Visualization of mass transport and heat transfer in the FAPA ambient ionization source

Abstract

Ambient desorption/ionization mass-spectrometry (ADI-MS) has shown tremendous potential for the direct analysis of materials in the open atmosphere. Unfortunately, processes governing analyte desorption and transport into the mass spectrometer, which ultimately limit sampling reproducibility and quantification, have not been investigated for most ADI-MS sources. For plasma-based ADI-MS sources, such studies are further complicated because the discharge support gas is optically transparent.Here, two methods were employed to probe these otherwise invisible phenomena. Specifically, schlieren imaging and infrared (IR) thermography were utilized to visualize plasma-gas flow and heat transfer, respectively, from a pin-to-capillary geometry flowing atmospheric-pressure afterglow (FAPA) ambient ionization source. The influence of operating conditions was investigated, including plasma-gas flow rate and source capillary diameter. Interactions of the desorption/ionization beam with a sample probe or a sample surface before it is captured by a mock mass spectrometer interface were also explored.These experiments showed that schlieren imaging is a viable means for visualization of the plasma transport gas, and that coupling with IR thermography yields information on gas and temperature distributions. Schlieren images revealed that the existence of a discharge can alter by up to 8 cm the location where the flowing afterglow transitions from laminar to turbulent flow. Addition of a sampleintroduction probe into the plasma gas perturbed the width of the gas beam by 1.5 cm. Additionally, helium impinging on a surface expanded rapidly, unless an interface was present to capture the gas, while local heating was confined to a small area (<1 cm 2 ), based on 75% of the maximum temperature, compared to background.

Measurement and Visualization of Mass Transport for the Flowing Atmospheric Pressure Afterglow (FAPA) Ambient Mass-Spectrometry Source

“…When the melting point capillary was positioned 1 mm from the FAPA source, the signal for caffeine showed a strong bimodal distribution, with analyte signals (M+H) + being strong only along the edges of the MPC (Figure 2d). This finding meshes with what has been seen previously [20] where the MPC blocks and deflects helium away from the MS inlet, so analyte could be transported from the MPC into the MS only along the edges of the MPC where the blocking effect is minimal. Moving the MPC equidistant between the FAPA source and MS inlet (Figures 2b and 2e) improves the mass transfer from the MPC, but still results in a bimodal distribution, albeit less dramatic than in the position closer to the FAPA source (Figure 2d).…”

“…The difference in refractive index between helium (1.000036) and air at 23° C (1.0002926) [19] is sufficient for schlieren imaging. Recent work by Pfeuffer et al proved that schlieren imaging can provide valuable insight into mass transport with the FAPA source [20]. Information about the transition from turbulent to laminar flow and how sample placement affects helium flow was obtained.…”

Schlieren visualization of fluid dynamics effects in direct analysis in real time mass spectrometry

“…In seminal works by Pfeuffer et al , the gas flow profile in the sample gap between a FAPA source and mock mass spectrometer orifice was studied using the Schlieren technique . This technique is a clever means of visualizing gas flow patterns based on the " Schlieren effect" , wherein gases can be distinguished by gradients in light intensity proportional to the change in density or refraction between media, and rendered " visible" using traditional optics .…”

Multimodal Vacuum-Assisted Plasma Ion (VaPI) Source with Transmission Mode and Laser Ablation Sampling Capabilities

“…Reproducible sampling and quantitation with plasma discharge sources have proven to be complicated, necessitating the use of internal standards [22,23] and automated sampling approaches [24]. Much effort has been dedicated to improving the sensitivity and sampling efficiency of these ionization techniques with the aid of simulations that model gas flow patterns [25,26] and real-time visualization of source gas flow profiles using the Schlieren technique [27][28][29]. The most successful attempts at enhancing the quantitative capability of ambient plasma-based analysis have been realized with DART-MS systems incorporating a Vapur interface, rail-mounted sampling probes, and auxiliary pumping, all of which help entrain divergent gas flows toward the mass spectrometer inlet and reduce variability [30].…”