Sec. 9.11] MASS SPECTROGRAPHS 293 



near the exit slit. This effect sometimes produces an undesirable back- 

 ground spectrum when light elements are used in the construction of the 

 ionization chamber, e.g., glass, tin, or aluminum. 



Carriers: Heavy metals sometimes can be transported by a stream of 

 chlorine or fluorine which forms halides of the metals. Most of these halides, 

 however, condense at room temperature, and it is therefore necessary to 

 maintain the sample and gas at an elevated temperature until it enters the 

 ionization chamber. 



9.9. Detector Requirements. The accuracy of measurements of relative 

 abundance of isotopes or of the constituents of a sample under analysis 

 depends largely upon the sensitivity, linearity, and stability of the device 

 used to measure the focused ion beams of different m/e. This places severe 

 requirements on the detector since the ion currents may range in magnitude 

 from less than 10 -16 up to 10 -9 amp. Hence, in addition to a high order of 

 sensitivity, the response of an accurate mass spectrograph should be repro- 

 ducible and linear over a range of beam intensity at least of the order of 10 3 . 



9.10. Photographic Plates. The applications that have required the 

 greatest accuracy have been determinations of isotope mass (relative to 

 O 16 = 16.00000). In nearly all such determinations photographic plates 

 have been employed for ion detection. Since they are essentially an inte- 

 grating device, exceedingly small ion currents can be detected by prolonged 

 exposure, and fluctuations in the current will not affect the accuracy of 

 measurements of relative beam intensities. Both abundance and mass 

 measurements are made from a microphotometric recording of the focal 

 images on the photographic plate. The relative area under the intensity 

 curve of a focal image gives directly the relative abundance, and the distance 

 between maxima gives the relative masses in the appropriate mass scale for the 

 instrument. When a wide range of beam intensities is analyzed, the response 

 of photographic plates cannot be considered linear. In such cases it is nearly 

 always necessary to make several exposures of different values of integrated 

 current on the same plate. For the greatest accuracy, calibration of the 

 emulsion response of the plates should be carried out for a wide range of 

 integrated currents and ion velocities. Since it is known that the emulsion 

 darkening, i.e., number of grains reduced, varies markedly with the ion 

 velocity, this latter effect may be appreciable over the length of the 

 spectrum usually photographed at any one time. 



Photographic emulsions should be uniform and show little or no fog in 

 development. Further, it is important in abundance measurements that no 

 solarization occur around the most dense images. 



9.11. Electrical Devices. The earliest electrical devices for detecting ion 

 beams in mass spectrographs were electroscopes and electrometers. These 

 instruments are current-integrating devices and hence unsuited to rapid 



