  We present a spectral line survey of the proto-planetary nebula AFGL 2688 in
the frequency ranges of 71-111 GHz, 157-160 GHz, and 218-267 GHz using the
Arizona Radio Observatory 12m telescope and the Heinrich Hertz Submillimeter
Telescope. A total of 143 individual spectral features associated with 32
different molecular species and isotopologues were identified. The molecules
C3H, CH3CN, H2CO, H2CS, and HCO+ were detected for the first time in this
object. By comparing the integrated line strengths of different transitions, we
are able to determine the rotation temperatures, column densities, and
fractional abundances of the detected molecules. The C, O, and N isotopic
ratios in AFGL 2688 are compared with those in IRC+10216 and the Sun, and were
found to be consistent with stellar nucleosynthesis theory. Through comparisons
of molecular line strengths in asymptotic giant branch stars, proto-planetary
nebulae, and planetary nebulae, we discuss the evolution in circumstellar
chemistry in the late stages of evolution.
