Introduction to Gas / Liquid Chromatography (GLC)
Welcome to the study of Gas / Liquid Chromatography (often abbreviated as GLC). While you may have already encountered Thin-Layer Chromatography (TLC), GLC is a more advanced technique used to separate and identify volatile organic compounds—liquids that can easily be turned into gases.
In the world of analytical chemistry, GLC is like a highly sensitive "chemical nose." It is used in everything from drug testing in athletes to checking for pollutants in the air or impurities in food. It allows us to take a complex mixture and find out exactly what is in it and how much of each component is present.
Note: This chapter focuses on the A Level specific technique of GLC. For other methods of analysis, see the chapters on Thin-layer chromatography, Carbon-13 NMR, and Proton (\(^{1}H\)) NMR.
The Components of GLC
To understand how GLC works, we need to look at its two main "phases." In any chromatography, components are separated because they distribute themselves differently between a moving phase and a still phase.
1. The Mobile Phase
The mobile phase is the substance that carries the sample through the machine. In GLC, this is an unreactive (inert) carrier gas.
Common examples include:
• Nitrogen (\(N_{2}\))
• Helium (\(He\))
• Argon (\(Ar\))
The gas must be unreactive so that it does not react with the sample we are trying to analyze.
2. The Stationary Phase
The stationary phase is the substance that stays put inside the chromatography column. In GLC, the stationary phase is a high boiling point non-polar liquid.
This liquid is usually coated onto the inside of a very long, thin, coiled tube (the column) or onto a solid support material inside the column. It must have a high boiling point so that it stays as a liquid even when the machine gets hot.
How Separation Occurs: The Principle of Partition
The fundamental principle behind GLC is partition. When your sample is injected, it is vaporized (turned into gas). The carrier gas then pushes it through the column. As the molecules travel, they move back and forth between the carrier gas and the liquid stationary phase.
The "Sticky" Analogy:
Imagine a hallway where the floor is covered in sticky glue (the stationary phase). You and a friend are being blown down the hallway by a giant fan (the mobile phase).
• If your shoes are very sticky, you will spend a lot of time stuck to the floor and move slowly.
• If your shoes aren't sticky at all, you will zip right through with the wind.
In GLC, "stickiness" is usually determined by how soluble a component is in the liquid stationary phase.
Key Takeaway: Components that are more soluble in the liquid stationary phase will travel slower. Components that are less soluble in the liquid stationary phase (and more soluble in the carrier gas) will travel faster.
Retention Time (\(R_{t}\))
The retention time is the time taken for a component to travel from the injection point to the detector at the end of the column. This is the "fingerprint" of a substance under specific conditions.
Factors affecting Retention Time:
• Boiling Point: A substance with a high boiling point will spend more time as a liquid (condensed in the stationary phase) and thus have a longer retention time.
• Solubility: A substance that is highly soluble in the stationary phase will have a longer retention time.
• Temperature: Higher temperatures generally make all components move faster, shortening the retention time.
Quick Tip: If the stationary phase is non-polar, then non-polar components in the mixture will be more soluble in it and will have longer retention times (remember "like dissolves like").
Interpreting the Chromatogram
When the components exit the column, they pass through a detector, which produces a graph called a chromatogram.
• The x-axis shows the retention time.
• The y-axis shows the detector response (the height/strength of the signal).
Identifying Components
Each peak represents a different substance. By comparing the retention time of a peak to the retention times of known "standard" substances (measured under identical conditions), we can identify what the substance is.
Calculating Percentage Composition
One of the most useful features of GLC is that it tells us how much of a substance is present. The area under each peak is proportional to the amount (moles) of that substance in the mixture.
To calculate the percentage composition of a specific component (let's call it Component \(A\)):
\(\text{Percentage of A} = \frac{\text{Area of peak A}}{\text{Total area of all peaks}} \times 100\)
Example:
If a chromatogram has three peaks with areas of \(10\text{ cm}^{2}\), \(30\text{ cm}^{2}\), and \(60\text{ cm}^{2}\):
• Total area = \(10 + 30 + 60 = 100\text{ cm}^{2}\)
• Percentage of the second component = \(\frac{30}{100} \times 100 = 30\%\)
Common Pitfalls and Summary
Don't forget:
• The mobile phase in GLC is always a gas (unreactive).
• The stationary phase in GLC is always a liquid (high boiling point, non-polar).
• Retention time is measured from injection to detection.
• Use Peak Area, not just peak height, to calculate the percentage composition. Peak height can be misleading if a peak is very wide.
Summary Checklist:
1. Can you define retention time?
2. Do you know the typical stationary and mobile phases?
3. Can you explain separation in terms of solubility/partition?
4. Can you calculate the percentage of a compound from the area of its peak?
Don't worry if the graph looks messy at first! In exam questions, they will usually give you the peak areas or a clear grid to work with. Just remember: Area = Amount!