Guidelines for descriptor assignments for the solvation parameter model by separation techniques.

The solvation parameter model uses six compound descriptors to model equilibrium properties in biphasic systems formally defined as excess molar refraction, E, dipolarity/polarizability, S, overall hydrogen-bond acidity, A, overall hydrogen-bond basicity, B, McGowan's characteristic volume, V, and the gas-liquid partition constant on hexadecane at 25 °C, L. The V descriptor can be assigned from structure and the E descriptor for compounds liquid at 20 °C can be calculated from its refractive index and characteristic volume. The E descriptor for compounds solid at 20 °C and the S, A, B, and L descriptors are assigned from experimental properties traditionally obtained by chromatographic, liquid-liquid partition, and solubility measurements. Here I report an efficient experimental design using the Solver method for the accurate assignment of descriptors for neutral compounds that simultaneously minimizes laboratory resources. This multi-technique approach requires 3 retention factor measurements in a 60 °C temperature range per compound on four columns by gas chromatography, 3 retention factor measurements in a 30 % (v/v) acetonitrile composition range per compound on two columns by reversed-phase liquid chromatography, and eight partition constant measurements by liquid-liquid partition in totally organic and aqueous biphasic systems for a total of 26 experimental measurements. The accuracy of the descriptor assignments was validated by comparison with the values in the Wayne State University (WSU) descriptor database taken as the best estimate of the true descriptor values. The E, S, A, B and L descriptors were assigned simultaneously by the Solver method using the above approach without significant bias and with an average absolute deviation (AAD) of 0.054, 0.018, 0.015, 0.013, and 0.040, respectively, compared with the WSU database values, corresponding to a relative absolute average deviation in percent (RAAD) of 7.2, 1.9, 3.6, 5.1, and 0.84 %, respectively, for 32 varied compounds. This streamlined approach represents a significant improvement on earlier single-technique approaches used as the starting point for the development of the multi-technique approach. For compounds of variable hydrogen-bond basicity modifications to the multi-technique approach were implemented while maintaining the same number of experimental measurements. Acceptable descriptor assignments for B/B° were obtained for compounds liquid at 20 °C for which the E descriptor was available by calculation. For solid compounds at 20 °C the E and B/B° descriptors are restricted to qualitative application where approximate values may be acceptable.

Assessment of liquid-liquid partition for the assignment of descriptors for the solvation parameter model.

The solvation parameter model uses five system independent descriptors to characterize compound properties defined as excess molar refraction, E, dipolarity/polarizability, S, hydrogen-bond acidity, A, hydrogen-bond basicity, B, and McGowan's characteristic volume, V, to model transfer properties between condensed phases. The V descriptor is assigned from structure. For compounds liquid at 20 °C the E descriptor can be assigned from the characteristic volume and its refractive index. The E descriptor for compounds solid at 20 °C and the S, A, and B descriptors are experimental properties traditionally assigned from chromatographic, liquid-liquid partition, and solubility measurements. In this report liquid-liquid partition constants in totally organic and aqueous biphasic systems are evaluated as a standalone technique for descriptor assignments. Using six totally organic biphasic systems the S, A, and B descriptors were assigned with an average absolute deviation (AAD) of about 0.04, 0.03, and 0.04, respectively, compared with the best estimate of the true descriptor values for 65 compounds. The E descriptor for compounds solid at 20 °C can only be estimated with an AAD of approximately 0.1. For six aqueous biphasic systems the B descriptor is assigned with a lower AAD of 0.028 and higher AAD of 0.08 and 0.05 for the S and A descriptors, respectively, than for the totally organic biphasic systems for compounds with a reliable value for the E descriptor. The preferred system for descriptor assignments utilizes both totally organic biphasic systems (heptane-1,1,1-trifluoroethanol, isopentyl ether-propylene carbonate, isopentyl ether-ethanolamine, heptane-ethylene glycol, heptane-formamide, and 1,2-dichloroethane-ethylene glycol) and aqueous biphasic systems (octanol-water, cyclohexane-water) with the possible substitution of some systems with alternative systems of similar selectivity. For 55 varied compounds this combination of eight organic and aqueous biphasic systems resulted in an AAD of approximately 0.03, 0.02, and 0.02 for the S, A, and B descriptors compared to the best estimate of the true descriptor value. For 30 compounds solid at 20 °C the AAD for the E descriptor of 0.11 is poorly assigned. The relative average absolute deviation in percent (RAAD) corresponds to 9.7 %, 3.1 %. 4.0 % and 8.3 % for E, S, A, and B, respectively, for the eight biphasic systems. Liquid-liquid partition is compared to reversed-phase liquid and gas chromatography as a standalone technique for descriptor assignments.

