please note that someone else answered this on Chegg saying the answers were: A,
ID: 521375 • Letter: P
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please note that someone else answered this on Chegg saying the answers were: A, D, and F--these are wrong (or there is another correct answer) thanks!
Shown below are three van Deemter curves for columns containing a microporous stationary phase with particle diameters of 5 Am, 3 Am, and 1.8 pm. Why does the van D curve shift downward as the particle size decreases? Once the particle size is small enough, the equilibration 18 5 pm time between the mobile and stationary phases becomes 16 very rapid which reduces the contribution of the Cterm to the plate height. E 14 Decreasing the particle size results in more possible paths 12 through which the solvent can flow, which increases the Su 10 3 pm contribution of the A term to the plate height. constant pressure, the flow rate through the column O aecreases as the particle size decreases which increases E 6 1.8 Am the contribution of the B term to the plate height. As the particle size decreases, the flow through the column C becomes more uniform which reduces the contribution of 10 the A term to the plate height. Linear flow rate (mm/s) As the particle size decreases, the analyte spends more O time in the stationary phase which increases the contribution of the C term to the plate height. The pressure required to push solvent through the column increases as the particle size decreases. Higher pressures mean the analyte spends less time in the column which reduces the contribution of the B term to the plate height.Explanation / Answer
The Van Deemter equation is an empirical formula describing the relationship between plate height (H, the length needed for one theoretical plate) which is a measure of column efficiency, and linear velocity (µ) .
The van Deemter equation is comprised of three terms:
The A term is plotted as a horizontal line. It is related to the particle size and how well a column is packed and is independent of linear velocity [mobile-phase speed]. As the particle size of the packing material is decreased, the H value also decreases [higher efficiency].
Decreasing particle size has been observed to limit the effect of flow rate on peak efficiency—smaller particles have shorter diffusion path lengths, allowing a solute to travel in and out of the particle faster. Therefore the analyte spends less time inside the particle where peak diffusion can occur. We notice that as the particle size decreases, the curve becomes flatter, or less affected by higher column flow rates. Smaller particle sizes yield better overall efficiencies, or less peak dispersion, across a much wider range of usable flow rates.
Particle size has a significant impact on the analyte band as it relates to the A term [eddy diffusion]. The path which analyte molecules take to transfer from the bulk mobile phase to the surface of the particle and around that particle takes less time as particle size is decreased. Larger particles cause analyte molecules to travel longer, more indirect paths. The differences in these paths result in different migration times for the analyte molecules within a population, resulting in a broader analyte band and resulting peak. As the particle size of the packing is decreased, the paths of the analyte molecules are encouraged to be more similar in length. This results in narrower analyte bands which translate into narrower peaks, higher efficiency and higher sensitivity
From above explanation we get options (A) and (F)
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