Method for producing a gradient elution

A method of producing an eluent containing a time varying concentration of two liquids, comprising the steps of:

• a. supplying a first liquid to a first operative portion of normally closed valving means;
• b. supplying a second liquid to a second operative portion of said normally closed valving means;
• c. alternately activating said first operative portion of said valving means for a first set period of time to allow a first set volume of said first liquid to enter a mixing region and then activating said second operative portion of said valving means for a second set period of time to allow a second set volume of said second liquid to enter said mixing region and mix with said first set volume of first liquid to form a mixture containing a set concentration of said first and second liquids , said first and second set periods of time being such that the volume of said first and second liquid entering said mixing region during each period when either operative portion of said valving means is activated is less than the volume of said mixing region;
• d. allowing a portion of said mixture to leave said mixing region thereby producing an eluent having a concentration of said first and second liquid, the instantaneous concentration of which is equal to the instantaneous concentration of said first and second liquids at the downstream end of said mixing region at the time said portion of said mixture leaves said mixing region;
• and
• e. varying said first and second set periods of time during which said first and second operative portions of said valving means are respectively activated, whereby the concentrations of said first and second liquids in said mixture are gradually changed so that an eluent having a time varying concentration of said first and second liquids is produced.