About the C term in the van Deemter's equation of plate height in monoliths.

Monolithic stationary phases provide high rates of mass transfer at low pressure drops and enable much faster separations. Experimental data from published H versus u plots obtained with monolithic columns are compiled with an emphasis on the mass tranfer (C term of the van Deemter equation). Silica-based monoliths of the C18 Chromolith type exhibit a very low C value that does not vary too much within a wide range of flow rates. The C term is a function of retention, temperature, molecular weight of the solute and mesopore size. Silica-based monoliths of capillary format exhibit a slightly higher C value which slightly increases with increasing flow rates. The key parameter is the through-pore size/skeleton size ratio. Experimental data on polymer-based monoliths retrieved from the literature mostly concern acrylate-based material in a capillary format. Due to the unique character of every column the scatter of published C values is wide. The observed C term are higher than those observed with silica monoliths and increase quite linearly with the flow rate. Low-density polymers exhibit better performances than high-density polymers. The majority of HETP versus u plots are drawn from the peak width at half height. Accurate data should be acquired through the second moment of the peak. Some equations for C have been derived. An equivalent dispersion particle diameter is a measure of band broadening in the mesopores. Mass transfer in silica monoliths is well accounted by the theory developed by Guiochon et al.

The behavior of some phenothiazines and their demethylated derivatives in reversed-phase liquid chromatography.

Three selected phenothiazines and their demethylated derivatives are chromatographed on different C18 bonded reversed-phase liquid chromatography columns. A quadratic equation fits the relationship log k versus the organic modifier percentage. When acetonitrile is the organic modifier, the demethylated derivative is eluted before the parent compound, whereas it is eluted after when methanol is the organic modifier. The log kw values are therefore different. Selectivity between the metabolite and the parent compound is higher with methanol.

Early steps of silica-based monolith fabrication as monitored by 29Si nuclear magnetic resonance spectroscopy.

We review here the contribution of 29Si NMR to monitor the early steps of fabrication of monoliths.

Optimization of the separation of some psychotropic drugs and their respective metabolites by liquid chromatography.

A chemometric procedure is described to optimize the separation of some drugs used in the treatment of psychotic disorders: haloperidol, levomepromazine, risperidone, venlafaxine, carbamazepine and their main metabolites: reduced haloperidol, 9-hydroxy risperidone, desmethyl levomepromazine, desmethyl venlafaxine. The purpose of the procedure is the unambiguous identification and detection in biological fluids. Isocratic reversed-phase liquid chromatography with diode array detection was utilized. An experimental design methodology was carried out in which the experimental response is selectivity. In this way the designs for mixture compounds and for process variables (five variables) was performed which produced 36 experiments to carry out. The desirability function was used to select optimum separation conditions. The procedure provides a chromatogram of well separated solutes.

Electrophoretic separations of twelve phenothiazines and N-demethyl derivatives by using capillary zone electrophoresis and micellar electrokinetic chromatography with non ionic surfactant.

We focused our work on the separation of phenothiazines that are important drugs used for the treatment of psychic diseases. For a better understanding of the metabolism of these solutes, we wanted to separate not only a mixture of 12 phenothiazines but also a mixture containing phenothiazines and their N-demethyl metabolites by capillary electrophoresis. Separations in capillary zone electrophoresis were performed using 3 x 10(-2) mol/L H3PO4 (pH 2.5) but the obtained resolutions were not entirely satisfactory especially with regard to phenothiazine -N-demethyl derivative pairs. To improve the obtained results, we have performed separations by using micellar electrokinetic chromatography. In this approach, we used a running electrolyte containing 3 x 10(-2) mol/L H3PO4 electrolyte (pH 2.5) and octaethylene glycol monododecyl ether (C12E8) as neutral surfactant. By introducing 2 x 10(-3) mol/L C12E8 in the electrolyte, 11 out of 12 phenothiazines have been baseline separated. With respect to the separation of a mixture containing 3 phenothiazines and their 3 demethyl derivatives, we obtained an excellent separation by using a running electrolyte prepared with 7.5 x 10(-4) mol/L C12E8 and 3 x 10(-2) mol/L H3PO4.

