Gradient high pressure syringe pump

A pump for moving fluids comprising:

• a housing having a wall, defining an internal cavity closed at one end;
• a first piston within said cavity;
• a motor for selectively moving said first piston linearly along a pump axis outwardly toward and inwardly away from said one end;
• a second piston floating intermediate said one end and said first piston, said second piston being linearly movable along the pump axis and separating the internal cavity into a first pumping chamber and a second pumping chamber;
• said housing defining a first cylinder section having a first diameter, said first piston being movable within said first cylinder section;
• said housing defining a second cylinder section having a second diameter greater than said first diameter, said second piston being movable within said second cylinder section between said first cylinder section and said closed end;
• a first pump inlet and a first pump outlet intermediate said second piston and said first piston communicating with said first pumping chamber;
• said first pump inlet and first pump outlet being disposed to remain open to fluid flow with said first pumping chamber at any linear position of said first piston;
• a second pump inlet and a second pump outlet intermediate said second piston and said one end communicating with said second pumping chamber;
• said second piston movable between a first position toward said first pump inlet and first pump outlet and a second position toward said second pump inlet and second pump outlet;
• a check valve upstream of said first pump inlet preventing backflow from said first pumping chamber and a check valve upstream of said second pump inlet preventing backflow from said second pumping chamber;
• a spring biasing said second piston axially toward said first piston;
• and,
• a 2-position valve to control fluid flow from the pumping chambers, said valve having first and second valve inlets and a valve outlet, said first valve inlet communicating with said first pump outlet, said second valve inlet communicating with said second pump outlet, said valve being selectively switchable between a first position opening said first valve inlet allowing fluid flow from said first pumping chamber and shutting off fluid flow from said second pumping chamber, and a second position opening said second valve inlet allowing fluid flow from said second pumping chamber and shutting off fluid flow from said first pumping chamber, and wherein switching said valve between said first and second positions delivers fluid through said valve alternately from said first and second pumping chambers.