Determination of solvation parameter model compound descriptors by gas chromatography.

The solvation parameter model uses five system independent descriptors to characterize compound properties defined as excess molar refraction, E, dipolarity/polarizability, S, hydrogen-bond acidity, A, hydrogen-bond basicity, B, and the gas-liquid partition constant at 25 °C on n-hexadecane, L, to model transfer properties in gas-condensed phase biphasic systems. The E descriptor for compounds liquid at 20 °C is available by calculation using a refractive index value while E for solid compounds at 20 °C and the S, A, B, and L descriptors are determined by experiment. As a single-technique approach, it is shown that with up to 20 retention factor measurements on four columns comprising a poly(siloxane) containing methyloctyl or dimethyldiphenylsiloxane monomers (SPB-Octyl or HP-5), a poly(siloxane) containing methyltrifluoropropylsiloxane monomers (Rtx-OPP or DB-210), a poly(siloxane) containing bis(cyanopropylsiloxane) monomers (HP-88 or SGE BPX-90), and a poly(ethylene glycol) stationary phase (DB-WAXetr or HP-INNOWAX) are suitable for assigning the S, A, and L descriptors. Using the descriptors in the updated WSU compound descriptor database as target values the average absolute error in the descriptor assignments for 52 varied compounds in the temperature range 60-140 °C was 0.072 for E, 0.016 for S, 0.008 for A, and 0.022 for L corresponding to 30 %, 3.5 %, and 0.6 % as a relative average absolute error for E, S, and L, respectively. For the higher temperature range of 160-240 °C and 34 varied compounds that are liquid at 20 °C the average absolute error for the S, A and L descriptors was 0.026, 0.020, and 0.031, respectively, with the largest relative average absolute error for S of 3.2 % (< 1 % for the L descriptor). For 35 varied compounds that are solid at 20 °C the relative absolute error for the E, S, A, and L descriptors in the higher temperature range was 0.068, 0.035, 0.020, and 0.020, respectively, with a relative average absolute error for E (6.5 %), S (3.5 %) and L (0.88 %). The S, A, and L descriptor can be accurately assigned on the four-column system over a wide temperature range. The E descriptor for solid compounds at 20 °C exhibits greater variability than desirable. The B descriptor cannot be assigned by the four-column system, which lack hydrogen-bond acid functional groups, and is only poorly assigned on the weak hydrogen-bond acid ionic liquid column SLB-IL100.

Determination of the hydrogen-bond basicity descriptor by reversed-phase liquid chromatography.

Except for alkanes, most organic compounds are hydrogen-bond bases. The B° descriptor of the solvation parameter model provides a convenient measure of the effective (or summation) hydrogen-bond basicity of organic compounds. A fast and convenient method to assign the B° descriptor is required to support studies of hydrogen-bonding in separation systems. A two-column system with acetonitrile-water mobile phase compositions and the measurement of up to eleven isocratic retention factors is proposed for this purpose. Several reversed-phase column chemistries and mobile phases were evaluated with the two-column system consisting of a pentafluorophenylpropylsiloxane-bonded and octadecylsiloxane-bonded silica columns recommended for this purpose. To assess the accuracy of the method values for B° were taken from the Wayne State University (WSU) compound descriptor database, which were assigned using conventional multi-technique methods and large datasets. The two-column systems provided an unbiased assignment of B° with an average deviation of 0.008 and an average absolute deviation of 0.021 compared with the target value for 55 varied compounds. The two-column system is unsuitable for assigning the other descriptors used in the solvation parameter model and results in erroneous assignments of B° for nitrogen-containing compounds capable of electrostatic interactions on silica-based reversed-phase columns.

Determination of descriptors for the principal flavor compounds of the cinnamons of commerce by gas chromatography and liquid-liquid partition.

