The present invention still further provides labeled compounds, of the structure Ar—S—C*D2−xHx—O—R where 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 {HEAD ~ a C1 C5 lower alkyl group} or an acyl group including {TAIL ~ a C1 C5 lower alkyl group substituent}, i.e., the acyl group is where R1 is a C1 C5 lower alkyl 
including
The present invention still further provides labeled compounds , of the structure Ar — S — C*D2−xHx — O — R where 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 { HEAD ~ a C1 C5 lower alkyl group } or an acyl group including { TAIL ~ a C1 C5 lower alkyl group substituent } , i.e. , the acyl group is where R1 is a C1 C5 lower alkyl
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