Klairs Freshly Juiced Vitamin C Drop. Klairs formulated their serum with the sole intention of being safe and irritant free. It’s the ideal serum for people with sensitive skin, as the Vitamin C content is only at 5%. The serum is made in Korea, making it a high quality product albeit extremely affordable. When it comes to choosing the best vitamin C serum, there are a few important factors to consider. In this buying guide, we will discuss what features to look for to ensure that you choose a high-quality product that meets your needs. 1. Vitamin C Concentration. The concentration of vitamin C in a serum is one of the most important factors to Vitamin C on its own is a potent antioxidant that soaks up free radicals and prevents the oxidative damage that results from UV light and other environmental stressors, as well as normal biological processes. Vitamin C is an essential cofactor that’s required for enzymes that crosslink and stabilise collagen (prolyl and lysyl hydroxylase) As vitamin C is well known for its instability, it’s important to keep an eye — and a nose — out for any changes in the serum’s color or smell from the time it was opened. “Vitamin C serum should be a clear liquid; when it turns orange/brown, that’s a sign that the serum has oxidized and has lost some of its efficacy,” Casey said. Retinol serum. What it is: Available over the counter, a retinol serum is a vitamin A derivative. Dr. Robinson says they contain a lower concentration of the active retinoic acid ingredient than the retinoid cream your dermatologist can prescribe. (Retinoids are higher strength, hence the need for a doctor's prescription.) Mistake #4: Applying Too Much. When it comes to Vitamin C serum a little goes a long way, and using too much of the potent ingredient can be irritating. You only need two to three drops of serum for your face, neck, and chest. Place the drops into the palm of your hand, then use fingertips to press the product onto skin. Furthermore, although the measurement of vitamin D concentrations in a mixture of irradiation products is analytically simple, vitamin D 3 levels in human serum are rarely used to estimate cutaneous synthesis of the vitamin, in part because of the transient nature of this blood parameter and the difficulty of measuring the low levels in serum This multi-tasking vitamin C serum combines several products into one, making you look and feel more vibrant than ever. NOT JUST WATER: When it comes to vitamin C face serum, the ordinary face serum is watered down and ineffective. Our formula, on the other hand, is a high-quality and highly concentrated vitamin C serum. vrXV3w.