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C2 3.8 Instrumental Analysis

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Gemma Boyson

on 23 November 2015

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Transcript of C2 3.8 Instrumental Analysis

C2 3.8 Instrumental Analysis
Analysing Mixtures
Gas Chromatography
Mass Spectrometry
Key Points
Samples to be analysed are often mixtures of different compounds.
Must be separated
Before identification by instrumental analysis
All done by gas chromatography-mass spectrometry (GC-MS)

1. Gas chromatography separates compounds that are easily vaporised.

2. Mass spectrometer produces a pattern of peaks which is used to identify both elements and compounds.
Similar to paper chromatography, but instead of a solvent moving over paper, gas moves through a column packed with a solid.
We can use mass spectrometry to identify the unknown substances separated by gas chromatography

It identifies substances quickly and accurately, and can detect very small quantities in the sample.

Can also accurately measure the relative formula mass of a compound.

The peak with the largest mass corresponds to an ion with just one electron removed. Called the molecular ion peak.
Always the last peak on the right.
Compounds in a mixture can be separated using gas chromatography.

Once separated, compounds can be identified using a mass spectrometer.

The mass spectrometer can be used to find the relative molecular mass of a compound from its molecular ion peak.
How can we use gas chromatography to separate compounds in a mixture?

How can we use a mass spectrometer to identify the compounds in a sample?

Chemistry laboratory: 1900
Modern chemistry laboratory
Why has instrumental analysis only recently developed?
1. Sample mixture is vaporised
2. A 'carrier' gas moves the vapour through the coiled column
The compounds in the sample have different attractions to the material in the column. The compounds with stronger attractions move through slower.
We say they have a longer "retention time".
The compounds with the weakest attractions leave the column first.
They have shorter retention times.
The separated compounds can be recorded on a chart as they leave the column.

We can identify the unknown substances in the sample by comparing the chromatograph with the results for known substances.

The analysis must have taken place in exactly the same conditions to compare retention times.
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