Normal-phase high-performance liquid chromatography of volatile compounds Selectivity and mobile phase effects on polar bonded silica

Descriptors for terpene esters from chromatographic and partition measurements: Estimation of human odor detection thresholds

We have used gas chromatographic retention data together with other data to obtain Abraham descriptors for 30 terpene esters. These include the air-water partition coefficient, as log Kw, for which no experimental values are available for any terpene ester. The other descriptors are the ester dipolarity, S, the hydrogen bond basicity, B, (the ester hydrogen bond acidity is zero for the esters studied), and L the logarithm of the air-hexadecane partition coefficient. Both S and B are larger than those for simple aliphatic esters, as expected from the terpene ester structures that include ring systems and ethylenic double bonds. These descriptors can then be used to obtain a large number of physicochemical and environmental properties of terpene esters. We have analyzed experimental results on human odor detection thresholds and have constructed another equation for the calculation of these thresholds, to go with a previous equation that we have reported. Then the descriptors for terpene esters can be used to estimate the important odor detection thresholds.

Normal-phase liquid chromatography retention behavior of polycyclic aromatic hydrocarbon and their methyl-substituted derivatives on an aminopropyl stationary phase

Retention indices for 124 polycyclic aromatic hydrocarbons (PAHs) and 62 methyl-substituted (Me-) PAHs were determined using normal-phase liquid chromatography (NPLC) on a aminopropyl (NH2) stationary phase. PAH retention behavior on the NH2 phase is correlated to the total number of aromatic carbons in the PAH structure. Within an isomer group, non-planar isomers generally elute earlier than planar isomers. MePAHs generally elute slightly later but in the same region as the parent PAHs. Correlations between PAH retention behavior on the NH2 phase and PAH thickness (T) values were investigated to determine the influence of non-planarity for isomeric PAHs with four to seven aromatic rings. Correlation coefficients ranged from r = 0.19 (five-ring peri-condensed molecular mass (MM) 252 Da) to r = -0.99 (five-ring cata-condensed MM 278 Da). In the case of the smaller PAHs (MM ≤ 252 Da), most of the PAHs had a planar structure and provided a low correlation. In the case of larger PAHs (MM ≥ 278 Da), nonplanarity had a significant influence on the retention behavior and good correlation between retention and T was obtained for the MM 278 Da, MM 302 Da, MM 328 Da, and MM 378 Da isomer sets. Graphical abstract NPLC separation of the three-, four-, five-, and six-ring PAH isomers with different number of aromatic carbon atoms and degrees of non-planarity (Thickness, T). The inserted figure plots the number of aromatic carbon atoms vs. the log I value for the 124 parent PAHs.

Simultaneous Determination of Various Isothiocyanates by RP-LC Following Precolumn Derivatization with Mercaptoethanol

Numerous isothiocyanates (ITCs) are poorly soluble in water which causes their precipitation in aqueous mobile phases used in reversed phase liquid chromatography (RP-LC), thus impacting the accuracy of the quantification. By comparing the amounts of ITCs injected and released from the column, losses could be estimated at 5-32% depending on polarities and concentrations. Results could be dramatically improved in terms of separation and quantification using RP-LC with a mercaptoethanol precolumn derivatization aimed at avoiding ITCs precipitation. The cancer chemoprotective allyl-ITC and sulforaphane were found in cabbage extracts at 1.2 and 2.7 μg g(-1) fresh weight, respectively.

Validation of a high-performance liquid chromatography method for the determination of (-)-alpha-bisabolol from particulate systems

A reversed-phase high performance liquid chromatography method has been developed and validated for determination and quantitation of the natural sesquiterpene (-)-alpha-bisabolol. Furthermore the application of the method was done by characterization of chitosan milispheres and liposomes entrapping Zanthoxylum tingoassuiba essential oil, which contains appreciable amount of (-)-alpha-bisabolol. A reversed-phase C(18) column and gradient elution was used with the mobile phase composed of (A) acetonitrile-water-phosphoric acid (19:80:1) and (B) acetonitrile. The eluent was pumped at a flow rate of 0.8 mL/min with UV detection at 200 nm. In the range 0.02-0.64 mg/mL the assay showed good linearity (R(2 )= 0.9999) and specificity for successful identification and quantitation of (-)-alpha-bisabolol in the essential oil without interfering peaks. The method also showed good reproducibility, demonstrating inter-day and intra-day precision based on relative standard deviation values (up to 3.03%), accuracy (mean recovery of 100.69% +/- 1.05%) and low values of detection and quantitation limits (0.0005 and 0.0016 mg/mL, respectively). The method was also robust for showing a recovery of 98.81% under a change of solvent in standard solutions. The suitability of the method was demonstrated by the successful determination of association efficiency of the (-)-alpha-bisabolol in chitosan milispheres and liposomes.

