Final Gas Chromatography Complete Test Bank Chapter 16

Chapter 16:

Problem 16.1: In the development of a GC method, list three experimental variables under the control of the analyst.

Problem 16.2:

(a) Use your knowledge of partition kinetics to speculate on how an analyte might behave if the temperature in a GC analysis was set too low.

(b) How might you exploit a “too low” temperature to your advantage in a GC analysis?

Problem 16.3: Using Examples 16.1, 16.2, and 16.3 as guides, propose a method for the separation of caffeine, catechin, and phenol. Justify your choice of columns and your temperature gradient.

Problem 16.4: Go online and use one of the GC column supplier’s “How to Select a Column” guides and select a column for the analysis of gasoline. (See Table 16.2 for a list of GC column suppliers.) Explain why you selected that particular column, and justify your selection based on chromatographic principles.

Problem 16.5: Go online and use one of the GC column supplier’s “How to Select a Column” guides and select a column for the analysis of metabolic steroids. Explain why you selected that particular column, and justify your selection based on chromatographic principles

Problem 16.6: As noted above, nitrogen gas exhibits a fairly low range of useful flow rates as a carrier. Hydrogen exhibits the widest range of useful flow rates among common carrier gases. Postulate on the relative advantages and disadvantages of using H₂ versus N₂ to explain why N₂ is more commonly used than H₂.

Problem 16.7: Do you think using CO₂ as a carrier gas in an instrument with an FID would be problematic? Explain.

Problem 16.8: In your job as a chemist for a regional analytical laboratory, you are expecting a large number of samples that you will need to test for potential herbicide contamination. Prepare a plan (including explanation) for which type of detector you will need for the trace-level GC analysis of (a) 2,4-D, (b) atrazine, and (c) RoundUp. Use an online search engine to find the structures of each of these substances.

Problem 16.9: You will be analyzing urine samples from patients taking low levels of thiazide diuretics. What type of GC detector would you choose? Explain.

Problem 16.10: Calculate the compressibility correction factor, j, as a function of changing ambient pressures. Assume that the column head pressure remains constant at 70 mm Hg and the ambient pressure (in mm Hg) is:

Problem 16.11: Given the following GC experimental parameters, your carrier gas flow at the column outlet measures 25.0 mL/min. The ambient temperature is 23°C. Ambient pressure is 758.8 mm Hg. The partial pressure of water at 23°C is 21.068 mm Hg. The column head pressure is 68.4 mm Hg. Calculate Fc for the follow­ing column temperatures:

Problem 16.12: Suppose we change the flow rate from 25.0 mL/min to 30.0 mL/ min. Recalculate Fc for each temperature in Problem 16.11.

Problem 16.13: Imagine you are running GC analysis of gases trapped in ice core samples in a lab in the Arctic Circle. The lab is kept at a temperature of 12°C. Your other GC parameters include carrier gas flow at the column outlet of 25.0 mL/min. Ambient pressure is 762.8 mm Hg and the column head pressure is 78.4 mm Hg. Calculate Fc for the following column temperatures: (Hint: You will need to look up the partial pressure of water at 12°C.)

Problem 16.14: Determine V_R^o for each peak in Figure 16.21.

Problem 16.15: Determine V_N for each peak in Figure 16.21.

Problem 16.16: Compare and contrast the ways a chromatographer performs a gra­dient separation in LC and GC.

EXERCISE 16.1: Define each of the following terms:

a. Eluate

b. Isothermal elution

c. Thermal gradient

d. Temperature programming

e. Selectivity factor

f. Retention factor

g. Carrier gas

h. Retention volume

i. Compressibility correction factor

j. van Deemter equation

k. Split injection

l. Hold-up volume

m. Adjusted retention volume

n. NET adjusted retention volume

Exercise 16.2: Calculate the theoretical plate height, H, and the number of theoretical plates, N, for peak 4 in Figure 16.21. You might need to review Section 15.2 to answer this question. It is acceptable to estimate the values of the variables you need for the calculation from the axes of the chromatogram.

Exercise 16.3: In designing a separation method, we use the theoretical plate as a representation of the separation efficiency of the column. So it might seem reasonable to assume that more theoretical plates is always better than fewer theoretical plates. Having said that, why do we not automatically increase the column length anytime we need to improve separation?

Exercise 16.4: Describe the effect of each variable in Equation 16.11 on column efficiency.

Exercise 16.5: Rank the following compounds in terms of the expected elution order for a capillary GC separation run under isothermal conditions.

(a) Ethanol

(b) n-Propanol

(c) Methanol

(d) n-Pentanol

(e) n-Butanol

Exercise 16.6: Sketch a schematic for a split flow injector and in your own words, describe each component.

Exercise 16.7: Sketch a schematic of an FID and in your own words, describe how it functions.

Exercise 16.8: Sketch a schematic of a TID and in your own words, describe how it functions.

Exercise 16.9: Sketch a schematic of an ECD and in your own words, describe how it functions.

Exercise 16.10: Sketch a schematic of a PID and in your own words, describe how it functions.

Exercise 16.11: Sketch a schematic of an FPD and in your own words, describe how it functions.

Exercise 16.12: Sketch a schematic of an AED and in your own words, describe how it functions.

Exercise 16.13: Sketch a schematic of a “light pipe” interface for a tandem GC-FTIR and in your

own words, describe how it functions.

Exercise 16.14: Sketch a schematic of a jet separator for a tandem GC-MS and in your own words, describe how it functions.

Exercise 16.15: Imagine the following two peaks coe­luted shortly after the hold-up volume. Which of the follow­ing actions would be the “best” way to improve the resolu­tion? Defend your answer and explain why you rejected the other options.

Exercise 16.16: Which of the various detector types dis­cussed in this chapter is the detector of choice for the detec­tion of halogenated organic compounds?

Exercise 16.17: In all chromatographic experiments, controlling the injection volume is an important consider­ation. Compare and contrast the methods used to control injection volume in LC and GC.

Exercise 16.18: Both the TCD and FID are considered universal GC detectors, but the TCD is considered “more” universal than the FID. If the TCD is so much more uni­versal, why use a FID at all?

Exercise 16.19: Compare and contrast the operation of an atomic emission spectrometer (Chapter 9) with the op­eration of an AED.

Exercise 16.20: Compare and contrast the features of the mobile phase in GC with that in LC. Describe the important properties of the mobile phase in each technique and its impact on the efficiency of the separation.

Exercise 16.21: Answer the following questions for the chromatogram given in Figure 16.21.

(a) Calculate the selectivity factor and resolution for the first two peaks (see Section 15.2 for a review on calculating resolution).

(b) Calculate the number of theoretical plates for the first and last peak.

Exercise 16.22: Which term of the van Deemter equa­tion is most influential in a GC separation? Explain how this factors into the general elution problem.

Exercise 16.23: The peak capacity is the total number of observable peaks in a chromatogram under a given set of experimental conditions. The equation for determining the peak capacity is:

Exercise 16.24: What type of detector would you select in the routine quantitative determination of Prozac in urine samples by GC?

Exercise 16.25: In the analysis of a series of n-alkanes, would you prefer to use a GC-FTIR or a GC-MS instru­ment? Explain.