  We present a molecular line study towards 31 extended green object (EGO)
clumps in the southern sky using data from MALT90 (Millimetre Astronomy Legacy
Team 90 GHz). According to previous multiwavelength observations, we divide our
sample into two groups: massive young stellar objects (MYSOs) and HII regions.
Our results seem to support that N2H+ and C2H emissions mainly come from the
gas inside quiescent clumps. In addition, we also find that the [N2H+]/[H13CO+]
and [C2H]/[H13CO+] relative abundance ratios decrease from MYSOs to HII
regions. These results suggest depletion of N2H+ and C2H in the late stages of
massive-star formation, probably caused by the formation of HII regions inside.
N2H+ and C2H might be used as chemical clocks for massive-star formation by
comparing with other molecules such as H13CO+ and HC3N.
