Electrochemically modulated liquid chromatographic separation of triazines and the effect of pH on retention

A novel switchable water stationary phase for supercritical fluid chromatography

A novel pH switchable water stationary phase is presented for use in supercritical fluid chromatography (SFC). By adding NH4OH to the water coating and system hydration, changes in CO2 pressure and temperature allow a wide range of stationary phase pH conditions (∼3-9) to be achieved, which impact analyte retention properties. For example, 100 atm and 50 °C produces an acidic water stationary phase (pH near 4.0) where octanoic acid readily elutes while the base caffeine does not. Conversely, at 80 atm and 120 °C a basic water stationary phase (pH near 8.0) is obtained and the opposite occurs. Further, under constant pressure and temperature conditions, simply adding or removing NH4OH from the system is also found to readily allow switching between the basic and acidic water stationary phase modes and demonstrates control over ionizable analyte elution. For instance, hexanoic acid elution is near 40 times more delayed on a basic water stationary phase and, as such, it can be eluted at later points in time as desired by removing the NH4OH and switching to an acidic stationary phase. Experiments indicate that stationary phase pH switching occurs uniformly across the 15 m column length within about 18 s and that analyte retention times are very reproducible upon performing a switch (1.4% RSD; n = 3). Results demonstrate the selectivity factor between acidic and neutral analytes can be reversed and increased about 35 times, while in other trials resolution also similarly increased near 40-fold. By rapidly switching the stationary phase pH back and forth between acidic and basic modes, the selectivity between ionizable analytes could also be increased as desired. Various applications with the system show that it can vastly increase the separation between target analytes and matrix components as required by the dynamics of a particular separation.

A Carbon Nanotube Packed Bed Electrode for Small Molecule Electrosorption: An Electrochemical and Chromatographic Approach for Process Description

Triggering the interaction of nanomaterials with molecules by means of electrical potentials in aqueous media remains challenging, especially if 3D through-flow systems are used as electrodes, as in potential-controlled liquid chromatography (PCC). In this paper, multi-walled carbon nanotubes (MWCNTs) function as a particulate packed bed electrode in order to study the system’s response to various applied potentials and electrolyte compositions. The process principle was analyzed using chronoamperometry and cyclic voltammetry. Applying an electrical potential to the hydrophilic MWCNTs induces the presence of both capacitive and faradaic currents. This leads, over time, to a degradation of the electrode due to structural changes of the MWCNT matrix and an increase in redox reactions on the surface. The role of the electrochemical double layer (EDL) is highlighted as a main player in the process, directly influencing the adsorption capability of the electrode. The EDL rearrangement time and coverage radius depend on the composition of the mobile phase and on the potential applied. The capacity of the electrode for the target (maleic acid) increases at high positive potentials (+800 mV vs. Ag/AgCl), while the presence of electrolytes leads to a capacity decrease. Our research enhances the understanding of capacitive through-flow cells.

Electrochemically Modulated Liquid Chromatography in Fused Silica Capillary Columns

Electrochemically modulated liquid chromatography (EMLC) uses electrical potentials, applied to a conductive chromatographic stationary phase (e.g., porous graphitic carbon [PGC]), to manipulate analyte retention. This paper reports the design of a capillary EMLC column with a smaller internal diameter (ID; 250 μm) than that of the standard bore predecessor (3.3 mm ID). The new capillary EMLC columns are configured so that the PGC stationary phase serves as the working electrode in a two-electrode electrochemical cell and simplifies electrode placement by obviating the need for a counter electrode. This configuration also eliminates the internal Nafion sleeve that is critical to operation for the standard bore columns, thereby avoiding Nafion deformation as a source of chromatographic band broadening and rupturing as a mode of column failure. Indeed, values for chromatographic efficiency obtained on the capillary columns meet or exceed those measured for the standard columns (20 000-40 000 vs 14 000 plates/m, respectively) with near symmetric elution bands (asymmetry factors of 1.1-1.4 for well-packed capillaries) that surpass band symmetries observed in all prior studies. A test suite of aromatic sulfonates was used to characterize the chromatographic performance of the capillary EMLC columns. Separations of this test mixture showed that retention factors for individual analytes could be manipulated by as much as 21× by changing the applied potential at the PGC stationary phase. Changes in retention behavior at different potential ranges, hypothesized to result from differences in adsorption orientation, were also observed and are consistent with past work. Collectively, the retention behavior unique to EMLC is operative in this new capillary configuration and promises to open new avenues in tuning LC separations.

Electrical field-induced extraction and separation techniques: promising trends in analytical chemistry--a review

Sample preparation is an important issue in analytical chemistry, and is often a bottleneck in chemical analysis. So, the major incentive for the recent research has been to attain faster, simpler, less expensive, and more environmentally friendly sample preparation methods. The use of auxiliary energies, such as heat, ultrasound, and microwave, is one of the strategies that have been employed in sample preparation to reach the above purposes. Application of electrical driving force is the current state-of-the-art, which presents new possibilities for simplifying and shortening the sample preparation process as well as enhancing its selectivity. The electrical driving force has scarcely been utilized in comparison with other auxiliary energies. In this review, the different roles of electrical driving force (as a powerful auxiliary energy) in various extraction techniques, including liquid-, solid-, and membrane-based methods, have been taken into consideration. Also, the references have been made available, relevant to the developments in separation techniques and Lab-on-a-Chip (LOC) systems. All aspects of electrical driving force in extraction and separation methods are too specific to be treated in this contribution. However, the main aim of this review is to provide a brief knowledge about the different fields of analytical chemistry, with an emphasis on the latest efforts put into the electrically assisted membrane-based sample preparation systems. The advantages and disadvantages of these approaches as well as the new achievements in these areas have been discussed, which might be helpful for further progress in the future.