Mass Spec 5&6 (Flo-3) Flashcards
(23 cards)
What are the two reasons for derivatisation?
1 - make molecule more volatile
2- direct fragmentation
Why do hydrophobic molecules work better in MS?
They are easier to ionise because they cannot form H-bonds are so are more volatile
Give an example of derivatisation to form a hydrophobic molecule.
trimethysiylation, addition of a S(CH3) group to an -OH group (O-S(CH3))
What general equation demonstrates radical elimination (radical ion eliminating radical molecules)?
A+. => B+ + C.
What general equation demonstrates neutral elimination (radical ion eliminating neutral molecules)?
A+. => B+. + C
Are neutral/radical species detected?
No, only charged species are detected
What is the even electron rule?
An even electron ion will not normally fragment to two odd electron species (two radicals)
Is radical elimination described as
a) one electron movement
b) two electron movement
a) one electron movement
Is neutral elimination described as
a) one electron movement
b) two electron movement
b) two electron movement
In radical elimination what is alpha cleavage?
Cleavage of carbonyl compound after ionisation of oxygen. The odd electron is donated to form a new bond to the adjacent atom and cleavage of the original bond to give:
R-C(=O+)-R –> R-C≡O+ + R.
Give an example of beta cleavage?
CH3-CH2-O+-CH3 –> CH2=O+-CH3 + .CH3
+N-CH2-R –> N+=CH2 + R.
Normally, the charged molecule is O/N
When does neutral elimination occur?
Occurs when a stable neutral molecule (eg. water) can be readily formed via 1,2 elimination mechanism
(often removal of water)
What is inductive cleavage?
Initiation of cleavage by the +ve charge involves attraction of electron pair.
e.g.
R-C(=O+)-R –> R-C.(=O) + R+
What is the name of the rearrangement when fragmentation occurs via bond breaking and forming?
McLafferty Rearrangement
What are CAD experiments?
Collisional activation decomposition - can select an ion and give it more energy to fragment it.
How does MS/MS create fragmentation?
Ions are separated and then selected by the first MA and selected ions are activated by exposure to collision gas (transfer of kinetic energy) in the collision chamber (between two MAs).
What gases could be used as collsion gases in MS/MS?
Helium, nitrogen, argon
What is the purpose of the second analyser in MS/MS
The second analyser separates based on their m/z ratios to produce a MS/MS spectrum
In the triple quadrapole MS/MS what does each QP do?
Q1 - separates molecular ions based on m/z
Q2- ions pass through collision chamber
Q3- resolves fragments based on m/z
Why is the MS/MS combination of QP-TOF used?
It is a very powerful tool in terms of sensitivity and resolution.
What are the advantages of using ESI instead of MALDI for MS/MS peptide sequencing?
- Multiply charged ions produced which requires less energy to fragment.
- Also majority of impurity derived signals are singly charged.
- Gives better signal to noise ratio if you can distinguish between singly and multiply charged ions.
Why do peptides digested with trypsin have a minimum of 2 charges?
N-terminus and C-terminal K or R
How are isotope clusters used to identify the charge on multiply charged ions?
The distance between the peaks with all 12C will be one mass lower than with 1 13C.
This mass difference (according to m/z) is divided by the charge. therefore if the difference is 1 the charge is 1. If the difference is 0.5 the charge is 2. If the difference is 0.33 the charge is 3.
