TLC sensitivity of six modifications of Dragendorff's reagent

The relative sensitivities of six modifications of Dragendorff's reagent were measured on TLC plates by spectrodensitometry, using compounds containing specific functional groups. A correlation between the structures of the compounds reactive to Dragendorff's reagents and the sensitivities of the reagents was made. Explanations for the variations in sensitivities between different modifications of Dragendorff's reagent are given.