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Study Notes: How does the GC work?

In GC, the sample is injected into the column via a syringe or sampling loop, and is converted to a vapour state (if not already a gas). The carrier gas moves the sample along the column past the stationary phase and into the detector to produce a chromatogram.

The column can be of two general types depending upon how the stationary phase is contained within it, as follows.

  • A packed column is filled with an inert material of high surface area and onto which the stationary phase is finely distributed.
  • A capillary, with a narrow opening running down its centre, has stationary phase coated directly onto the wall to form a thin film or is contained in other formats.

The carrier gas percolates through a packed column (around and over the coated particles) or flows though the narrow opening of a capillary column past the stationary phase.

The sample is often a mixture of different components. Their separation will be achieved based on boiling points (lower boiling point components coming off earlier), the size of the molecules (larger molecules might take longer) as well as the chemical characteristics of components and the stationary phase in the column.

In effect, separation occurs when components move at different speeds through the column, depending upon how strongly they are attracted to the stationary phase.

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The stationary phase is selected to perform different types of separations (in combination with the right operating conditions). For the interaction of the component with the stationary phase, the greater the interaction the longer the interaction time and the better the resolution.

A detector detects the bands of component coming off the column and converts these into an electrical signal that is converted by the chart recorder into a chromatogram showing a peak for each component detected.

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