A chelate comprising a lanthanide ion selected from europium, terbium, samarium and dysprosium and a chelating ligand of formula wherein X is selected from phenylethynyl, furyl, thienyl, phenyl, and pyrrole, and their substituted derivatives and wherein said substituents are selected from alkyl groups and alkoxy groups, L is a linker formed from one to ten moieties, each moiety being selected from the group consisting of phenylene, alkylene containing 1-12 carbon atoms, ethynydiyl (-C=C-), ethylenediyl (-C=C-), ether (-O-), thioether (-S-), amide (-CO-NH-, -CO-NR'-, -NH-CO- and -NR'-CO-), carbonyl (-CO-), ester (-COO- and -OOC-), disulfide (-SS-), sulfonamide (-SO 2 -NH-, -SO 2 -NR'-), sulfone (-SO 2 -), phosphate (-O-PO 2 -O-), diaza (-N=N-), and tertiary amine, wherein R' represents an alkyl group containing less than 5 carbon atoms, and L replacing a hydrogen anywhere in the parent compound, or not present, A is a reactive group selected from isothiocyanate, bromoacetamido, iodoacetamido, maleimido, 4,6-dichloro-1,3,5-triazin-2-ylamino, pyridyldithio, thioester, aminooxy, azide, hydrazide, amino, alkyne, a methacroyl group, carboxylic acid, acid halide, aryl ester, vinyl ester, and hydroxyamine ester, wherein A' is cleaving group selected from Cl, (CH 3) 2 SO, H 2 O, and NO 3 wherein - is the position of the linker L and wherein - is the position of linker L, and A replacing a hydrogen of the group X when the linker L is not present, and R a is selected from -CH 2 COO- and -(CH 2) n N(CH 2 COO -) 2, wherein n is 2 or 3, and R b is -(CH 2) m N(CH 2 COO -) 2, wherein m is 2 or 3, and R c is selected from CH 2 COO -, and -(CH 2) l N(CH 2 COO -) 2, wherein l is 2 or 3. X is selected from phenylethynyl, furyl, thienyl, phenyl, and pyrrole, and their substituted derivatives and wherein said substituents are selected from alkyl groups and alkoxy groups, L is a linker formed from one to ten moieties, each moiety being selected from the group consisting of phenylene, alkylene containing 1-12 carbon atoms, ethynydiyl (-C=C-), ethylenediyl (-C=C-), ether (-O-), thioether (-S-), amide (-CO-NH-, -CO-NR'-, -NH-CO- and -NR'-CO-), carbonyl (-CO-), ester (-COO- and -OOC-), disulfide (-SS-), sulfonamide (-SO 2 -NH-, -SO 2 -NR'-), sulfone (-SO 2 -), phosphate (-O-PO 2 -O-), diaza (-N=N-), and tertiary amine, wherein R' represents an alkyl group containing less than 5 carbon atoms, and L replacing a hydrogen anywhere in the parent compound, or not present, A is a reactive group selected from isothiocyanate, bromoacetamido, iodoacetamido, maleimido, 4,6-dichloro-1,3,5-triazin-2-ylamino, pyridyldithio, thioester, aminooxy, azide, hydrazide, amino, alkyne, a methacroyl group, carboxylic acid, acid halide, aryl ester, vinyl ester, and hydroxyamine ester, wherein A' is cleaving group selected from Cl, (CH 3) 2 SO, H 2 O, and NO 3 wherein - is the position of the linker L and wherein - is the position of linker L, and A replacing a hydrogen of the group X when the linker L is not present, and R a is selected from -CH 2 COO- and -(CH 2) n N(CH 2 COO -) 2, wherein n is 2 or 3, and R b is -(CH 2) m N(CH 2 COO -) 2, wherein m is 2 or 3, and R c is selected from CH 2 COO -, and -(CH 2) l N(CH 2 COO -) 2, wherein l is 2 or 3.