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Chapter 23: QEE (page 628)

The three chromatograms shown here obtained with 2.5, 1.0 and 0.4 渭Lof ethyl acetate injected on the same column under the same conditions. Explain why the peak becomes less symmetrical with increasing sample size.

2.5渭L 1.0L 0.4渭L

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Here we see in the given figure, the peak jumps up quickly and then gradually falls. This isotherms arise from an overloaded column in which too much solute or sample has been applied to the column. If the column has limited retention capacity, injection of concentrated solute saturates a significant portion of sorption sites, leaving fewer sites available for retention and resulting in a lower k. Such saturation causes the peak maximum to shift to a shorter retention time. The shift forward in retention time increases with increased overload. That鈥檚 why the peak becomes less symmetrical with increasing sample size.

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Most popular questions from this chapter

A Gas chromatogram of a mixture of toluene and ethyl acetate is shown here:

(a)Use the width of each peak (measured at the base)to calculate the number of theoretical plates in the column. Estimate all lengths to the nearest 0.1 mm.

(b)Using the width of the toluene peak at its base, calculate the width expected at half height. Compare the measured and calculated values. When the thickness of the the line is significant relative to the length being measured, it is important to take the pen line width into account. You can measure from the edge of one line to the corresponding edge of the other line,as shown here.

In chromatography, resolution is governed by (a) the number of plates, (b) relative retention, and (c) retention factor. Construct a set of graphs to show the dependence of resolution on each of these three parameters. Comment on the nature of the dependence of resolution on each of these three parameters.

(a) For the asymmetric chromatogram in Figure 23-14, calculate the asymmetry factor, BA.(b) The asymmetric chromatogram in Figure 23-14 has a retention time equal to 15.0 min and a w0.1of 44s . Find the number of theoretical plates. (c) The width of a Gaussian peak at a height equal to110 of the peak height is 4.297蟽. Suppose that the peak in (b) is symmetric with A=B=22s. Use Equations 23-30 and 23-32 to find the plate number.

A chromatographic procedure separatesof unknown mixture on a column with a length of 40cmand a diameter of 0.85cm . (a) What size column would you use to separate 100mgof the same mixture? (b) If the flow is 0.22mL/minon the small column, what volume flow rate should be used on the large column? (c) The mobile phase occupies 35%of the column volume. Calculate the linear velocity for the small column and the large column.

Consider a chromatography experiment in which two components with retention factors k1=4.00and k2=5.00are injected into column with N=1.00脳103theoretical plates. The retention time for the less-retained component is tr1=10.0min.

(a)Calculate tmandt12.Find w12(width at half height) and w (at the base) for each peak.

(b)Using graph paper , sketch the chromatogram analogous to figure 23-7,supposing that two peaks have same amplitude(height). Draw the half widths accurately.

(c)Calculate the resolution of the two peaks and compare this value with those drawn in Figure 23-10.
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