140 MEASUREMENTS OF GAS EXCHANGE 



sionally, one of the ground glass joints will leak, causing pressure changes 

 that appear too low. Sometimes the temperature of the bath will drift 

 slightly, leading to large thermobarometer corrections. Rarely, the tem- 

 perature in the bath is not uniform from place to place. In this case, 

 the thermobarometer correction may not truly represent the change that 

 would have occurred in the experimental manometers if they had had 

 no living material. 



These difficulties are less bothersome than the annoying problems that 

 are associated with many other laboratory instruments. The manometric 

 technique has proved its advantage over many years. The literature con- 

 tains only a few papers where manometry has been pushed beyond its 

 limitations, a fact which also attests to the value of the method. 



Several new physical methods of measuring various gases have been 

 developed recently. Some of these are much more convenient for certain 

 measurements, and some provide a specific analysis for a particular gas. 

 Although they will be used to advantage, it is doubtful that they will 

 ever completely replace the old reliable manometers. 



The infrared gas analyzer 



One of the more popular of the recently developed instruments for 

 gas analysis is the infrared analyzer. Most organic compounds have 

 infrared absorption bands, arising from the vibrations of the C— O, 

 C— H, O— H, and other bonds. Any individual molecule has a definite 

 collection of such heteroatomic bonds and, consequently, a defined spec- 

 trum of absorption in the infrared region. This principle is the basis of 

 the infrared spectrophotometry mentioned in Chapter 9. 



The Beckman InfraRed Gas Analyzer is essentially a colorimeter de- 

 signed to operate in the infrared region. It can be used for any of sev- 

 eral gases, including carbon dioxide, water, and methane. The instru- 

 ment responds to the amount of a particular gas in a stream of air, 

 providing a continuous record of its concentration. 



The essential features and the operating principle can be seen by 

 referring to Fig. 10-4. This is a simplified diagram of the analysis unit. 

 The air or gas to be analyzed is drawn continuously through the sample 

 tube. Infrared from the two sources (S) passes through the sample tube 

 and the reference tube to the detector chambers. The windows (W) 

 transmit infrared but prohibit the free movement of gas. The two detec- 

 tor chambers are filled with the gas to be analyzed, for example, CO2. 



