Fluid Dynamics of the Open Port Interface for High-Speed Nanoliter Volume Sampling Mass Spectrometry

The open port interface (OPI) coupled to an atmospheric pressure ion source is used to capture, dilute, focus, and transport nanoliter volume sample droplets for high-speed mass spectrometric analysis. For typical applications, the system has been optimized to achieve 1 Hz nanoliter volume sample transfer rates while simultaneously diluting the sample >1000-fold to minimize sample matrix-induced ionization suppression. Geometric, flow, and dispensing alterations to the system presented here demonstrate that sample transfer rates for the OPI of at least 15 Hz are possible. The fluid dynamic processes that enable sampling rates of 1 Hz and greater are examined in detail by correlating computational fluid dynamics simulations, analytic calculations, experimental data, photographic footage, and reference to the fluid dynamics literature. The resulting models and experimental results provide the rationale underlying the design and tuning of the system as well as information for developing optimized analytical methods. In combination with acoustic droplet dispensing, referred to as acoustic ejection mass spectrometry (AEMS), this system can be considered to be a special case of flow injection analysis with unique features that control the peak width, symmetry, and segregation of the samples transported in a fluid while simultaneously enabling their mixing and dilution with carrier fluids. In addition, conditions are established to prevent direct contact of the sample with a surface enabling, in combination with a contact-free dispenser like acoustic ejection, a dramatic reduction in sample-to-sample carry-over.