Descriptors for fourteen semivolatile organic compounds associated with the authenticity, botanical origin, and flavor potential of the cinnamons of commerce were determined using the Solver method and experimental retention factors determined by gas chromatography at several temperatures on a minimum of seven selectivity-selected, open-tubular columns and liquid-liquid partition constants in up to twenty totally organic biphasic systems. The six descriptors that encode the solvation properties of the compounds were used to predict water-gas, octanol-gas, and octanol-water partition constants commonly employed to assess environmental distribution properties. For octanol-water partition constants, log KOW, the predicted partition constants exhibited an average absolute deviation of 0.12 for log KOW experimental - log KOW predicted (n = 14). Soil-water, soil-air, urban aerosol-air, skin-water permeation, and non-specific toxicity to the fathead minnow were predicted for the same compounds to assess their potential environmental impact. The product terms of the solvation parameter model provide a useful insight into the contribution of individual intermolecular interactions to the distribution properties of the cinnamon compounds and their environmental impact.

Revised descriptors for polycyclic aromatic and related hydrocarbons for the prediction of environmental properties using the solvation parameter model.

Revised descriptors for twenty-five polycyclic aromatic and related hydrocarbons (PAHs) forming a component of the Wayne State University (WSU) descriptor database are provided for use with the solvation parameter model. The descriptors are determined by the Solver method using experimental data for calibrated gas-liquid and reversed-phase liquid chromatographic retention factors and liquid-liquid partition constants in totally organic biphasic systems. The characteristic solvation properties of the PAHs are accounted for mainly by the additional dispersion interactions (E descriptor) and dipole-type interactions (S descriptor) resulting from the availability of easily polarizable electrons that complement typical dispersion interactions for saturated hydrocarbons. The descriptors afford acceptable prediction of the water-air partition constant (average absolute deviation AAD = 0.17, n = 22), octanol-air partition constant (AAD = 0.12, n = 20), and water-octanol partition constant (AAD = 0.10, n = 23). A two-parameter model containing only the V and B descriptors provides an unbiased prediction of aqueous solubility for the PAHs with an AAD = 0.26 (n = 22). The descriptors estimated by convenient chromatographic and partition constant measurements are demonstrated to be a viable alternative to the experimental determination of environmental properties otherwise only available by tedious, expensive, and low data throughput experimental techniques.

The effect of the assigned descriptors for phthalate esters on the characterization of their separation properties using the solvation parameter model.

Revised descriptors are determined for fifteen phthalate esters for use in the solvation parameter model and form part of the Wayne State University (WSU) compound descriptor database. For thirteen phthalate esters a comparison is made with the same compounds in the Abraham descriptor database. Gas chromatographic retention factors on poly(methyloctylsiloxane), SPB-Octyl, and poly(cyanopropylphenyldimethylsiloxane), DB-225, stationary phases are used to facilitate an assessment of the contribution of cavity formation and dispersion interactions, L descriptor, and dipole-type interactions, S descriptor, to the experimental retention factors (log k) for the phthalate esters with minimum interference from competing intermolecular interactions. The results indicate a systematic overprediction of the cavity and dispersion interaction term and underprediction of dipole-type interactions for the Abraham descriptors compared with the WSU descriptors for the phthalate esters. The average absolute deviation (AAD) for 13 phthalate esters on SPB-Octyl is 0.039 (WSU descriptors) compared with 0.252 (Abraham descriptors) and for 9 phthalate esters on DB-225 0.030 (WSU descriptors) compared with 0.167 (Abraham descriptors). The results for dipole-type interactions are confirmed and extended to include the hydrogen-bond basicity of the phthalate esters, B descriptor, by evaluation of partition constants in aqueous biphasic systems and the n-heptane-2,2,2-trifluoroethanol biphasic system. Differences in the contribution of the hydrogen-bond basicity of the phthalate esters to the experimental partition constants are largely random with respect to database selection but important for the accurate prediction of the partition constants. The AAD for the partition constant for 15 phthalate esters is 0.063 (WSU descriptors) compared with 0.320 (Abraham descriptors) for the heptane-2,2,2-trifluoroethanol biphasic system and 0.13 (WSU descriptors) compared with 0.25 (Abraham descriptors) for 9 phthalate esters in the octanol-water biphasic system. The WSU descriptors for the phthalate esters exhibit a better fit with the experimental data for separation systems and are free of the extreme values predicted for the Abraham descriptors for several phthalate esters.

