Prevalence and morphometric analysis of the retromolar canal in a Spanish population sample: a helical CT scan study.

BACKGROUND: The retromolar canal (RMC) is an anatomical variation of the mandibular canal (MC) whose identification and study should be considered given its implication in the surgical procedures of the retromolar area. The prevalence of the RMC widely varies according to previous studies and may be influenced by the followed study method. This work aimed to evaluate the prevalence of the RMC in a Spanish population sample. MATERIAL AND METHODS: For this purpose, 225 CT scan images (with a higher resolution than the cone beam CT used in other previous studies) from the Hospital Clínico Universitario de Valencia were analyzed. The Osirix MD® radiological image analysis system was applied to analyse the dimensions, location in the retromolar area and morphologic characteristics of the RMC by classifying them according to their typology. Furthermore, the relations between the RMC and gender, age and laterality were studied. RESULTS: RMC prevalence was 23.1%. No significant relation between the presence of the canal and gender, age or laterality was found. Type Ia was the commonest type with a prevalence of 40.8%. CONCLUSIONS: Based on the results of this study, the RMC should be considered a frequent anatomical variation whose complete study is very important in daily clinical practice.

Considerations on the modelling and optimisation of resolution of ionisable compounds in extended pH-range columns.

The problems associated to the modelling and optimisation of the chromatographic resolution of mixtures involving ionisable solutes at varying pH and acetonitrile content are discussed. Several retention models that separate the contributions of solute, column and stationary phase, were used. The retention was predicted with low errors in large pH domains (2-12), which was an essential requirement to face the optimisation of resolution. The selected mixture was particularly problematic under the viewpoint of resolution, owing to the excessively diverse acid-base behaviour of solutes. This variety led to sudden drops in retention at different pH for each solute, yielding numerous peak crossing, which made finding shared regions of high resolution especially difficult. Conventional resolution diagrams for these situations are scarcely informative, since both the overall and the worst elementary resolutions drop to zero if at least two compounds remain overlapped, even when all the others are baseline resolved. A new chromatographic objective function is proposed to address this drawback. This function, called "limiting peak count", is based on the limiting peak purity concept, and measures the success in the resolution focusing on the resolved solutes, in contrast to conventional resolution assessments that attend mainly to the least resolved solutes. Limiting peak count yields the same result as conventional assessments when full resolution is possible, but it is also able to discriminate the maximal resolving power in low-resolution situations. It offers a different perspective to that given by the complementary mobile phases approach, and the computation is far simpler.

Prediction of the retention in reversed-phase liquid chromatography using solute-mobile phase-stationary phase polarity parameters.

A previously reported algorithm, based on the equation: log k = (log k)o + p(PN(m) - PN(s)), that relates the retention in reversed-phase liquid chromatography with solute (p), mobile phase (PN(m)) and stationary phase (PN(s)) relative polarity parameters, is improved. The retention data reported by several authors for different sets of compounds, eluted with acetonitrile-water and methanol-water mixtures, are used to test the algorithm and elaborate a database of p values. The methodology is successfully applied to predict the retention using PN(m), values calculated as PN(m) = 1.00 - (2.13phi)/(1+1.4phi) for acetonitrile-water and PN(m) = 1.00 - (1.33phi)/(l1 + 0.47phi) for methanol-water, phi being the organic solvent volumetric fraction. The polarity parameters are demonstrated to be useful to transfer retention data between solvent systems and between columns. Accordingly, the retention in a solvent system is predicted by characterising the working column with a small training set of compounds having diverse polarities, and using the p values known for another solvent system or column. The p polarity parameter is found to be a good descriptor of the retention, allowing the prediction of the expected elution order and peak overlaps.

Retention of ionizable compounds on HPLC. 8. Influence of mobile-phase pH change on the chromatographic retention of acids and bases during gradient elution.

