Highly sensitive determination of N-acyl dihydrosphingosine using liquid chromatography-electrospray ionization mass spectrometry. Application to human hair lipids.

An analytical method for highly sensitive determination of four N-acyl dihydrosphingosines (NDSs) of all ceramides (CERs) in human hair, such as N-palmitoyl dihydrosphingosine (N16DS18), N-stearoyl dihydrosphingosine (N18DS18), N-lignocerol dihydrosphingosine (N24DS18) and N-nervonoyl dihydrosphingosine (N24:1DS18), has been developed using electrospray ionization (ESI) MS connected to reversed-phase LC with selected ion monitoring (SIM). The selection of negative ESI under optimal conditions of in-source collision-induced dissociation was determined based on the simplicity of molecular-related ions and their intensities. Of all ESI-MS parameters tested, the flow of dry nitrogen gas strongly affected the sensitivity of molecular-related ions, particularly in N24DS18 and N24:1DS18, while the capillary voltage elicited significantly different effects on the signal-to-noise ratio between N16DS18/N18DS18 and N24DS18/N24:1DS18. This newly developed method to determine the NDSs is the most sensitive of all existing methods, as shown in the limits of detection and quantification being in the range of 0.06-0.29 and 0.18-0.98fmol, respectively. The linearity, precision and accuracy were all sufficient to determine the NDSs in ca. 0.1mg of a hair fiber ( approximately 1cm in length). This method has been used to characterize levels of the NDSs from the proximal root end to the distal tip of each of six hair fibers obtained from two different females. Characteristic changes were observed between both females as well as among fibers derived from each female. This method will be useful not only for clarifying the roles of the CERs in human hair but also for investigating the physiology of CERs relevant to signal transduction and cell regulation in human cells/tissues.