7: Modern analytical techniques 1 Flashcards
What is a mass spectrum produced by?
Mass spectrometer
What are mass spectra used for?
Showing relative isotopic abundances to calculate relative atomic mass
Determining molecular masses of compounds
What is a molecular ion denoted by?
M+(g)
How does a molecular ion form?
Molecule is bombarded with electrons which removes an electron from the molecule
Where is the molecular peak found?
Peak with the highest m/z value
(ignoring any small peaks to the right - M+1)
What is the M+1 peak?
Caused by the presence of 13C
Very small as it is not very abundant
What is on the y-axis of a mass spectra?
Abundance of ions
%
What is the on the x-axis of a mass spectra?
Mass/charge ratio
What is the typical charge of molecules or fragments in a mass spectra?
+1
How do fragments of compounds form in a mass spectra?
Bombarding of electrons make then break up into fragments
What is a fragmentation pattern?
Fragments which are the ions which show up on the mass spectrum
Why are fragmentation patterns useful?
Allows for the identification of molecules and their structure
When a molecule fragments in mass spectrometry what forms?
Molecule → positive ion + free radical
Only positive ion is identified
Learn these common fragment molecular masses
CH3+ - 15
C2H5+ - 29
OH+ - 17
What should you assume about the m/z value?
Same as the mass of the ion that made it
As charge is +1
How can you tell two different compounds with the same molecular formula using mass spectrometry?
Produce different fragmentation patterns
What occurs in infrared spectroscopy?
Infrared raditation is passed through a sample of the chemistry
Transmission of the light is measured
What absorbs IR radiation in molecules?
Absorbed by covalent bonds in molecules
This increases their vibrational energy
Why can IR be used to differentiate between different bonds?
Bonds between different atoms absorb different frequencies of radiation
Bonds in different places absorb different frequencies
What is the peak for C=O in infrared spectroscopy?
Strong and sharp absoption at about 1700 cm-1
What is on the x and y axis from infrared spectroscopy?
x axis - wavenumber (cm-1)
y axis - transmittance (%)
What does a peak on an infrared spectra show you?
At which frequency the radiation is being absorbed
What is wavenumber?
1/wavelength in cm
This is proportional to the frequency of the radiation
When is infrared spectroscopy most used?
To determine if a reaction has occured
Because it shows which functional groups are present
