2:45 PM - 3:00 PM
[PCG20-17] Development of an ion species identification method utilizing both LEF and ST paths in a TOF ion mass spectrometer
Keywords:ion, ion mass spectrometer, space exploration, planetary atmosphere
The time-of-flight (TOF) ion mass spectrometer is one of the analytical techniques with high mass resolution used for ion mass analysis and is widely utilized for plasma and ion observations in space exploration. However, when designed for spacecraft applications, the need for miniaturization results in reduced mass resolution. While low mass resolution is sufficient for detecting monoatomic ions, distinguishing molecular ions with the same mass number requires a high-resolution, large-scale instrument such as ROSINA onboard the Rosetta mission. In this study, we develop a method to enhance ion species identification accuracy in a low-mass-resolution TOF ion mass spectrometer by utilizing both the Linear Electric Field (LEF) and Straight Through (ST) paths.
In a TOF ion mass spectrometer, incident ions generate start electrons upon passing through a carbon foil, which serves as the trigger for measuring ion flight times that vary depending on ion mass. The carbon foil also induces ion separation and charge exchange, leading to two distinct trajectories: ions that become positively charged follow the LEF path, while those that lose charge and neutralize take the ST path. The LEF path allows determination of the post-separation ion mass based on the trajectory influenced by the linear electric field, whereas the ST path provides the pre-separation ion mass based on the direct flight time. Since mass resolution is depends directly on time resolution, the miniaturization of the instrument tends to degrade it.
Previous studies analyzed data from these two paths separately to estimate ion species and compositions. Using only LEF path data allows the identification of elements such as hydrogen and oxygen; however, it does not provide sufficient information to determine their molecular ion origins. On the other hand, relying solely on ST path data results in lower mass resolution and difficulties in distinguishing ions with the same mass, such as H2O+ and NH4+. To address these limitations, we propose an approach that integrates data from both LEF and ST paths to improve the accuracy of ion species identification.
Our method begins by simulating ion count distributions for different ion species with assumed ion number densities based on planetary atmospheres. We generate estimated mass spectra by calculating the count distributions of mass-to-charge ratios (m/z) for both LEF and ST paths, considering the mass resolution of the ion mass spectrometer used in the Comet Interceptor mission, which is m/Δm = 45 for the LEF path and m/Δm = 5 for the ST path. In this process, we define a matrix A to represent the transformation from the original ion number densities to the estimated mass spectra.
Next, to reconstruct the original ion abundance ratios from the estimated mass spectra, we use the inverse of matrix A or employ a non-negative least squares method for both LEF and ST datasets. To assess the accuracy of the reconstruction, we introduce random errors into the estimated mass spectra to simulate noise in actual measurements and analyze the resulting errors in reconstructed ion number densities.
Furthermore, for ion species where either the LEF or ST path provides reliably low error, we adopt those values and use them as fixed constraints. By incorporating these constraints into the least squares method, we improve the accuracy of ion abundance estimation. This iterative refinement allows us to preserve the advantages of both paths while improving the accuracy of other ion species' abundance ratios, ultimately leading to a more precise reconstruction of the original ion number densities.
While this study primarily focuses on developing a numerical analysis model, future work will involve experimental measurements of ion separation and charge exchange rates for various molecular ions relevant to planetary atmospheres. These experimental results will further refine and enhance the accuracy of our model.
In a TOF ion mass spectrometer, incident ions generate start electrons upon passing through a carbon foil, which serves as the trigger for measuring ion flight times that vary depending on ion mass. The carbon foil also induces ion separation and charge exchange, leading to two distinct trajectories: ions that become positively charged follow the LEF path, while those that lose charge and neutralize take the ST path. The LEF path allows determination of the post-separation ion mass based on the trajectory influenced by the linear electric field, whereas the ST path provides the pre-separation ion mass based on the direct flight time. Since mass resolution is depends directly on time resolution, the miniaturization of the instrument tends to degrade it.
Previous studies analyzed data from these two paths separately to estimate ion species and compositions. Using only LEF path data allows the identification of elements such as hydrogen and oxygen; however, it does not provide sufficient information to determine their molecular ion origins. On the other hand, relying solely on ST path data results in lower mass resolution and difficulties in distinguishing ions with the same mass, such as H2O+ and NH4+. To address these limitations, we propose an approach that integrates data from both LEF and ST paths to improve the accuracy of ion species identification.
Our method begins by simulating ion count distributions for different ion species with assumed ion number densities based on planetary atmospheres. We generate estimated mass spectra by calculating the count distributions of mass-to-charge ratios (m/z) for both LEF and ST paths, considering the mass resolution of the ion mass spectrometer used in the Comet Interceptor mission, which is m/Δm = 45 for the LEF path and m/Δm = 5 for the ST path. In this process, we define a matrix A to represent the transformation from the original ion number densities to the estimated mass spectra.
Next, to reconstruct the original ion abundance ratios from the estimated mass spectra, we use the inverse of matrix A or employ a non-negative least squares method for both LEF and ST datasets. To assess the accuracy of the reconstruction, we introduce random errors into the estimated mass spectra to simulate noise in actual measurements and analyze the resulting errors in reconstructed ion number densities.
Furthermore, for ion species where either the LEF or ST path provides reliably low error, we adopt those values and use them as fixed constraints. By incorporating these constraints into the least squares method, we improve the accuracy of ion abundance estimation. This iterative refinement allows us to preserve the advantages of both paths while improving the accuracy of other ion species' abundance ratios, ultimately leading to a more precise reconstruction of the original ion number densities.
While this study primarily focuses on developing a numerical analysis model, future work will involve experimental measurements of ion separation and charge exchange rates for various molecular ions relevant to planetary atmospheres. These experimental results will further refine and enhance the accuracy of our model.