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Chapter 26: Q27P (page 748)
What is electroosmosis?
Electroosmosis (electroendosmosis) is the pumping process in which charged particles tend to migrate to a less charged area.
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26-D. Consider a capillary electrophoresis experiment conducted near pH9, at which the electroosmotic flow is stronger than the electrophoretic flow.
(a) Draw a picture of the capillary, showing the placement of the anode, cathode, injector, and detector. Show the direction of electroosmotic flow and the direction of electrophoretic flow of a cation and an anion. Show the direction of net flow.
(b) Using Table 15-1, explain why has a shorter migration time than Predict whether will have a shorter migration time than or a greater migration time than
(c) Why is the mobility of greater than that of?
To obtain the best separation of two weak acids in capillary electrophoresis, it makes sense to use the pH at which their charge difference is greatest. Prepare a spreadsheet to examine the charges of malonic and phthalic acid as a function of pH. At what pH is the difference greatest?
Material balance. If you intend to measure all the anions and cations in an unknown, one sanity check on your results is that the total positive charge should equal the total negative charge. The table lists concentrations of anions and cations measured in pond water in an undergraduate experiment and expressed in mg/mL.
Find the total concentration of negative and positive charge (mol/L) to assess the quality of the analysis. What do you conclude about this analysis?
Figure 26-24 shows the effect on resolution of increasing voltage from 28 to120kV.
(a) What is the expected ratio of migration times in the two experiments? Measure the migration times for peak 1 and find the observed ratio.
(b) What is the expected ratio of plates in the two experiments?
(c) What is the expected ratio of bandwidths?
(d) What is the physical reason why increasing voltage decreases bandwidth and increases resolution?
Figure 26-24 shows the effect on resolution of increasing voltage from 28 to120kV
(a) What is the expected ratio of migration timesin the two experiments? Measure the migration times for peak 1 and find the observed ratio.
(b) What is the expected ratio of plates in the two experiments?
(c) What is the expected ratio of bandwidths?
(d) What is the physical reason why increasing voltage decreases bandwidth and increases resolution?
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