CHEMISTRY 465 PROBLEM SET TWO Fall 2010 Name date

1. Skoog 26-2: The general elution problem arises whenever chromatograms are obtained on samples that contain species with widely different distribution constants. When conditions are such that good separations of the more stongly retained species are realized, lack of resolution among the weakly held species is observed. Conversely when conditions are chosen that give satisfactory separations of the weakly retained compounds, severe band broadening and long retention times are encountered for the strongly bound species. The general elution problem is often solved in liquid chromatography by gradient elution and in gas chromatography by temperature programming

2. Skoog 26-6: Variables that affect the selectivity factor α include the composition of the mobile phase, the column temperature, the composition of the stationary phase, and chemical interactions between the stationary phase and one of the solutes being separated

3. Skoog 26-8: The number of plates in a column can be determined by measuring the retention time tR 2 and the width of a peak at its base W. The number of plates N is then N = 16(tR/W)

4. Skoog 26-10: Longitudinal diffusion is much more important in GC that in LC because gaseous diffusion coefficients are orders of magnitude larger than liquid values. Longitudinal diffusion is a large contribution to H at low flow rates in GC. The initial decreases in H in plots of plate height vs. flow rate are thus largely the result of longitudinal diffusion

5. Skoog 26-11: Gradient elution is a method of performing liquid chromatography in which the composition of the mobile phase is change continuously or in steps in order to optimize separations.

6. Given the following information obtained from a gas chromatogram, predict the retention time for the next two higher compounds in a homologous series. analyte adjusted retention time propane 1.29 n-butane 2.21 n-pentane 4.10 n-hexane 7.21 n-heptane 12.86

7. Skoog 26-14:

8. Skoog 26-17

2[(tR )B − (tR )A ] 2[21.6 −14.1] Rs = = = 5.2 WA −WB (1.72 +1.16)

(R )2 2 S 1 (1.5) € N1 = N2 2 = 2534x 2 = 210 (Rs)2 (5.21) L=HN = (0.009749 cm) X 210 plates = 2.0 cm

€ 9. Skoog 27-3: Temperature programming involves increasing the temperature of a GC column as a function of time. This technique is particularly useful for samples that contain constituents whose boiling points differ significantly. Low boiling point constituents are separated initially at temperatures that provide good resolution. As the separation proceeds, the column temperature is increased so that the higher boiling constituents come off the column with good resolution and at reasonable lengths of time.

10. Skoog 27-16: Film thickness influences the rate at which analytes are carried through the column, The rate increases as the thickness is decreased. Less band broadening is encountered with thin films.

11. Skoog 27-19: The retention index for an analyte is a measure of the rate at which it is carried through a column compared with the rate of movement of two normal alkanes, one that moves faster than the analyte and the other that moves more slowly. To obtain the retention index of an analyte on a given column, the log of the adjusted retention times for the two alkanes and the analyte are determined. The retention index for butane is always 400 and for pentane 500. The retention index for the analyte is then derived by interpolation between the two logarithmic retention indexes of the alkane (see solution to Problem 6)

12. Skoog 28-5: In adsorption chromatography on an alumina packing, it is generally best to increase the polarity of the mobile phase as the elution proceeds. Thus the ratio of to hexane should be increased as the elution proceeds

13. Skoog 28-10: (a) diethyl ether, , n-hexane (b) acetamide, acetone, dichloroethane

14, Skoog 28-11: (a) ethyl , dimethylamine, (b) hexane, propylene, benzene, dichlorobenzene

15. Why are UV-absorption detection limits so much worse in capillary electrophoresis (CE) than in HPLC? Recall Beer’s law: A = εbc; pathlengths in CE are on the order of several microns compared to mm in HPLC

16. How can capillary electrophoresis be used to separate neutral compounds? Neutral compounds can be separated by micellar electrokinetic capillary chromatography. This is accomplished by adding a surfactant such as SDS above its critical micelle concentration to the running buffer. Chiral compounds can be separated in CE by adding a chiral additive to the running buffer (e.g. a cyclodextrin).

17. Skoog 20-2: The most fragmentation and thus the most complex spectra are encountered with electron impact ionization. Field ionization produces the simplest spectra (very soft ionization). Chemical and electron impact ionization result in higher sensitivities than does field ionization.

18. Skoog 20-9: The resolution of a single focusing mass spectrometer is limited by the initial kinetic energy spread of the sample . This spread is minimized in a double focusing instrument by accelerating the sample through an electrostatic analyzer, which limits the range of kinetic energies of ions being introduced into the magnetic sector analyzer. Significantly narrower peaks are the result

19. Skoog 20-11: Resolution = m/Δm (a) m = (28.0187 +28.0061)/2 = 28.012 m/Δm = 28.012/(28.0187 – 28.0061 = 2.22 × 103

(b) m/Δm = 28.013/(28.0313 – 27.9949) = 770

(c) m/Δm = 85.0647/(85.0653 – 85.0641) = 7.09 × 104

(d) m/Δm = 286.158/(286.1930 – 286.1240) = 4.15 × 103

20. Skoog 30-1: Electroosmotic flow occurs when a mobile phase in a capillary tube is subjected to a high potential difference between one end of the tube and the other. For a silica tube, the flow is generally away from the positive electrode towards the negative. The flow occurs because of the attraction of positively charge species toward the negative silica surface. This layer of positive charge is mobile and is attracted toward the negative electrode carrying with it the mobile phase molecules.

21. Skoog 30-4: Under the influence of an electric field, mobile ions in solution in a capillary are attracted or repelled by the negative potential of one of the electrodes. The rate of movement toward or away from the negative electrode is dependent on the net charge on the analyte and the size and shape of the analyte molecules. These properties vary from analyte to analyte. Thus, the rate at which molecules migrate under the influence of the electric field vary, and the time it takes them to traverse the column varies, making separation possible.

22. Skoog 30-8: The major advantages of MECC include higher column efficiencies and the ease with which the pseudostationary phase can be altered.