Application of Non-Dispersive Infrared (NDIR) Spectroscopy to the Measurement of Atmospheric Trace Gases
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Gaseous pollutants have been an environmental concern since 1956, when the first clean air act was established in the United Kingdom. Monitoring of gaseous emissions is a legal requirement in most countries, and this has generated a large demand for inexpensive, portable, and versatile gas analysers for the measurement of gaseous emissions. Many of the current commercial gas analysers have differing advantages and disadvantages, however, high cost is an important factor. Instruments with low detection limits and the ability to measure multiple gases tend to be very expensive, whereas, single gas analysers tend to be much more affordable. A non-dispersive infrared (NDIR) spectrometer, originally developed for a previous M.Sc. project, has been further developed in order to increase the sensitivity and to extend the instrument to the measurement of multiple gases. This type of instrument would be useful for environmental, industrial, and research applications. The instrument was inexpensive to construct when compared with the cost of current commercial gas analysers, is robust, and is partially portable around the laboratory. Infrared radiation from two infrared sources, pass through adjacent sample and reference cells and into corresponding detector cells. A sample comprising the analyte gas is contained in the sample cell, a non-absorbing gas, such as argon, is contained in the reference cell, and pure analyte gas of interest is contained in the detector cells. The two identical detector cells, which follow the reference and sample cells in the infrared optic paths, communicate only through a differential capacitance manometer which accurately measures small pressure differences between the otherwise identical cells. Any trace amount of the analyte gas in the sample cell absorbs radiation, depleting the appropriate infrared frequencies. This results in lower energy incident on the sample detector cell, reducing the infrared induced pressure rise in that detector cell compared to the reference side detector cell. The pressure difference is ii proportional to the concentration of absorbing gas in the sample cell, which is then determined using a calibration graph. Carbon dioxide, methane, and nitrous oxide calibration graphs from 40 ppm to 1000 ppm have been successfully established, and detection limits of 10.33 ppm for CO₂, 8.81 ppm for N₂O and 9.17 ppm for CH₄ were determined. Dried air samples measured using the spectrometer gave an average value of 382 ± 9.6 ppm which can be compared to the latest global atmospheric loading of 382.4 ppm.