Reversed phase behaviour of high molecular mass polystyrenes in ethyl acetate solvent mixtures

Chromatographia - The reversed phase behaviour of high molecular mass polystyrenes was investigated on C18 bonded phase columns with ethyl acetate-methanol and ethyl acetateacetonitrile mobile...     

Two dimensional reversed-phase-reversed-phase separations isomeric separations incorporating C18 and carbon clad zirconia stationary phases

Informational theory and a geometric approach to factor analysis were employed to evaluate the degree of orthogonality of a two-dimensional reversed-phase-reversed-phase chromatographic system. The system incorporated a C18 column as one dimension and a carbon clad zirconia column as the second dimension. In order to study the resolving power of this system, the separation of a sample matrix containing an artificial mix of 32 isomers (structural and diastereoisomers) was evaluated. Using this system, between 25 and 28 of the 32 isomers could be separated, depending on the mobile phase combinations--with resolution that could not possibly be achieved in a single one dimensional separation. The results from this study indicate that in order to fully evaluate the resolving power of a 2D system multiple methods of analysis are most appropriate. This becomes increasingly important when the sample contains components that are very closely related and the retention of solutes is clustered in one quadrant of the 2D space. Ultimately, the usefulness of the 2D separation is determined by the goals of analyst.     

Evaluation of the secondary consolidation of columns for liquid chromatography by ultrasonic irradiation

The consolidation of packed analytical chromatography columns was carried out under ultrasonic irradiation. Columns were first packed using a conventional high pressure downward slurry method. Then, they were subjected to further bed consolidation in the presence of ultrasonic vibration. This process of further bed consolidation is referred to as secondary consolidation. Secondary consolidation was observed to occur more readily in solvents of low viscosity and at low flow-rates (low pressures). Column efficiency was not observed to be a factor affecting the process of secondary consolidation of the packed bed.     

Personal Sun Exposure and Serum 25‐hydroxy Vitamin D Concentrations

Solar ultraviolet‐B radiation (UVB) is essential for epidermal vitamin D production. We aimed to quantitate the relationship between personal solar UV exposure and serum 25hydroxy vitamin D (25[OH]D) concentration. Blood was collected for 25(OH)D analysis in 207 South Australian adults aged 27–61 years. At the time of blood collection, each participant completed a questionnaire, which included a calendar for recall of sun exposure in the preceding 16 weeks. We examined the association between solar UV exposure and serum 25(OH)D graphically from smoothed scatter plots, and modeled it using multiple linear regression, with age, sex and body mass index as covariates. Estimated erythemal solar UV exposure in the 6 weeks before blood collection best predicted serum 25(OH)D concentrations. Serum 25(OH)D rose with increasing personal solar UV exposure to a maximum of about 89 nmol L^?1 at an estimated mean weekly solar erythemal UV exposure of about 1230 mJ cm^?2. The maximum was the same after accounting for clothing coverage and was reached at an estimated whole body equivalent exposure to ambient UV of ca 700 mJ cm^?2. These results suggest that an average maximum serum 25(OH)D of ca 89 nmol L^?1 is achieved from sun exposure in a healthy Australian adult population.     

Evaluation of the two-dimensional reversed-phase-reversed-phase separations of low-molecular mass polystyrenes

The resolving power of four reversed-phase-reversed-phase two-dimensional (2D) chromatographic systems was evaluated using information theory (IT) and a geometric approach to factor analysis. The first separation dimension employed a C18 column, while the second separation dimension employed a carbon clad zirconia column. Mobile phases in each dimension were either methanol or acetonitrile. The sample matrix that was employed in this study contained a mixture of 58 components, comprised of stereoisomers and structural isomers of a systematic variation in molecular weight. Each of the components were oligostyrenes, with between two and five configurational repeating units having either n-butyl, sec-butyl or tert-butyl end groups. In the two-dimensional systems employed in this study, between 46 and 49 of the 58 components could be separated, depending on the mobile phase combinations--with apparent resolution that could not possibly be achieved in a single one-dimensional separation. The results from this study indicate that in order to fully evaluate the resolving power of a 2D system multiple methods of analysis that evaluate the separation potential are most appropriate. This becomes increasingly important when the sample contains components that are very closely related and the retention of solutes displays a high degree of solute crowding.     

