A MALT90 study of the chemical properties of massive clumps and
  filaments of infrared dark clouds
  We attempt to characterise the chemical properties of a sample of massive
clumps of IRDCs through multi-molecular line observations. We also search for
possible evolutionary trends among the derived chemical parameters. The clumps
are studied using the MALT90 line survey data obtained with the Mopra
telescope. The spectral-line data are used in concert with our previous LABOCA
870-$\mu$m dust emission data. Most of the detected species (SiO, C$_2$H, HNCO,
HCN, HCO$^+$, HNC, HC$_3$N, and N$_2$H$^+$) show spatially extended emission
towards many of the sources. The fractional abundances of the molecules are
mostly found to be comparable to those determined in other recent similar
studies of IRDC clumps. We found that the abundances of SiO, HNCO, and HCO$^+$
are higher in IR-bright clumps than in IR-dark sources, reflecting a possible
evolutionary trend. A hint of such a trend is also seen for HNC and HC$_3$N. An
opposite trend is seen for C$_2$H and N$_2$H$^+$. Moreover, a positive
correlation is found between the abundances of HCO$^+$ and HNC, and between
those of HNC and HCN. The HCN and HNC abundances also appear to increase as a
function of the N$_2$H$^+$ abundance. The HNC/HCN and N$_2$H$^+$/HNC abundance
ratios are derived to be ~1 on average, while that of HC$_3$N/HCN is ~10%. The
N$_2$H$^+$/HNC ratio appears to increase as the clump evolves, while the
HNC/HCO$^+$ ratio shows the opposite behaviour. The detected SiO emission is
likely caused by outflow shocks in most cases, although shocks resulting from
the cloud formation process could also play a role. Shock-origin for the HNCO,
HC$_3$N, and CH$_3$CN emission is also plausible. The average HNC/HCN ratio is
in good agreement with those seen in other IRDCs. Our results support the
finding that C$_2$H can trace the cold gas, and not just the PDRs. The
HC$_3$N/HCN ratio appears to be comparable to that in T Tauri disks and comets.
