Evaluation of three temperature- and mobile phase-dependent retention models for reversed-phase liquid chromatographic retention and apparent retention enthalpy.

Predicting retention and enthalpy allows for the simulation and optimization of advanced chromatographic techniques including gradient separations, temperature-assisted solute focusing, multidimensional liquid chromatography, and solvent focusing. In this paper we explore the fits of three expressions for retention as a function of mobile phase composition and temperature to retention data of 101 small molecules in reversed phase liquid chromatography. The three retention equations investigated are those by Neue and Kuss (NK) and two different equations by Pappa-Louisi et al., one based on a partition model (PL-P) and one based on an adsorption model (PL-A). More than 25 000 retention factors were determined for 101 small molecules under various mobile phase and temperature conditions. The pure experimental uncertainty is very small, approximately 0.22% uncertainty in retention factors measured on the same day (2.1% when performed on different days). Each of the three equations for ln(k) was fit to the experimental data based on a least-squares approach and the results were analyzed using lack-of-fit residuals. The PL-A model, while complex, gives the best overall fits. In addition to examining the equations' adequacy for retention, we also examined their use for apparent retention enthalpy. This enthalpy can be predicted by taking the derivative of these expressions with respect to the inverse of absolute temperature. The numerical values of the fitted parameters based on retention data can then be used to predict retention enthalpy. These enthalpy predictions were compared to those obtained from a modified van 't Hoff equation that included a quadratic term in inverse temperature. Based on analysis of 1 211 van 't Hoff plots (solute-mobile phase-day combinations), ninety-eight percent showed a significantly better fit when using the modified van 't Hoff expression, justifying its use to provide apparent enthalpies as a function of mobile phase composition and temperature. The foregoing apparent enthalpies were compared to the apparent enthalpies predicted by the three models. The PL-A model, which contains a temperature dependent enthalpy, provided the best enthalpy prediction. However, there is virtually no correlation between the overall lack of fit to experimental ln(k) for each model and the corresponding lack of fit of the linear (in 1/T) van 't Hoff expression. Thus, the temperature-dependent enthalpy is apparently not the cause of a model's ability to fit ln(k) as a function of mobile phase composition and temperature. The value in these expressions is their ability to predict chromatograms, allowing for optimization of an advanced chromatographic technique. The two simpler models NK and PL-P, which do not contain a temperature dependent enthalpy, have their merits in modelling retention (NK being the better of the two) and enthalpy (PL-P being the better of the two) if a simpler expression is required for a given application.

Synthesis, Structure, and Acidity Constants of Ligated α-Boryl Acetic Acids.

Basic hydrolyses of various ligated α-boryl acetic acid esters provided the first ligated derivatives of the unknown compound boroacetic acid (BH2 CH2 CO2 H). Four monoacids (L-BH2 CH2 CO2 H) and one diacid (L-BH(CH2 CO2 H)2 ) were prepared with N-heterocyclic carbene, amine, and pyridine ligands (L). The stable acids were characterized by X-ray crystallography and acidity constant (pKa ) measurements. They rank among the least acidic of all known carboxylic acids. In turn, their conjugate bases are among the strongest of all carboxylates.

Aptamer-functionalized neural recording electrodes for the direct measurement of cocaine in vivo.

Cocaine is a highly addictive psychostimulant that acts through competitive inhibition of the dopamine transporter. In order to fully understand the region specific neuropathology of cocaine abuse and addiction, it is unequivocally necessary to develop cocaine sensing technology capable of directly measuring real-time cocaine transient events local to different brain regions throughout the pharmacokinetic time course of exposure. We have developed an electrochemical aptamer-based in vivo cocaine sensor on a silicon based neural recording probe platform capable of directly measuring cocaine from discrete brain locations using square wave voltammetry (SWV). The sensitivity of the sensor for cocaine follows a modified exponential Langmuir model relationship and complete aptamer-target binding occurs in < 2 sec and unbinding in < 4 sec. The resulting temporal resolution is a 75X increase from traditional microdialysis sampling methods. When implanted in the rat dorsal striatum, the cocaine sensor exhibits stable SWV signal drift (modeled using a logarithmic exponential equation) and is capable of measuring real-time in vivo response to repeated local cocaine infusion as well as systemic IV cocaine injection. The in vivo sensor is capable of obtaining reproducible measurements over a period approaching 3 hours, after which signal amplitude significantly decreases likely due to tissue encapsulation. Finally, aptamer functionalized neural recording probes successfully detect spontaneous and evoked neural activity in the brain. This dual functionality makes the cocaine sensor a powerful tool capable of monitoring both biochemical and electrophysiological signals with high spatial and temporal resolution.

