It took some scratching around to find out who she was, but Olga's Gallery provided the breakthrough. She was married to Artemiy Ivanovich Vorontsov. There is a miniscule amount of information about her here.
From Wikimedia.
Countess Praskovia wears an extremely simple version of a late ancien régime dress with a fichu and scoop neckline framed by ruching in this 1790 Levitsky portrait.
Keywords: 1790, Levitsky - Dmitri, Praskovia Vorontsova, Kvashin Samarin family, Vorontsov family, Countess, Russian, long straight coiffure, fichu, scoop neckline, ruching, close sleeves