Optimization of artificial neural network for retention modeling in high-performance liquid chromatography

An artificial neural network (ANN) model for the prediction of retention times in high-performance liquid chromatography (HPLC) was developed and optimized. A three-layer feed-forward ANN has been used to model retention behavior of nine phenols as a function of mobile phase composition (methanol-acetic acid mobile phase). The number of hidden layer nodes, number of iteration steps and the number of experimental data points used for training set were optimized. By using a relatively small amount of experimental data (25 experimental data points in the training set), a very accurate prediction of the retention (percentage normalized differences between the predicted and the experimental data less than 0.6%) was obtained. It was shown that the prediction ability of ANN model linearly decreased with the reduction of number of experiments for the training data set. The results obtained demonstrate that ANN offers a straightforward way for retention modeling in isocratic HPLC separation of a complex mixture of compounds widely different in pK_a and log K_ow values.     

Effect of ammonia carrier gas on the response of the flame ionization detector

The effect of using ammonia as a carrier gas on the response of the flame ionization detector (FID) has been investigated. It was found that the FID response, calculated as the effective carbon number (ECN), increased for all the compounds studied when ammonia, rather than helium, was used. The change was 0–0. 9 carbon atom for hydrocarbons, one carbon atom for alcohols and diphenyl ether, and 0.4–1 carbon atom for phenols and ketones. The increase in ECN was larger for amines (0. 8–5 carbon atoms), but these numbers also reflected an improvement in chromatographic performance as a result of reduced adsorption on the column. The largest change in signal-to-noise ratio, a six-fold increase, was obtained for octyl-amine; ratios for hexyl methyl ketone, diisobutyl ketone, dihexyl-amine, dibutylamine, and N-methyloctylamine increased by a factor of 2–3 when ammonia was used as carrier gas. To determine the extent to which the effect on detector response was solely attributable to ammonia, a mixture of 5 % ammonia in nitrogen was used as detector make-up gas with helium as carrier gas. Under these conditions the noise in the FID increased but for most of the compounds studied the signal-to-noise ratio also increased.     

Silica triflate as an efficient reagent for the solvent-free synthesis of coumarins

Silica triflate,as a new silica-based reagent,can be used for the efficient synthesis of 4-substituted coumarins via a Pechmann reaction under solvent-free reaction conditions.All reactions were performed at 80℃in good to high yields.     

Efficient lipase-catalyzed synthesis of new lipid antioxidants based on a catechol structure

Lipid antioxidants phenolic saturated fatty acid esters were synthesized in high yields and short reaction times using the corresponding ethyl fatty acid esters, lipase from Candida Antarctica, vacuum and no solvent. Phenolic esters with mono- and polyunsaturated fatty acids (EPA and DHA) were also prepared.     

Regioselective construction of polysubstituted phenols from Baylis-Hillman adducts via formal [4+2] annulation strategy

Polysubstituted phenol derivatives were synthesized regioselectively starting from the Baylis-Hillman adducts via the formal [4+2] annulation protocol as the key step.     

Direct Analysis of Phenol, Catechol and Hydroquinone in Human Urine by Coupled-Column HPLC with Fluorimetric Detection

Phenol, catechol, and hydroquinone, are urinary end-products of the metabolism of benzene, nutrients, drugs, and endogenous substances. Recent research demonstrated that phenol, catechol, and hydroquinone, may have themselves a role in the carcinogenicity of benzene and in mechanisms that lead to leukemia. In this respect there is the need of rapid, low-cost, and possibly direct methods to quantitate these phenolic metabolites. Three single-residue coupled-column HPLC methods with fluorimetric detection (LC-LC-FLD) are described for the direct quantitation of phenol, catechol, and hydroquinone, in human urine. After enzymatic hydrolysis of the corresponding beta-glucuronoconjugates and sulfates, urine was directly injected into the LC-LC analyzer. The LC-LC-FLD procedure allowed base-to-base separation of the target compounds from urine interferents and good linearity (r² = 0.998) within the ranges studied (0.5–50 mg L⁻¹ for phenol, 0.35–35 mg L⁻¹ for catechol and 0.2–10 mg L⁻¹ for hydroquinone). Despite the high background levels of these metabolites in human urine, within- and inter-session precision expressed as RSD% was better than 20% on spiked and on authentic urine samples obtained from benzene-exposed workers. Accuracy expressed as the recovery ratio between measured and nominal concentration in spiked urine was comprised between 93% and 115% for the three metabolites. The column switching system was fully automated and computer-controlled, and was applied to the determination of phenol, catechol, and hydroquinone in urine samples showing a sample throughput of at least 20–30 samples per day.Revised: 21 February and 7 April 2005     

Ionic Liquids as Reagent and Reaction Medium: Preparation of Alkyl Aryl Ethers

Summary. Room temperature ionic liquid, [bmIm]OH, is used as a green recyclable reaction medium and reagent for the alkylation of phenols in excellent yields. The recovered ionic liquid was reused five to six times with consistent activity.     

Lactamomethylation of alkylphenols: Synthesis and quantum-chemical study of the reaction pathway

Six novel lactamomethyl derivatives of 2,5-dimethylphenol and 2,3,5-trimethylphenol were prepared with moderate yields by the reaction of corresponding phenols with 1-(hydroxymethyl)lactams in the presence of an acid catalyst. In all cases, the substitution occurred at position 4 to the phenolic hydroxyl group. The structures of all synthesized compounds were confirmed by FT-IR, ¹H and ¹³C NMR, 2D NMR and elemental analysis. The selectivity and possible pathways of the lactamomethylation reaction were studied by quantum-chemical methods. In silico calculations showed that the substitution at para-position to the hydroxyl group of the corresponding phenols was more preferable due to the higher stability of forming intermediates.     

Palladium-Nafion SAC-13 catalysed depolymerisation of lignin to phenols in formic acid and water

Guaiacol, pyrocatechol and resorcinol, were isolated and quantified as the major products from catalytic pyrolysis of lignin of various origin. The novel procedure utilises formic acid as the hydrogen source in combination with a palladium catalyst and Nafion^® SAC-13 as a solid acid catalyst with water as reaction medium. The reaction is conducted at moderate pyrolysis temperature and short reaction time. Several palladium based catalysts and different types of lignin were investigated. The lignins and the model compounds demonstrated various chemical behaviour, under identical reaction conditions; various phenols were formed from the lignins, while the model compounds were hydrodeoxygenated, demethylated and demethoxylated.