Luisa Pasti

Recent applications in chiral high performance liquid chromatography: A review

The most important and broadly used chiral stationary phases (CSPs) for high performance liquid chromatography (HPLC) are reviewed. After a short description of the state of the art, for each kind of CSP the most important contributions published in the last couple of years are summarized. For the sake of classification, these works have been divided into studies on enantiorecognition mechanisms, new materials, and new applications. Emphasis is given to new, emerging CSPs that seem to possess all requisites to be considered potentially successful chiral separation media in the next future.

Silica-supported 5-(pyrrolidin-2-yl)tetrazole: development of organocatalytic processes from batch to continuous-flow conditions

5-(Pyrrolidin-2-yl)tetrazole functionalized silica prepared by photoinduced thiol–ene coupling is packed into a short stainless steel column. The resulting packed-bed microreactor is conveniently heated to perform environmentally benign continuous-flow aldol reactions with good stereoselectivities, complete conversion efficiencies, and long term stability of the packing material.

Pirkle-type chiral stationary phase on core-shell and fully porous particles: Are superficially porous particles always the better choice toward ultrafast high-performance enantioseparations?

Pirkle-type Whelk-O1 chiral stationary phase (CSP) was prepared on 2.6μm superficially porous particles (SPPs). The chromatographic behavior of columns packed with this new CSP was compared with that of columns packed respectively with 1.8 and 2.5μm Whelk-O1 fully porous particles (FPPs). In the comparison, both thermodynamic and kinetic aspects were considered. Contrary to initial expectations, chiral columns packed with 2.6μm SPPs were quasi-comparable to those packed with 2.5μm FPPs, apparently due to larger contributions to band broadening from both eddy dispersion and, especially for the second eluted enantiomer, adsorption-desorption kinetics. These findings raise the question if SPPs, in spite of the undeniable advantages of their morphology to speed up mass transfer, are always the best choice for high-efficient ultrafast chiral separations. The last part of the work focuses on the use of short columns (10mm long) and very high flow rates to realize the separation of the enantiomers of trans-stilbene oxide (TSO) in normal phase mode in less than 1s.

Factors affecting drug adsorption on beta zeolites

The adsorption behaviour of three commonly used drugs, namely ketoprofen, hydrochlorothiazide and atenolol, from diluted aqueous solutions on beta zeolites with different SiO2/Al2O3 ratio (i.e. 25, 38 and 360) was investigated by changing the ionic strength and the pH, before and after thermal treatment of the adsorbents. The selective adsorption of drugs was confirmed by thermogravimetry and X-ray diffraction. The adsorption capacity of beta zeolites was strongly dependent on both the solution pH and the alumina content of the adsorbent. Such a remarkable difference was interpreted as a function of the interactions between drug molecules and zeolite surface functional groups. Atenolol was readily adsorbed on the less hydrophobic zeolite, under pH conditions in which electrostatic interactions were predominant. On the other hand, ketoprofen adsorption was mainly driven by hydrophobic interactions. For undissociated molecules the adsorption capability increased with the increase of hydrophobicity.

Experimental validation of the stochastic theory of size-exclusion chromatography: Retention on single and coupled columns

SummaryThe experimental validation of the recently developed stochastic model of size-exclusion chromatography (SEC), interpreted as a random process in which macromolecules undergo a continuous exchange process between the moving zone between the packing particles and the stagnant zone inside the particles, is attempted on the basis of an extended data set of experimental chromatograms of polystyrene standards, with molar masses ranging from 0.16 to 2200 kDa, on five different columns with widely different pore sizes. Several features of experimental chromatograms of standards eluting within or outside the permeation range, such as band broadening, peak tailing and peak splitting are qualitatively interpreted at the light of the stochastic theory of SEC. Furthermore, some aspects such as the influence of sample polydispersity on peak broadening and of the exclusion effect in the moving zone (hydrodynamic chromatographic effect) on retention are discussed.A new chemometric procedure was developed for estimating theKSEC equilibrium distribution constant without a priori assumptions regarding the extraparticle column volume,V0, and the total volumeVp. This procedure allowed to obtain an unified description of the large retention data set over different columns. All retention data belonging to calibration ranges of the various columns, followed quite similar calibration plots, under the form of KSEC=(1-ρ)m, where ρ is the ratio of macromolecule size to pore size, withm values close to 3 for all columns. A similar approach was able to represent the behaviour of two connected column in series, with consistent values of the pertinent parameters, which validates the stochastic theory of SEC with two pore types of different sizes. Finally, this two-pore model and a mixed SEC-hydrodynamic chromatography model were compared to interpret retention data on single columns around the exclusion limits. The practical application of these approaches in routine analysis, e.g. for monitoring the column degradation during its use, is mentioned.

