Mass Spec Intro Flashcards
Why Mass Spec ? (MS)
- Very high sensitivity; used to detect small amounts of samples (pg-pikogram)
- To determine; Elements present
> Molecular weight of molecules (this required soft ionisation)
> Molecular formula (High resolution MS may be required)
> Molecular structure (of biomolecules, sequence peptides and proteins) - this process may require fragment patterns which requires MS/MS process
> High spatial resolution label-free imaging
State some limitations of mass spec
- Complex equipment required (although portable/handheld versions becoming available)
- Not all molecules are easily susceptible to MS
> MS requires molecules to be in vapour phase although not all molecules easily transition to this phase)
> MS requires molecules to form ions but not all molecules easily form ions - Not inherently quantitative
- Mixture analysis can be challenging
What are the 5 basic steps of MS. State them
1) Generation of Molecules in gas phase
2) Ionisation of molecules
3) Mass separation of ions
4) Ion detection
5) Data intrerpretation
Some molecules are already in Gas Phase. TRUE or FALSE
TRUE
Describe the generation of molecules into the Gas Phase
Molecules is introduced and heated until evaporation occurs. The molecule has now entered the gas phase and can be analysed using MS
State a problem that can occur in the gas phase and give examples
Lots of molecules will react or break down before they evaporate.
e.g: Sugars will caramelise
Upon heating, starchy foods will form Acrylamide
What is the most common type of ionisation ?
Electron Ionisation (EI)
During the ionisation process, when electrons bump into gas phase molecules and exchange energy, what is the energy of the gas phase molecules. Give answer in eV and kjmol-1
Energy of gas phase molecules = 70eV = 6700kjmol-1
How much energy is required to break a C-C bond ?
about 350kjmol-1
EI-MS is only used for molecules that can readily be in gas phase without decomposing. TRUE or FALSE
TRUE
State 2 properties of molecules that tend to survive the gas phase without decomposing
1) Size limited molecules
2) Polarity limited molecules
In EI-MS, electron beams interact with the evaporated molecules (and produce ions). TRUE or FALSE
TRUE
In EI-MS, ions are put into a mass filter then in a detector (in order to count charged particles). TRUE or FALSE
TRUE
EI mostly forms anions (negatively charged ions). TRUE or FALSE
FALSE, cations (positively charged ions) are formed
Where is the preferred location to knock off an electron in gas phase molecule ?
From a lone pair
e.g: ether with charge (positive!) and radical located on O
State some locations of missing elcetrons
- Bonding electron e.g: CH4
- Lone pair
- Alkene: pi orbital is HOMO orbital therefore loses e-: with resonance structures that can switch position of charge and radical
If there is fragmentation of the molecular ion, what happens to the ion ?
- one part of ion has even no of e- and positive charge
- other part has odd no of e- and no charge
Therefore, charge ad electron separate on fragmentation
In Mass separation, ions should be accelerated to move in a straight line into a ‘mass filter’. TRUE or FALSE
TRUE
The mass filter region is in a Vacuum, therefore there re o collisions of ions. TRUE or FALSE
TRUE
State and explain the 2 basic methods used in mass separation
1) Deflection of ions in electromagnetic field;
Ions deflect more if charge (z) higher or mass (m) lower
2) Time-of-flight:
Ions fly faster if charge (z) higher or mass (m) lower
In common: m/z is measured rather than just m
(especially important for “soft” ionisation techniques)
Describe Electromagnetic field mass filter
– Oldest type, conceptually easiest
– Detector in fixed position which ions can only reach if they are deflected into it
– Vary magnetic field
Only specific m/z deflected precisely into detector
– Maths: convert strength magnetic field → m/z value
-– Graph: m/z on x-axis and number of detected ions (Intensity) on y-axis: the mass spectrum
Quadrupole filter only detects ions that carry straight on. TRUE OR FALSE
TRUE
Quadrupole filter detect 1 m/z value at a time. TRUE or FALSE
TRUE
Quadrupole Analyser
- Mass filter: only 1 mass at a time reaches the detector
- Low mass resolution
- Cheap, robust
- Works well for ions up to 1000m/z
- Diff systems can goup to m/z ~ 70,000
- Can be used to analyse complex (biological) mixtures (when combined with a separation technique)
- very sensitive (~2.5 10-18 mol peptide)
- Couple to chromatographic separation (GC, LC)
Name an alternative method to analyse ions
ToF (Time of flight)
Describe ToF
- All ions reach the detector!
- But separated by time
- Give all ions same kinetic energy (Ekin) at the same time and place
- Basic physics: Ekin = mv2
- So if mas) is different, but Ekin the same, v must be
different. - v is velocity of the moving ions
-Heavier ions take longer to reach the detector
– Depends on charge, so again linked to m/z rather than m - Convert time to m/z, counted ions at each time on y-axis
- Higher mass resolution possible
Time-of-Flight analyser is based on what equation ?
E kinetic = 1/2 m v2
Electron Ionisation is known as ‘Hard ionisation’. TRUE or FALSE
TRUE
State 2 problems associated with EI
1) Diffuclt to identify fragile ions
2) Difficult to analyse composition of mixtures
The most intense peak in a mass spec is known ?
Base peak