Performance of columns packed with the new shell particles,Kinetex-C18

The performance of the new Kinetex-C₁₈ column was investigated. Packed with a new brand of porous shell particles, this column has an outstanding efficiency. Once corrected for the contribution of the instrument extra column volume, the minimum values of the reduced plate heights for a number of low molecular weight compounds (e.g., anthracene and naphtho[2,3-a]pyrene) were between 1.0 and 1.3, breaking the legendary record set 3 years ago by Halo-C₁₈ packed columns. The liquid-solid mass transfer of proteins (e.g., insulin and lyzozyme) is exceptionally fast on Kinetex-C₁₈ much faster than on the Halo-C₁₈ column. The different contributions of dispersion and mass transfer resistances to the column efficiency were determined and discussed. The possible reasons for this extremely high column efficiency are discussed.     

The Development of the In Situ Modification of 1st Generation Analytical Scale Silica Monoliths

Analytical scale silica monoliths are commercially limited to three column selectivities (bare silica, C8 and C18). An in situ modification is reported in detail to overcome this barrier and allow for any functionality of choice to be bonded to the silica surface of the monolithic stationary phase support. The modification method was conducted on a commercial bare silica column to bond the C18 moiety to the silica surface through a silylation reaction. The C18 type of stationary phase was chosen, as this is the most commonly bonded functionality for the majority of stationary phases used for high-performance liquid chromatography (HPLC) separations. The C18-modified monolith’s performance was compared to a commercial C18 monolithic and a particle packed column of the same analytical scale column dimensions (100 × 4.6 mm). The modified C18 monolith proved to be of high quality with an efficiency of 73,267 N m^?1, fast analysis times (operated at flow rates up to 3 mL min^?1 using a conventional 400 bar HPLC system) and improved resolution of a set of polar and non-polar substituted aromatics in comparison to a commercial C18 monolith.     

The design of a new concept chromatography column

Active Flow Management is a new separation technique whereby the flow of mobile phase and the injection of sample are introduced to the column in a manner that allows migration according to the principles of the infinite diameter column. A segmented flow outlet fitting allows for the separation of solvent or solute that elutes along the central radial section of the column from that of the sample or solvent that elutes along the wall region of the column. Separation efficiency on the analytical scale is increased by 25% with an increase in sensitivity by as much as 52% compared to conventional separations.     

Anomalies in evaporative light scattering detection

A two-dimensional (2-D) “heart-cutting” HPLC system was used to fractionate oligostyrenes into the respective diastereoisomers. For samples of known composition, the response of an ultraviolet (UV) absorbance detector followed the anticipated pattern. The response of an evaporative light-scattering (ELSD) detector on the other hand indicated quite different concentrations for the two diastereoisomers, relative to what was anticipated and what was indicated by the UV detector. Whereas approximately the same concentration was indicated by UV, ELSD in some cases indicated no detection of the later eluting isomer. The magnitude of the errors depended on both the molecular weight and the tacticity of the diastereomers. These anomalies appear to be an artifact of power transform functions imbedded within the firmware processor of the ELSD, invisible to the user.     

Study of the selectivity of reversed-phase columns for the separation of polycarboxylic acids and polyphenol compounds

A systematic investigation was undertaken into the relative separation performance of five reversed-phase chromatography columns including some commercially new hybrid packed columns for a series of polycarboxylic acids and polyphenol compounds. Information theory (IT) and factor analysis (FA), together with a basic evaluation of retention information (band shape, retention factor and elution order) were used to compare four columns to a conventional C18 column. The results revealed very little difference in retention behaviour between the Phenomenex Aqua C18 column, the Waters XTerra RP C18 column, and the conventional Phenomenex Luna C18 column. However, there were notable differences in the retention processes between the Phenomenex Synergi polar-RP column, which is an ether-linked phenyl base with polar endcapping, and the Luna C18 column. The most significant differences were observed between the Luna C18 column and a Phenomenex Luna Cyano column. However, the limited degree of retention of the polycarboxylic acids and polyphenol compounds on the Luna Cyano column permits only limited use for the separation of these types of compounds. Overall, the Phenomenex Synergi polar-RP column exhibited the best performance for the separation of the test solutes compared to that of the conventional C18 column, with IT yielding an Informational Similarity of 0.99 and FA a moderate correlation coefficient of 0.70. The Phenomenex Synergi polar-RP column gave the best peak shape and offered substantial selectivity differences thereby providing a good alternative over the conventional C18 column for separating polycarboxylic acids and polyphenols.