I have a fiddley little problem! I am trying to get a superscript (i.e. the -1 in "Mg:Ca(µmol mol-1)" into a text grob but can't find the correct annotation of the script:

Axis labels

   yleft<- textGrob("Mg:Ca(µmol mol-1)",rot = 90, gp = gpar(fontsize = 12))
   bottom <- textGrob("Distance from Core (μm))", gp = gpar(fontsize = 12))

I'm a bit of a newbie, any help would be greatly appreciated!