The effect of descriptor database selection on the physicochemical characterization and prediction of water-air, octanol-air and octanol-water partition constants using the solvation parameter model.

The distribution of neutral compounds in biphasic separation systems can be described by the solvation parameter model using six solute properties, or descriptors. These descriptors (McGowan's characteristic volume, excess molar refraction, dipolarity/polarizability, hydrogen-bond acidity and basicity, and the gas-liquid partition constant on n-hexadecane at 298.15 K) are curated in two publicly accessible databases for hundreds (WSU compound descriptor database) or thousands (Abraham compound descriptor database). These databases were developed independently using different approaches resulting in descriptor values that vary for many compounds. Previously, it was shown that the two descriptor databases are not interchangeable, and the WSU descriptor database consistently demonstrated improved model performance for chromatographic systems where the uncertainty in the dependent variable was minimized by suitable quality control and calibration procedures. In this report we wish to evaluate whether the same conclusions are true for models with a dependent variable containing significant measurement uncertainty. To evaluate this hypothesis, we assembled databases for water-air, octanol-air, and octanol-water partition constants reported by multiple laboratories using various measurement methods. It was found that database selection has little effect on model quality or model predictive capability but significantly affects the assignment of the contribution of individual intermolecular interactions to the dependent variable. The latter information is database specific, and a quantitative comparison of system constants should be restricted to models using the same compound descriptor database.

Selectivity evaluation of extraction systems.

Extraction is the most common sample preparation technique prior to chromatographic analysis for samples which are too complex, too dilute, or contain matrix components incompatible with the further use of the separation system or interfere in the detection step. The most important extraction techniques are biphasic systems involving the transfer of target compounds from the sample to a different phase ideally accompanied by no more than a tolerable burden of co-extracted matrix compounds. The solvation parameter model affords a general framework to characterize biphasic extraction systems in terms of their relative capability for solute-phase intermolecular interactions (dispersion, dipole-type, hydrogen bonding) and within phase solvent-solvent interactions for cavity formation (cohesion). The approach is general and allows the comparison of liquid and solid extraction phases using the same terms and is used to explain the features important for the selective enrichment of target compounds by a specific extraction phase using solvent extraction, liquid-liquid extraction, and solid-phase extraction for samples in a gas, liquid, or solid phase. Hierarchical cluster analysis with the system constants of the solvation parameter model as variables facilitates the selection of solvents for extraction, the identification of liquid-liquid distribution systems with non-redundant selectivity, and evaluation of different approaches using liquids and solids for the isolation of target compounds from different matrices.

Sample preparation for planar chromatography.

High-performance thin-layer chromatography has favorable properties for high-throughput separations with a high matrix tolerance. Sample preparation, however, is sometimes required to control specific matrix interferences and to enhance the detectability of target compounds. Trends in contemporary applications have shifted from absorbance and fluorescence detection to methods employing bioassays and mass spectrometry. Traditional methods (shake-flask, heat at reflux, Soxhlet, and hydrodistillation) are being challenged by automated instrumental approaches (ultrasound-assisted and microwave-assisted solvent extraction, pressurized liquid extraction, and supercritical fluid extraction) and the quick, easy cheap, efficient, rugged, and safe extraction method for faster and streamlined sample processing. Liquid-liquid extraction remains the most widely used approach for sample clean-up with increasing competition from solid-phase extraction. On-layer sample, clean-up by planar solid-phase extraction is increasingly used for complex samples and in combination with heart-cut multimodal systems. The automated spray-on sample applicator, the elution head interface, biological detection of target and non-target compounds, and straightforward mass spectrometric detection are highlighted as the main factors directing current interest toward faster and simpler sample workflows, analysis of more complex samples, and the determination of minor contaminants requiring high concentration factors.

The influence of descriptor database selection on the solvation parameter model for separation processes.

