However, it can be difficult to select a membrane that will bind all substances of interest, and removing sample from the membrane requires additional handling. Alternative 3 uses coaxial-flow capillaries downstream of UV detectors. Electrolyte is pumped through a capillary that surrounds the separation capillary, constantly bathing it to maintain electrical contact without the need to immerse it in a buffer reservoir. Collection devices, in this case probes, are positioned under the capillary outlet. Samples are collected through computer control when peaks are detected by UV absorbance.
Electroosmotic flow causes liquid to form small droplets, which are deposited on probes.
Fractions as small as 50 nanoliters can be collected reproducibly. CE separation systems using a variety of buffers lead to usable spectra. However, some buffers work better than others for signal intensity and resolution. Sodium adduct peaks result in degradation of resolution of the instrument. Alternative separation systems such as isotachophoresis or isoelectric focusing require changing the fraction collecting system because it is difficult to predict when components reach the end of the capillary.