The invention provides a microfluidic system, including an optional separation system for separating and preparing an analyte solution, a microfluidic device downstream from the separation system for dispensing and analyte solution, comprising a substrate having a channel defining a portion of a microfluidic channel; a polymeric substrate having a channel for contacting the substrate to define the second portion of the microfluidic channel; a cooling element associated with the substrate and channel for cooling an analyte solution in the microfluidic channel; and a heating element adjacent to the microfluidic channel for heating the analyte solution, wherein the cooling element operates to maintain the channel in a closed state by cooling the analyte solution in the channel and wherein the heating element may be activated to place the channel in an open state by heating the analyte solution in the channel; and a detector for detecting the dispensed analyte solutionThe invention also provides a microfluidic device and/or valve, including a substrate having a micro fluidic channel for carrying an analyte solution; a cooling element associated with the substrate and micro fluidic channel for cooling the analyte solution in the channel; and a heating element adjacent to the channel for heating the analyte solution in the channel wherein the cooling element operates to maintain the channel in a closed state by cooling the analyte solution in the channel and wherein the heating element may be activated to place the channel in an open state by heating the cooled analyte solution in the channel.The invention also provides a method of valve control in a microfluidic device, including maintaining a cooling element in an active state to freeze an analyte solution in a microfluidic channel and close the microfluidic channel; and engaging a heating element to thaw the analyte solution in the microfluidic channel and open the microfluidic channel to allow fluid flow through the channel.