Ramia Z. Al Bakain

Comparison of different statistical approaches to evaluate the orthogonality of chromatographic separations: Application to reverse phase systems

Selectivity of phase system is of primary concern when designing a bidimensional chromatographic system and looking for the highest degree of orthogonality between the two separations. Several statistical or geometrical criteria can potentially be used to measure the degree of orthogonality. A comparison of eight candidate criteria has been carried out in this study. Analysis of variance (ANOVA) was used to evaluate the relevance of each criterion and its ability to reveal the significance of the influence of factors like pH, stationary phase, and organic modifier. Experimentally, a set of 32 chromatographic systems was evaluated by the same generic gradient with 63 probe solutes, likely to be present in biological and/or environmental samples and covering a wide range of physico-chemical properties: acidic, basic and neutral compounds with different pKa, molecular mass and hydrophobicity (logP). Each chromatographic system was defined by the nature of the stationary phase (8 different silica or grafting chemistries), the pH of the aqueous fraction of the mobile phase (2.5 or 7.0) and the nature of the organic modifier (acetonitrile or methanol). The orthogonality of the 496 couples of chromatographic systems was evaluated and ranked using the eight different approaches: the three correlation coefficients (Pearson, Spearman and Kendall), two geometric criteria characterizing the coverage of the 2D separation space, Sloneckers information similarity and two chi-square statistics of independence between normalized retention times. In fact, there were only seven distinct criteria, since we established the analytical equivalence between the rankings with the likelihood ratio statistics and Sloneckers information similarity. Kendalls correlation coefficient appeared to be the best measure of orthogonality since, according to ANOVA, it exhibited the highest sensitivity to all experimental factors. The chi-square measures, and hence Sloneckers information similarity, performed equally well provided the discretization of the separation space was carried out appropriately. Finally, from the compared study of the factors acting upon orthogonality carried out by ANOVA, it is possible to draw the conclusion that the pH of the mobile phases has the highest impact on the selectivity followed by the type of stationary phase and finally by the organic modifier.

Synthesis and characterization of new derivatives of alginic acid and evaluation of their iron(III)-crosslinked beads as potential controlled release matrices

Abstract Context: The excellent gelling and safety profiles of alginic acid combined, however, with drawbacks of its ionotropically crosslinked beads (i.e. their quick release of loaded drugs) prompted us to chemically modify alginic acid. Objective: Alginic acid was chemically conjugated with four amines of varying hydrophilic-hydrophobic properties (i.e. tris(hydroxymethyl)methyl-, allyl-, benzyl- or pentyl-amines) in an attempt to enhance the drug release profiles from respective metal crosslinked beads. Materials and methods: Chemical conjugation procedures were performed using dicyclohexylcarbodiimide as a coupling agent and the resulting new derivatives were characterized using proton nuclear magnetic resonance (1H NMR), infrared (IR) spectroscopy and differential scanning calorimetry (DSC). These modified polymers were used to prepare iron (III)-crosslinked beads loaded with folic acid as model drug, which were tested in vitro to assess their folic acid release profiles. Results and discussion: Interestingly, the resulting beads accessed enteric release kinetics, with tris(hydroxymethyl)methyl-amide alginic conjugate producing most pronounced enteric profile. Conclusion: The results suggest the possibility of achieving controlled drug release from alginate-based beads via facile chemical modification of alginic acid.

Combination of partial least squares regression and design of experiments to model the retention of pharmaceutical compounds in supercritical fluid chromatography

This work presents a first attempt to establish a model of the retention behaviour for pharmaceutical compounds in gradient mode SFC. For this purpose, multivariate statistics were applied on the basis of data gathered with the Design of Experiment (DoE) methodology. It permitted to build optimally the experiments needed, and served as a basis for providing relevant physicochemical interpretation of the effects observed. Data gathered over a broad experimental domain enabled the establishment of well-fit linear models of the retention of the individual compounds in presence of methanol as co-solvent. These models also allowed the appreciation of the impact of each experimental parameter and their factorial combinations. This approach was carried out with two organic modifiers (i.e. methanol and ethanol) and provided comparable results. Therefore, it demonstrates the feasibility to model retention in gradient mode SFC for individual compounds as a function of the experimental conditions. This approach also permitted to highlight the predominant effect of some parameters (e.g. gradient slope and pressure) on the retention of compounds. Because building of individual models of retention was possible, the next step considered the establishment of a global model of the retention to predict the behaviour of given compounds on the basis of, on the one side, the physicochemical descriptors of the compounds (e.g. Linear Solvation Energy Relationship (LSER) descriptors) and, on the other side, of the experimental conditions. This global model was established by means of partial least squares regression for the selected compounds, in an experimental domain defined by the Design of Experiment (DoE) methodology. Assessment of the models predictive capabilities revealed satisfactory agreement between predicted and actual retention (i.e. R2=0.942, slope=1.004) of the assessed compounds, which is unprecedented in the field.

