Chapter 17 - Mass Spectrometry
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What is mass spectra used for?
Used to identify the molecular mass of an organic compound and to gain further information about its structure
What happens in a mass spectrometer?
• organic compound loses an electron and forms a positive ion, the molecular ion (M^+) • the mass spectrometer detects the mass-to-charge ratio of the molecular ion which gives the molecular mass of the compound
How can we find the mr from the mass spectrometer?
It is the molecular ion peak located at the far right on the mass spectrum
Why is there a very small peak next to the molecular mass peak sometimes?
This is the M+1 peak It exists because of the 1.1% of carbon-13
What is fragmentation and what are fragment ions?
When some molecular ions break down into smaller pieces and the other peaks in the mass spectrum are caused by these fragment ions formed as the molecular ion breaks down
What is the simplest way molecular ions can break down for a mass spectra?
Two species - a positively charged fragment ion and a radical The positive ions are detected by the mass spectrometer
How can we identify organic molecules from their mass spectrum?
The molecular ion peak to find the molecular mass, the different peaks representing different fragments which tells us about any functional groups within the compound
What do covalent bonds in molecules possess?
Energy and vibrate naturally about a central point
What increases the amount for vibration of covalent bonds?
Increasing temperature
What type of motion do atoms in molecules have?
Constant motion
What happens when bonds absorb infrared radiation?
Makes the bonds bend or stretch more
What is a stretch of a bond like?
A rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases
What is the second type of vibration, a bend like?
Results in a change in bond angle
What does the amount of a bond stretch or bend depend on?
• mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter • strength of the bond - stronger bonds vibrate faster than weaker
What type of radiation can each bond absorb?
Only radiation that has the same frequency as the natural bond frequency of the bond
What scale do scientist use for frequency and why?
Wave number -> proportional to frequency As frequency values are very large, for more convenience
Why does water vapour, carbon dioxide and methane contribute to global warming?
As they absorb the longer wavelength IR radiation as it has the same frequency as the natural frequency of the bonds and eventually the bonds re emit the radiation near the Earth’ surface increasing the temperature
What is infrared spectroscopy used for?
Identifying functional groups present in organic molecules
How does infrared spectroscopy work?
• The sample under investigation is placed inside an IR spectrometer • A beam of IR radiation in the range 200-4000cm-1 is passed through the sample • The molecule absorbs some of the IR frequency and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample • The IR spectrometer is usually connected to a computer that plots a graph of transmittance against wave number
What does each “peak” in the IR spectrum represent?
Can be related to a particular bond in the molecule
What is the region of peaks below 1500cm-1 referred to as?
The fingerprint which is unique to the compound which can be analysed by a computer software or by physical comparison to other spectrums
What type of peak is common to many organic compounds?
The C-H bonds at 2850-3100
What is the key peak of an alcohol on an IR spectrum
The O-H peak and the C-O peak
What are the key peaks for an aldehyde or ketone?
Peak for the C=O bond