Gas Chromatography: Principles and Techniques

Homework Solutions for Chem 422 Chapter 27 3. Temperature programming in gas chromatography involves increasing the temperature of the column as a function of time. This technique is particularly useful for samples that contain components with significantly different boiling points. Low boiling point components are separated first at temperatures that provide good resolution. As the run proceeds, the column temperature is increased so that the higher boiling point components come off the column with good resolution and in a reasonable time. 6. The thermal conductivity detector is based upon the decrease in thermal conductivity of the carrier gas (He or H) brought about by the presence of analyte molecules. The electron-capture detector is based upon the decrease in the standing current generated in the carrier gas (by ionization with a β emitter) brought about by the presence of analyte molecules. It is particularly sensitive to electrophilic functional groups. The flame ionization detector is based on the collection of ions and electrons when substances containing oxidizable carbon atoms are burned in a hydrogen flame. 7. Thermal Conductivity Detector Advantages: general applicability, large linear range, simplicity, nondestructive Disadvantages: low sensitivity Electron-Capture Detector Advantages: high sensitivity and selectivity for analytes with electrophilic functional group, nondestructive Disadvantages: limited linear response range Homework Solutions for Chem 422 Chapter 27 7. (continued) Flame Ionization Detector Advantages: high sensitivity for hydrocarbons, large linear range, low noise, detector response independent of mobile phase flow rate Disadvantages: destructive 11. Diatomaceous earth particles having diameters ranging from 250 to 150 µm are used. 13. Megabore columns are open tubular columns with an inside diameter of 0.53 mm instead of the more typical 0.2 - 0.32 mm diameters. They are more convenient to handle and can tolerate larger sample sizes than the smaller bore columns. 14. Fused silica columns have greater physical strength and flexibility than glass capillary columns and are less reactive than either glass or metal columns. 15. The stationary-phase liquid should have: low volatility, good thermal stability, chemical inertness, characteristics that provide suitable k’ and α values for the analytes of interest. 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. 17. Liquid stationary phases are generally bonded and/or cross linked in order to provide thermal stability and a more permanent stationary phase that will not leach off the column. Bonding involves attaching a monomolecular layer of the stationary phase to the packing surface by means of chemical bonds. Crosslinking involves treating the stationary phase while it is in the column with a chemical reagent that creates cross-links between the stationary phase molecules. Homework Solutions for Chem 422 Chapter 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 normal alkanes under the same conditions. To obtain the retention index of an analyte on a given column, the log of the adjusted retention time for at least two n-alkanes (one that moves faster than the analyte and one that moves more slowly) and the analyte is determined. The retention index for each n-alkane is, by definition, 100 times the number of carbon atoms in the molecule. The retention index for the analyte is gotten by interpolation on a plot of log(tR’) vs. retention index. 20. The polar compound will have a greater affinity for the polar carbowax 20M column, so its distribution constant will be larger for this column than for the nonpolar column. 21. Sample Adjusted Retention Time, tR’ Retention Index, I n-pentane 1.59 500 n-hexane 3.66 600 1-hexene 2.58 A linear interpolation for the adjusted retention time for 1-hexene yields 24. The retention times for the three compounds would decrease because these polar substances would have less affinity for a nonpolar silicone oil stationary phase. Homework Solutions for Chem 422 Chapter 27 27. a) Increasing the weight of the stationary phase relative to the packing weight leads to an increase in the film thickness df. This increases the mass transfer term Cs (more rapid equilibrations) and thus increases H. b) Decreasing the rate of sample injection will result in band broadening and an increase in H because all of the molecules do not start down the column at the same time. c) Increasing the injection port temperature will cause the sample to vaporize more rapidly and be introduced onto the column in a narrower band. This leads to a decrease in H. d) Increasing the flow rate (u) may cause H to increase or decrease depending upon the flow rate. (Recall that there is a minimum in the plot of H vs. u.) e) Reducing the particle size of the packing decreases the mass transfer term Cm (less eddy diffusion and fewer stagnant pools) and thus decreases H. f) Decreasing the column temperature will lower the diffusion rates Dm and Ds, which will increase the mass transfer terms Cs and Cm, but lower the longitudinal diffusion term B. These offsetting effects generally result in an increase in H.