Described herein are methods and an apparatus to determine 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 method relies on trial-and-error mixing of the liquid ingredients into a single container. At the end of the trial-and-error procedure the formed blend becomes an exact replica of the target blend. The method can also be used to build calibration curves without employing sets of previously prepared standard solutions.