A Combined Kinetic and Thermodynamic Approach for the Interpretation of Continuous‐Flow Heterogeneous Catalytic Processes

The heterogeneous proline-catalyzed aldol reaction was investigated under continuous-flow conditions by means of a packed-bed microreactor. Reaction-progress kinetic analysis (RPKA) was used in combination with nonlinear chromatography for the interpretation, under synthetically relevant conditions, of important mechanistic aspects of the heterogeneous catalytic process at a molecular level. The information gathered by RPKA and nonlinear chromatography proved to be highly complementary and allowed for the assessment of optimal operating variables. In particular, the determination of the rate-determining step was pivotal for optimizing the feed composition. On the other hand, the competitive product inhibition was responsible for the unexpected decrease in the reaction yield following an apparently obvious variation in the feed composition. The study was facilitated by a suitable 2D instrumental arrangement for simultaneous flow reaction and online flow-injection analysis.

Rationale behind the optimum efficiency of columns packed with new 1.9μm fully porous particles of narrow particle size distribution

Columns packed with new commercially available 1.9 fully porous particles of narrow particle size distribution (nPSD) are characterized by extremely high efficiency. Under typical reversed phase conditions, these columns are able to generate very high number of theoretical plates (in the order of 300,000plates/m and more). In this paper, we investigate the origin of the high performance of these nPSD columns by performing a series of measurements that include, in addition to the traditional determination of the van Deemter curve, peak parking, pore blocking and inverse size exclusion experiments. Two nPSD columns (both 100×3.0mm) have been considered in this study: the first one, packed with particles of 80Å pore size, is commercially available. The second one is a prototype column packed with 1.9 fully porous particles of 120Å pore size. The main conclusion of our study is that these nPSD columns are characterized by extremely low eddy dispersion, while longitudinal diffusion and mass transfer kinetics are substantially equivalent to those of other fully porous particles of similar chemistry.

Evaluation of saline tracer performance during electrical conductivity groundwater monitoring.

Saline solutions are the most commonly used hydrological tracers, because they can be easily and economically monitored by in situ instrumentation such as electrical conductivity (EC) loggers in wells or by geoelectrical measurements. Unfortunately, these low-cost techniques only provide information on the total concentration of ions in solution, i.e., they cannot resolve the ionic composition of the aqueous solution. This limitation can introduce a bias in the estimation of aquifer parameters where sorption phenomena between saline tracers and sediments become relevant. In general, only selected anions such as Cl(-) and Br(-) are recognised to be transported unretarded and they are referred to as conservative tracers or mobile anions. However, cations within the saline tracer may interact with the soil matrix through a range of processes such as ion exchange, surface complexation and via physical mass-transfer phenomena. Heterogeneous reactions with minerals or mineral surfaces may not be negligible where aquifers are composed of fine alluvial sediments. The focus of the present study was to examine and to quantify the bias between the aquifer parameters estimated during model-based interpretation of experimental data of EC measurements of saline tracer relative to the aquifer parameters found by specific measurements (i.e. via ionic chromatography, IC) of truly conservative species. To accomplish this, column displacement experiments with alluvial aquifer materials collected from the Po lowlands (Italy) were performed under water saturated conditions. The behaviour of six selected, commonly used saline tracers (i.e., LiCl, KCl, and NaCl; LiBr, KBr, and NaBr) was studied and the data analysed by inverse modelling. The results demonstrate that the use of EC as a tracer can lead to an erroneous parameterisation of the investigated porous media, if the reactions between solute and matrix are neglected. In general, errors were significant except for KCl and KBr, which is due to the weak interaction between dissolved K(+) and the sediment material. The study shows that laboratory scale pre-investigations can help with tracer selection and to optimise the concentration range targeted for in situ multilevel monitoring by unspecific geoelectrical instrumentation.

Experimental evidence of the kinetic performance achievable with columns packed with new 1.9μm fully porous particles of narrow particle size distribution

Fully porous particles of narrow particle size distribution (nPSD) are now commercially available. In this paper, the kinetic performance of columns packed with these particles (1.9μm, 80Å pore size) has been investigated under typical reversed phase conditions by using a mixture of benzene derivatives as probes. The columns exhibited remarkably high efficiency (in the order of 300,000 theoretical plates per meter) and the possibility to be used at relatively high flow rates without loss of performance. These results contrast with previous studies on the same columns. Indeed we have found column efficiency comparable to that reported in previous work but, on the other hand, we could not observe the same dramatic loss of performance when columns were operated at high flow rates. The results presented in this paper, based on a set of six columns with different geometries (2.1 and 3.0 internal diameter×50, 75 and 100mm length), are not consistent with the previously proposed hypothesis that the unusually low intraparticle diffusion, which would characterize these particles, is the origin of the high efficiency of the columns. In a companion paper [1], a detailed investigation of the different terms leading to band broadening will be performed to point out the major contribution to plate height on nPSD columns.

Determination of adsorption isotherms by means of HPLC : Adsorption mechanism elucidation and separation optimization

The purpose of this review is to illustrate the most important techniques for isotherm determination by means of HPLC. Starting on the traditional Frontal Analysis approach, Frontal Analysis by Characteristic Point, Elution by Characteristic Point, Perturbation Method in its different applications will be considered to conclude with the most recent Inverse Method approach. Since many of these techniques are based on the fundamentals of nonlinear chromatography, a short overview of the theory of nonlinear chromatography is presented. Emphasis is given to the most recent applications of these techniques for pharmaceutical applications, characterization of binding mechanisms, bioaffinity studies, molecular and chiral recognition processes.