Disclosed is a compound of Formula (I), wherein: L is a linking agent; B is a binding agent; X is an atom or group suitable for attaching L to the glycerol chain; and R is a straight chain saturated or unsaturated alkyl group having from 8 to 30 carbon atoms, substituted with M or M" wherein at least one of M and/or M" is a detectable label. The compound can be used as a lipase substrate in a solid phase-based assay system, such as a scintillation proximity assay, to detect lipase enzyme activity.