Evaluation of automated isocratic and gradient nano-liquid chromatography and capillary electrochromatography

An automated liquid nano-separation system has been developed for nano-liquid chromatography (nano-LC) and capillary electrochromatography (CEC) using both isocratic and gradient elution. One fused-silica nanocolumn, typically 75 μm i.d. × 39 cm (25 cm effective packed length), packed with Spherisorb ODS 1, 3 μm particle size, can be used with either technique without having to remove the column upon switching from one mode to the other. The mobile phase is delivered by two reciprocating micro-LC pumps at a flow rate of 30 μL/min to a postinjection splitter that houses the nanocolumn inlet. The splitter is directly connected to a micro-injection valve with a 0.5 μL injection volume. In the CEC mode, pressure is not applied (no restriction on splitter) to the column inlet or outlet and the voltage is continuously applied during sample injection and mobile phase delivery. In the nano-LC mode, the restrictor is coupled to the splitter. Using the same nanocolumn under isocratic conditions, the repeatabilities of retention time and peak area for nano-LC were better than 0.2% and 4%, respectively, and those for CEC were better than 0.6% and 6%, respectively. On average, column efficiency was 57% higher in CEC compared to nano-LC. Gradient elution separations of parabens and polynuclear aromatic hydrocarbons (PAHs) were accomplished by CEC.