Flashcards in Mass Spectrometry Deck (35):
What are the main components of a MS
Data acquisition and analysis
Which elements of an MS are required to be in a vacuum
Mass analyser and Detector
What is the level of pressure needed for an 'ultra-high' vacuum
< 10 ^-7
What equipment is needed to achieve an ultra-high vacuum
Why is a vacuum necessary for MS
To prevent species from colliding into an air molecule (?)
What are the units of the mass/charge ratio
What are the two modes of mass measurement and how are they achieved
Positive-ion mode (proton added)
Negative-ion mode (proton taken away)
Define an isotope
An atom with the same number of protons/electrons but a different number of neutrons
Why are bromine ions annoying
Because the two isotopes of bromine are equally abundant, so mass spec gives two equal peaks
What is the relationship between the distance between isotope peaks and charge state
+1 charge gives peaks 1 Dalton apart.
+2 charge gives peaks 0.5 Dalton apart.
+4 charge gives peaks 0.25 Dalton apart
What is the equation to work out resolution
Resolution = Mass of 2nd peak / Resolving power
R = M / delta M
What is better; higher or lower resolution
Higher - allows for better individual peak identification
What are the main ionisation techniques
Electrospray ionisation (ESI)
Matrix-assisted laser desorption/ionisation (MALDI)
Electron Ionisation (EI)
Chemical Ionisation (CI)
Fast-Atom Bombardment (FAB)
What is the main idea behind Electrospray Ionisation (ESI)
Adding voltage to a liquid forces the atoms (charged) to separate and become nanoparticles (which can be run through the MS)
What are the main advantages of Electrospray Ionisation (ESI)
'soft' ionisation technique - used in solution. Allows analysis of biological samples that are defined by non-covalent interactions.
Able to ionise samples with large masses.
Can easily be coupled with separation techniques (e.g. nano-LC)
What are the main disadvantages of Electrospray Ionisation (ESI)
Not good at analysing mixtures.
Becomes contaminated easily, and difficult to clean.
Extra charges added can give skewed results
What is the equation to work out molecular mass from an Electrospray Ionisation-MS graph
n = [M(n+1) - H] / [M(n) - M(n+1)]
where n = molecular mass; M = m/z values for different peaks
What does MALDI stand for
Matrix-assisted Laser Desorption/Ionisation
How does MALDI work
Sample 'co-deposited' with 'Matrix'. Laser excites matrix. Matrix transfers energy to sample. Produces singly charged species.
What is necessary for a MALDI matrix
Strong absorption of laser wavelength
Low sublimation temp
Good mixer/solvent compatibility with sample
Able to participate in a photochemical reaction
Give examples of Matrices used in MALDI
What is the normal ratio of sample/matrix in MALDI
Lots of matrix, very little sample.
Name some applications of MALDI
Find mass of an intact protein
Find molecular weight distribution of polymers
What are the main advantages of MALDI
Samples with a high MW can be analysed
Molecules don't need to be volatile
Easy to get sensitive results
Can give a variety of charge states (1-3; +ve or -ve)
What are the main disadvantages of MALDI
Low m/z ions can be obscured
Sample must have low vapour pressure
Pulsed therefore some mass analysers wont work with it
Difficult to couple with chromatography
Can have problems when samples absorb laser light
What are the main types of Mass Analysers
Sector instruments (magnetic (B) and electrostatic (E))
Ion Trap (IT)
Ion Cyclotron Resonance (ICR)
How do sector instruments work
Ionised sample curves through an electrostatic sector. Hits a spectrometer lens which splits the beam. Curves through the magnetic sector and exits.
What are the pros and cons of sector instruments
Resolving power up to 100,000
Mass range up to 15,000
Not suitable for ESI or MALDI ionisation.
What is the main idea behind Time-of-Flight (TOF) (mass analyser)
Analyser tests how quickly ions travel a certain distance. Smaller ions get there first, larger ions take longer.
How can the resolution of TOF be increased
Longer flight time = higher resolution. Longer flight time from the use of electrostatic mirrors
What is the mass range of TOF
Theoretically unlimited, but realistically >250k
What are the pros and cons of TOF
Pros. Fast, sensitive, simple, cheap, unlimited mass range,
Cons. Requires pulsed ionising (MALDI or pulser ESI). Variation within one species can cause problems.
What is the main idea behind Quadrupole (Q) (mass analyser)
4 parallel metal rods. Rods oscillate DC potential (charge), which makes the ions oscillate. At a fixed DC potential, only a certain m/z passes through. Varying the DC/Rf allows scanning for ions with various m/z 's
What are the main advantages of the Quadrupole (Q)(mass analyser)
No high voltage needed
High dynamic range