The distribution of neutral compounds in biphasic separation systems can be described by the solvation parameter model using six solute properties, or descriptors. These descriptors characterize the size (McGowan's characteristic volume), V, excess molar refraction, E, dipolarity/polarizability, S, hydrogen-bond acidity and basicity, A and B, and the gas-liquid partition constant on n-hexadecane at 298.15 K, L. McGowan's characteristic volume and the excess molar refraction for liquids are available by calculation (E requires and experimental refractive index). The other descriptors and excess molar refraction for solids are experimental quantities and subject to greater variation or are estimated using computational or empirical models. Solute descriptors for several thousand compounds are available in the Abraham descriptor database and for several hundred compounds in the WSU descriptor database. These publicly accessible databases were developed independently using different approaches and for many compounds provide different descriptor values. In this report we evaluate the effect of mixing descriptors from the two databases on modeling chromatographic retention factors and liquid-liquid partition constants. It is shown that the two descriptor databases are not interchangeable. The WSU descriptor database consistently demonstrates improved model quality as determined by statistical parameters. Model system constants exhibit a general dependence on database selection with an approximately linear trend as a function of the fraction of compounds assigned descriptors from either database. There is no general model performance advantage to using mixed descriptor datasets and no real cause for concern for relatively large datasets containing < 15 % of compounds with descriptors assigned from the other database. For small datasets, descriptor quality is an important variable for adequate model performance.

Planar chromatography - Current practice and future prospects.

Planar chromatography, in the form of thin-layer or high-performance thin-layer chromatography (TLC, HPTLC), continues to provide a robust and widely used separation technique. It is unrivaled as a simple and rapid qualitative method for mixture analysis, or for finding bioactive components in mixtures. The format of TLC/HPTLC also provides a unique method for preserving the separation, enabling further investigation of components of interest (including quantification/structure determination) separated in both time and space from the original analysis. The current practice of planar chromatography and areas of development of the technology are reviewed and promising future directions in the use of TLC/HPTLC are outlined.

Recent advances for estimating environmental properties for small molecules from chromatographic measurements and the solvation parameter model.

The transfer of neutral compounds between immiscible phases in chromatographic or environmental systems can be described by six solute properties (solute descriptors) using the solvation parameter model. The solute descriptors are size (McGowan's characteristic volume), V, excess molar refraction, E, dipolarity/polarizability, S, hydrogen-bond acidity and basicity, A and B, and the gas-liquid partition constant on n-hexadecane at 298.15 K, L. V and E for liquids are accessible by calculation but the other descriptors and E for solids are determined experimentally by chromatographic, liquid-liquid partition, and solubility measurements. These solute descriptors are available for several thousand compounds in the Abraham solute descriptor databases and for several hundred compounds in the WSU experimental solute descriptor database. In the first part of this review, we highlight features important in defining each descriptor, their experimental determination, compare descriptor quality for the two organized descriptor databases, and methods for estimating Abraham solute descriptors. In the second part we focus on recent applications of the solvation parameter model to characterize environmental systems and its use for the identification of surrogate chromatographic models for estimating environmental properties.

Analysis of the solvent strength parameter (linear solvent strength model) for isocratic separations in reversed-phase liquid chromatography.

The solvent strength parameter (slope) and intercept (log kw) of the linear solvent strength model are determined for methanol-water (17 columns), acetonitrile-water (15 columns), acetone-water (7 columns), tetrahydrofuran-water (6 columns), and 2-propanol-water (4 columns) for varied compounds. It is shown that the linear region of the plots of the retention factor (log k) against the volume fraction of organic solvent (ϕ) is largely a system property independent of solute type for compounds with accessible retention factors. Solvent strength parameters for methanol (3.12 ± 0.12), acetonitrile (2.78 ± 0.18), acetone (2.71 ± 0.11), and tetrahydrofuran (2.95 ± 0.24) are assigned for siloxane-bonded silica and porous graphitic carbon columns as an average of the compound-specific experimental values. Statistical analysis indicates that the compound-specific solvent strength parameters are column dependent and limited as a general solvent property. Evaluation of the column-specific solvent strength parameters provides further evidence that the average solvent strength parameter is column dependent. The column-specific solvent strength values are compared with the hypothetical water-organic solvent distribution systems using the system constants of the solvation parameter models as variables confirming the column dependence of the solvent strength parameter. The column-specific solvent strength parameters can be predicted by the solvation parameter model with a typical deviation of about 0.12 over a range of 1.69 to 6.33 for the experimentally determined values. The intercept of the linear solvent strength model is shown to be both column and organic solvent dependent. Compound column-specific values are adequately described by the solvation parameter model. Log kw cannot be recommended as a descriptor of solute properties since it has no clear connection to a real distribution system.

