Real-time, high-resolution quantitative measurement of multiple soil gas emissions : selected ion flow tube mass spectrometry

Abstract

A new technique is presented for the rapid, high-resolution identification and quantification of multiple trace gases above soils, at concentrations down to 0.01 µL L⁻¹ (10 ppb). The technique, selected ion flow tube mass spectrometry (SIFT–MS), utilizes chemical ionization reagent ions that react with trace gases but not with the major air components (N₂, O₂, Ar, CO₂). This allows the real-time measurement of multiple trace gases without the need for preconcentration, trapping, or chromatographic separation. The technique is demonstrated by monitoring the emission of ammonia and nitric oxide, and the search for volatile organics, above containerized soil samples treated with synthetic cattle urine. In this model system, NH₃ emissions peaked after 24 h at 2000 nmol m⁻² s⁻¹ and integrated to approximately 7% of the urea N applied, while NO emissions peaked about 25 d after urine addition at approximately 140 nmol m⁻² s⁻¹ and integrated to approximately 10% of the applied urea N. The monitoring of organics along with NH₃ and NO was demonstrated in soils treated with synthetic urine, pyridine, and dimethylamine. No emission of volatile nitrogen organics from the urine treatments was observed at levels >0.01% of the applied nitrogen. The SIFT method allows the simultaneous in situ measurement of multiple gas components with a high spatial resolution of <10 cm and time resolution <20 s. These capabilities allow, for example, identification of emission hotspots, and measurement of localized and rapid variations above agricultural and contaminated soils, as well as integrated emissions over longer periods.