Difficulty: Intermediate | Prerequisites: Dehydration of 2-Methyl-2-Butanol study notes (Part 1), basic understanding of chromatography concepts
Gas chromatography is the primary analytical tool used in this course to identify and quantify volatile organic products. In the dehydration lab, you have a mixture of two isomeric alkenes and need to know which one dominates and by how much. GC answers both questions in a single injection. Understanding how GC works, how to read a chromatogram, and how to confirm peak identity (e.g., via the peak enhancement method) are skills you will use repeatedly in CHEM 2311 and beyond.
GC separates volatile compounds by boiling point and polarity as they travel through a heated column in a stream of inert carrier gas. Each compound exits at a characteristic retention time, letting you identify it (by comparing to standards) and measure how much of it you have (from peak area). The peak enhancement method is an alternative way to confirm a peak's identity without running a separate standard.
Gas chromatography (GC)
An analytical technique for separating and quantifying volatile compounds. A sample is vaporised, carried through a column by an inert gas, and detected as it exits. Think of it as a race through a tube: each compound finishes at a different time depending on how strongly it interacts with the column coating.
Retention time
The time (in minutes) from injection to when a compound's peak reaches the detector. Each compound has a characteristic retention time under a given set of conditions (column type, temperature, flow rate). In simple terms, it is each compound's "finish time" in the race.
Carrier gas (mobile phase)
The inert gas that pushes the sample through the column. Common examples: helium (most common), nitrogen, hydrogen. It does not interact chemically with the sample.
Stationary phase
The coating on the inside of the GC column that interacts with sample compounds. It can be polar (e.g., polyethylene glycol, sold as Carbowax) or nonpolar (e.g., polydimethylsiloxane, sold as OV-1 or DB-1). Polar compounds spend more time in a polar stationary phase, and vice versa.
Chromatogram
The output plot from a GC run: detector response (y-axis) vs time (x-axis). Each compound appears as a peak at its retention time.
Peak enhancement method
A technique for confirming a peak's identity without a separate standard injection. You add a small amount of a known pure compound (a "spike") to your product sample and re-inject. If the suspected peak grows taller without broadening or splitting into two peaks, the identity is confirmed.
Response factor
A correction factor that accounts for the fact that different compounds may give different detector signals per unit mass. When the response factor is assumed to be the same for both products (as in this lab), peak area percentages directly represent mass percentages.
Air peak
A small peak near the start of a chromatogram caused by air injected with the sample. It is not a product peak and should be ignored in analysis.
Injector: the sample (a tiny amount, often < 1 µL) is injected and immediately vaporised.
Column: a long, thin tube (capillary columns can be 15 to 60 m) coated on the inside with the stationary phase. Held in an oven at a controlled temperature.
Carrier gas: flows continuously through the column, pushing vaporised sample along. Helium is the most common choice. Nitrogen and hydrogen are also used.
Detector: sits at the column exit and measures each compound as it elutes. Common detectors include flame ionisation detectors (FID) and thermal conductivity detectors (TCD).
Compounds separate based on their boiling points and their affinity for the stationary phase. A compound with a lower boiling point or weaker interaction with the stationary phase moves through faster and has a shorter retention time. On a nonpolar column, compounds elute roughly in order of increasing boiling point.
Nonpolar example: polydimethylsiloxane (PDMS), also known by trade names such as OV-1, SE-30, or DB-1. Good general-purpose phase for separating compounds by boiling point.
Polar example: polyethylene glycol (PEG), sold as Carbowax. Better for separating compounds that differ in polarity but have similar boiling points.
This is an alternative to running a separate standard chromatogram.
Run your product sample and note the retention times of the peaks.
Add a small, known amount of one suspected compound (e.g., pure 2-methyl-2-butene) to your product sample.
Re-inject the spiked sample.
If the peak you suspected grows taller (larger area) without broadening or splitting into a shoulder or two separate peaks, the identification is confirmed.
If a new peak appears or the suspected peak splits, the compound was not what you thought.
In this lab you run two GC injections.
Standard injection: a purchased mixture of known 2-methyl-1-butene and 2-methyl-2-butene. This gives you reference retention times for each compound on your specific instrument.
Product injection: your actual reaction product. By comparing your product's peak retention times to the standards, you identify which peak is which.
