50 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



The primary technical problems are the avoidance of contamination in 

 the handling of samples and the precise measurement of the low levels of 

 carbon 14. Details of procedure have been published by various labora- 

 tories (115 to 118). The main considerations may be listed as follows: 



(a) reduction of background by anticoincidence counters and shields, 



(b) avoidance of traces of radioactivity in shield and counter materials, 



(c) correction or elimination of drifts in sensitivity or background, (d) 

 elimination of spurious counts, and (e) cleanliness in handling of samples. 

 Present techniques permit age determinations in the range of 500 to 

 30,000 yr. 



Several laboratories in addition to Chicago are now reporting radio- 

 carbon age results: Lamont (119, 120), Yale (121), Michigan (116, 117, 

 122), Copenhagen (123, 124), and Mexico (125). The results are usually 

 based on a C'^ half-life of 5568 ± 30 yr and contain an error term that is 

 calculated from the standard deviation of the counting data. It must be 

 remembered that there may be other errors, both laboratory and non- 

 laboratory, which may have to be taken into account (121, 122, 126). 

 The nonlaboratory errors are concerned with the mode of deposition of the 

 material to be dated and the circumstances of its preservation. For 

 example, thousands of years after an organic deposit had been laid down, 

 it could be enriched with C^^ from new organic material such as plant 

 roots and burrowing animals. There are also problems of isotope effects 

 and equilibria in the sea and in carbonates. 



Criteria for Blood Preservation. When whole blood or packed red 

 cells are stored for transfusion purposes, the cells tend to lose their capac- 

 ity for survival in the blood stream of the recipient. It is very difficult 

 to estimate by conventional methods whether or not the stored cells have 

 lost their viability. Procedures have been developed by Ross et al. (127) 

 and Gibson et al. (128), using radioiron, which have been convenient and 

 satisfactory for this determination. In principle, tracer amounts of 

 radioiron are administered to normal individuals so as to label their red 

 cells. Blood samples are then taken from them for storage studies and 

 subsequent transfusion into the recipient. After transfusion the non- 

 viable cells are rapidly removed from the circulation, so that the isotope 

 content or, more particularly, the specific activity of the recipient's blood 

 at various times after transfusion gives a refiable measure of the value of 

 the blood sample for transfusion purposes. The primary practical appli- 

 cation has been in the improvement of preservation methods and the 

 determination of maximum storage times. 



Study of Rooting Patterns. It is important to have knowledge of the 

 extent and activity of the root systems of crop plants so as to gain an idea 

 of the volume of soil from which the plant can obtain the nutrients and 

 water that it requires. Also, such information is most helpful in deter- 

 mination of the placement of fertilizer for optimum utilization by the 



