Figure 6. Product ion spectrum of m/z 904 obtained directly from the tissue surface (negative ion detection mode).

In the obtained product ion spectrum, the fragment ions derived from polar head group were nearly identical to the product ion spectrum of m/z 888. Above the region of m/z 600, clear unique peak patterns derived from products of the ceramide moiety were also observed. One specific difference between m/z 888 and m/z 904 was the detection of high-intensity peak at m/z 540. This characteristic peak was derived from neutral loss of a hydroxy-fatty-acid.