SAMPLING ERROR IN SMALL-BORE SHEATHLESS CAPILLARY ELECTROPHORESIS ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

Two previously unreported sources of systematic error in electrokineti c injection caused by induced (hydrodynamic) flow in sheathless capill ary electrophoresis/electrospray have been characterized for an interf ace constructed from 5 mu m i.d. capillary column(s) with a 2-5 mu m i .d. electrospray tip(s), Tbe 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 sam pling by as much as 50%, This bias can be eliminated during the inject ion process by temporarily immersing the tip in buffer solvent, The se cond source of hydrodynamic flow results from the electrostatic pull e xerted by the electrospray process on the buffer, reducing migration t imes. Reduction in migration time causes an overestimation of sampling volume if there are no electrospray processes during the sampling eve nt, The magnitude of this effect is a complex function of electrophore tic and electrospray parameters and has been measured to be as high as 15% for 5 mu m i.d. capillaries. The possible deterioration of electr ophoretic resolution caused by these processes is not yet clarified.