XVI. STABLE ISOTOPES AS TRACERS 587 



dance only, so that precise measurement of their absolute concentra- 

 tion is not essential. 



Since the rate of diffusion of gas molecules depends on their mass, 

 it is to be expected that'the lighter of two isotopic compounds would 

 enter the spectrometer more readily than the heavier. Unless 

 counterbalanced, this preferential effect would result in a relatively 

 higher reading for the concentration of the lighter isotope. As a 

 matter of fact, a similar but compensatory effect does occur in the 

 diffusion pump, which is constantly evacuating the spectrometer, 

 thus tending to reduce the observed concentration of the lighter iso- 

 tope component. Hence these two factors tend to counterbalance 

 each other and the net error introduced is probably not large. Dis- 

 crimination between particles of different masses may also occur in 

 the ion source of a mass spectrometer (2, p. 539). As a consequence 

 the concentration ratios may be affected even to the extent of a few 

 per cent ; the effect is greater for higher mass ratios. While such dis- 

 crimination can be reduced by employing an ion source with a more 

 complicated design, it maj^ not be profitable to do so in the routine 

 analytical spectrometer since an accurate absolute measurement of 

 the concentration is not necessary in tracer experiments. 



A number of other sources of error that affect the accuracy of the 

 absolute abundance ratio will be discussed in the sections that follow, 

 since they also influence the relative accuracy of isotope determina- 

 tions (see also 3, p. 101). 



2. Relative Accuracy 



Mass spectrometric analyses of biological materials usually com- 

 prise comparisons between the concentration of an element used as a 

 tracer and that of its naturally more abundant isotope. As pointed 

 out in preceding paragraphs, this ratio may not be correct in an abso- 

 lute sense to within a few per cent. However, if it is used only with 

 reference to similar ratios all of w^hich are based on the same standard 

 gas sample, as discussed later (Sect. D8), it yields entirely adequate 

 information for use in tracer studies. Thus, the relative accuracy of 

 a pair of ratio determinations may be independent of a number of 

 sources of error that affect adversely the absolute determination of 

 either one. Still, the relative ratio does depend on the quality of the 

 equipment and on the technical skill of the operator in a number of 

 ways. These will now be briefly discussed. 



