinone t-butyldimethylsilyl ethers was achieved in high yields by the use of potassium fluoride in the presence of catalytic amounts of aqueous 48% hydrobromic acid (HBr), or basic aluminium oxide (Al₂O₃), and tetra-n-butylammonium fluoride (Bu₄NF) as cleaving agents. Copyright © 1998 John Wiley & Sons, Ltd.     

Novel Substituted Purine Isosteres: Synthesis,Structure-Activity Relationships and Cytotoxic Activity Evaluation

A number of pyrrolo[2,3-c]pyridines, pyrrolo[3,2-d]pyrimidines and pyrazolo[4,3-d]pyrimidines were designed and synthesized as antiproliferative agents. The target compounds possessed selected substituents in analogous positions on the central scaffold that allowed the extraction of interesting SARs. The cytotoxic activity of the new derivatives was evaluated against prostatic (PC-3) and colon (HCT116) cell lines, and the most potent analogues showed IC₅₀ values in the nM to low µM range, while they were found to be non-toxic against normal human fibroblasts (WI-38). Flow cytometric analysis of DNA content revealed that the most promising derivative 14b caused a statistically significant accumulation of PC-3 cells at G₂/M phase and induced apoptosis in PC-3 cells.     

Synthetic,structural and spectroscopic studies of copper(II) complexes derived from the combination of the succinamate(−1) ligand with aromatic N,N′-chelates

The use of succinamic acid (H₂sucm)/N,N′-chelate (2,2′-bipyridine, bpy; 4,4′-dimethyl-2,2′-bipyridine, dmbpy; 1,10-phenanthroline, phen) ‘ligand blends’ in CuX₂·yH₂O (X = NO₃, y = 3; X = Cl, y = 0) chemistry has yielded the new complexes [Cu₂(Hsucm)₃(bpy)₂](NO₃)·0.5MeOH (1·0.5MeOH), [Cu₂(Hsucm)(OH)Cl(bpy)₂](OH)·3.6H₂O (5·3.6H₂O) and [Cu₂(Hsucm)₂Cl₂(phen)₂] (6). The succinamate(−1) ion behaves as a carboxylate ligand and exists in two different coordination modes in the structures of the above complexes, i.e., the common syn, syn μ₂-κO:κO′ in 1, 5 and 6, and the μ₂-κ²O:κO′ in 1. The primary amide group of Hsucm⁻ remains uncoordinated and participates in intermolecular hydrogen bonding interactions leading to 1D, 2D and 3D networks. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the Hsucm⁻ ligands.     

Benzene elimination from reformate gasoline by high pressure hydrogenation in a fixed-bed reactor

The reduction of benzene from benzene-rich real gasoline fractions has been studied in a high-pressure fixed-bed reactor using a Pt/TiO₂ catalyst. It was found that the yield of this process decreases with the toluene content in the feedstock, but it is independent of the kind of the saturated hydrocarbons. This revised version was published online in August 2006 with corrections to the Cover Date.