The relationships between retention and mobile-phase pH in gradient elution are studied for acids and bases. The apparent pH shift caused by the increasing amount of acetonitrile and methanol has been determined starting from a wide range of pH values. It is shown that good relationships between the retention of ionizable compounds and the pH of the aqueous buffer can be established if the same type of buffer (ammonium acetate in this work) is used for all pH points. Equations are proposed to fit the gradient retention data to the pH of the aqueous buffer. The proposed equation gives an account of the relative variation of the pKa of the compound in the reference to the variation of the pH of the buffer as both parameters change during gradient elution.

Retention of ionizable compounds on HPLC. 6. pH measurements with the glass electrode in methanol-water mixtures.

The relationship, delta values, between the two rigorous pH scales, S(S)pH (pH measured in a methanol-water mixture and referred to the same mixture as standard state) and S(W)pH (pH measured in a methanol-water mixture but referred to water as standard state), in several methanol-water mixtures was determined (delta = S(W)pH-S(S)pH). Delta values were measured using a combined glass electrode and a wide set of buffer solutions. The results are consistent with those obtained with the hydrogen electrode. This confirms the aptness of the glass electrode to achieve rigorous pH measurements in methanol-water mixtures. An equation that relates delta and composition of methanol-water mixtures, and allows delta computation at any composition by interpolation, is proposed. Therefore, S(S)pH can be achieved from the experimental S(W)pH value and delta at any mobile phase composition. S(S)pH (or S(W)pH) values are related to the chromatographic retention of ionizable compounds through their thermodynamic acid-base constants in the methanol-water mixture used as mobile phase. These relationships were tested for the retention variation of several acids and bases with the pH of the mobile phase. Therefore, the optimization of the mobile phase acidity for any analyte can be easily reached avoiding the disturbances observed when W(W)pH is used.

Retention of ionizable compounds on high-performance liquid chromatography. VII. Characterization of the retention of ionic solutes in a C18 column by mass spectrometry with electrospray ionization.

The elution of ions from a C18 column with mobile phases containing methanol (60%, v/v) and aqueous buffers is studied by mass spectrometry. It is demonstrated that the anions are excluded from the stationary phase by the ionized silanols. However, the ionized silanols interact strongly with cations, which are retained in the column. These cations are later eluted from the column by ion exchange with the cations present in the pH buffered mobile phase. The size of the ions, the mobile phase cation concentration and the mobile phase pH are the main parameters that affect elution of the retained cations. It is also demonstrated that there are at least two different types of ionizable silanols, with different acidities, that contribute to the retention of cations. An estimate of the pKa values of these two groups of silanols in 60% methanol is given.

Solute-solvent interactions in micellar electrokinetic chromatography. Selectivity of lithium dodecyl sulfate-lithium perfluorooctanesulfonate mixed-micellar buffers.

The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate as surfactant. The variation in MEKC surfactant composition results in changes in the coefficients of the correlation equation, which in turns leads to information on solute-solvent and solute-micelle interactions. Lithium perfluorooctanesulfonate is more dipolar and hydrogen bond acidic but less polarizable and hydrogen bond basic than lithium dodecyl sulfate. Therefore mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate cover a very wide range of polarity and hydrogen bond properties, which in turn results in important selectivity changes for analytes with different solute properties.

Unique selectivity of perfluorinated stationary phases with 2,2,2-trifluoroethanol as organic mobile phase modifier.

The selectivity of Luna C18 Xterra C18 and Fluophase (perfluorinated C6) stationary phases has been investigated with aqueous acetonitrile, methanol and 2,2,2-trifluoroethanol mobile phases using linear solvation equations. The gradient retention times of a set of 60 compounds with known molecular descriptors have been determined. Linear solvation equations have been set up to describe the relationship between the gradient retention times and the molecular properties. The selectivity of the stationary phase/mobile phase systems was characterised by the regression coefficients of the molecular descriptors. The perfluorinated stationary phase showed very different selectivity using 2,2,2-trifluoroethanol (TFE) as co-solvent. Compounds with H-bond donor functionality were retained much less than in the other investigated high-performance liquid chromatography (HPLC) systems. This unique selectivity can be explained by the stronger adsorption of trifluoroethanol on the perfluorinated stationary phase surface, than on the hydrocarbon surface. It suggests the importance of the adsorbed organic modifiers in the separation mechanism during reversed-phase HPLC.

