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Negative Concentrations in a Chromatographic Peak
Posted 02.12.2015, 18:31 GMT-5 Version 4.4 0 Replies
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I have a model for a microfluidic separation device composed of four microchannels of the same cross section and differing lengths that intersect at a single point (a cross injection chip). When running the Transport of Diluted Species and Creeping Flow physics in Comsol 4.4, I have all the inlets at specific flow rates and outlets as open boundaries to cause the top microchannel stream to be pinched off in a stationary study (attached as HelpPicture1.tif). I then change one of the boundary conditions to an open boundary in a second 1s long time-dependent study and use the results of the first study as the starting point (attached as HelpPicture2.tif). The fluid flow is then turned back on in the side channel (same fluid flow profile as in the first stationary study) for a third time-dependent study using the results of the 2nd study as a starting point to create a small sample plug that can then be separated downstream (attached as HelpPicture3.tif).
This all looks well and good in a 2D plot, but when I look at the peak as it traverses down the channel, I see some large overshoots and undershoots at the baseline of 0[mol/L] concentration of the surrounding buffer that become taller and broader as the peak continues to travel downstream (attached as HelpPicture.tif). I've tried a few tricks to get rid of the negative concentrations by having a smoothed step function boundary condition, doing a logarithmic transform of the concentration variable, and having a very dense mesh with very small elements. Nothing seems to work. I don't think it is a problem with the boundary conditions, because the problem I am seeing is deep in the model away from the fixed concentration boundary. Also, all the literature I have found claim that the initial negative concentrations should be the worst at the onset and quickly smooth out to a realistic answer. However, this obviously does the opposite by getting worse as time continues. Has anyone run into something similar? Any help would be appreciated.
Regards,
Erick
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Hello Erick Foster
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