XVI. STABLE ISOTOPES AS TRACERS 593 



mass 17 in the form of C'^H4. Corrections for this type of interfer- 

 ence are often quite difficult, but even here satisfactory analyses can 

 be made provided the offending radical is recognized. First, stand- 

 ardization measurements are made covering each of the various com- 

 pounds present and also their disintegration products under carefully 

 controlled conditions of operation (18). Of primary importance is 

 the energ}^ of the bombarding electrons that produce the radicals 

 and bring about their ionization. Corrections for the contributions 

 of numerous molecules and radicals to a given mass peak are con- 

 sidered routine when mass spectrometers are used for ciuantitative 

 analyses of organic mixtures such as are encountered in the petroleum 

 industry (7) . 



It may be well also to mention here the type of self-interference 

 that may result from leaks in the apparatus. Perhaps the most an- 

 noying illustration occurs in the use of molecular nitrogen for N'^ 

 measurements; any leak of atmospheric air into the apparatus is 

 troublesome. The same is true when isotopic oxygen determinations 

 are made with molecular oxygen. 



8. Standard Samples for Comparison 



As already pointed out, it is very difficult and quite unnecessary to 

 determine absolute values of concentration of the stable isotopes with 

 great accuracy. Biological requirements are amply satisfied provided 

 accurate relative determinations of isotope ratios can be obtained. 

 However, it is highly desirable always to relate such measurements to 

 some permanent standard sample. 



For a standard sample in the case of C^^, an adequate reserve of a 

 carbonate salt should be set aside. For each run, a standard test 

 sample is admitted to the mass spectrometer so that all calculations 

 of the isotope concentrations in terms of excess per cent can be based 

 on the same standard. This procedure serves as a check on the opera- 

 tion of the mass spectrometer too, not only during each set of meas- 

 urements but also from day to day and from month to month. With 

 equipment that is functioning satisfactorily, daily variations in the 

 recorded ratios for the standard sample should lie within rather narrow 

 limits. The preservation of a permanent laboratory standard will 

 permit later comparisons with other laboratories should the occasion 

 arise. Perhaps eventually all data could be related to the same na- 

 tional or international standard. 



