MALDI Mass Spectrometry Imaging and Semi‐Quantification of Topically Delivered Lactic Acid

BACKGROUND: Lactic acid is a common active ingredient in many topical skincare products; however, measuring its delivery into the skin is challenging due to the presence of a large level of endogenous lactic acid. In this study, matrix‐assisted laser desorption/ionization mass spectrometry imaging (MALDI‐MSI) was used to quantitatively and qualitatively measure the delivery of lactic acid into the skin from a range of topical skincare products. MATERIALS AND METHODS: Porcine skin samples were treated with various skincare products containing lactic acid. After 24 h, skin samples were sectioned and treated via H&E staining or prepared for MALDI‐MSI using chemical derivatization. Samples were then analyzed by MALDI‐MSI imaging to obtain lactic acid distribution in the entire skin section. RESULTS: Due to the high level of endogenous lactic acid in the skin, a “triple isotope” of lactic acid (L‐Lactic acid‐(13)C(3)), was needed to provide full resolution from the skin's background signal with MALDI‐MSI. With this approach, the topically delivered lactic acid could be quantitatively and qualitatively analyzed from a variety of skincare products. CONCLUSIONS: The combination of L‐Lactic acid‐(13)C(3) and MALDI‐MSI was successfully used to quantitatively and qualitatively measure the topical delivery of lactic acid from a variety of skincare products. This approach could be used in future work to better understand the mode of action of lactic acid as an active ingredient in skincare products.