298 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 9 



Among the various recording mechanisms that have been developed, the 

 most straightforward is direct recording of the galvanometer light beam on a 

 moving strip of photosensitive paper. With several identical galvanometers 

 connected in series and each shunted for a different sensitivity, a wide range 

 of ion currents can be recorded on a narrow paper strip since the shunts or 

 sensitivities can be chosen so that at least one galvanometer trace remains 

 on the paper for the largest expected beam currents. More complex record- 

 ing systems now in use employ standard recording instruments, usually 

 recording potentiometers such as the Speedomax.* Scale contraction in 

 these instruments is accomplished by automatic shunt selection which 

 provides a stepwise sensitivity approximating a logarithmic scale over the 

 width of the recording paper [20]. A further modification of the automatic 

 shunt-selection system enables the top of a peak to be recorded on a linear 

 scale while a large portion of the center of the peak is contracted by a non- 

 linear scaling factor which depends upon the amplitude of the peak [21]. 



9.13. Mass-spectrometer Errors. Relative mass-abundance measure- 

 ments made with a mass spectrometer are subject to many possible errors, 

 some of which are inherent in the instrument and others in the isotopic 

 species themselves. The observed ion currents of several isotopes do not 

 always represent exactly the mass abundance but must be corrected for one 

 or more of the effects that lead to errors. The more important sources of 

 errors are described below. 



a. Inlet-sample Flow Rate. The lighter of two molecules flows through the 

 leak or capillary at a higher rate than the heavier component in accordance 

 with the laws of diffusion. If the system allows free molecular flow, i.e., 

 if the mean free path is very much greater than the capillary radius and the 

 pressure gradient through the leak is small, then the flow rate is inver- 

 sely proportional to the square root of the molecular weight. This effect 

 is particularly important in isotopes of very light mass, e.g., H2 and HD. 



b. Pump-out Rates. As in a, the rates at which substances are pumped out 

 of the spectrometer tube favor the lighter masses and, with a liquid nitrogen 

 or oxygen trap, also the condensible vapors. To some extent this may tend 

 to compensate a. 



c. Voltage Discrimination. The ion current varies somewhat with accel- 

 erating voltage, i.e., electric field strength at the source. If several mass 

 peaks are measured by voltage scanning, the lighter masses are enhanced 

 progressively by the higher accelerating voltage required to focus them 

 on the collecting electrode. Among the isotopes of light elements the effect 

 can be quite large. It can be avoided, however, by magnetic field scanning 

 or by using several collecting electrodes properly located to receive the ion 

 beams of the different isotopes simultaneously. 



* Leeds and Northrup Company. 



