The Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory
is suffering a gradual loss of low energy sensitivity due to a buildup of a
contaminant. High resolution spectra of bright astrophysical sources using the
Chandra Low Energy Transmission Grating Spectrometer (LETGS) have been analyzed
in order to determine the nature of the contaminant by measuring the absorption
edges. The dominant element in the contaminant is carbon. Edges due to oxygen
and fluorine are also detectable. Excluding H, we find that C, O, and F
comprise >80%, 7%, and 7% of the contaminant by number, respectively. Nitrogen
is less than 3% of the contaminant. We will assess various candidates for the
contaminating material and investigate the growth of the layer with time. For
example, the detailed structure of the C-K absorption edge provides information
about the bonding structure of the compound, eliminating aromatic hydrocarbons
as the contaminating material.