INSTITUTE OF LASER MEDICINE

Prof. Dr. P. Hering

 
The analysis of human breath is a very useful tool for noninvasive diagnostics. Usually ¹³C-labelled substrates were given. The course of the ¹³CO2/¹²CO₂ ratio in breath allows the diagnostic of diseases. The usual ¹³CO₂/¹²CO₂ ratio in human breath is 1.095 %. The metabolism of ¹³C labelled substrate lead to a different isotop ratio. For example the human body is not able to split urea. Instead the bacterium helicobacter pylori splits by the enzyme urease urea into CO₂ an NH₄. Therefore the increase of isotop ratio after taking ¹³C labelled urea is the definite sign for the infection by helicobacter pylori. 
In table 1 are some ¹³C breath tests listed. All measurements of isotop ratio in breath were done by nondispersive infrared spectroscopy with the precision of 0.7 ‰. 

The typical setup of an nondispersive infrared spectrometer is shown in figure 1. The light of an filament is chopped, passes the measurement cell for gas and finally the light is detected by an acustooptic detector. The acustooptic detector consists of two chambers, filled with CO₂, and one microphone, which measures the pressure between these two chambers. If the chopper opens the ray, the absorption of light by the gas of the detector causes the increase of pressure. If the chopper closes the ray, the gas cools down and the pressure decreases. The amplitude of pressure is proportional to the intensity of light. For the measurement of the ¹³CO₂/¹²CO₂ ratio there are two measurement cells and two detectors, one for ¹³CO₂ and one for ¹²CO₂.

 

 Figure 1: Setup of an nondispersive infrared spectrometer.

For measurement of concentration, the gas is filled in the measurement cell. As higher the concentration of gas, as more light is absorbed in the cell. Therefore less intensity reaches the acustooptic detector, and the amplitude of pressure decreases. The concentration of gas can be calculated by the decrease of the amplitude. The concentration of ¹²CO₂ and ¹³CO₂ can be measured by nondispersive infrared spectroscopy better than 0.3 ‰ and therefore the isotop ratio can be calculated with an error better than 0.7 ‰. 
For better comprehension of acustooptic detectors, the filament was replaced by tuneable diode laser with an bandwidth less than 10-4 cm-1. The signal of detector was examined resolved by wavelength. In figure 2 is the amplitude of the detector shown, while the laser is tuned over one absorption line. Several detectors were examined. The curve depends on the filling of the detector and the length of the two detector-chambers. The curve can be excellently calculated by an model, which bases on heat conduction. 
 
 

             Figure 2: Amplitude of the microphone signal depending on the wavelength of the laser.

 
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