A method of separating at least first and second species in a sample mixture, the first species having a lower electrophoretic mobility than the second species, the method comprising: providing a device having at least first, second, and third channel segments fluidly coupled at a first fluid junction; characterised by either: (ai) applying a pressure gradient between said first and second channel segments; and (aii) applying a voltage gradient between said second and third channel segments whereby all of the first species flows into the second channel segment while all of the second species flows into the third channel segment; or (bi) applying a voltage differential across at least the second and third channel segments; flowing the sample mixture through the first channel segment and into the first fluid junction; and (bii) applying a pressure gradient across at least the first and second channel segments such that all of said second species in the fluid junction has an electrophoretic velocity component that exceeds a pressure-driven velocity component of the portion of the second species in the fluid junction whereby said second species flows into the third channel segment, while all of the first species in the fluid junction has a pressure-driven velocity component that exceeds an electrophoretic velocity component of the portion of the first species in the fluid junction such that said all of the first species flows into the second channel segment. providing a device having at least first, second, and third channel segments fluidly coupled at a first fluid junction; characterised by either: (ai) applying a pressure gradient between said first and second channel segments; and (aii) applying a voltage gradient between said second and third channel segments whereby all of the first species flows into the second channel segment while all of the second species flows into the third channel segment; or (bi) applying a voltage differential across at least the second and third channel segments; flowing the sample mixture through the first channel segment and into the first fluid junction; and (bii) applying a pressure gradient across at least the first and second channel segments such that all of said second species in the fluid junction has an electrophoretic velocity component that exceeds a pressure-driven velocity component of the portion of the second species in the fluid junction whereby said second species flows into the third channel segment, while all of the first species in the fluid junction has a pressure-driven velocity component that exceeds an electrophoretic velocity component of the portion of the first species in the fluid junction such that said all of the first species flows into the second channel segment. (ai) applying a pressure gradient between said first and second channel segments; and (aii) applying a voltage gradient between said second and third channel segments whereby all of the first species flows into the second channel segment while all of the second species flows into the third channel segment; or (bi) applying a voltage differential across at least the second and third channel segments; flowing the sample mixture through the first channel segment and into the first fluid junction; and (bii) applying a pressure gradient across at least the first and second channel segments such that all of said second species in the fluid junction has an electrophoretic velocity component that exceeds a pressure-driven velocity component of the portion of the second species in the fluid junction whereby said second species flows into the third channel segment, while all of the first species in the fluid junction has a pressure-driven velocity component that exceeds an electrophoretic velocity component of the portion of the first species in the fluid junction such that said all of the first species flows into the second channel segment.