THALLIUM 



313 



Intake Levels. Thallium does not occur naturally in biological mate- 

 rials, but animals and humans may be exposed to it on account of its use 

 as a vermin poison. The lethal oral dose for mammals is about 25 mg/kg 

 body weight, which amounts to 3 to 6 mg per rat. The LD50 of TI2SO4 

 given intravenously was 12 mg/kg for the adult male rat (Tl-1). About 

 0.12 mg per rat causes loss of hair. Thallium is poisonous to plant life at 

 about 5 ppm in the soil. 



Radioassay. The higher-specific-activity preparation appears to be 

 the one of choice. Reliance is placed on beta counting, with correction 

 for or elimination of self-absorption. 



Chemistry. The method for the handling of tissues described in the 

 next section is the same as used for chemical estimation, except that for 

 the latter no carrier is added and the chloroform solution is evaporated, 

 the organic matter destroyed, and the thallium oxidized to thallic chloride 

 with bromine in the presence of NH4CI. Excess bromine is removed, 

 and KI added; this reacts with the thallic chloride to liberate iodine, 

 which is determined colorimetrically (Tl-2). Methods of analysis for 

 thallium from a toxicological standpoint have been reviewed (Tl-3), and 

 an analytical procedure described which is based on activation analysis 

 (Tl-4). 



Typical Methods. Following is a description of methods used for 

 studies of distribution in the chick embryo, rat, and man (Tl-1) : Labeled 

 TINO3 with a specific activity of 0.218 mc/mg was used. Rats were 

 injected with 5 fxc, fertile eggs were injected with 25 fxc, and a patient 

 received 500 fic orally. The tissues were digested with concentrated 

 HNO3 and concentrated H2SO4, the digest cooled, and a pinch of sodium 

 bisulfite added to reduce thallic to thallous ion. The solution was neu- 

 tralized with 50 per cent KOH, and 0.15 ml of a 10~^ M TI2SO4 solution 

 added as carrier, followed by 2 ml of a solution of 12.5 g KCN and 12.5 g 

 ammonium citrate per liter. The mixture was extracted four times with 

 1-ml portions of a 0.1 per cent solution of dithizone in chloroform, and the 

 chloroform extracts placed on planchets and evaporated under infrared. 

 The planchets were counted with a Geiger tube with a 2.2-mg/cm^ 

 window. The assays showed an average loss of 10 per cent which was 

 applied as a correction factor. 



