Separation of Ammonia Isotopologues by Benchtop Drift Tube Ion Mobility Spectrometry and Chemometric Modeling

This research study assesses the applicability of drift tube ion mobility spectrometry (DTIMS) to resolve ammonia (NH₃) from its isotopologue, ammonia-d3 (ND₃). DTIMS, known for its rapid response at ambient pressure, was employed to conduct real-time analysis of gaseous samples. Two ammonia introduction methods were evaluated, evaporation of a 3 ppm(v) aqueous ammonia solution and dry gas synthesis. The study tested these methods for the mixture analysis of ammonia and examined their impact on resolution. The resolution of isotopologues and the influence of moisture in the carrier gas on separation were assessed. Individual analysis by evaporation method provided drift times of 6.03 ms for NH₃ and 6.17 ms for ND₃, from which reduced mobility (K₀) values of 2.52 cm² V^-1 s^-1 and 2.46 cm² V^-1 s^-1 were calculated, respectively. The presence of moisture in the carrier gas was found to significantly reduce the resolution of isotopologue separation. To address this limitation and enhance signal clarity, the application of wavelet denoising was conducted, with universal thresholding based on Symlet, Daubechies, and Coiflet wavelet families systematically evaluated. The Daubechies wavelet (db9) at level 9 was identified as the optimal denoising approach. Following this, multivariate curve resolution alternating least-squares (MCR-ALS) was employed for the decomposition of the complex overlapping signals into their pure ion mobility spectra and corresponding concentration profiles over time. The integration of wavelet-based denoising and MCR-ALS offers a practical strategy for the enhancement of DTIMS performance in complex isotopologue separation, particularly for future studies focused on quantitative analysis. Future work will involve the integration of steady-state isotopic transient kinetic analysis (SSITKA) for a comprehensive kinetic framework for kinetic models.