[en] [123-I]-Iodomethyl tyrosine, an analog of tyrosine, is used as a radiopharmaceutical to detect malignant tissue in vivo. Initial synthesis report removal of the starting material using HPLC reversed phase chromatography as well as a simple method using a C18-SepPak cartridge when an HPLC system is not available. Small amounts of residual starting material have not been reported to interfere with tumor uptake following biodistribution in vivo. However, in vitro tissue culture studies do require the final product to be free of un-reacted methyl tyrosine. Our goal was to quantify the amount of residual methyl tyrosine after C18-SepPak purification to confirm that ¹²³I methyl tyrosine purified in this manner would be suitable tissue culture studies. A preconditioned (rinsed with 2 ml Ethanol and 8 ml PBS) C18-SepPak cartridge is loaded with the ¹²³I-iodomethyl tyrosine reaction mixture and washed with 8 mL of PBS to remove the un-reacted methyl tyrosine and free ¹²³I-iodide. The cartridge is then eluted with a 20% alcohol/PBS mixture to recover the ¹²³I-iodomethyl tyrosine. Paper chromatography confirmed the removal of un-reacted ¹²³I-iodide. A parallel study with a methyl tyrosine standard was used to confirm the removal of the methyl tyrosine from the SepPak cartridge during the washing with 8 mL of PBS. Fractions were collected and UV absorbance was recorded. A standard curve was prepared using the UV absorbance of serial dilutions of methyl tyrosine. The detection limit was in the order of ng/mL. An elution profile of both ¹²³I methyl tyrosine and methyl tyrosine was obtained and shows that traces of methyl tyrosine can still be present after an 8 mL PBS wash