Range of molecular column densities (upper panel) and fractional abundances (lower panel) for our sources as compared with similar samples of other studies. The y-axes are shown on a logarithmic scale. The abbreviation TW refers to this work. The references are coded as follows: FCC06  =  Fontani et al. (2006), SSH10  =  Sakai et al. (2010), BLL95  =  Butner et al. (1995), GPB09  =  Gibson et al. 2009, SSK08  =  Sakai et al. (2008), BH09  =  Beuther & Henning (2009), CLS10  =  Chen et al. (2010), FPC11  =  Fontani et al. (2011), RBP06  =  Ragan et al. (2006), TF08  =  Thomas & Fuller (2008), VLH11  =  Vasyunina et al. (2011). The ¹³CS abundance of  ~ 6.2 × 10^-9 we derived towards I18102 MM1 is shown by a slightly stretched bar for clarity.