Influence Of Organic Modifiers On The Retention Characteristics Of Graphitized Carbon Column

Carbon nanotube based stationary phases for microchip chromatography

The objective of this article is to provide an overview and critical evaluation of the use of carbon nanotubes and related carbon-based nanomaterials for microchip chromatography. The unique properties of carbon nanotubes, such as a very high surface area and intriguing adsorptive behaviour, have already been demonstrated in more classical formats, for improved separation performance in gas and liquid chromatography, and for unique applications in solid phase extraction. Carbon nanotubes are now also entering the field of microfluidics, where there is a large potential to be able to provide integrated, tailor-made nanotube columns by means of catalytic growth of the nanotubes inside the fluidic channels. An evaluation of the different implementations of carbon nanotubes and related carbon-based nanomaterials for microfluidic chromatography devices is given in terms of separation performance and ease of fabrication.

Assessment of supporting electrolyte contributions in electrochemically modulated liquid chromatography

This paper reports the results of an investigation on the role of the supporting electrolyte in separations using electrochemically modulated liquid chromatography (EMLC) with a porous graphitic carbon stationary phase. With respect to the identity of the supporting electrolyte, the elution strength of the electrolyte anion increased as F- < OH- < BF4- < ClO4- < PF6- for injections of negatively charged aromatic molecules, whereas a 10-fold increase in electrolyte concentration induced a 60% change in retention for the same solutes. Furthermore, both the concentration and composition of the supporting electrolyte affected retention in a manner that varied with the charge of the analyte and applied potential. This behavior is explained using Gouy-Chapman diffuse double layer theory, coupled with comparisons of this theory with closely related models for ion-pair chromatography. Insights into the retention mechanism reveal that an ion-exchange mechanism controls the retention of negatively charged solutes at applied potentials removed from the potential of zero charge (PZC). At potentials close to the PZC, the electrostatic model is less effective with the predominant retention mechanism likely involving hydrophobic interactions with the carbonaceous stationary phase. The combined effects of these findings are demonstrated by using a temporal gradient in supporting electrolyte concentration to optimize an EMLC separation.

Retention characteristics and practical applications of carbon sorbents

Reversed-phase separations provides a versatile technique in high-performance liquid chromatography. Porous graphitized carbon (PGC) support shows unique retention characteristics. Separations on PGC columns use typical reversed-phase eluents (water and organic modifiers miscible with water), however, the retention order of solutes generally does not follow their hydrophobicity order. Molecular hydrophobicity influences but not determines the elution order of any set of solutes. The properties of these supports, mechanisms of retention and application are discussed, along with correlations which can guide the choice of solvent combinations for typical separations.

Separation of steroidal drugs on porous graphitized carbon column

The retention time of 11 steroidal drugs was determined on a porous graphitized carbon (PGC) column using water tetrahydrofuran mixtures as eluents. Linear correlations were calculated between the logarithm of the capacity factor (k') and the tetrahydrofuran concentration in the eluent. To find the molecular parameters significantly influencing the retention the intercept and slope values of the above relationship were correlated with the physicochemical characteristics of steroidal drugs using principal component analysis (PCA). Both the slope and intercept values show high differences proving that steroidal drugs can be successfully separated on a PGC column. Five principal components explained > 90% of the total variance. Calculations indicated that both electrostatic interactions and sterical conditions can influence the retention of steroidal drugs on PGC column.