Analytical techniques Flashcards
What happens when a molecule absorbs infrared radiation?
It makes the covalent bond vibrate more in a stretching or bending motion
What factors affect the amount of vibration of a bond
Bond strength
Bond length
Mass of each atom in the bond
How does IR spectroscopy work?
Every bond has a unique vibration frequency in the IR region of the EM spectrum
Bonds absorb radiation that has the same frequent as their frequency vibration
IR radiation emerged from a sample is missing the frequencies that have been absorbed, this information can be used to identify the compound’s functional group
What do the peaks on an infrared spectrum represent?
Absorbance of energy from the IR radiation
What are the two peaks that must be present to identity a substance as carboxylic acid
Very broad peak at 2500- 3300 cm-1 –> O-H group
Sharp peak at 1680-1750 cm-1 –> C=O group
What happens inside a mass spectrometer
Organic compound is vaporised and passed through the spectrometer
Some molecule lose an electron and form molecular ions
Excess energy from the ionisation makes the bonds vibrate more
Vibration causes bond to weaken
Molecular ions splits by fragmentation
What is the symbol of molecular ion?
M+
Is fragmentation predictable? Why?
No, because it can happen anywhere in the molecule
In a mass spectrometry how is a molecular ion represented
Peak with the highest mass/charge ratio
The molecular mass of the molecular ions is equal to what
Relative molecular mass of the compound
Will the molecular ion peaks of two isomers of the same compound be same or different?
Same
What is the m/z value of CH3+
15
What is the m/z value of OH- from alcohol
17
What is the m/z value of C2H5+
29
What is the m/z value of C3H7+
43
What is the m/z value of C4H9+
57
What are the advantages of using MS
Cheap
Small quantities of sample required
What is the main disadvantage of using mass spectrometry
The sample is completely destroyed
What do the troughs on an IR spectrum show?
Frequencies where radiation has been absorbed- match to table to find out which bonds they represent
What is the fingerprint region?
Area of the IR spectrum below wavelength of 1500cm-1
Many peaks caused by complex vibrations of the whole molecule
Unique to every compound so can be used to identify compounds
How is the fingerprint region used to identify compounds?
The fingerprint regions pattern is matched to a database on a computer to identify the compound accurately
