574 FRED M. UBEH 



3. Apparatus for Detecting Stable Isotopes 



Measurements on all of the commonly employed stable isotopes, 

 with the possible exception of deuterium, are best made with the mass 

 spectrometer. Owing to the fact that the mass of deuterium is just 

 double that of ordinary hydrogen, pure samples of heavy water (D2O) 

 possess a density about 10% greater than ordinary water. Inasmuch 

 as density measurements on liquids can be made with an accuracy of 

 a few parts per million, it is feasible to deduce deuterium concentra- 

 tions from the density of water samples. The advantages lie in the 

 direction of simplicity and economy of apparatus. The latter may 

 consist either of equipment for falling-drop determinations or of a 

 gradient tube. The use of the equipment has been described ade- 

 quately elsewhere {1, pp. 51-65). 



There are several disadvantages to the method of density deter- 

 minations on water as a means of determining deuterium concentra- 

 tion. Perhaps the greatest difficulty lies in achieving adequate puri- 

 fication of a water sample, since clearly any trace of impurity might 

 influence the results very markedly. Purification then, and not 

 density determination, is the main problem. A disturbing factor is 

 the possible occurrence of any one of the three oxygen isotopes in a 

 given water molecule. Since the heavy oxygen isotopes are usually 

 concentrated in varying degree in commercial heavy water samples, 

 this problem is one of S3me importance. A third disadvantage to 

 the density determination method resides in the relatively large test 

 sample required. Usually measurements are made on drops of 100 

 mg. This quantity exceeds that required with the mass spectrom- 

 eter by roughly two orders of magnitude. However, where a re- 

 search program involves only the deuterium isotope, and where sam- 

 ples of sufficient size can be made available for analysis, selection of 

 the density method would seem to be logical. 



The type of mass spectrometer employed in most biological studies 

 to date was developed by Nier (5, p. 80 ; 4) at the University of Minne- 

 sota and is illustrated schematically in Figure 2. 



The gas to be analyzed is contained in the 50 or 100 ml. flask, X, which is 

 fitted with a stopcock and an interchangeable ground glass joint. (For an 

 alternative device, see 11.) After evacuation of the capillary connecting 

 tubes and the reservoir, Y, the test sample is transferred into Y and then 

 compressed to the desired pressure (a few centimeters of mercury) as indi- 

 cated by the attached manometer. Air is pumped into V by the rubber 

 atomizer bulb, Z, if the height of the mercury in Y needs to be raised above 



