7. Modern analytical techniques (MASS SPECTROMOTERY+ IR) Flashcards
Give the relative mass and charge of protons, electrons and neutrons
Particles Relative mass Charge
Proton: 1 + 1
Neutron: 1 0
Electron: 1/1840 -1
Define nucleons
Total number of protons and neutrons in an atoms nucleus
Define atomic (proton) number
Number of protons in the nucleus
–> given the letter Z
Define mass number
Number of protons and neutrons in an atom
–> given the letter A
Define isotope
Atoms of the same element with the same number of protons but different number of neutrons
Define relative isotopic mass
The average mass of an atom of an isotope relative to one twelfth of the mass of one atom of carbon -12
Define relative atomic mass (Ar)
The average mass of an atom of an element relative to one twelfth of the mass of one atom of carbon -12
Uses of mass spectrometry
1) identifying elements on other planets and their isotopes
2) drug testing in sports to identify chemicals in the blood
3) quality control in the pharmaceutical industry
4) radioactive dating (carbon - 14) to determine age of fossils etc.
Give the 5 stages of mass spectrometry
Vaporisation
Ionisation
Acceleration
Deflection
Detection
Describe stage one of mass spectrometry
Ionisation - Heated metal coil gives off electrons that are attracted to the positively charged plate above it. Gaseous sample is bombarded by electrons to form 1+ ions.
Describe stage two of mass spectrometry
Accelerated using an electric field
Describe stage 3 of mass spectrometry
Charged particles deflected by a magnetic field - with an electromagnet
What does the size of deflection in mass spectrometry depend on
Mass of ion - lighter ions are deflected more than heavier ones
Charge on the ion- 2+ ions are deflected more than a +1 ion. This halves the m/z (mass/charge) value but the abundance stays the same.
What does the m/z ratio show
The mass of the isotope
–> mass / charge but charge is often just +1 so its just mass
Describe stage 4 of mass spectrometry
The beam of ions passing through the machine is detected electrically.
–> when the beam of ions hits a negative plate its charge is neutralised by an electron jumping from the metal on to the ion. That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it.
–> A flow of electrons in the wire is detected as an electric current which can be amplified and recorded. The more ions arriving, the greater the current.
What does the mass spectrum for monatomic elements show
Lines represent different isotopes in that element
What is fragmentation
When a molecule becomes ionised to form a molecular ion (M+) which is unstable and can fragment into different pieces
what does the mass spectrum for a molecule with different elements show
It shows the different fragments produced when the molecular ion breaks up
Describe the mass spectrum for chlorine.
In the ionisation chamber, an electron is knocked off the molecule to form a molecular ion - Cl2+. This can then form a chlorine atom and cl+ ions. As the atom has no charge it is not included in mass spectra. The cl+ atom will form lines at 35 and 37 in the ratio 3:1. However, you will also get lines for the unfragmented (cl2) +. This ion could either be from the isotopes:
chlorine 35 and chlorine 35
chlorine 35 and chlorine 37
chlorine 37 and chlorine 37
Therefore, you get lines in the ratio: 9:6:1
Which peak is the molecular ion
M+ ion is the heaviest so it is furthest to the right
What does the molecular ion peak ( M+) show on a mass spec
The relative molecular mass of the molecule
Why must a vacuum be used in a mass spectrometer
no interference from atoms or molecules in air
- Molecular fragments could not exist otherwise
When calculating relative atomic mass using the mass spectra, do you always divide by 100
No, not always. You divide by the total abundance which is not always 100%. Check by adding up total abundance either from a table or graph
Give the equation to work out relative atomic mass
(isotopic mass x abundance) + (isotopic mass x abundance)…..
/
total abundance ( normally 100 but not always)
How is emission spectra evidence for quantum shells?
1) the radiation emitted has a fixed frequency
2) electrons can only exist in shells, not between them
3) each shell has a fixed energy
4) defined lines of emission spectra prove electrons exist in shells only and not in between
Describe how you would predict a mass spectra
1) Write the % of each isotope given as a decimal
2) Write the different combinations of the isotope and times the now decimal abundances
3) Divide the abundances by the smallest value to get a whole number ratio which can be used to predict the spectra
Write the equation for the formation of the molecular ion eg: for ethane
C2H6 + e- → C2H6+ + 2e-
Write the eqution for the formation of frgaments
C2H5OH+ → C2H5O+ + H.
Give the reason for the existence of a small peak after the molecular ion peak
The presence of a small amount of the isotope 13C in the compound
Describe how IR spectroscopy works
When radiated with infrared radiation, certain bonds (of functional groups) vibrate at certain frequencies so you can see weather these bonds are present or absent in an IR spec
–> you compare the absorbance to the ones listed in data booklet
In IR spec, where is the fingerprint zone
below 1500cm^-1
What does the frequency of absorbed IR depended on
The bond strength
The masses of the atoms
Describe the shapes you would see on an IR spec for:
O-H (alcohol)
O-H (carboxylic)
C-O (carboxylic acid
O-H (alcohol) - broad, cucumber shaped
O-H (carboxylic) - very broad, thumb shaped
C-O (carboxylic acid, narrow, finger