The present invention still further provides labeled compounds, of the structure Ar—S—C*D2−xHx—O—{HEAD ~ R} where {TAIL ~ Ar} is an aryl group, C* is a 13C labeled carbon atom, D is a 2H, x is 0 or 1, and R is selected from a C1 C5 lower alkyl group or an acyl group including a C1 C5 lower alkyl group substituent, i.e., the acyl group is where R1 is a C1 C5 lower alkyl 