Reagent exchange in an instrument

An instrument comprising:

• a reagent management system, positioned in the instrument, the reagent management system comprising a plurality of reagent wells, the reagent wells having at least a first, a second and a third reagent well operable to contain an incorporation reagent, a cleavage reagent and a buffer reagent positioned respectively therein, the reagent management system to select a flow of reagent from one of the plurality of reagent wells;
• and
• a flow cell, positioned in the instrument, the flow cell comprising a flow channel in fluid communication with the reagent management system, the flow channel to route the flow of reagent over a first polynucleotide positioned in the flow channel;
• wherein the instrument is operable to:
• flow the incorporation reagent, through the reagent management system and flow cell, to add a first base onto a sequence of bases, the sequence of bases comprising a second polynucleotide complementary to the first polynucleotide;
• capture an image, of an identification signal emanating from the first base, after the first base has been added onto the second polynucleotide;
• flow the cleavage reagent, through the reagent management system and flow cell, to remove a first terminator from the first base in order to enable a subsequent base in the sequence of bases to be added to the second polynucleotide;
• and
• flow the buffer reagent, through the reagent management system and flow cell in a plurality of cycles of consecutive forward and reverse flow directions to remove the cleavage reagent from the flow cell.