The GC instrument was programmed to suppress the solvent heptane peak ("workup"), so it does not appear.
On both chromatograms, you should identify: the air peak (earliest, near time zero), the 2-methyl-1-butene peak (shorter retention time of the two alkenes, lower bp of 31 °C), the 2-methyl-2-butene peak (slightly longer retention time, bp 38 °C), and any impurity peaks.
Assuming equal response factors (as stated in the worksheet), the percentage of each alkene in the product mixture equals its peak area divided by the total alkene peak area, multiplied by 100.
% of alkene = (peak area of that alkene / total peak area of both alkenes) × 100
The expected result: 2-methyl-2-butene is the major product (roughly 60 to 70%), 2-methyl-1-butene is the minor product (roughly 30 to 40%), consistent with Zaitsev's rule.
GC is used extensively in forensic science (drug testing, blood-alcohol analysis), environmental monitoring (detecting pollutants in air and water), the petroleum industry (analysing crude oil fractions), and food science (flavour and fragrance profiling). The same principles you learn here scale directly to these professional applications.
Students often think the air peak is a product. It is not. It appears because a small amount of air enters the syringe during injection and passes through the column unretained.
A common error is assuming that retention time alone is proof of identity. Two different compounds can have the same retention time on a given column. Confirmation requires either a second method (e.g., GC-MS) or the peak enhancement method.
Students sometimes think a taller peak means more compound. Peak height alone does not equal quantity: peak area is the correct measure. A narrow, tall peak may contain less material than a broad, shorter one.
Some students assume changing the carrier gas flow rate changes the order of elution. It changes absolute retention times (everything comes out faster or slower), but the relative order stays the same.
⚠️ Be able to explain the peak enhancement method step by step. This is a favourite exam question.
⚠️ Know the difference between the mobile phase (carrier gas) and the stationary phase (column coating), and be able to give a named example of each.
⚠️ Be able to calculate % composition of a product mixture from GC peak areas, assuming equal response factors.
⚠️ Understand why 2-methyl-1-butene elutes before 2-methyl-2-butene (lower boiling point).
⚠️ Know what the air peak is and why it appears.
Fill in the blank: the inert gas that pushes sample through a GC column is called the ______. (carrier gas / mobile phase)
True or false: in peak enhancement, if the spiked peak splits into two, the identity is confirmed. (False: splitting means the suspected compound is not the same as the peak.)
Fill in the blank: peak ______ (not height) is used to calculate the percentage of each compound in the mixture. (area)
True or false: on a nonpolar GC column, compounds generally elute in order of increasing boiling point. (True)
Fill in the blank: polyethylene glycol (Carbowax) is an example of a ______ stationary phase. (polar)
Q: Provide an example of a mobile phase carrier gas used in a GC instrument.
A: Helium is the most common carrier gas. Nitrogen and hydrogen are also used.
Q: Give an example of both a polar and a nonpolar stationary liquid phase used in GC columns, with chemical names.
A: Nonpolar: polydimethylsiloxane (PDMS). Polar: polyethylene glycol (PEG, also called Carbowax).
Q: Explain how the peak enhancement method works to identify a peak in a GC chromatogram.
A: Add a small amount of a known pure compound to your product sample and re-inject. If the suspected peak increases in area without broadening or splitting, the compound in that peak is confirmed to be the same as the known compound you added. If the peak splits or a new peak appears, the identification is incorrect.
Q: How closely should the product retention times compare to those of the standards?
A: On the same GC instrument under the same conditions, the product retention times should match the standard retention times very closely (within a few seconds). Small differences can arise from slight variations in injection technique or column temperature.
Q: Summarise the results of the dehydration reaction as determined by GC. Is the result expected?
A: GC analysis shows that 2-methyl-2-butene is the major product and 2-methyl-1-butene is the minor product. This is expected: Zaitsev's rule predicts that the more substituted (trisubstituted) alkene forms preferentially because it is more thermodynamically stable than the less substituted (disubstituted) alternative.
GC analysis connects to every lab in CHEM 2311 that produces volatile products. The separation principles (boiling point, polarity interactions) are the same ones behind distillation and column chromatography, just applied in the gas phase. Understanding retention time and peak area will also help when you encounter HPLC (high-performance liquid chromatography) later, which uses the same logic but with a liquid mobile phase for non-volatile compounds.
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