Aperture Size Influences Oxidation in Positive-Ion Nitrogen Direct Analysis in Real Time Mass Spectrometry

Direct Analysis in Real Time (DART) mass spectrometry commonly uses helium as the DART gas. With the looming helium shortage, other gases are being evaluated for DART. Nitrogen is inexpensive and readily available, making it a desirable alternative. However, NO⁺ reagent ions present in positive-ion nitrogen DART result in extensive oxidation for many compounds. The DART source uses a ceramic insulator cap to protect the operator from electrical shock. The most common cap has an aperture with a 2.5 mm inner diameter, through which the gas exits the DART source. By using a cap with a narrow (0.5 mm) ID, oxidation can be significantly reduced for nitrogen DART. The 0.5 mm cap is hypothesized to reduce back-diffusion of atmospheric oxygen into the DART source, with a reduction in the relative abundance of NO⁺ and increase in the relative abundance of [(H₂O)₂ + H]⁺ as the reactive species responsible for ionization of the analytes.