Sampling Error in Small-bore Sheathless Capillary Electrophoresis/Electrospray-ionization Mass Spectrometry

Two previously unreported sources of systematic error in electrokinetic injection caused by induced (hydrodynamic) flow in sheathless capillary electrophoresis/electrospray have been characterized for an interface constructed from 5 μm i.d. capillary column(s) with a 2–5 μm i.d. electrospray tip(s). The tip of a sheathless interface is usually exposed to the atmosphere, resulting in evaporation of buffer solvent and inducing flow inside the column; such flow can cause a significant underestimation of injection size for quantitative electrokinetic sampling by as much as 50%. This bias can be eliminated during the injection process by temporarily immersing the tip in buffer solvent. The second source of hydrodynamic flow results from the electrostatic pull exerted by the electrospray process on the buffer, reducing migration times. Reduction in migration time causes an overestimation of sampling volume if there are no electrospray processes during the sampling event. The magnitude of this effect is a complex function of electrophoretic and electrospray parameters and has been measured to be as high as 15% for 5 μm i.d. capillaries. The possible deterioration of electrophoretic resolution caused by these processes is not yet clarified.