Mass Spectrometry
Mass Spectrometry (MS)
Purpose: We study molecules by inducing a charge on a molecule and its fragments
and studying the mass-to-charge ratios of generated ions.
Common Use: To study structural features through mass and stability through
fragment stability
Overview:
● How it works/theory (EI and how it fragments)
● What is m/z values
● Types of fragmentation
● Relative abundance (cation stability)
● Isotope stuff/M+#
● Rule of 13 + Nitrogen Rule (odd m/z)
● Briefly discuss other non-ionizing forms?
Theory
There are many techniques involved in Mass Spectrometry, but the one you’ll likely
encounter involves electron ionization (EI). Electron Ionization involves using a
high-power energy beam to remove one electron from a molecule to form a radical.
Radical species are generally less stable and will fragment into various different
smaller parts as a result. The net result is that this process also generates cations,
molecules with a positive charge. Equipment is then used to separate the various
fragments based on their mass-to-charge ratio before being sent to a detector to
be recorded
, Mass-to-charge Ratios: m/Z (x-axis)
As the name sounds like, this quantity represents how much a molecule weighs and
the charge associated with that molecule.
A mass spectrometry detector is set up to notice any positive charges. This means
that m/z ratios apply to all charged species and can be measured. Most fragments
will have a charge of +1, meaning m/z values effectively are just the mass of a
molecule or its fragments.
Since m/z values often just reflect the mass of a fragment, these fragments are
separated within the analyzer in order to properly record every fragment.
Relative Abundance (%) - Fragment Stability (y-axis)
When your molecule fragments, how can you predict which ones to expect to see
more frequently? We can make predictions based on how stable the positive
charge is. This generally follows your general carbocation rules with one important
note:
● More substituted carbocations are more stable
● In molecules with heteroatoms, the radical usually forms on the heteroatom
● If the positive charge can resonate, that will stabilize it as well
Mass Spectrometry (MS)
Purpose: We study molecules by inducing a charge on a molecule and its fragments
and studying the mass-to-charge ratios of generated ions.
Common Use: To study structural features through mass and stability through
fragment stability
Overview:
● How it works/theory (EI and how it fragments)
● What is m/z values
● Types of fragmentation
● Relative abundance (cation stability)
● Isotope stuff/M+#
● Rule of 13 + Nitrogen Rule (odd m/z)
● Briefly discuss other non-ionizing forms?
Theory
There are many techniques involved in Mass Spectrometry, but the one you’ll likely
encounter involves electron ionization (EI). Electron Ionization involves using a
high-power energy beam to remove one electron from a molecule to form a radical.
Radical species are generally less stable and will fragment into various different
smaller parts as a result. The net result is that this process also generates cations,
molecules with a positive charge. Equipment is then used to separate the various
fragments based on their mass-to-charge ratio before being sent to a detector to
be recorded
, Mass-to-charge Ratios: m/Z (x-axis)
As the name sounds like, this quantity represents how much a molecule weighs and
the charge associated with that molecule.
A mass spectrometry detector is set up to notice any positive charges. This means
that m/z ratios apply to all charged species and can be measured. Most fragments
will have a charge of +1, meaning m/z values effectively are just the mass of a
molecule or its fragments.
Since m/z values often just reflect the mass of a fragment, these fragments are
separated within the analyzer in order to properly record every fragment.
Relative Abundance (%) - Fragment Stability (y-axis)
When your molecule fragments, how can you predict which ones to expect to see
more frequently? We can make predictions based on how stable the positive
charge is. This generally follows your general carbocation rules with one important
note:
● More substituted carbocations are more stable
● In molecules with heteroatoms, the radical usually forms on the heteroatom
● If the positive charge can resonate, that will stabilize it as well
