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We attempt to characterise the chemical properties of a sample of massiveclumps of IRDCs through multi-molecular line observations. We also search forpossible evolutionary trends among the derived chemical parameters. The clumpsare studied using the MALT90 line survey data obtained with the Mopratelescope. The spectral-line data are used in concert with our previous LABOCA870-$\mu$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 emissiontowards many of the sources. The fractional abundances of the molecules aremostly found to be comparable to those determined in other recent similarstudies 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 possibleevolutionary trend. A hint of such a trend is also seen for HNC and HC$_3$N. Anopposite trend is seen for C$_2$H and N$_2$H$^+$. Moreover, a positivecorrelation is found between the abundances of HCO$^+$ and HNC, and betweenthose of HNC and HCN. The HCN and HNC abundances also appear to increase as afunction of the N$_2$H$^+$ abundance. The HNC/HCN and N$_2$H$^+$/HNC abundanceratios are derived to be ~1 on average, while that of HC$_3$N/HCN is ~10%. TheN$_2$H$^+$/HNC ratio appears to increase as the clump evolves, while theHNC/HCO$^+$ ratio shows the opposite behaviour. The detected SiO emission islikely caused by outflow shocks in most cases, although shocks resulting fromthe 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 isin good agreement with those seen in other IRDCs. Our results support thefinding that C$_2$H can trace the cold gas, and not just the PDRs. TheHC$_3$N/HCN ratio appears to be comparable to that in T Tauri disks and comets.

A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds