Statistical Model for Organic Chromatographic Trace Analysis of Complex Samples. A Case Study: Plant Extracts

Abstract

The use of pooled plant extracts is described in the estimation of matrix interference in HPLC (UV and EC) determinations of organic compounds in plant extracts. An extract from freeze dried leaves of 134 different plant species was used for this purpose. It was split in different subgroups with solid extraction clean-up procedures. UV, EC and chromatographic data of the subgroups were used in the calculation of minimum concentrations of organic compounds which are still accurately determinable in plant samples with HPLC methods. The UV and/or EC characteristics of the compound must be known. The contribution of the solid phase extraction procedures and of the analytical system to the selectivity of the method can be estimated. Information is also supplied which allows rapid comparison of the selectivity of the UV and EC (single, or dual parallel) detectors for the determination of a specified compound.     

Space as a “bucket of dust”

A pregeometry of space is considered, in which the only structure imposed on individual points is a uniform probability of adjacency between any two arbitrarily chosen points. This probability is not specified (but only assumed small) and neither is the total number of points specified (but is assumed large). It is shown that the most likely distribution of points has similarities with a closed three-dimensional space of constant positive curvature, in particular, the number of points at a given distance from an arbitrary origin is proportional to sine-squared of the distance from that origin.     

Eine neue Doppelmesspipette

Ohne Zusammenfassung     

Potentiometric detection of organic acids in liquid chromatography using polymeric liquid membrane electrodes incorporating macrocyclic hexaamines

Potentiometric detection employing coated-wire electrodes was applied to the determination of organic acids in liquid chromatography (LC). Poly(vinyl chloride)-based liquid membranes, incorporating lipophilic macrocyclic hexaamines as neutral ionophores were used as electrode coatings. The selectivity and sensitivity of the macrocycle-based electrodes were found to be superior to an electrode based on a lipophilic anion exchanger (a quaternary ammonium salt). Sensitive detection was obtained for the di- and tricarboxylic acids tartaric, malonic, malic, citric, fumaric, succinic, pyruvic, 2-oxoglutaric and maleic acids after separation in reversed-phase LC. Detection limits (signal/4sigmanoise=3) of 6 pmol for malonic acid and 2 pmol for maleic acid were attained. The detection was explained using a molecular recognition model. The hexaamine-based potentiometric electrodes had a 1-s response time at 1 ml min(-1) flow-rates. They were stable for at least 4 months, with an intra-electrode variation of 3.2% (n=5).     

Determination limits in high-performance liquid chromatography of plant phenolic compounds with an ultraviolet detector

The capabilities of high-performance liquid chromatography (h.p.l.c.) for the determination of phenolic compounds in 80% ethanol extracts from plant material are described. A reversed-phase column was used and elution was done with a linear gradient from 0.01 M phosphoric acid up to methanol. The efficiency of the method was studied via determination limits, defined as the minimum concentration of a compound (μg of compound per gram of extracted dry plant material) necessary to provide 90% probability that the relative error on the determination of the compound in an extract from a plant sample taken at random is < 10%. These limits take into account matrix interferences as a source of error, and were calculated with a minicomputer for the determination of 19 phenolic compounds in plant extracts. For good determinations, the concentrations of the components should be in the range 1–10 mg g^?1 of dry plant material. Separating the extracts into different chemical groups (on ion-exchange materials) prior to h.p.l.c. decreases the determination limits about five times. The dependence of determination limits on the u.v. characteristics of the compound, the sample clean-up, and the column characteristics are discussed quantitatively by means of a simple empirical equation.     

On the ν1/2 ν2 Resonance in the Complex of Carbon Dioxide with Dimethyl Ether

Infrared and Raman spectra of solutions in liquid argon and krypton containing dimethyl ether or its fully deuterated isotopomer and ¹²CO₂ or ¹³CO₂ are investigated. The spectra lead to new data on the ν₁/2 ν₂ resonances appearing in the complex of CO₂ with the ether. The experimental data, and their interpretation, is supported by MP2/6‐311++G(2d,2p) calculations of the cubic and quartic force constants and of the first and higher order dipole moment derivatives required for the modelling of the Fermi and Darling‐Dennison resonances observed.     

Potentiometric detection of amines in ion chromatography using macrocycle-based liquid membrane electrodes

Potentiometric detection employing solid-state electrodes was applied to the determination of organic amines in cation-exchange, and in ion-interaction chromatography. The amines included nine industrial alkylamines, the biogenic amines putrescine and cadaverine, the neurotransmitter acetylcholine and choline. Three PVC-based liquid membrane electrode coatings were used, incorporating respectively the lipophilic cation-exchanger tetrakis(p-chlorophenyl)borate, a combination of tetrakis(p-chlorophenyl)borate and dibenzo-18-crown-6, or a combination of tetrakis(p-chlorophenyl)borate and a calix[6]arene. The three solid-state electrodes allowed sensitive detection of all amines. The addition of macrocycles with molecular recognition properties resulted in a superior sensitivity when compared to the tetrakis(p-chlorophenyl)borate-based electrode. The effect was most significant for the smaller amines which form the most stable complexes with the macrocycles. Detection limits of the order of 10⁻⁶ M (injected concentration) were measured for mono- and diamines for the macrocycle-based electrodes.