instrumental analysis Flashcards
What is the first step when beginning to analyze an unknown substance?
Samples are often mixtures of different compounds, so they need to be separated first
What happens in gas chromatography–mass spectrometry (GC–MS)?
- firstly gas chromatography is used to separate the compounds that are easily vaporized
- then they are passed into the mass spectrometer, which can identify each of them
- the pattern of peaks it produces identifies the sample
How is the mass spectrometer useful?
It can identify both elements and compounds
What happens during gas chromatography?
Gas moves through a column packed with a solid to separate a mixture
What is the method for gas chromatography?
- the sample mixture is vaporized
- a ‘carrier’ gas moves the vapour through the coiled column
- the compounds with a longer retention time take longer to pass through the column
- the compounds with a shorter retention time travel more quickly and leave first
How is a compound’s retention time determined?
The compounds in the sample have different attractions to the material in the column. The compounds with stronger attractions will take longer to get through the column (longer retention time); whereas the ones with weak attractions will move quicker and leave first (shorter retention time)
How are the separated compounds recorded?
They can be recorded as peaks at different times on a chart as they leave the column
How can the unknown substances be identified?
By comparing their retention times on the chromatograph with the results for known substances
How does attaching a mass spectrometer directly to the gas chromatography apparatus help?
This ensures that the unknown substances are identified, and it does so very quickly and accurately. It can also detect very small quantities in the sample
What does a mass spectrometer do?
It provides an accurate way of measuring the relative molecular (formula) mass of a compound
What is the molecular ion peak?
It is the peak with the largest mass and so is the one that corresponds to the positive ion formed when the original molecule has just one electron removed
Where is the molecular ion peak found on the mass spectrum?
It is found as the last peak on the right