Solvation parameter model: Tutorial on its application to separation systems for neutral compounds.

The solvation parameter model affords a useful tool to model distribution properties of neutral compounds in biphasic separation systems. Common applications include column characterization and method development in gas chromatography; reversed-phase, micellar and hydrophilic interaction liquid chromatography; supercritical fluid chromatography; and micellar electrokinetic chromatography. The characterization of the distribution properties of liquid-liquid partition systems is another major application of this model. This tutorial is aimed at establishing good practices for the application of the model to separation systems. Suitable experimental protocols to determine system constants by multiple linear regression analysis and descriptors by the Solver method are presented; statistical tools to evaluate model quality are discussed; and model-specific data analysis tools based on system maps and correlation diagrams are described.

Determination of physicochemical properties of ionic liquids by gas chromatography.

Over the years room temperature ionic liquids have gained attention as solvents with favorable environmental and technical features. Both chromatographic and conventional methods afford suitable tools for the study of their physicochemical properties. Use of gas chromatography compared to conventional methods for the measurement of physicochemical properties of ionic liquids have several advantages; very low sample concentrations, high accuracy, faster measurements, use of wider temperature range and the possibility to determine physicochemical properties of impure samples. Also, general purpose gas chromatography instruments are widely available in most laboratories thus alleviating the need to purchase more specific instruments for less common physiochemical measurements. Some of the main types of physicochemical properties of ionic liquids accessible using gas chromatography include gas-liquid partition constants, infinite dilution activity coefficients, partial molar quantities, solubility parameters, system constants of the solvation parameter model, thermal stability, transport properties, and catalytic and other surface properties.

Selection of calibration compounds for selectivity evaluation of siloxane-bonded silica columns for reversed-phase liquid chromatography by the solvation parameter model.

For the faster evaluation of selectivity in reversed-phase liquid chromatography of siloxane-bonded silica columns using the solvation parameter model a minimal set of calibration compounds is described suitable for mobile phase composition from 20-70% (v/v) methanol-, acetonitrile-, or tetrahydrofuran-water. The Kennard-Stone uniform mapping algorithm is used to select the calibration compounds from a larger database of compounds with known retention properties used earlier for column selectivity evaluation. Thirty-five compounds are shown to be necessary to minimize the standard deviation of the system constants and to minimize the difference between the system constants determined by conventional calibration and the values obtained for the reduced calibration compounds. The models for SunFire C18 with methanol-, acetonitrile- and tetrahydrofuran-water mobile phase compositions and XBridge Shield RP18, XBridge C8, XBridge Phenyl and Discovery HS F5 with methanol- and acetonitrile-water mobile phase compositions had an average coefficient of determination of 0.996 (standard deviation = 0.003, n = 11) and average standard error of the estimate 0.025 (standard deviation = 0.005, n = 11) for the reduced calibration compounds. Some octadecylsiloxane-bonded silica stationary phases with a high bonding density and methanol-water mobile phase compositions containing ≤ 30% (v/v) methanol exhibit extreme retention factors (log k > 2.5) for the low-polarity, two-ring aromatic compounds in the thirty-five compound calibration set. Alternative calibration compounds with more favorable retention properties are suggested as replacements in these cases. The predictive capability of the calibration models is validated using external test sets characterized by the average error, average absolute error and root mean square error of prediction. For the thirty-five calibration compounds sets the average absolute error 0.026 (standard deviation = 0.009, n = 11) and root mean square error of prediction 0.032 (standard deviation = 0.010, n = 11) confirm the suitability of the calibration models for column selectivity evaluation. System maps for XBridge Shield RP18 for 20-70% (v/v) methanol-water and Synergi Hydro-RP and 50% (v/v) methanol-water at temperatures from 25-65 °C together with a correlation diagram for XBridge Shield RP18 and SunFire C18 are presented as representative applications of the reduced calibration compounds for column selectivity evaluation.

Selectivity evaluation of core-shell sislica columns for reversed-phase liquid chromatography using the solvation parameter model.

