580 FRED M. UBER 



For intermediate concentrations, the ionized particles appearing in 

 both columns must bo considered. Hydrogen samples for analysis 

 are obtained by coml)ustion of the biological (compound (/, p. 56) 

 and subsequent reduction of the resultant water is produced either 

 by the use of a magnesium amalgam (20, p. 421) or by electrolysis 

 (17, p. 130). 



For oxygen isotopes, analyses can be made conveniently with 

 molecular oxygen. The ions produced most plentifully are listed in 

 the fifth column of Table II . Ions resulting from traces of water in the 

 instrument interfere with atomic oxygen particles at mass positions 17 

 and 18. Consequently isotopic ratios are preferably based on the 

 relative heights of the ion current peaks for masses 33 and 32, and 

 for 34 and 32. 



Carbon dioxide can also be used as the test gas for oxj^gen isotope 

 analyses. Not all of the possible ions produced in carbon dioxide 

 are shown in Table II. But in concentrated test samples one would 

 observe peaks corresponding to all mass combinations of C^^ and C^^ 

 with O^^, O'^, and 0^^ that are possible in a triatomic molecule. 

 Where the O^^ concentration is constant and the O^^ is variable, 

 abundance ratios can be based on measurements at mass numbers 44 

 and 46. 



Determinations of N^^ can be made fairly satisfactorily with the 

 diatomic nitrogen molecule as the test gas. The detectable ionized 

 particles that may be expected to appear will be found listed in Table 

 II. The use of the atomic ions of masses 14 and 15 is counterindi- 

 cated, owing to the presence of a peak in the 14^/2 position. This 

 results from doubly charged N"N^^ ions and obviates the anticipated 

 improvement in the spectral separation that would otherwise accrue 

 from the lower masses of the atomic ions compared with the molecular 

 ions. Other disadvantages of the atomic ions for ratio measurements 

 have been discussed by Nier (i, p. 21). Accordingly, nitrogen analy- 

 ses are normally based on measurements of the 28 and 29 ion current 

 peaks. Some interference at the 28 and 29 positions is to be expected 

 from atmospheric nitrogen contamination; also carbon monoxide 

 and carbon dioxide must be excluded. In addition, Rittenberg (1, p. 

 32) cautions against still another impurity that can be present at 

 mass 29 unless the Kjeldahl digestion is carried out according to defi- 

 nite, prescribed rules. To obtain nitrogen gas from biological organic 

 compounds, the latter must undergo either an alkaline hydrolysis or a 

 Kjeldahl combustion to yield an ammonium salt. Subjecting the 



