40K primordial radioisotope has been extensively used in Mexico in measuring K concentration in foodstuff, since it is essential in the diet of vertebrates. The used method is a not-destructive one requiring to detect gamma rays being emitted by 40K (0.0118% of elementary K, 1.28 × 109 years (half life) in a low background NaI (Tl) scintil- lation detector during several hours. Samples are condi- tioned in Marinelli containers, having enough weight as to fill them. Suitable time detection might be overnight. Specific total radioactivity R of elementary K is 31.19 Bq/gK is calculated according to the following equation: