16 Testing your understanding

Define and describe chromatography using the following key terms:

  • Mobile phase
  • Stationary phase
  • Gas Chromatography

Answer

Chromatography is a laboratory technique for separating mixtures.

The mixture to be analysed travels in a mobile phase (which may be a liquid or gas) which passes through a stationary phase. The interaction that compounds in the mixture have between the mobile phase and the stationary phase determines the speed at which they travel. This separates the different compounds.

In gas chromatography the mobile phase is an inert gas such as helium which flows through a column coated with the stationary phase.

The figure below shows an example chromatogram.

Figure 9 Example chromatogram

What should the label on the horizontal axis be?

Answer

Retention time/minutes (Remember units!)

Which of the three compounds is the most volatile?

Answer

Compound 1 is the most volatile because the retention time is the shortest.

Which is of the three compounds has the highest concentration?

Answer

Compound 1 has the highest concentration because the peak area is largest.

Use your results or return to the GC-MS laboratory to answer the following question.

GC analysis of a water sample from a lake showed a peak with a retention time of 11.26 minutes and a peak area of 322.4. Identify the pesticide present and determine its concentration.

Answer

The pesticide is endosulfan and the concentration in the water sample is 2.6 ng l−1.

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Concentration of pesticide in initial sample equals (peak area of sample times response factor) all divided by (15 times percentage recovery). Concentration of endosulfan in initial sample equals (3.22 times 10.9 nanograms per litre) all divided by (15 times 89.9) equals 2.6 nanograms per litre.

Extra materials for interested pupils

Video clip on mass-spectroscopy (this takes you to an external website)

https://www.youtube.com/ watch?v=J-wao0O0_qM [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

15 What do your results tell you?

17 To summarise