Procedure for the quantification of rider peaks.

Rider peaks are small peaks which are not well resolved from a large and asymmetrical neighbour but sit on its trailing side. The usual case is a large, tailed peak which is eluted just in front of the small peak, although the opposite situation can also occur (a small peak in front of a large peak with fronting). The common integration techniques. i.e. separating the peaks by vertical drop or by a tangent and determining area or height, give erroneous results. We propose a method for their quantification with low error. It is necessary to set up a "two-dimensional" calibration by varying both concentrations, i.e. of the large peak and of the rider. This leads to a series of linear equations which describe the rider size, as found by the integrator, as a function of the size of the large peak. The y-axis intercepts i of these equations show a linear relationship with the concentration x of the rider analyte, whereas the slopes s follow a quadratic relationship. These equations can be used to solve the equation y = s(x) x z + i(x) for x (y and z are the integrated peak size of the rider and the large peak, respectively). The procedure was tested with computer-generated peak pairs as well as with HPLC separations of 2,3-dimethylaniline (large tailing peak) and 2,3-dimethylphenol (symmetrical rider peak).

Argentometric titration for the determination of liquid chromatographic injection reproducibility.

The reproducibility of autosamplers for high-performance liquid chromatography can be checked by the injection of a sodium chloride solution without an installed column. The dispensed chloride plug is analyzed quantitatively by argentometric titration. If the titration can be performed with high enough precision, the autosampler performance is investigated without additional effects. Experiments with a 20-microL loop were performed. We found the following repeatabilities (relative standard deviations): titration, 0.07%; 10-microL "pull loop" injection (partial filling of the loop), 0.6%; and 20-microL "full loop" injection, 0.1%. In contrast to these data, the relative standard deviations obtained from chromatographic peaks include the steps of injection, separation, detection, and integration without any knowledge of the individual contributions. Such values are higher; we found for chromatographic peak areas of parabens 2% under pull loop conditions and 0.17% by full loop. The titration approach allows the determination of the contribution of the injection to the uncertainty budget of a liquid chromatographic analysis.

Cloud-point extraction for the determination of the free fraction of antiepileptic drugs in blood plasma and saliva.

A method for the determination of the free fraction of antiepileptic drugs in plasma and saliva was developed. The separation of free and protein-bound fractions was obtained by cloud-point extraction under optimum conditions of pH, surfactant type, and concentration. It is shown that the free fraction of carbamazepine and of phenobarbital in plasma coincides with the drug concentration in saliva. The dependence of the free fraction in plasma on the administered dose was studied. The influence of the simultaneous administration of the two drugs on their free concentrations was revealed: In the presence of carbamazepine the free fraction of phenobarbital is decreased markedly whereas phenobarbital does not influence the behavior of carbamazepine.

News from the Autumn Meeting of the New Swiss Chemical Society: Laser MS reveals urban air pollution.

Veronika R. Meyer reports from Basel, Switzerland.

The kinetics of diastereomeric amino acids with o-phthaldialdehyde.

The kinetics of the reaction of the amino acid epimers L-isoleucine, D-allo-isoleucine, L-threonine, and D-allo-threonine with o-phthaldialdehyde and mercaptoethanol were determined at 25 degrees C. L-Isoleucine reacts faster than its D-epimer whereas L-threonine reacts slightly slower than its D-epimer. In the case of isoleucine, the consequence can be an allo/iso ratio which in the worst case is 25% too low if these amino acids are quantified by liquid chromatography and o-phthaldialdehyde fluorescence detection. The effect on dating of fossils by amino acid racemization is discussed.

A vivid model illustrating chiral recognition induced by achiral structures.

If diastereomeric complexes are adsorbed on a surface or if they include a molecule of solvent, two points of attractive interaction between the chiral species of the complex can be sufficient for mutual chiral recognition of these species. In special cases even one single point of interaction is sufficient. This extension of the three-point contact rule of Dalgliesh, first observed in chiral ligand-exchange chromatography, can be demonstrated by using hands.

A vivid model of chiral recognition.

Hands can be used to demonstrate the three-point model of chiral recognition. The points of attachment are thumb, forefinger, and middle finger. This vivid model has the advantages of simplicity, perspicuity, and availability at any time, although two persons are necessary. It can be shown that two interactions are not sufficient for chiral recognition but that three attractive or two attractive and one repulsive attraction are needed. It can also be used to explain some possibilities of weakening or elusion of the three-point model.