Development of a 1.0 mm inside diameter temperature-assisted focusing precolumn for use with 2.1 mm inside diameter columns.

On-column solute focusing is a simple and powerful method to decrease the influence of precolumn band spreading and increase the allowable volume injected increasing sensitivity. It relies on creating conditions so that the retention factor, k', is transiently increased during the injection process. Both solvent composition and temperature control can be used to effect solute focusing. In the case of temperature, the release of the transiently delayed solute band requires increasing the temperature rapidly and with a minimum of radial thermal gradients. Thus, the focus of attention in temperature-based efforts to carry out on-column focusing has been on capillary columns. As a result, the benefits of this simple and reliable approach, temperature-assisted solute focusing or TASF, are not available to those using larger diameter columns, in particular the highly successful 2.1mm inside diameter columns. Based on considerations of thermal entrance length at the volume flow rates used with 2.1mm inside diameter columns, TASF would not be effective with such columns. However, we determined that the thermal entrance length for a 1.0mm inside diameter precolumn is sufficiently short, about 2mm, that it could work as a precolumn before a 2.1mm inside diameter analytical column. Finite element calculations demonstrate that a 1.0×20mm precolumn packed with 5µm reversed phase particles is effective at a flow rate of 250µL/min, suitable for the 2.1mm inside diameter column. Eight 1-cm2 Peltier devices are used to heat (and cool) the precolumn. The computed axial temperature profile shows that the center of the column heats more rapidly than the ends. Based on the changes in back pressure, the full temperature transient from 5°C (focus) to 80°C (release) takes about 10s. Experimental van Deemter curves indicate that the reduced velocity in the precolumn at 250µL/min flow rate is about 50. Nonetheless, about 1000 theoretical plates are generated. When operating as a precolumn, clear advantages are seen for solutes across a range of modest k' values (2.2-23.4 at the separation conditions at 65°C) using TASF alone (5°C) with 50µL injection volumes of methyl through n-butyl parabens, and with 100µL injections that also include solvent-based focusing (90:10 aqueous/acetonitrile sample, 80:20 mobile phase).

Correction: Aptamer-functionalized neural recording electrodes for the direct measurement of cocaine in vivo.

Correction for 'Aptamer-functionalized neural recording electrodes for the direct measurement of cocaine in vivo' by I. Mitch Taylor et al., J. Mater. Chem. B, 2017, 5, 2445-2458.

Temperature-based on-column solute focusing in capillary liquid chromatography reduces peak broadening from pre-column dispersion and volume overload when used alone or with solvent-based focusing.

On-column focusing is essential for satisfactory performance using capillary scale columns. On-column focusing results from generating transient conditions at the head of the column that lead to high solute retention. Solvent-based on-column focusing is a well-known approach to achieve this. Temperature-assisted on-column focusing (TASF) can also be effective. TASF improves focusing by cooling a short segment of the column inlet to a temperature that is lower than the column temperature during the injection and then rapidly heating the focusing segment to the match the column temperature. A troublesome feature of an earlier implementation of TASF was the need to leave the capillary column unpacked in that portion of the column inside the fitting connecting it to the injection valve. We have overcome that problem in this work by packing the head of the column with solid silica spheres. In addition, technical improvements to the TASF instrumentation include: selection of a more powerful thermo-electric cooler to create faster temperature changes and electronic control for easy incorporation into conventional capillary instruments. Used in conjunction with solvent-based focusing and with isocratic elution, volumes of paraben samples (esters of p-hydroxybenzoic acid) up to 4.5-times the column liquid volume can be injected without significant bandspreading due to volume overload. Interestingly, the shapes of the peaks from the lowest volume injections that we can make, 30nL, are improved when using TASF. TASF is very effective at reducing the detrimental effects of pre-column dispersion using isocratic elution. Finally, we show that TASF can be used to focus the neuropeptide galanin in a sample solvent with elution strength stronger than the mobile phase. Here, the stronger solvent is necessitated by the need to prevent peptide adsorption prior to and during analysis.