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The Determination of Carbon Dioxide Concentration Using Atmospheric Pressure Ionization Mass Spectrometry/Isotopic Dilution and Errors in Concentration Measurements Caused by Dryers

^a Science Applications International Corporation, P.O. Box 68, Gunpowder Branch, Aberdeen Proving Ground, MD 21010-0068
^b Dep. of Chemistry, Drexel Univ., 3141 Chestnut St., Philadelphia, PA 19104

^* Corresponding author (brendan.delacy{at}us.army.mil).

Received for publication May 31, 2007. An atmospheric pressure ionization mass spectrometry/isotopically labeled standard (APIMS/ILS) method has been developed for the determination of carbon dioxide (CO₂) concentration. Descriptions of the instrumental components, the ionization chemistry, and the statistics associated with the analytical method are provided. This method represents an alternative to the nondispersive infrared (NDIR) technique, which is currently used in the atmospheric community to determine atmospheric CO₂ concentrations. The APIMS/ILS and NDIR methods exhibit a decreased sensitivity for CO₂ in the presence of water vapor. Therefore, dryers such as a nafion dryer are used to remove water before detection. The APIMS/ILS method measures mixing ratios and demonstrates linearity and range in the presence or absence of a dryer. The NDIR technique, on the other hand, measures molar concentrations. The second half of this paper describes errors in molar concentration measurements that are caused by drying. An equation describing the errors was derived from the ideal gas law, the conservation of mass, and Dalton's Law. The purpose of this derivation was to quantify errors in the NDIR technique that are caused by drying. Laboratory experiments were conducted to verify the errors created solely by the dryer in CO₂ concentration measurements post-dryer. The laboratory experiments verified the theoretically predicted errors in the derived equations. There are numerous references in the literature that describe the use of a dryer in conjunction with the NDIR technique. However, these references do not address the errors that are caused by drying.

Abbreviations: APIMS, atmospheric pressure ionization mass spectrometry • APIMS/ILS, atmospheric pressure ionization mass spectrometry/isotopically labeled standard • NDIR, nondispersive infrared