CHM1006
Mass Spec
Lecture 1
Functions of a mass spectrometer
- It is essentially a molecular top-pan balance - it weighs molecules
- Generates ions from samples introduced directly or from a GC or HPLC
- Separates ions according to their mass-to-charge rations, m/z. Often, z = 1, but you can
have multiply charged ions
- Collects ions at each m/z and records relative abundances
- A data system can be used to process the recorded data
- Plots normalised relative abundance against m/z - this is the mass spectrum
Normalised - divided all the data by 1 (that value is 100%) and others are relative to that peak
Essentials of a mass spectrometer
● Sample inlet source
● Ion source
● Mass analyser
● Detector
● Vacuum generator
● Software to control instrument and t do data processing
● On-line or off-line database search algorithms
,Advantages of mass spectrometry
Near universal applicability - almost all substances give a mass spectrum
Selectivity - high resolving power allows selection of 1 component from a complex mixture
Specificity - exact molecular weight is often specific to the compound under investigation;
observation of a chosen fragmentation in MS often identifies a given component in a mixture
Sensitivity - detection levels as low as 1 femtomole (10-15 moles). Complete structure from less
than 100 femtomoles
Speed
Limitations of mass spectrometry
Can sometimes have difficulty in distinguishing isomers from each other
It can distinguish isobaric compounds, only if the resolving power is sufficiently high
May decompose or isomerise compounds during the ionisation process. Secondary or higher
structure observed in solution may be lost during ionisation process
Usually needs very, very, very pure samples - so sample preparation is the key to obtaining a
good spectra
Detailed mechanism of ionisation and fragmentation processes are not fully understood;
sometimes difficult to predict the mass spectrum of a particular molecule from first principles
Getting detailed structure information from the spectra requires a solid understanding of
fragmentation mechanisms
Quantitation requires use of of an internal standard such as deuterated analog
Isobaric compounds - same m/z ratio (ie. N2 CO)
, Mass
Mass - 1/12th of the mass of the carbon 12C isotope as carbon. This is 1 Dalton (Da)
Mass = ∑me x ne
me = mass
ne = abundance
Isotope distributions
Monoisotopic ‘A’ elements: F, P, Ce, Na, I
‘A + 1’ elements: C, N, H
‘A + 2’ elements: O, Cl, Br, Si, S
How isotopic distributions change with mass
Molecular mass and other things
Nominal mass: me is the integer mass value for the most abundant isotope (H = 1, etc)
Monoisotopic mass: me is the exact mass value for the most abundant isotope (H = 1.00782510,
etc)
Average mass: mw is the chemical (average) atomic mass value (H = 1.00974, etc)
Mass Spec
Lecture 1
Functions of a mass spectrometer
- It is essentially a molecular top-pan balance - it weighs molecules
- Generates ions from samples introduced directly or from a GC or HPLC
- Separates ions according to their mass-to-charge rations, m/z. Often, z = 1, but you can
have multiply charged ions
- Collects ions at each m/z and records relative abundances
- A data system can be used to process the recorded data
- Plots normalised relative abundance against m/z - this is the mass spectrum
Normalised - divided all the data by 1 (that value is 100%) and others are relative to that peak
Essentials of a mass spectrometer
● Sample inlet source
● Ion source
● Mass analyser
● Detector
● Vacuum generator
● Software to control instrument and t do data processing
● On-line or off-line database search algorithms
,Advantages of mass spectrometry
Near universal applicability - almost all substances give a mass spectrum
Selectivity - high resolving power allows selection of 1 component from a complex mixture
Specificity - exact molecular weight is often specific to the compound under investigation;
observation of a chosen fragmentation in MS often identifies a given component in a mixture
Sensitivity - detection levels as low as 1 femtomole (10-15 moles). Complete structure from less
than 100 femtomoles
Speed
Limitations of mass spectrometry
Can sometimes have difficulty in distinguishing isomers from each other
It can distinguish isobaric compounds, only if the resolving power is sufficiently high
May decompose or isomerise compounds during the ionisation process. Secondary or higher
structure observed in solution may be lost during ionisation process
Usually needs very, very, very pure samples - so sample preparation is the key to obtaining a
good spectra
Detailed mechanism of ionisation and fragmentation processes are not fully understood;
sometimes difficult to predict the mass spectrum of a particular molecule from first principles
Getting detailed structure information from the spectra requires a solid understanding of
fragmentation mechanisms
Quantitation requires use of of an internal standard such as deuterated analog
Isobaric compounds - same m/z ratio (ie. N2 CO)
, Mass
Mass - 1/12th of the mass of the carbon 12C isotope as carbon. This is 1 Dalton (Da)
Mass = ∑me x ne
me = mass
ne = abundance
Isotope distributions
Monoisotopic ‘A’ elements: F, P, Ce, Na, I
‘A + 1’ elements: C, N, H
‘A + 2’ elements: O, Cl, Br, Si, S
How isotopic distributions change with mass
Molecular mass and other things
Nominal mass: me is the integer mass value for the most abundant isotope (H = 1, etc)
Monoisotopic mass: me is the exact mass value for the most abundant isotope (H = 1.00782510,
etc)
Average mass: mw is the chemical (average) atomic mass value (H = 1.00974, etc)
