Since the samples are tested under the same condition: under a set wavelength (which is the analytical wavelength: the wavelength of maximal absorbance chosen from the absorbance spectrum of the substance) and incident light distance, e and l are the same for all samples, thus A varies linearly with c.
The first step is to construct a curve relating known analyte quantities to their measured absorbance. OD recorded from the spectrophotometer (absorbance) is plotted on the Y-axis and the analyte amounts are plotted on the X-axis. A series of solutions with known analyte quantity (the "standards") are prepared, the absorbance values are recorded and plotted versus the solution amounts; a straight line can be best fitted. The first spectrophotometer cuvette is the "blank": it contains no analyte, the OD is recorded.