Instrument for determination of the base sequence of nucleic acid

An apparatus for determination of a base sequence of a nucleic acid, comprising:

• an electrophoretic matrix having a migration lane and buffer therein;
• means for introducing sample containing nucleic acid fragments into a first end of said electrophoretic matrix;
• means for applying an electrical potential between positive and negative electrodes for electrophoresis so that a negative potential is applied to said first end of said electrophoretic matrix and a positive potential is applied to a second end of said electrophoresis matrix;
• detection means located at said migration lane and substantially separated from said first end of said electrophoretic matrix for detecting in real time the nucleic acid fragments successively arriving at a detection part;
• and
• the ionic strength of said buffer in said electrophoretic matrix is higher at least at one position in a region spaced from the detection part towards said first end than at the detection part.