4.5.5.1 Electro-Spray Ionization (ESI)

Electro-spray ionization is an ideal technique for interfacing CE and MS, as it accepts small volumes at a low flow rate and results in gaseous, ionized particles (for a detailed description go to section 5.3.1). Additionally, ESI is well-suited to larger, biological molecules, which are frequently analyzed by CE. However, CE and ESI are both dependent on electric fields. In order to couple them in the simplest of cases, the end electrode of the CE instrumentation is shared with the ESI electrode. However, in some situations the difference in the magnetic field can be several orders of magnitude and the electrodes may be of the opposite sign.

Some situations can be remedied by grounding the end electrolyte of the CE system, and providing voltage to the electro-spray system from the MS. If the potential applied to the ESI is negative, then positive ion will enter the MS and this is called positive ion mode. If the CE voltage is much greater than the ESI voltage, a ground path for the current is provided by a resistor sink. In general, when the ESI voltage is much different from the CE voltage, the variability in the electric field can affect the sample separation in the CE system.