A 20% Pure Vitamin C Serum that brightens, tightens and firms. Our Universal C Skin Refiner Brightening Vitamin C Serum is the first 20% vitamin C serum containing stabilized L-ascorbic acid (the purest form of vitamin C) through a patented encapsulated system promising maximum potency and delivering the fastest age rejuvenation results. First time shopping on poshmark? Use code "DANAREIK" to save $10 on your first order.