GENERAL PROCEDURES FOR RADIOASSAY 19/ 



of biological knowledge whether taking the body size into account will 

 produce more error in the results than neglecting it. Unless there is 

 information available upon which a decision can be based, it may be wiser 

 to neglect this correction factor. In studies with radioisotopes of Na, Ca, 

 etc., w^hich may be distributed in proportion to body weight, this correc- 

 tion is justified. However, in an iodine distribution study, for example, 

 the size of the thyroid gland will probably be the governing factor, and 

 the gland size may well be inversely related to the body size. In cases 

 where excretion values are know^n, a partial correction can be made with 

 confidence by calculation of the tissue concentrations in terms of the per- 

 centage that was absorbed. Similarly, if there is retention at a site of 

 injection, this may be measured and corrections applied. These consid- 

 erations have been illustrated for animal studies but might well apply to 

 plants also. 



So far the discussion has been concerned with the expression of results 

 for gross tissue distribution. It is often desirable, especially in turnover 

 comparisons, to express results in terms of specific activities. It will be 

 recalled from Chap. 1 that specific activity was used to designate the 

 amount of radioactive element per unit weight of the element present, 

 this weight to include both active and stable isotopes. Specific activity 

 may then be expressed in terms of percentage of dose per unit mass of 

 element. In comparisons between tissues of the same animal, the body- 

 w^eight factor does not enter. However, in comparisons between animals 

 the same considerations as discussed for concentration values of dose and 

 body weight apply also to the specific-activity values. If various ele- 

 ments are to be compared, the specific activity may be expressed as per- 

 centage of dose per millimole of element. Specific activity may also be 

 used for pure compounds isolated from the tissue, in which case the 

 specific activity is expressed as percentage of administered activity per 

 millimole or per milligram of compound. 



In some experiments it may be of value to make comparisons such as 

 tissue concentration vs. average concentration in the w^hole animal, or 

 tissue concentration vs. some reference tissue concentration. In man}- 

 such cases the units cancel out and are unimportant. 



GENERAL REFERENCES 



1. Furman, N. Howell, ed.: "Scott's Standard Method of Chemical Analysis," 

 5th ed., vols. 1 and 2, D. Van Nostrand Company, Inc., New York, 1939. 



2. Sandell, E. B.: "Colorimetric Determination of Traces of Metals," 2d ed.. Inter- 

 science Publishers, Inc., New York, 1950. 



3. Win ton, Andrew L., and Kate Barber Winton: "The Analysis of Foods," John 

 Wiley & Sons, Inc., New York, 1945. 



4. Piper, C. S.: "Soil and Plant Analysis," Interscicnce Publishers, Inc., New York, 

 1947. 