Solvent excess adsorption on the stationary phases for reversed-phase liquid chromatography with polar functional groups

The measurement of acetonitrile and methanol adsorption was carried out on stationary phases with specific functionalities. The results were compared with the adsorption of those solvents on alkyl-modified adsorbents. This comparison allows us to describe the effect of polar groups on the adsorption of the organic modifiers. Our results clearly demonstrate how the functional groups modify the chromatographic properties of the homogeneous hydrophobic adsorbents.

Structure-retention and mobile phase-retention relationships for reversed-phase high-performance liquid chromatography of several hydroxythioxanthone derivatives in binary acetonitrile-water mixtures

The reversed-phase high-performance liquid chromatographic (RP-HPLC) behavior of some newly synthesized hydroxythioxanthone derivatives using binary acetonitrile-water mixtures as mobile phase has been examined. First, the variation in the retention time of each molecule as a function of mobile phase properties was studied by Kamlet-Taft solvatochromic equations. Then, the influences of molecular structure of the hydroxythioxanthone derivatives on their retention time in various mobile phase mixtures were investigated by quantitative structure-property relationship (QSPR) analysis. Finally, a unified model containing both the molecular structure parameters and mobile phase properties was developed to describe the chromatographic behavior of the systems studied. Among the solvent properties, polarity/polarizability parameter (pi*) and hydrogen-bond basicity (beta), and among the solute properties, the most positive local charge (MPC), the sum of positive charges on hydrogen atoms contributing in hydrogen bonding (SPCH) and lipophilicity index (logP) were identified as controlling factors in the RP-HPLC behavior of hydroxythioxanthone derivatives in actonitrile-water binary solvents.

In situ adsorption studies at silica/solution interfaces by attenuated total internal reflection fourier transform infrared spectroscopy: examination of adsorption models in normal-phase liquid chromatography

ATR-FT-IR spectroscopy was employed to the study the adsorption of ethyl acetate and 2-propanol to the surface of thin silica sol-gel films in contact with n-heptane solutions. In situ vibrational spectra of silica-adsorbed species provided information regarding the mechanisms of solute retention and elution in normal-phase chromatography. Previous normal-phase chromatographic studies of ethyl acetate adsorption revealed nonlinear isotherms which were explained by both bilayer and adsorbate delocalization models. Infrared spectra of ethyl acetate at the silica surface versus concentration showed that nonlinear adsorption can be attributed to site heterogeneity, where adsorption to free silanols and surface-adsorbed water can be distinguished. Least-squares modeling of the data produced resolved spectra for the two sites and adsorption equilibrium constants that differed by about an order of magnitude. Adsorption of 2-propanol was best modeled by a single Langmuir isotherm showing no significant difference in adsorption energy for the two sites; 2-propanol was shown to easily displace ethyl acetate from the silica surface. Ethyl acetate could also displace 2-propanol from the silica, and least-squares modeling again revealed two-adsorbed-component spectra for ethyl acetate that were indistinguishable from spectra obtained when ethyl acetate adsorbed directly onto the surface.

Development of a new cyano-bonded column for high-performance liquid chromatography

A unique cyano-bonded column for high-performance liquid chromatography was developed by chemically bonding cyanopropyl groups to the silica gel support, and its chromatographic performance was described. Eight cyanopropyl-bonded silica gels were prepared under different conditions. These packing materials were packed into a stainless steel column, the chromatographic properties and durability of which were investigated in both normal- and reversed-phase partition modes. The separating selectivity and the durability of cyanopropyl-bonded silica (CN) columns were dependent on the preparation conditions. Particularly, the relationship between the density of cyanopropyl groups on the surface of the silica gel and the irreversible adsorption of basic compounds was examined. Also, the experimental results indicated that endcapping resulted in the poor separating selectivity and durability of the CN columns.