Procedure Title: Medium Pressure Liquid Chromatography (MPLC)
Instrumentation: Medium Pressure Liquid Chromatography (MPLC) Apparatus
Theory of Operation:
• Medium Pressure Liquid Chromatography (MPLC) is an analytical column chromatographic technique. This method is used for separating the saturate, aromatic and polar fractions of a crude oil or an extract through a pre-column, containing thermally deactivated silica, and a main column of activated silica as the stationary phase with n-hexane as the mobile phase.
• Oil and extracts are first deasphaltened by adding n-pentane to the oil or extract (n-pentane dissolves the maltene fraction (Saturates, Aromatics and NSO)), then agitated well and centrifuged to make sure that all the maltene is dissolved in the n-pentane and the asphaltenes precipitated forming a dark layer at the bottom of the vial.
• The maltene sample passes through the pre-column and the main column. The aromatics are retained near the head of the main column, while the saturates are eluted from the column and collected. Then, a back flush occurs, reversing the flow of n-hexane in the main column and the aromatics are eluted and collected. Resins (NSO compounds) are adsorbed by the pre-column silica and subsequently removed by a mixture of dichloromethane and methanol.
Analytical Procedure:
Weigh the oil in an 8 ml vial, add n-pentane, and place the vial in the ultrasonic bath, then in the centrifuge.
Filter the deasphaltened oil (maltene fraction) from the asphaltenes through a glass wool-containing pipette.
Record the weights of the asphaltene and maltene fractions.
Inject the maltene fraction into the MPLC.
Collect the saturate, aromatic and NSO fractions into labeled collection tubes.
Evaporate them to a minimum solvent volume using a turbo vap, and then transfer the fractions with some solvent into pre-weighed 4 ml vials.
Evaporate the vials on the nitrogen evaporator carefully, just to remove all solvent.
Record the fraction weights to get their percentages.