For the selectivity evaluation of core-shell silica, siloxane-bonded reversed-phase columns by the solvation parameter model a minimal set of thirty-five calibration compounds were identified for applications with mobile phases containing from 20-70 % (v/v) methanol- or acetonitrile-water. The Kennard-Stone uniform mapping algorithm was employed to select the calibration compounds from a larger database of compounds with known retention properties used previously for column selectivity evaluation. The number and identity of the calibration compounds was optimized by considering the statistical quality of the calibration models, the absolute value for the standard deviation of the individual system constants, and the closeness of the estimated system constants for the calibration models to the values for the full data set on seven core-shell columns with different morphology for mobile phases containing 50 % (v/v) methanol- or acetonitrile-water. The predictive capability of the calibration models was evaluated for an external test set for 50 % (v/v) methanol-water for the same columns with a typical root mean square error of prediction (log k) of 0.028 for the thirty-five calibration compounds (n = 35) models. For all columns and mobile phase compositions the calibration models with n = 35 had a range for the coefficient of determination from 0.999 to 0.970 with < 5 % smaller than 0.99 (84 models) and a standard error of the estimate from 0.011 to 0.057 with 75 % < 0.030. System constants determined at 10 % (v/v) increments of organic solvent were utilized for the construction of system maps and correlation diagrams on Kinetex columns with octadecylsiloxane-bonded (C18 and EVO C18), diisobutyloctadecylsiloxane-bonded (XB-C18) octylsiloxane-bonded (C8), biphenylsiloxane-bonded (Biphenyl), phenylhexylsiloxane-bonded (Phenyl-Hexyl) and pentafluorophenylpropylsiloxane-bonded (F5) silica stationary phases for 20-70 % (v/v) methanol- and acetonitrile-water mobile phases. System maps for the Kinetex Phenyl-Hexyl column and correlation diagrams for columns of similar selectivity (Kinetex C8 and Phenyl-Hexyl) and dissimilar selectivity (Kinetex EVO C18 and Biphenyl) is used to illustrate the general approach to selectivity evaluation.

Recent advances in analytical methods for the determination of citrinin in food matrices.

Citrinin is a toxic small organic molecule produced as a secondary metabolite by fungi types Penicillium, Monascus and Aspergillus and is known to contaminate various food commodities during postharvest stages of food production. During the last 10 years, most reported methods for citrinin analysis employed enzyme-linked immunosorbent assays or high-performance liquid chromatography. Over this same time period, liquid extraction, solid-phase extraction, dispersive liquid-liquid microextraction and QuEChERS were the most cited sample preparation and clean-up methods. In this review the advantages and disadvantages of the various sample preparation, separation and detection methods for citrinin analysis over the last decade are evaluated. Furthermore, current trends, emerging technologies and the future prospects of these methods are discussed.

Evaluation of the solvation parameter model as a quantitative structure-retention relationship model for gas and liquid chromatography.

The Wayne State University (WSU) experimental descriptor database is utilized to bench mark the current capability of the solvation parameter model for use as a quantitative structure-retention relationship tool for estimating retention in gas and reversed-phase liquid chromatography. The prediction error for the retention factors of varied compounds on six open-tubular columns for gas chromatography (Rtx-5 SIL MS, DB-35 ms, RtxCLPesticides, HP-88, HP-INNOWAX and SLB-IL76) and three packed columns for reversed-phase liquid chromatography (SunFire C18, XBridge Shield RP18, and XBridge Phenyl) is used to establish expectations related to current practices. Each column data set was divided into a training set for calibration and a test set for validation employing a roughly 1 to 2 split, such that each test set contained about 40 to 80 varied compounds. The average absolute error for the prediction of retention factors by gas chromatography varied from about 0.1 to 0.4 on the retention factor scale with the larger error typical of stationary phases ranked as the most polar (or cohesive). For reversed-phase liquid chromatography the average error for the prediction of retention factors was 0.3 to 0.5 and generally larger than for gas chromatography. Statistical filters where utilized to identify a group of polycyclic aromatic compounds without hydrogen-bonding functional groups with a larger prediction error on the SunFire C18 column than for other compounds of smaller size, flexible structure or containing hydrogen-bonding functional groups. The heterogeneity of the retention mechanism is speculated to be the main contribution to the prediction error for both gas and liquid chromatography using the solvation parameter model.