  X-ray absorption spectroscopy provides a potentially powerful tool in
determining the metal abundances in various phases of the interstellar medium
(ISM). We present a case study of the sight line toward 4U 1820-303 (Galactic
coordinates l, b=2.79, -7.91 and distance = 7.6 kpc), based on Chandra Grating
observations. The detection of OI, OII, OIII, OVII, OVIII, and NeIX Kalpha
absorption lines allows us to measure the atomic column densities of the
neutral, warm ionized, and hot phases of the ISM through much of the Galactic
disk. By comparing these measurements with the 21 cm hydrogen emission and with
the pulsar dispersion measure along the same sight line, we estimate the mean
oxygen abundances in the neutral and total ionized phases as 0.3(0.2, 0.6) and
2.2(1.1, 3.5) in units of Anders & Grevesse (1989) solar value. This
significant oxygen abundance difference is apparently a result of molecule/dust
grain destruction and recent metal enrichment in the warm ionized and hot
phases. We also measure the column density of neon from its absorption edge and
obtain the Ne/O ratio of the neutral plus warm ionized gas as 2.1(1.3, 3.5)
solar. Accounting for the expected oxygen contained in molecules and dust
grains would reduce the Ne/O ratio by a factor of ~1.5. From a joint-analysis
of the OVII, OVIII, and NeIX lines, we obtain the Ne/O abundance ratio of the
hot phase as 1.4(0.9, 2.1) solar, which is not sensitive to the exact
temperature distribution assumed in the absorption line modeling. These
comparable ISM Ne/O ratios for the hot and cooler gas are thus considerably
less than the value (2.85+-0.07; 1sigma) recently inferred from corona emission
of solar-like stars (Drake & Testa 2005). (abridged)
