Injection Valve for Ultrahigh-Pressure Liquid Chromatography

The increased interest in HPLC at elevated pressures, beyond the conventional 6000 psi (400 bar), has created a demand for injection systems capable of withstanding pressures beyond the 20 000 psi (1380 bar). To achieve high-resolution separations, an appropriate length of columns packed with sub 2-m packing materials, a 30 000-40 000 psi (2070-2760 bar) pressure range is desirable. A new air-actuated needle valve injection system rated to withstand pressures of up to 40 000 psi (2760 bar) has been evaluated. Under isocratic chromatographic conditions, injecting 200 nL and operated at ~20 000 psi (1380 bar), the system showed a peak area reproducibility of ~2.5% RSD, contrasting the 5% RSD of a pressured-balanced injection system operated under similar conditions. Programmed for partial loop injections using injection times of 300-700 ms (injection volumes in the range of 1-2.5 L) and operated at pressures close to 30 000 psi (2070 bar), the reproducibility in peak area for the amounts injected was ~1.5% RSD or lower, while an injection time of 100 ms resulted in a reproducibility of 3-4% RSD. The new injection system did not show any significant carryover, and after thousands of injections, the system has not shown sign of wear, loss of pressure during injection, or loss in chromatographic performance. We report on the potential-dependent behavior of the zwitterionic molecule p-aminobenzoic acid (PABA) at a Ag(111) electrode surface. Infrared-visible sum frequency generation spectroscopy (SFG) in tandem with electrochemical capacitance and CV measurements are used to study the effects of applied potential on the adsorption and orientation of PABA. Changes in the SFG signal indicate that PABA changes orientation in response to the charge on the electrode surface, orienting one way positive of the potential of zero charge (pzc) and oppositely negative of the pzc. At positive potentials, a phase change is observed associated with the formation of a condensed layer. PABA is observed to remain on the surface at all potentials examined. These results show that the orientation of small molecules with large dipoles, like zwitterions, can be controlled by applied potential. so, siderophore-complexed Th interacted strongly with a cation-exchange resin suggesting that, even when complexed by trianionic siderophores, Th mobility will be impeded by interactions with negatively charged binding sites in subsurface environmental matrixes. These results have important implications regarding siderophore-enhanced actinide(IV) mobility in the terrestrial environment.