The present invention still further provides labeled compounds, of {TAIL ~ the structure Ar—S}—C*D2−xHx—O—R where Ar is an aryl group, C* is a 13C labeled carbon atom, D is {HEAD ~ 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 