A study on removal characteristics of o-, m-, and p-nitrophenol from aqueous solutions by organically modified diatomaceous earth

AbstractThe natural diatomaceous earth (DAT) was modified with surfactant of hexadecyltrimethylammonium bromide (HDTMABr) to form organic-modified diatomaceous earth (DAT-HDTMABr). The DAT and DAT-HDTMABr were characterized by X-ray fluorescence, X-ray diffraction, FT-IR and DTA-TG. The surface area was determined using Brunauer, Emmett, and Teller adsorption method. Cation exchange capacity was estimated using an ethylenediamine complex of copper method, and the modifier loading was calculated from the total carbon analysis. The ability of raw and organomodified diatomaceous earth to remove o-nitrophenol (ONP), m-nitrophenol (MNP), and p-nitrophenol (PNP) from aqueous solutions has been carried out at predetermined equilibration time, adsorbent amount, pH, and temperatures using a batch technique. The removal of ONP, MNP, and PNP from aqueous solutions by modified clay seems to be more effective than unmodified sample. The experimental equilibrium adsorption data were analyzed by four widely used two par...

Impact of the probe solutes set on orthogonality evaluation in reverse phase chromatographic systems

Two-dimensional liquid chromatography (2DLC) is a very attractive technique for the characterization of complex samples due to its separation power obtained via the coupling of two separation modes exhibiting different mechanisms, i.e. orthogonality. In reverse phase, orthogonality is mainly governed by three factors: the pH of the mobile phase, the structure of the stationary phase and the nature of the organic modifier. In the present paper, we studied the impact of the nature of the probe solutes on orthogonality evaluation. A set of 63 compounds with various physicochemical properties was used to evaluate 32 reverse phase chromatographic systems (2 pH × 8 stationary phases × 2 organic modifiers). Principle component analysis revealed that the solutes could be split into three subsets according to their charge in the experimental conditions. The factors affecting orthogonality and the magnitude of their effect were shown to depend on the charge of the compounds. For positively charged (basic) compounds, the pH was the most important factor, followed by the nature of the stationary phase. For negatively charged (acidic) compounds, the nature of the stationary phase had the highest impact. For neutral compounds, only the nature of the stationary phase and, to a smaller extent, the organic modifier had an influence. The present study also showed that a reduced set of only 9 test compounds instead of whole set of the 63 could enable an appropriate orthogonality evaluation.

Spectrophotometric Determination of Melamine in Liquid Milk by Multivariate Second Order Calibration

Melamine was quantified in liquid milk without performing the usual liquid-liquid or solidphase extraction steps. Following a simple clean up procedure (extraction by acetonitrile/water mixture, centrifugation and filtration by 0.45 m membrane), melamine was accurately quantified in fullfat liquid milk down to 0.057 mg kg using second order multivariate calibration. Second order data was created from absorbance-pH measurements (batch-experiment method) taking the advantage of acid/base properties of melamine. The study was carried out over the spectral domain 200-250 nm and pH 2.0-8.0. Although the number of pH points was lower than the spectral points (4 vs. 51), the data was satisfactorily analyzed by second order multivariate calibration methods. Quantification of melamine in spiked milk samples (0.5-2.0 mg kg) is carried out using unfolded partial least squares-residual bilinearization U-PLS/RBL, parallel factor analysis PARAFAC and multivariate curve resolution-alternating least squares MCR-ALS. U-PLS/RBL outperformed other methods with mean recoveries and relative errors of prediction of 100.1 (2.3), 99.3 (6.2), and 93.8 (16.4) for U-PLS/RBL, MCR-ALS, and PARAFAC, respectively. The reason for success of U-PLS/RBL is attributed to its effectiveness for resolving melamine-signal (the basic form) from the signal of milk-extract. PARAFAC and MCR-ALS showed a modest performance and this is attributed to the rank-deficiency in the collected matrices (for test samples), linear dependency (in pH-dimension), and intense spectral-overlapping in the spectral mode. The performance of the proposed method is critically compared with other published methods.