5:15 PM - 6:30 PM
[MTT28-P05] Development of on-site neon measurement system using a portable ultra-high resolution mass spectrometer (MULTUM)
Keywords:MULTUM, dissolved noble gas, on-site measurement, ultra-high resolution mass spectrometry
Present measurement of dissolved noble gases requires a comprehensive purification and separation of each noble gases from sample gases extracted from seawater (e.g., Sano and Takahata, 2005, Stanley, et al., 2009) to avoid that residual atmospheric gases interfere subsequent mass spectrometric quantification of the target trace noble gases using a quadruple mass spectrometer (QMS) with a low mass resolution. The pretreatment system is quite large and complicate for its field operation and it is practically impossible to conduct the purification and quantification on-site along with seawater sampling. Water samples, then need to be brought back to a land-based laboratory. The taking seawater samples back to the land-based laboratory without suffering contamination of highly permeable atmospheric noble gases requires to store the samples in copper tubes with the both ends fully squashed and completely sealed, which is quite cumbersome and requires man power. Such time-consuming and labor-intensive sampling and pretreatment in both field and laboratory have limited available number of samplings and measurements, and subsequently resulted in a quite limited number of observed data of noble gases in water environments.
Here we introduce a new method for the direct measurement of atmospheric neon concentration without cumbersome pretreatment and sampling treatment by combining multi-turn time of flight mass spectrometer (MULTUM) and a simple cryo-gas trap as a preliminary stage of the development for the online analysis of dissolved noble gases. The ultra-high mass resolution of the MULTUM (10000~30000) compared to that of ordinary QMS (100~500) allows our Ne+ (m/z 19.9919) ion detection to be free from interference ions, such as H218O+ and HF+ (m/z 20.0057 and 20.0151, respectively) which have close m/z to that of Ne+ (Fig. 1) and cannot be detected separately with commonly used QMS. Prior to the ultra-high resolution mass spectrometric analysis, most of the major atmospheric gases (e.g., N2, O2, Ar and CO2) were removed by a cryo-gas trapping to avoid a saturation effect during ionization in the mass spectrometer and further optimize the mass spectrometer for the detection of trace Ne+ ion. Such a simple and compact pretreatment system, compared to the ordinary large and complicated one, enables us to bring the pretreatment system to the field and conduct on-site analysis of Ne.
As a test case, atmospheric Ne was analyzed by directly injecting 0.5~5 cm3 of atmospheric air into the developed system. Atmospheric Ne (18 ppmv) was successfully measured in every 10 minutes by simply injecting sample atmospheric air into the system (Fig. 1-2). An obtained good linearity between injected air volume and Ne+ ion peak (Fig. 2) shows that the simple cryo-gas trap also can work as a pre-concentrator, which is another advantage for trace gas analysis. The preliminary result shows that the developed method can be applied for the continuous and direct measurement of dissolved Ne and other noble gases with further modification.