Appendix IX F. Determination of Carbon Dioxide in Medicinal Gases

Gases absorb light at unique wavelengths. This property is widely used to allow highly selective measurement of their concentrations.

Description and principle of measurement The concentration of carbon dioxide in other gases can be determined using an infrared analyser.

The infrared analyser generally consists of a light source emitting broadband infrared radiation, an optical device, a sample cell and a detector. The optical device may be positioned either before or after the sample cell and it consists of one or several optical filters, through which the broadband radiation is passed. The optical device in this case is selected for carbon dioxide. The measurement light beam passes through the sample cell and may also pass through a reference cell if the analyser integrates such a feature (some use an electronic system instead of a reference cell).

When carbon dioxide is present in the sample cell, absorption of energy in the measurement light beam will occur according to the Beer-Lambert law and this produces a change in the detector signal. This measurement signal is compared to a reference signal to generate an output related to the concentration of carbon dioxide. The generated signal is linearised in order to obtain the carbon dioxide concentration. To prevent the entry of particles into the sensors, which could cause stray-light phenomena, the apparatus is fitted with a suitable filter.

Required technical specifications When used for a limit test, the infrared analyser meets the following technical specifications:

• — limit of detection: (generally defined as a signal-to-noise ratio of 2) maximum 20 per cent of the maximum admissible concentration;

• — repeatability: maximum RSD of 10 per cent of the maximum admissible concentration, determined on 6 measurements;

• — linearity: maximum 10 per cent of the maximum admissible concentration.

The technical specifications must be met in the presence of the other gas impurities in the sample.