Analysis of terpenoic composition of conifer needle oils by steam distillation/extraction,gas chromatography and gas chromatography-mass spectrometry

Summary The essential oils of Estonian spruce and pine needles have been analysed by a combination of steam distillation/extraction, GC and GC-MS. According to their monoterpene composition Estonian pines belong to a high carene chemotype with monoterpenes representing up to 85% of total oil. Estonian spruce needle oil contains up to 70% of oxygenated terpenes and more limonene and 1,8-cineole than the same spruce species [Picea abies (L.) Karst.] from other localities.A total of 60 components were identified. Coefficients of variation describe the range of component contents in individual trees within the population.     

Selecting parameters and limits for equipment operational qualification

While operational qualification (OQ) is a well-established term within equipment qualification, users of equipment often become unsure when it comes to implementation. The biggest problem is how to select procedures and acceptance criteria. Should these be the vendor's specifications or should the users define their own limits, and, if so, how? Should all instruments of the same type have the same values or should these be optimized for each individual instrument? This article will provide an overall strategy and specific examples for HPLC on how to select procedures and acceptance limits that are based on efficient use of resources, on practicality and on the intended use of the equipment.     

Thermodynamic study of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated beta-cyclodextrin stationary phase

Thermodynamic studies were performed on 12 pairs of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated beta-cyclodextrin stationary phase (CP Chirasil-Dex CB). The influence of ester alkyl group structure on interaction with permethylated beta-cyclodextrin (Me-CD) and enantioselectivity was studied. The types of alkyl groups studied included n-alkyl (C1-C5) and groups containing branching at differing locations relative to the chiral center of the molecule. The results show that for a given molecular weight, the n-alkyl esters have stronger interactions with Me-CD than esters containing branched alkyl groups. However, although having weaker interactions with Me-CD, esters containing alpha-branched alkyl groups exhibit higher enantioselectivity than the corresponding n-alkyl or beta-branched isobutyl esters. From the retention data, thermodynamic parameters were estimated using the retention increment method and enthalpy-entropy compensation plots (ln R' versus deltaH) were constructed. The results suggest that ester enantiomers with branching at the alpha-carbon of the ester alkyl group have additional and/or different types of enantioselective interactions with Me-CD than the C1-C5 n-alkyl esters or beta-branched isobutyl ester. In order to obtain a qualitative sense of the interaction with Me-CD, structures of the diastereomeric complexes formed between Me-CD and some of the ester enantiomers were modeled using simulated annealing molecular dynamics.     

Computational Modelling of the Behaviour of Potentiometric Membrane Biosensors

This paper presents a mathematical model of a potentiometric biosensor based on a potentiometric electrode covered with an enzyme membrane. The model is based on the reaction–diffusion equations containing a non-linear term related to theMichaelis–Menten kinetics of the enzymatic reaction. Using computer simulation the influence of the thickness of the enzyme membrane on the biosensor response was investigated. The digital simulation was performed using the finite difference technique. Results of the numerical simulation were compared with known analytical solutions.      

Ion-Pair Solvent Extraction of EDTA Anions with Tetraalkylammonium Ions in Various Organic Solvents

Summary.  The ion-pair solvent extraction behavior of ethylenediaminetetraacetate (EDTA) anion by various tetraalkylammonium ions was investigated at 25.0 ± 0.1°C. The extraction of EDTA exceeded 90% from the basic aqueous solution into the organic solvents such as n-hexane and benzene derivatives containing tri-n-octylmethylammonium chloride, but EDTA was hardly extracted from acidic solution. Among the chemical species of EDTA in aqueous solution, edta ⁴⁻ is the most extractable one. On the other hand, the extraction of EDTA was less than 1% into chloroform and 1,2-dichloroethane even from the basic aqueous solution. The effect of the structure of alkylammonium ion was also examined. Tetra-n-hexylammonium and tetra-n-octylammonium ions could not extract EDTA even from the basic aqueous solution, while the use of tri-n-octylmethylammonium and di-n-lauryldimethylammonium ions enhances the extraction of EDTA. These results suggest that the steric hindrance in the ion-pair of alkylammonium and EDTA anion in the organic phase affects the extractability of EDTA containing ion-pair. The solution structure of ion-pair in the organic phase was calculated by MMFF force field and the steric effect in the ion-pair was also suggested. From the extraction constants obtained, the possibility of the extraction separation of EDTA has been shown. Present address: Chemistry Department, Heilongjiang University, Harbin, China     

Evaluation of a complete HPLC solvent optimization system involving piece-wise quadratic modelling

Summary Interpretive methods are accepted to give the best possible results for selectivity optimization in HPLC. However the methods are very complex, and most work so far has been detailed academic studies. This paper describes an evaluation of a complete integrated system incorporating peak labelling, modelling of retention behaviour and calculation of response surfaces, with particular emphasis on the retention modelling. The peak labelling section has been discussed previously.A piece-wise quadratic function is investigated for the modelling of retention times across an isoeluotropic plane to effect selectivity optimization in HPLC. This requires 10 data-points on the isoeluotropic plane. The predicted global optimum and local optima are evaluated by comparison of calculated and experimental retention data, for a nine component sample. Seven interstitial points, distributed across the whole plane between the data-points, are similarly evaluated for a related sample. The typical error (in retention time) is less than 2%, often 1%, and the maximum error is 4.2%. At the global optimum the error was found be less than 1.3% for all 9 peaks.     

Solventless injecction for packed column supercritical fluid chromatography

The adverse effects of injection solvent strength on microbore packed column SFC band broadening are demonstrated and a solventless injection system that eliminates these effects is introduced. The injection system removes solvent in a GC-like manner using a retention gap and an on-column capillary GC syringe. The analyte is delivered to the analytical column in a solvent-free plug of supercritical fluid mobile phase.     

A quantitative evaluation of the electrostatic theory for ion pair chromatography

Summary A quantitative model for ion pair chromatography based on the electrostatic theory is described. The model is based on the solution of the linearised Poisson-Boltzmann equation in a cylinder. The obtained equations are compared with experimental data from a number of different systems. The agreement between theory and experiments is satisfactorily. Systematic deviations due to the use of the linearised equation and ion correlation effects are discussed.     

Modelling Dynamics of Amperometric Biosensors in Batch and Flow Injection Analysis

A mathematical model of amperometric biosensors has been developed. The model is based on non-stationary diffusion equations containing a non-linear term related to Michaelis–Menten kinetic of the enzymatic reaction. Using digital simulation, the influence of the substrate concentration as well as maximal enzymatic rate on the biosensor response was investigated. The digital simulation was carried out using the finite difference technique. The model describes the biosensor action in batch and flow injection regimes.     

Role of the solvent in thin-layer chromatography of steroids on alumina

The effects of different binary solvents on the retention behaviour of some steroids in thin-layer chromatography on alumina were studied. The slope of the linear relationship between the retention constant of the steroid and the logarithm of the volume fraction fo the polar component in the binary solvent mixture depends predominantly on the diluent. Linear relationships between the axis intercepts and slopes of particular steroids exists for all chromatographic systems examined. The slope of this relationships is a function of the retention constant of the steroid hydroxyl group.