Mass Spec mid term Flashcards
Why would we want to change the end product of the CI I/M reactions of the reagent gas?
For our analyte to ionize, the reaction needs to be exothermic (at least 2 kcal/mol - or else too slow) so changing what is interacting with our analyte will change Delta H (moderate means can see M+H, but if we want to fragment - can tune for a reagent that gives a larger delta H).
How to tune for different reagent end products in CI?
Change pressure (change the amount of collisions - changes which reactions more likely)
Nitrogen Rule
The nitrogen rule states that any molecule (with all paired electrons) that contains an odd number of nitrogen atoms will have an odd nominal mass.
What is mean free path dependant on
Collisional Cross section and density of neutrals (also temp and radii)
Sack rule for Mean free path
4.95/pressure (mtorr) (give MFP in cm)
What is mass defect and how does it play into monoisotopic mass?
The equivalent of the energy of binding (and from e = mc^2 we know this has a mass that adds to our analyte)
So when we talk about our exact mass of isotopes – it’s not just the sum of proton , neutron electron etc its that + mass defect (EVERY ISOTOPE HAS UNIQUE MASS DEFECT)
Resolving power formulas
IUPAC – M/Delta M – M is mass we’re at and delta M is the difference between two adjacent peaks at equal abundance with a specific overlap defined (50%)
Common(?) – Full Width Half Mass (FWHM) at a specific x hiegh at a specific M
Mass accuracy formula
(Mexp -Mcalc)/Mcalc (exp is experimental, calc is true)
Expressed in terms of PPM – (multiply by 1 million)
What is efficiency
16) EFFICIENCY – product of transmission of analyzer and duty cycle
a. Duty cycle - % of ions of INTEREST ionized
-measurement dependant (SIM high duty cycle, scan - low)
List operating pressures for the following:
EI, CI,
10^-5 for EI, 0.1-0.5 for CI
Pressure regimes
Low - <10^-5 (most everything else); Moderate 10^-5 – 10torr (CI) ; > 10 torr (AP)
4 ionization mechs for EI
1) gets hit - M+ dot
2) Gets hit - fragments
3)gets hit loses multiple electrons (M n+)
4) Picks up electrons (M- dot)
What is ionization cross section
is a measure of the probability that a given ionization process will occur when an atom or molecule interacts with a photon. - it’s dependant on pi * b (impact parameter)
How does ionization occur in EI
The wavelength of the electron is close to molecule bond length – the wave becomes disturbed
-if one of the frequencies has an energy matching a transition in the molecule – energy transfer occurs – which can lead to electronic excitation and if this has enough energy (Ionization potential) – an electron can be expelled
Draw the mclafferty rearrangment
look it up
What are the pressures of the CI parts
sample is at 10^-5 , reagent gas is at 0.1 ish analyzer at 10^-5 so there’s a lot more reagent than sample
What exothermicity does our reaction need to be at and why for CI?
2 kcal/mol at least - otherwise reaction too slow
What are the units of kcoll
in solution its: 1/Ms (in gas phase its cm^3 / molecule *s
For a CI reaction (given our rate equation) what is it dependant on
N and T
What determines fragmentation in CI
internal energy distribution, observational window, collisional cooling (pressure)
How does electron energy compare from CI to EI
Higher to get through the many more neutral molecules
Alternative reaction can see in CI
Adducts, e- transfer, hydride extraction, cluster formation, e- capture (negative), metal ion attachment
EI vs CI in terms of ionization
Soft vs hard
-ability to generate negative ions for CI
-DIFFERENT FRAGMENTATION PATHS due to odd vs even electron species being formed
Can tune CI reagent gas
-M+H
-less fragmentation pathways – more predictable
-ionization flexibility – can pick types of ions you want and pathway H transfer, e- transfer, adducts etc
steps in apci
liquid goes through - nebulized -heated for vaporization- corona discharge to ionize via chemical ionization so most of the initial ions are the solvent