Disclosed herein are methods for determining and replicate unknown ratios of original target liquid blends, such as hydrocarbon fuel blends or contaminants, by using an in-process fluorescence-monitored procedure. The methods rely on trial-and-error mixing of the fuel ingredients into a single container. At the end of the trial-and-error procedure, the formed blend becomes an exact replica of the target fuel blend. The methods can also be used to build calibration curves without employing sets of previously prepared standard solutions.