Silica gel-based monoliths prepared by the sol-gel method: facts and figures.

There is a great deal of interest in continuous beds as stationary phases for both HPLC and CEC. There are various ways to prepare monoliths, by polymerization of organic species or by polymerization of silicon alkoxides. The former method has recently been reviewed, while silica based monoliths are now commercially available. The purpose of this paper is to deal with the problems associated with silica based monoliths. The most important problem is obviously the cracking and the shrinkage of the bed during drying. The second problem is monolith cladding. Much literature has been published but no definitive solution is available and thus a wide research area remains open. Monoliths are a compromise between loadability, permeability and mass transfer kinetics. Due to the better mass transfer properties of a monolithic skeleton over distinct particles, high flow rates and high speed separations are possible.

Behavior of haloperidol and various phenothiazines on several alkyl bonded phases.

Haloperidol and phenothiazines are present in psychiatrical treatments. An analysis in body fluids is tedious because of the presence of demethylated (DM) derivatives of phenothiazines. The behavior of some interfering solutes on alkyl bonded phases has been studied. Phenothiazines and DM derivatives exhibit a very similar behavior with a binary eluent (phosphate buffer-acetonitrile), which precludes an optimization with this system. When a ternary phase is used (phosphate buffer-acetonitrile-methanol), haloperidol and reduced haloperidol behave differently as compared with phenothiazines. In this mode it is possible to unambiguously detect haloperidol that would otherwise interfere. Phenothiazine peaks are characterized by a large tailing. An interesting feature is the comparison between cyclohexyl bonded and octadecyl bonded phases, the former being much more efficient.

Optimization methods in chromatography and capillary electrophoresis.

Many methods have been developed in order to optimize the parameters of interest in either chromatography or capillary electrophoresis. In chemometric approaches experimental measurements are performed in such a way that all factors vary together. An objective function is utilized in which the analyst introduces the desired criteria (selectivity, resolution, time of analysis). Simplex methods and overlapping resolution maps are declining. Factorial designs and central composite designs are more and more popular in electrodriven capillary separations since the number of parameters to master is much larger than in either GC or LC. The use of artificial neural networks is increasing. The advantage of chemometrics tools is that no explicit models are required, conversely the number of experiments to perform may be high and boundaries of the domain are not straightforward to draw and the approach does more than is required. When models are available optimization is easier to perform by regression methods. Computer assisted methods in RPLC are readily available and work well but are still in infancy in CE. Linear solvation energy relationships seem a very valuable tool but estimates of coefficients still require many experiments.

Planar chromatographic techniques in biomedicine: current status.

In planar chromatography (PLC), the solvent flows through a layer either by means of capillary forces [conventional thin-layer chromatography (TLC)] or by a forced-flow system (over-pressured layer chromatography). Phases and instrumentation currently available are briefly examined. The main applications in biomedicine are reviewed. Although silica gel TLC plates still predominate, interest in other phases is increasing. Unique detection features such as immunostaining are emphasized. Although gas chromatography and high-performance liquid chromatography have superseded TLC in the analysis of carbohydrates, amino acids and indole derivatives, interest in PLC continues to be high in lipid analysis.

Determination of nitrosamines by liquid chromatography with post-column photolysis and electrochemical detection.

The liquid chromatographic detection of nitrosamines at the ppb (10(9)) and sub-ppb levels is still a difficult problem. A significant improvement was achieved when a nitrosamine eluted from the column was photolysed by a UV lamp, the charged nitrite species being detected amperometrically. N-Nitrosodimethylamine (NDMA) was selected as a model solute to test the applicability of the detection device. The calibration graphs were linear over a wide concentration range. Optimal detection depended on the nature of the buffer used and the UV spectrum of the nitrosamine to be detected. An interesting application of the detection of a high-molecular-weight nitrosamine is given as an example.