This blog is part of our separation science primer series which provides an easy-to-understand overview of key topics in solid-phase extraction and chromatography.
This blog focuses on Electrochemically Modulated liquid chromatography (EMLC)
Let us start with what is common. An electric field is applied in electrophoretic and electrochemical separation, generating a positive or negative pole on the electrodes. A charged atom or molecule (let’s call it an analyte) placed in this field moves in the direction of these poles. A negatively charged analyte moves toward the positive pole, and a positively charged analyte moves toward the negative pole.
The electric field drives the flow of the analyte during electrophoretic separation. For example, a negatively charged protein is introduced near the negatively charged electrode and moves toward the positively charged electrode (anode). The higher the negative charge on the protein, the stronger its pull towards the anode. Thus, two negatively charged proteins with the same mass but different charges will move at different speeds to the anode. Generally, the speed at which any charged analyte moves an electrode depends on its charge-to-mass ratio. This relationship drives the separation of the charged analytes in a mixture.
The analytes may pass through a supporting media to further improve the separation. These support media may separate out the analyte flowing in the electric field by serving as a sieve and separating it according to its molecular weight. It may serve as an adsorption site and separate according to how much the analyte interacts with this adsorption site (see below). agar, agarose, starch, and polyacrylamide are commonly used media.
The electrophoresis procedure is broadly classified into:
(a) Capillary electrophoresis.
(b) Slab electrophoresis.
The electric field does not drive the flow of the analyte during electrochemical separation. The flow is driven by positive or negative pressure (vacuum) that is typically used in solid phase extraction or chromatography techniques. Here, the electric field manipulates the adsorptive separation processes.
Adsorptive separation is one of the key mechanisms in liquid chromatography. The analyte in the liquid (mobile) phase passes through and interacts with the stationary phase. Adsorption is one of these interactions. The nature and strength of the adsorptive interaction at the solid (stationary phase)–liquid (mobile phase) interface depend on multiple factors. Key factors include polarity and charge of the stationary phase surface and analyte (e.g., Van der Waals, electrostatic). Different analytes in the mobile phase exhibit distinct interactions with the stationary phase and can either be physically or chemically adsorbed.
Liquid chromatography theory describes this phenomenon as the partitioning of the analyte between mobile and stationary phases, and uses the term distribution coefficient (Kd), This term is the ratio of concentrations of an analyte sorbed on the stationary phase (Cs) and dissolved in the mobile phase (Cm)
Kd = Cs/Cm.
Electrochemical separation employs the electric field (Ea) to stimulate this adsorption process. Thus, electrochemical separation uses Ea to manipulate Kd, consequently, the adsorption time of the analytes on the stationary phase. Electrochemical separation can be performed in two ways; by generating capacitive or faradaic currents to alter the stationary phase properties.