142 MEASUREMENTS OF GAS EXCHANGE 



Magnetic oxygen analyzer 



Another instrument offering a specific test for a certain gas is the mag- 

 netic oxygen analyzer. Oxygen is about the only gas which is to any 

 extent paramagnetic; that is, it is attracted into a magnetic field. Carbon 

 dioxide, water vapor, and most other gases likely to be encountered in 

 biological experiments do not respond appreciably to a magnetic field. 

 Several models of oxygen analyzers are available. 



The instruments consist of a chamber surrounded by a magnetic field. 

 A stream of the air to be analyzed flows past this chamber, but only 

 oxygen is drawn in. The pressure in the chamber is related to the amount 

 of oxygen in the gas stream. In the Beckman instrument, changes in 

 amounts of oxygen cause a rotation of a small dumbbell suspended on a 

 fine wire. The rotation of the dumbbell is measured directly in some 

 models or indirectly, through a null-point optical and electrical system, 

 in other models. Another instrument, made by Siemens in Germany, 

 determines oxygen concentration by measuring heat conductivity in the 

 chamber. Changes in the amount of oxygen affect the conduction of 

 heat away from a wire, which in turn influences the electrical resistance 

 of the wire. 



In our experience, the Beckman Magnetic Oxygen Analyzer has 

 proved to be slightly slow to respond because the gas must diffuse into 

 the magnetic chamber. Otherwise, the instrument is very reliable, yield- 

 ing reproducible measurements of oxygen concentration very con- 

 veniently. 



SELECTED REFERENCES 



Dixon, Malcolm, Manometric Methods as Applied to the Meastire- 

 ment of Cell Respiration and Other Processes, 3rd ed. New York: 

 Cambridge University Press, 1951. An especially valuable discussion 

 of the theory of manometry. 



Umbreit, W. W., R. H. Burris, and J. F. Stauffer, Manometric 

 Techniques, 3rd ed. Minneapolis: Burgess Publishing Company, 

 1957. The standard laboratory handbook on the subject. 



