Abstract: The residue of K_2.2.2 was one of the key metric in ¹⁸F-FDG quality control. The specificity of the spectrophotometry for K_2.2.2 was compared with others. Methods: The 14 samples were determined for the K_2.2.2 by the spectrophotometry and iodine vapor TLC stain method, respectively. The results were compared with LC-MS/MS. The spectrophotometry showed that the 9 of negative samples were false positive, 2 of positive samples were higher than its actual K_2.2.2 contents, 3 of ¹⁸F-FDG injections contained 14.3-19.2 μg/mL for K_2.2.2. The iodine-vapor TLC stain method showed that 9 of negative samples were negative, 2 of positive samples were the same with its K_2.2.2 contents, 3 of ¹⁸F-FDG injections were lower than 10 μg/mL with K_2.2.2. The iodine vapor TLC stain results were confirmed by the LC-MS/MS. Results: The specificity of spectrophotometry was poor for K_2.2.2. The iodine-vapor TLC stain was simple and good specificity method for K_2.2.2 in ¹⁸F-FDG injections.