Sec. 0.14] MASS SPECTROGRAPHS 299 



d. Background. The presence of a background of peaks due to residual 

 gas can introduce serious errors when they appear at mass positions of the 

 less abundant sample components. The only reliable correction is elimina- 

 tion of the background in the region of the spectrum in which mass peaks 

 of the sample components will appear. Except for certain organic com- 

 pounds, water vapor is the most difficult of the common substances to outgas. 

 Once the system has been exposed to water vapor, a prolonged bake-out 

 with the entire spectrometer tube maintained at an elevated temperature is 

 necessary to remove the water vapor peaks at masses 16, 17, and 18. Also 

 frequently observed are mercury or hydrocarbon peaks due to vapors from 

 the diffusion pump. 



e. Incomplete Resolution. Peaks of large amplitude may be sufficiently 

 wide at the bottom to overlap the adjacent mass position, thus making a 

 correction necessary for the amount of overlap. This effect arises from poor 

 slit- and magnetic-field adjustment, inadequate collimation, metastable 

 ions [28], or a long time constant in some part of the detecting circuits. 



/. Nonlinearity of Detecting Circuits. This can be avoided by the use of 

 100 per cent feedback in the electrometer-amplifier system or by use of a 

 null method. 



g. Instability of Electrical Circuits. This includes primarily the emission 

 current and voltage and the detecting circuits. If drift and short period 

 fluctuations cannot be avoided, it is necessary to take a sufficient number of 

 readings to give a statistically reliable average. 



h. Overlapping Spectra. Molecules containing more than one atomic species, 

 each with several isotopes, frequently form combinations of different isotopes 

 but with the same total mass, e.g., carbon dioxide. Determination of the 

 abundance of the separate isotopes or of the molecule is sometimes possible 

 only by analysis of its entire spectrum. 



9.14. Hydrogen. The hydrogen isotopes are measured in the form of 

 hydrogen gas. Water vapor is impracticable because of the great difficulty 

 in ridding the system of it, the inconsistency of results, and the needless 

 complication by the oxygen isotopes. Attempts to use hydriodic acid have 

 met with consistent failure. 



The hydrogen gas sample is usually obtained by reduction of water derived 

 from the original labeled substance. This is most conveniently done by 

 passing the water vapor through a quartz tube containing zinc grains main- 

 tained at a temperature of 350°C. The collected hydrogen is then toeplered 

 into a small glass sample bottle for transfer to the gas sampling-system of the 

 spectrometer. 



Hydrogen-isotope analysis is somewhat complicated by the formation of H 3 

 in the ionization chamber under conditions that depend upon the pressure, the 

 source geometry, and the accelerating field strength [24]. The formation of 



