A method of radiolabelling a biological targeting moiety which comprises: (i) provision of a biological targeting moiety functionalized with an aminooxy group of Formula: [BTM-L 1 -Y-NH 2] wherein: BTM is a biological targeting moiety; L 1 is a synthetic linker group of formula - m- wherein each A is independently -CR 2 -, -CR=CR-, -CC-, -CR 2 CO 2 -, -CO 2 CR 2 -,-NRCO-, -CONR-, -CR=N-O-, -NR(C=O)NR-, -NR(C=S)NR-, - SO 2 NR-, -NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, -CR 2 NRCR 2 -, a C4-8 cycloheteroalkylene group, a C4-8 cycloalkylene group, -Ar-, -NR-Ar-, -O-Ar-, -Ar- -, an amino acid, a sugar or a monodisperse polyethyleneglycol building block, wherein each Ar is independently a C 5-12 arylene group, or a C 3-12 heteroarylene group; each R is independently chosen from H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 1-4 alkoxyalkyl or C 1-4 hydroxyalkyl; m is an integer of value 1 to 20; Y is -O-; (ii) making a composition which comprises a solution of a radioactive aldehyde of Formula together with multiple non-radioactive aldehydes of Formula: wherein: X 1 is 18 F; X 2 is a combination of -N + (CH 3) 3, -N(CH 3) 2, and -OH; wherein the total concentration of derivatives of Formula present in the composition is less than 150 μg/mL; using a method comprising dilution of a crude labelling mixture with ammonium hydroxide solution and purification on an MCX mixed mode solid phase extraction cartridge; (iii) reaction of the functionalized- biological targeting moiety from step (i) with the radioactive aldehyde composition of step (ii), such that the radioactive aldehyde of Formula condenses with said aminooxy group, to give the radiolabelled biological targeting moiety of Formula: wherein: BTM, L 1, and Y are as defined for Formula and X 1 is as defined for Formula. (i) provision of a biological targeting moiety functionalized with an aminooxy group of Formula: [BTM-L 1 -Y-NH 2] wherein: BTM is a biological targeting moiety; L 1 is a synthetic linker group of formula - m- wherein each A is independently -CR 2 -, -CR=CR-, -CC-, -CR 2 CO 2 -, -CO 2 CR 2 -,-NRCO-, -CONR-, -CR=N-O-, -NR(C=O)NR-, -NR(C=S)NR-, - SO 2 NR-, -NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, -CR 2 NRCR 2 -, a C4-8 cycloheteroalkylene group, a C4-8 cycloalkylene group, -Ar-, -NR-Ar-, -O-Ar-, -Ar- -, an amino acid, a sugar or a monodisperse polyethyleneglycol building block, wherein each Ar is independently a C 5-12 arylene group, or a C 3-12 heteroarylene group; each R is independently chosen from H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 1-4 alkoxyalkyl or C 1-4 hydroxyalkyl; m is an integer of value 1 to 20; Y is -O-; BTM is a biological targeting moiety; L 1 is a synthetic linker group of formula - m- wherein each A is independently -CR 2 -, -CR=CR-, -CC-, -CR 2 CO 2 -, -CO 2 CR 2 -,-NRCO-, -CONR-, -CR=N-O-, -NR(C=O)NR-, -NR(C=S)NR-, - SO 2 NR-, -NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, -CR 2 NRCR 2 -, a C4-8 cycloheteroalkylene group, a C4-8 cycloalkylene group, -Ar-, -NR-Ar-, -O-Ar-, -Ar- -, an amino acid, a sugar or a monodisperse polyethyleneglycol building block, wherein each Ar is independently a C 5-12 arylene group, or a C 3-12 heteroarylene group; each R is independently chosen from H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 1-4 alkoxyalkyl or C 1-4 hydroxyalkyl; m is an integer of value 1 to 20; Y is -O-; (ii) making a composition which comprises a solution of a radioactive aldehyde of Formula together with multiple non-radioactive aldehydes of Formula: wherein: X 1 is 18 F; X 2 is a combination of -N + (CH 3) 3, -N(CH 3) 2, and -OH; wherein the total concentration of derivatives of Formula present in the composition is less than 150 μg/mL; using a method comprising dilution of a crude labelling mixture with ammonium hydroxide solution and purification on an MCX mixed mode solid phase extraction cartridge; X 1 is 18 F; X 2 is a combination of -N + (CH 3) 3, -N(CH 3) 2, and -OH; wherein the total concentration of derivatives of Formula present in the composition is less than 150 μg/mL; using a method comprising dilution of a crude labelling mixture with ammonium hydroxide solution and purification on an MCX mixed mode solid phase extraction cartridge; (iii) reaction of the functionalized- biological targeting moiety from step (i) with the radioactive aldehyde composition of step (ii), such that the radioactive aldehyde of Formula condenses with said aminooxy group, to give the radiolabelled biological targeting moiety of Formula: wherein: BTM, L 1, and Y are as defined for Formula and X 1 is as defined for Formula.