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Chapter 25: Q29P (page 710)
What does it mean for a separation procedure to be 鈥漴ugged鈥� and why is it desirable?
A separation procedure to be rugged and why is it desirable.
It is desirable because that small changed conditions do not change the final outcome of separation.
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Question: what are the general steps in developing an isocratic separation for reversed-phase chromatography?
(a) Make a graph showing retention times of peaks 6, 7, and 8 in Figure 25-12 as a function of %acetonitrile (%B). Predict the retention time of peak 8 at 45% B.
(b) Linear-solvent-strength model: In Figure 25-12, tm = 2.7 min. Compute k for peaks 6, 7, and 8 as a function of %B. Make a graph of log k versus 桅, where 桅= %B/100. Find the equation of a straight line through a suitable linear range for peak 8. The slope is -S and the intercept is log kw. From the line, predict tr for peak 8 at 45% B and compare your answer with (a).
(c) Gradient elution: A linear eluent gradient from 40 to 80% acetonitrile over 30 min is performed on the column in Figure 25-12. Assuming a dwell volume of 0 mL, use your data from (b) to plot the retention factor of peaks 6 and 8 during the gradient. What are the general characteristics of the plot?
(d) Why are the peaks in a gradient separation sharp?
Why are silica stationary phase height limited to operating in the pH range ?why does the silica in figurehave improved stability at low pH?
(a) Nonpolar aromatic compounds were separated by HPLC on an octadecylbonded phase. The eluent was 65 vol% methanol in water. How would the retention times be affected if 90% methanol were used instead?
(b) Octanoic acid and 1-aminooctane were passed through the same column described in (a), using an eluent of 20% methanol/80% buffer (pH 3.0). State which compound is expected to be eluted first and why.
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(c) Polar solutes were separated by hydrophilic interaction chromatography (HILIC) with a strongly polar bonded phase. How would retention times be affected if eluent were changed from 80 vol% to 90 vol% acetonitrile in water?
(d) Polar solutes were separated by normal-phase chromatographyon bare silica using methyl t-butyl ether and 2-propanol solvent. How would retention times be affected if eluent were changed from 40 vol% to 60 vol% 2-propanol? (Hint: See Table 25-4.)
After poisonous melamine and cyanuric acid appeared in milk in China (Box) in , there was a flurry of activity to develop methods to measure these substances. An analytical method for milk is to treat 1volume of milk with 9 volumes of to precipitate proteins. The mixture is centrifuged for to remove precipitate. The supernatant liquid is filtered through a filter and injected into a HILIC liquid chromatography column (TSK gel Amide-stationary phase) and products are measured by mass spectrometry with selected reaction monitoring (Section ). Melamine is measured in positive ion mode with the transition. Cyanuric acid is measured in negative ion mode with the transition .
(a) Write the formulas for the four ions and propose structures for all four ions.
(b) Even though milk is a complex substance, only one clean peak is observed for melamine and one for cyanuric acid spiked into milk. Explain why
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