Analyses of radiochemical purity

Measurement of labelling yield and subsequent radiochemical purity requires a suitable analytical technique, and the method of choice for radio-labelled peptides is reversed phase HPLC with on-line UV and radiometric detection. It is important to use as stringent a separation method as possible with isocratic or slow mobile phase composition gradients over the peptide peak. Ideally, more than one mobile phase system should be used (e.g. a phosphate buffer-methanol system in addition to the standard water-acetonitrile system), since these may show the presence of new impurities. It is important to recognize that HPLC analyses only measure those components that elute from the column. Insoluble, highly lipophilic or positively charged species may bind to the solid phase. It is very important to verify the absence of these species by a complimentary technique such as thin layer chromatography (TLC) and to ensure that the two techniques produce similar results. 

Although not an absolute requirement at this stage of development, consideration should be given to identifying the structure of the radioconjugate formed. A good indication of the structure can be obtained using electrospray 

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Analyses of radiochemical purity were performed by solid phase extraction (SepPak C18 cartridge, Waters), TLC and reversed phase HPLC. An aliquot of 0.1 mL of the labelled peptide was loaded on the preconditioned SepPak cartridge, followed by addition of 5 mL of ImM HCl to elute LuCls or Tc. The labelled peptide was eluted with 3 mL of ethanoksaUne (1 1). 

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