Robert J. Cooper, whose unusual elderflower liqueur, St-Germain, introduced in 2007, was so completely embraced by the cocktail community that it became known as “bartender’s ketchup,” died on Monday in Santa Barbara, Calif. He was 39.

His death was confirmed by Robyn Greene, the senior vice president for marketing and innovation at the Cooper Spirits Company, who said the cause was not immediately known.

Mr. Cooper was a scion of the family that owns Charles Jacquin et Cie, an old cordials and liqueurs house based in Philadelphia. By his account, his father, Norton J. Cooper, gave him the cold shoulder when he suggested creating an elderflower liqueur like the ones he had encountered in bars in London.

The younger Mr. Cooper persisted, however, going so far as to leave the family business to pursue his dream.