Retention of ionizable compounds on HPLC. 5. pH scales and the retention of acids and bases with acetonitrile-water mobile phases

The pH calibration procedures that lead to the different pH scales in acetonitrile-water mixtures used as mobile phases in reversed-phase liquid chromatography are discussed. Appropriate buffers of known pH value in acetonitrile-water mixtures are selected and used to establish the relationship (delta values) between the two rigorous acetonitrile-water pH scales: sspH and wspH (pH measured in acetonitrile-water mixtures and referred to acetonitrile-water or water, respectively, as standard state). These delta values allow one to convert pH values measured in acetonitrile-water with electrode systems calibrated with aqueous buffers (wspH scale) to sspH values, which are directly related to the thermodynamic acid-base constants. This offers an easy way to measure the pH of acetonitrile-water mobile phases and to relate this pH to the chromatographic retention of acids and bases through the thermodynamic acid-base constants. The relationships are tested for the variation of the retention of acids and bases with the pH of the mobile phase at several mobile-phase compositions and favorably compared with the relationships obtained with the common wwpH scale (pH measured in the aqueous buffer before mixing it with the organic modifier). The use of the rigorous sspH and wspH scales allows one to explain the retention behavior of bases, which in many instances cannot be justified from the pH measurement in the ill-founded wwpH scale.

Interpretive optimisation strategy applied to the isocratic separation of phenols by reversed-phase liquid chromatography with acetonitrile-water and methanol-water mobile phases.

An optimisation protocol is presented for the resolution of complex mixtures in isocratic RPLC with binary mobile phases of organic solvent and water, which is based on the prediction of peak position and shape of the individual compounds. A good description of the retention was achieved through the application of statistical weights to the widely used linear or quadratic relationships between the logarithm of the retention factor (log k) and the organic solvent concentration in the mobile phase. The maximisation of the product of peak purities for each compound is shown as a competitive resolution strategy versus the worst value of a selectivity parameter. Peak purities allow one to associate a single resolution value to each compound, which is not affected by the identity of the interfering peaks. It is shown how when full resolution is not achieved with a single mobile phase, the same experimental data set (retention factors, asymmetries and efficiencies) can be used for finding two or three optimal complementary mobile phases (CMPs). Each CMP resolves fully some compounds in the mixture, while the remaining compounds can overlap among them. The elementary limiting resolutions, which measure the maximal separation degree for each compound, are also given as a useful guide in the selection of the elution conditions. A mixture of 13 phenols (phenol, chloro-, bromo-, nitro- and methyl-derivatives), eluted with acetonitrile-water or methanol-water mobile phases, is used to show the proposed methodology.

Retention of ionizable compounds on HPLC. 4. Mobile-phase pH measurement in methanol/water

The different procedures used in HPLC to measure the pH of a mobile phase are evaluated in terms of the rigorous IUPAC definition of pH. The three procedures evaluated are as follows: measurement of the pH of the aqueous HPLC buffer before mixing it with the organic modifier, measurement of the pH of the HPLC buffer after mixing it with the organic modifier using a pH electrode system calibrated with aqueous buffers, and measurement of the pH of the HPLC buffer after mixing it with the organic modifier but calibrating the electrode system with reference buffers prepared in the same mixed solvent used as mobile phase. Following IUPAC definitions and recommendations, the three pH values can be related with the pH scales: w(w)pH, s(w)pH, and s(s)pH, respectively. The relationships between these three pH scales are also presented. The retention of several compounds with acid/base behavior in a C-18 and a polymeric column with buffered methanol/water as mobile phase is related to the mobile phase pH value measured in the three pH scales. It is demonstrated that the s(w)pH and s(s)pH scales give better relationships than the w(w)pH scale (pH measured in the aqueous buffer before mixing it with the organic modifier), commonly used on HPLC. The s(w)pH scale is specially recommended because of its simplicity of measurement: the pH is measured after mixing the aqueous buffer with the organic modifier, but the pH calibration is performed with the common aqueous reference buffers.

