✨Module 4: Analytical techniques Flashcards
What is infrared spectroscopy?
A technique used to identify compounds/functional groups based on changes in vibrations of atoms when they absorb IR of certain frequencies.
Describe the infrared spectroscopy process
- sample placed in the IR spectrometer
- a beam of IR radiation (200-4000 cm-1) is passed through the sample
- molecule absorbs some of the frequencies and the emerging beam is studied to see which frequencies have been absorbed
- a computer plots the wave of transmittance on a graph
IR energy is only absorbed if …
A molecule has a permanent dipole that changes as it vibrates.
What is the fingerprint region?
In an IR spectrum, the region of 1400cm-1 is unique to every molecule. It has many peaks that can be difficult to assign. These peaks represent the complex vibrational interactions that occur between different bonds within a molecule.
Uses of IR radiation?
- IR breathalysers to test for alcohol consumption
- Sensors identify pollutants in vehicle emissions in the air.
What is the typical sequence for identification?
- Elemental analysis
- Mass spectrometry
- Infrared spectroscopy
What is elemental analysis?
To derive empirical formulae.
Mass spectrometry.
This determines molecular mass and fragment ions from the whole molecule:
=> When a compound is analysed in a mass spectrometer, vaporised molecules are bombarded with a beam of high-speed electrons.
=> These knock off an electron from some of the molecules, creating molecular ions.
What is the [M+1] peak?
A smaller peak due to the natural abundance of the isotope carbon-13.
The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule. The more carbon atoms, the larger the [M+1] peak is.
