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.
Kinetic studies of biological macromolecules increasingly use microfluidic mixers to initiate and monitor reaction progress. A motivation for using microfluidic mixers is to reduce sample consumption and decrease mixing time to microseconds. Some applications, such as small-angle x-ray scattering, also require large (>10 micron) sampling areas to ensure high signal-to-noise ratios and to minimize parasitic scattering. Chaotic to marginally turbulent mixers are well suited for these applications because this class of mixers provides a good middle ground between existing laminar and turbulent mixers. In this study, we model various chaotic to marginally turbulent mixing concepts such as flow turning, flow splitting, and vortex generation using computational fluid dynamics for optimization of mixing efficiency and observation volume. Design iterations show flow turning to be the best candidate for chaotic/marginally turbulent mixing. A qualitative experimental test is performed on the finalized design with mixing of 10 M urea and water to validate the flow turning unsteady mixing concept as a viable option for RNA and protein folding studies. A comparison of direct numerical simulations (DNS) and turbulence models suggests that the applicability of turbulence models to these flow regimes may be limited.
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