How is the sugar quantified???

Hi, Hayat
As said in the protocol, first you inject into the column a mixture of sugars (threalose, galactose, glucose, fructose and sucrose) at different known concentrations (1, 0.5, 0.1, 0.05 and 0.01 mM; you can see the chromatogram of 0.5 mM mixture in figure 1A). For each sugar, you record the peak area at each concentration and with these data you can generate a calibration curve, that is, the mathematical function that relates both variables (peak area (x) and concentration (y). Then, once you inject your sample and record the area of the peak of your sugar(s) of interest, all you have to do is to use that value as “x” in the calibration curve you have obtained.
I hope I understood your question properly. If not, please feel free to contact me.