Gas Chromatography Principles and Analysis Techniques

Gas Chromatography Instrumentation and Analysis

1. Draw a block diagram of a GC instrument, labeling all the important components.

The components of a Gas Chromatography (GC) system follow this sequence:

• Mobile Phase Reservoir: Pressurized Helium (He) cylinder
• Sample Injector: Micro-syringe (located inside the oven)
• Stationary Phase: Column (located inside the oven)
• Detector: Thermal Conductivity Detector (TCD) or Flame Ionization Detector (FID)
• Readout: Computer system

2. What quantities are plotted on the x- and y-axes of (a) a chromatogram and (b) a calibration curve?

• a. Chromatogram: The x-axis represents time, and the y-axis represents the detector signal.
• b. Calibration Curve: The x-axis represents concentration, and the y-axis represents the peak area.

3. Define the following common chromatography terms:

• a) Stationary phase: A solid or liquid substance coating the interior of the column.
• b) Mobile phase: The gas that carries the analyte through the column.
• c) Partition coefficient (equation): A type of equilibrium constant defined as: K = [A]_stationary / [A]_mobile.
• d) Retention time: The length of time it takes for a compound to pass through the column and appear as a peak on the chromatogram.

4. In general, how would you determine which of several peaks in a chromatogram obtained for a mouthwash sample might correspond to ethanol? (Is your suggested method 100% reliable?) How would you use that peak to determine how much ethanol is present in the mouthwash?

To identify the ethanol peak, you should run a pure ethanol standard under identical conditions and see which peak in the mouthwash sample matches its retention time most closely. This method is not 100% reliable because different compounds can have the same retention time. To determine the quantity, you would calculate the area under the peak and compare it to a calibration curve to find the concentration of ethanol in the mouthwash sample.

5. In general, will an increase in operating temperature cause a given peak to have a longer or a shorter retention time? Why?

An increase in temperature will result in a shorter retention time. This occurs because the analyte's vapor pressure increases, causing the molecules to move more quickly through the mobile phase.

6. In general, will an increase in mobile phase flow rate cause a given peak to have a longer or a shorter retention time? Why?

An increase in flow rate will result in a shorter retention time because the carrier gas moves the analyte through the column at a higher velocity.

Methods to Decrease Retention Time

7. Suppose that you need to “speed up” a chromatogram (i.e., decrease the retention times of all the peaks). Suggest three specific GC conditions that you could change in order to accomplish this.

1. Reduce the column length.
2. Increase the operating temperature of the column.
3. Increase the velocity or flow rate of the mobile phase.

Analytical Calculations

8. In a GC experiment, a student injects 2.0 μL of a 20% ethanol/water solution into the GC. Calculate the weight of ethanol that has been injected. (The density of ethanol is 0.789 g/mL at 20° C.)

To find the weight of the injected ethanol:

• Volume of solution = 2.0 μL
• Volume of ethanol = 2.0 μL × 0.20 = 0.4 μL
• Convert volume to mL: 0.4 μL / 1000 = 0.0004 mL
• Calculate mass: 0.0004 mL × 0.789 g/mL = 3.156 × 10^-4 g