Dissociation constants of phenols in methanol--water mixtures.

A preferential solvation model that relates solute properties with solvent composition in binary mixtures has been applied to the dissociation pKa values of a set of 28 substituted phenols in methanol-water mixtures. The parameters of the model allow estimation of the pKa value of each phenol for any methanol-water composition. Moreover, it is demonstrated that the pKa values of the whole set of phenols at any methanol-water composition are linearly related to the pKa values of the phenols in water. Equations that relate the correlations' slope and intercept values with the solvent composition have been derived and tested with the set of phenols. The general parameters obtained for these equations allow an accurate calculation of the pKa value of any phenol, even of those not included in the original set, at any methanol-water composition solely from the pKa value of the phenol in water. These calculated pKa values can be used for quantitative structure-HPLC retention relationships. The method is tested by comparison of the calculated pKa values with the HPLC determined pKa values of 26 phenols in a polymeric column with a 50% methanol as mobile phase.

Inorganic salts as hold-up time markers in C(18) columns.

A systematic study of the retention behaviour in a C(18) reversed phase liquid chromatography (RPLC) system of several inorganic salts, namely NaNO(2), NaNO(3), KBr, Bu(4)NBr and K(2)Cr(2)O(7), is presented. The behaviour of the markers in unbuffered mobile phases and the marker retention dependence on the pH and ionic strength of the buffered mobile phases have been analyzed. In addition, a comparison between the retention behaviour of the markers and several ionizable analytes in buffered eluents is presented.

Solute-solvent interactions in micellar electrokinetic chromatography. Characterization of sodium dodecyl sulfate-Brij 35 micellar systems for quantitative structure-activity relationship modelling.

The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of sodium dodecyl sulfate and Brij 35 as surfactant. The variation in MEKC surfactant composition results in changes in the coefficients of the correlation equation, which in turns leads to information on solute-solvent and solute-micelle interactions. Since the same solvation model can be used to describe many biological processes, particular MEKC surfactant compositions can be selected that model the solute-solvent interactions of some of these processes. Two different MEKC systems have been selected to model the solute-solvent interactions of two processes of biological interest (octanol-water partition and tadpole narcosis).

Low-cost safe water for the world: a practical interim solution.

A very large segment of the world's population is without a microbiologically safe water supply. It is estimated that in Latin America more than 40% of the population is utilizing water of dubious quality for human consumption. This figure is probably even higher in Africa and areas of southeast Asia. Water used for drinking and food preparation can be an important route of transmission for many of the most widespread and debilitating of the diseases that afflict humans. The cholera pandemic which struck Latin America in January 1991, and has become endemic in many of the countries, continues to exemplify the public health significance of contaminated drinking water. Ideally, this neglected segment of the world's population should be served with piped water systems that provide a continuous supply of microbiologically safe water, but this would require such enormous investments of financial and human resources that it is not reasonable to expect that it will be accomplished. Interim practical measures to assure microbiologically safe water are necessary. The public health intervention to accomplish this is described in this paper and has an annual per family cost of which ranges between $1.50 and $4. It consists of providing individual households with one or preferably two suitable water containers in which to disinfect and store the essential quantities of water that need to be free of pathogens, with the containers of a design that will preclude recontamination of the contents and enable the production and distribution of the water disinfectants to be managed at the local level. It includes the necessary component of public education, promotion and involvement to establish the sustainability of the measures as a community-based endeavor. Investigation and demonstration projects are being carried out in II countries to determine and perfect and appropriate intervention, and it has been proven that it is economically, technically and socially feasible to assure microbiologically safe water for the world's population that is threatened by waterborne diseases. Carefully controlled microbiological analysis of the untreated and treated water shows that waterborne pathogens can be destroyed or inactivated, and carefully controlled epidemiological studies being carried out by the Centers for Disease Control and Prevention show that this intervention achieves considerable reduction in the incidence of water borne disease. It is recommended that all developing countries initiate programs to replicate the health measure described in this paper in order to test its validity and to adapt it to their local conditions.

