Mass Spectrometry Flashcards
What does an odd m/z (mass:charge ratio) indicate about the composition of the compound?
It indicates that the compound contains an odd number of nitrogen atoms. (Nitrogen rule: states that the molecular ion of a compound with an odd number of nitrogen atoms has an odd m/z ratio; if there are zero or an even number of nitrogen atoms, the molecular ion has an even m/z ratio.)
What are the axis labels on a mass spectrum?
x-axis - m/z (mass to charge ratio) y-axis - relative abundance (0 to 100)
What is a molecular ion (M+)?
The radical cation formed by removal of a single electron from a parent molecule in a mass spectrometer.
What effect do common isotopes have a mass spectrum?
They give rise to peaks shifted at higher m/z values. An isotope two atomic mass units heavier will give rise to an M+2 peak that will have a value 2 m/z higher than the lower mass isotope.
What are the most common elements giving rise to M+2 peaks?
Chlorine, bromine, and oxygen. Chlorine (3:1), bromine (1:1), Oxygen (low relative abundance of isotope, but additive effect of many oxygens can contribute significantly to the M+2 peak).
How to calculate the relative intensity of an M+1 peak.
% (M+1) = Σ (% abundance of heavier isotope x # of atoms in the formula) Hydrogen and carbon both have M+1 isotopes so you would obtain the sum of the two probabilities.
If you have the relative abundance of an M+1 peak what conclusions can you draw about the molecule?
Most likely the M+1 peak is dominated by 13C isotopes (relative abundance of 1.11%). Using the relative abundance on the spectrum compared with 12C you can divide out how many carbons there are. (Ex: an M+1 peak 5.74% of the molecular ion: 5.74% / 1.11% = ~ 5 carbon atoms)
What is characteristic of alkene and alkyne mass spectrums?
They show a strong peak for the molecular ion (probably formed by removal of one pi electron from the double/triple bond)
Aside from the molecular ion, what is one of the most prominent peaks in the mass spectrum of most alkynes?
The delocalization stabilized 3-propynyl (propargyl) cation (m/z 39).
What is the m/z of a ethyl cation?
29
What is a common fragment observed in the mass spectrum of alkenes?
An resonance-stabilized allylic cation (m/z 41)
What is characteristic of the mass spectrum of an alcohol?
There will be a low intensity molecular ion peak from primary and secondary alcohols, and no molecular ion peak detectable from tertiary alcohols.
What common fragmentation pattern is observed in the mass spec of alcohols?
The loss of a water molecule (M -18) - this is useful when the M+ (molecular ion) peak is detectable. Another common pattern is loss of an alkyl group from the carbon bearing the -OH group (forms an alkyl radical and a resonance-stabilized oxonium ion).
What are the typical alkane fragmentation patterns?
Fragmentation tends to occur toward the middle of unbranched chains forming a series of fragments differing by 14 amu (CH2). Branched chains give fragments at the branches to generate secondary and tertiary cations.
What are the characteristic fragmentation patterns of aliphatic (non-aromatic) aldehydes and ketones?
α-cleavage (cleavage of one of the bonds to the carbonyl group; fragments will occur for loss of either group), and McLafferty rearrangement.
What is a McLafferty rearrangement?
Occurs in aldehydes and ketones - the carbonyl oxygen abstracts a hydrogen five atoms away to give an alkene and a new radical cation. Molecules at even m/z will give a fragment at even m/z due to McLafferty rearrangement b/c it involves cleavage of 2 bonds. Results in enol of acetone (C=COHOH)
How do you distinguish between an aldehyde and ketone on a mass spectrum?
α-cleavage of an aldehyde proton will give an often quite distinct M-1 peak, a ketone will not have this.
What are the most common fragmentation patterns of a carboxylic acid?
α-cleavage of the carboxyl group [COOH]+, and McLafferty rearrangement.
What is the m/z ratio resulting from α-cleavage of a carboxylic acid?
45 - [COOH]+
What are the common fragmentation patterns observed on the mass spectrum of esters and amides?
α-cleavage, and McLafferty rearrangement
What m/z is generally observed for α-cleavage of an ester?
59 [O=C-O-CH3]+
What are the characteristics of an aromatic hydrocarbon mass spectrum?
Strong molecular ion peak, and most alkylbenzenes fragment to give the tropylium cation.
What peak is observed for most alkylbenzenes?
m/z 92 - due to fragmentation to give the tropylium cation.
What characteristics are common on the mass spectrum of amines?
Usually a weak molecular ion peak for aliphatic amines (though valuable if present: if the m/z is odd, there is an odd # of nitrogens in the compound), and an even m/z resulting from β-cleavage.
What fragmentation pattern is commonly observed in the mass spectrum of amines.
β-cleavage of the C-C bond with the largest R group around the nitrogen.
What peak is commonly seen on the mass spec of amines?
m/z 30 from all amines (primary, secondary, tertiary) due to complex rearrangement and fragmentation.
What fragment corresponds to peak at m/z 57?
tert-butyl cation
What fragment corresponds to a peak at m/z 43?
propyl/isopropyl cation
What fragment corresponds to a peak at m/z 41?
allyl cation [H2C=CHCH2+]
What fragment corresponds to a peak at m/z 45?
Carboxyl cation {COOH]+
What fragment corresponds with a peak at m/z 60?
McLafferty rearrangment of carboxylic acid [C=COHOH] - becomes enol of acetone.
What does an M-31 peak indicate?
Loss of a methyl ester radical (OCH3 radical)
What fragment is indicated by a peak at m/z 30?
A primary amine most likely (can be secondary or tertiary). [H2C=NH2 - resonance stabilized]
What would cause an M-17 for a carboxylic acid?
Loss of -OH; fairly common, strong peak
