Lecture 3 Flashcards
State the stages of a mass spec experiment
- Sample introduced into instrument
- Ionisation e.g. EI, ESI, MALDI
- gas phase ions
(- fragmentation)
-mass analysis- ions are separated by mass/charge ratio and detected
e.g. TOF, quadrupole
State the ionisation methods
- Electron impact
- Chemical ionisation
- ElectroSpray ionisation
- Fast atom bombardment
- Matrix assisted laser desorption ionisation
Describe electron impact ionisation
- example of hard ionisation method
- involves ionising gas-phase sample molecules with an electron beam, and generally produces a molecular ion M+* (a “radical cation”), along with numerous fragment
ions. - A disadvantage of EI is that the molecular ion may be entirely fragmented before detection.
Describe chemical ionisation method
- example of soft ionisation method- more likely to observe the molecular ion
- the gas-phase sample molecules are mixed with a reagent gas (e.g. methane, ammonia) before the electron beam is introduced.
- The electrons ionise the reagent gas, and the ionised reagent gas collides with the sample molecules, producing
ions (often a protonated molecule, MH+, although it depends on the nature of the reagent gas).
Describe the ElectroSpray ionisation method
- example of soft ionisation method
- ionisation takes place from solution phase – hence, it is useful for fragile/involatile species. - Typically generates a protonated molecule MH+, and does not generate fragment ions.
Describe the Fast atom bombardment method
- example of soft ionisation method
- the sample is mixed with a viscous liquid (e.g. glycerol). This is bombarded with high energy atoms/ions (e.g. Kr, Xe) to generate ions of the sample.
- It is useful for fragile/involatile species. Typically generates MH+, and does not generate fragment ions.
Describe the Matrix assisted laser desorption ionisation method
- example of a soft ionisation method
- The sample is mixed with an acidic organic molecule (e.g. benzoic acid) and co
crystallised to form a matrix – ions of both are then generated with an incident laser pulse.
What is the purpose of mass analysers?
Once the ions have been generated, they are separated according their m/z ratio. The
principle works for positive ions and negative ions.
State the different methods of mass analysis
- Magnetic sector
- Quadruple
- Quadruple ion trap
- Time of flight
Describe the magnetic sector mass analysis method
- this involves passing ions through a magnetic field, in which the ion flight path
is deflected proportionally to its m/z value. Familiar – but less commonly used.
Describe the Quadruple mass analysis method
- ions pass between four parallel metal rods, to which electric currents are applied,
which allows ions with selected m/z ratios to pass through. - Cheap, compact.
Describe the Quadruple ion trap mass analysis method
- ions pass into a metal box, to which electric currents are applied, causing the ions to move in a way which depends on their m/z ratio.
- Cheap, very compact.
Describe the time of flight mass analysis method
- this involves accelerating ions with an electric field, and then measuring
the time taken for the ions to reach a detector. - Excellent mass accuracy.
What are present on the axis of a mass spec?
Y-axis= Ion intensity
X-axis= mass/charge ratio
What is the tallest peak called?
- peak with 100% intensity is the base peak, most stable ion
- sets the top of the scale at 100%
Define mass/charge ratio
- the mass of the ion, divided by its charge. In many circumstances, the ions formed will have a single charge, and so the mass/charge ratio of the ion will be numerically equal to
the mass of the ion.
Define ion intensity
measure of the number of ions that are detected
What factors influence ion intensity?
- Stability of ions formed in ionisation process
- very stable= high intensity as alot to be detected
- less stable= lower intensity
- very unstable= dont appear in spec as to few to detect - Chemical formula of ions
- different elements have different
isotope characteristics. e.g. Carbon atoms may be present in the ion as 12C or 13C
- These isotopes combine statistically, with the result that any individual ion could be made up of a different combination of isotopes, and so have a different
mass, and therefore a different mass/charge ratio – leading to the observation of distinct groups of peaks (sometimes called clusters) in the mass spectrum.
- a single ion will usually give rise to a group of peaks, with each individual peak arising from ions which all have exactly the same formula and structure, but
are composed of different sets of isotopes (these are called “isotopologues”).
What is the purpose of relative peak heights?
The relative peak heights in the cluster are determined by the combination of the chemical formula and the isotope ratios. The isotope ratios are known (p1) – so we can (in principle) use the relative peak heights to make inferences about the chemical formula of an unknown molecule.
What types of molecules will show up in a mass spec?
- Only cations will show up in the mass spec
- Radicals will be invisible
In what situation do you use EI ionisation?
for molecules that are relatively volatile as they need to be in the gas phase
Explain how the process of EI ionisation
- The sample is either in the gas phase already, or heated to volatilise it.
- electrons are ejected from a heated filament, and accelerated towards the electron trap.
- the high energy electrons interact with the sample vapour, causing ionisation.
- The ionisation process converts the molecule into a radical cation: M + e− → M+* + 2e−
- the ionised sample molecules are guided out of the ionisation chamber by the ion repeller.
The molecular ions typically have excess energy, and this leads to their fragmentation hence, EI spectra do not always contain a peak for the molecular ion.
Explain the process of Quadruple mass analysis
- A quadrupole mass analyser separates ions by passing them through the space between four electrically charged metal rods. It can accept and analyse a constant stream of ions.
- The ions move into the space between the
four parallel metal rods. Opposing poles
have voltages of opposite polarity. - Variable DC and AC currents are applied to
the metal rods. The combination of these
currents constrains the direction of the ion
movement in a way that depends on m/z.
Explain the Time of flight mass analysis process
- A ToF mass analyser admits ions from the ionisation source (e.g. MALDI), then accelerates the ions with an electric field.
- The ions then enter the “flight tube”, where there is no electric field. In the flight tube, they drift towards the detector with a fixed speed. Given that the ions experience the same initial acceleration, the drift speed depends on their m/z ratio.
- Heavier species travel more slowly than light species, giving a different time of flight. Typically generates very accurate results.
Give the equation for kinetic energy
E= zeV
z= charge on ion
e= electron charge (on datasheet)
V= potential difference
Give the equation for kinetic energy (normal)
1/2mv^2
m= mass of ion
v= velocity of ion
Give the equation for velocity of ion
v= D/t
D= distance (length of flight tube)
t= time of flight
Give the equation for m/z
m/z= 2eV (t/D)^2
