Direct quantitative analysis of the natural moisturizing factor (NMF) in the stratum corneum by direct analysis in real time mass spectrometry (DART-MS)
Abstract
Analyzing the natural moisturizing factor (NMF) in the stratum corneum (SC), such as amino acids, pyrrolidone carboxylic acid, and urocanic acid, is important in dermatology research. Here, we report a rapid and simple analytical technique for quantifying the amino acids in SCs by using a combination of direct analysis in real time-mass spectrometry (DART-MS) and a stable isotope-labeled internal standard (SIL-IS). Among the twenty amino acids that we targeted, twelve were detected with a high selectivity. By using appropriate internal standards, the calibration curves for the twelve amino acids showed relatively good linear correlations between the peak area ratios of the amino acids to the internal standard and the amount of amino acids on each tape strip. The calibration curves obtained based on the standard addition technique showed good agreement with the calibration curves that were obtained with the internal standard over a range of concentrations, which indicated that the matrix effects were reduced by choosing appropriate internal standards. Amino acids in SCs taken from the forearms of volunteers were quantified using the calibration curves, and a depth profile of the amino acids was obtained using DART-MS. We also compared this DART-MS technique with the validated conventional gas chromatography-mass spectrometry method and found no significant differences in the amounts of glycine and alanine measured by each method. We concluded that the DART-MS technique developed here has a great potential to directly quantify amino acids in the SC.