{HEAD ~ 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 a C1 C5 lower alkyl group or an acyl group including a C1 C5 lower alkyl group substituent, i.e., {TAIL ~ the acyl group} is where R1 is a C1 C5 lower alkyl 
{ HEAD ~ 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 a C1 C5 lower alkyl group or an acyl group including a C1 C5 lower alkyl group substituent , i.e. , { TAIL ~ the acyl group } is where R1 is a C1 C5 lower alkyl
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