Retention of Ionizable Compounds on HPLC. 2. Effect of pH, Ionic Strength, and Mobile Phase Composition on the Retention of Weak Acids.

A new model that relates the retention of a weak acid in HPLC columns with the pH and ionic strength of the mobile phase is derived and tested for different benzoic acids in methanol-water mobile phases. The proposed model uses the pH value in the mobile phase instead of the pH value in water, takes into account the effect of the activity coefficients, and considers different holdup times for the neutral and ionic species. The dependence of the holdup time of the ionic species on the mobile phase properties (pH, solvent composition, and ionic strength) is evaluated. It is demonstrated that the holdup time of the neutral species does not depend on the mobile phase properties, but the holdup time of the ionic species depends on the particular buffer used. The proposed equations can be combined with previously derived equations that relate the retention with the solvent composition of the mobile phase to establish a general model that relates the retention of the solute with the significant mobile phase properties: composition, pH, and ionic strength.

The role of parent stress and coping and family functioning in parent and child adjustment to Duchenne muscular dystrophy.

This study found 57% of parents of 35 children with Duchenne Muscular Dystrophy (4 to 14 years of age) to have self-reported poor psychological adjustment. The hypothesized mediational variables of parent appraisal of stress, relative use of palliative coping methods, and level of family conflict accounted for 58% of the variance in general distress, 50% in depressive symptoms, and 31% in anxiety symptoms. In terms of parent-reported child adjustment, 89% of the children were classified by parent report as having a behavior problem pattern. The internalizing profile pattern was most frequent (37%). The parent mediational variables accounted for 26% of the variance in parent-reported internalizing behavior problems and 29% in externalizing behavior problems.

Ionic equilibria in neutral amphiprotic solvents: Structural effects on dissociation constants of several substituted phenols and mercaptopyrimidines in isopropyl alcohol.

The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair formation the incomplete dissociation of the tetrabutylammonium salt has been taken into account in the calculation of pK(a). The dissociation constants of the salts were previously measured conductometrically. The resolution of acid strength in isopropyl alcohol relative to that in water has been determined for each series of acids by plotting the pK(a) values in isopropyl alcohol vs. those in water. The results show greater resolution in isopropyl alcohol than in water. The resolution of acid strength in tert-butyl alcohol relative to that in isopropyl alcohol has also been determined.

Ionic equilibria in neutral amphiprotic solvents of low dielectric constant: Buffer solutions.

The ionic equilibria in neutral amphiprotic solvents (isopropyl and tert-butyl alcohols) have been established, and equations to calculate pH values in solutions of acids, bases, salts or their mixtures, developed. The effect, on the dissociation equilibria, of the presence of small quantities of water or other solvents in the bulk solvent used has been taken into account in the proposed equations. On the basis of these equations some buffer solutions have been studied and recommended for electrode standardization. The results, tested by experimental work, show the importance of the incompleteness of dissociation of salts in these solvents, which decreases the pH of acid buffers and increases the buffer capacity.

Ionic equilibria in neutral amphiprotic solvents of low dielectric constant: Titration curves.

Titration curves have been simulated for the titration of acids, bases and salts in neutral amphiprotic solvents such as isopropyl and tert-butyl alcohols. Ranges of pK values and acid concentrations have been examined. The incomplete dissociation of salts, which increases the acid or basic strength, has been found to be the major factor modifying the shape of the curve and the pH break. The theoretical predictions have been checked by titrating several series of acids and good agreement has been obtained between computed and experimental results.