Download 29 A Principles of mass spectrometry

Chapter 29
Mass Spectrometry
29 A Principles of mass spectrometry
In the mass spectrometer, analyte molecules are converted to ions by applying
energy
to them.
The ions formed are separated on the basis of their mass-to-charge ratio (m/z)
and directed to a transducer that converts the number of ions (abundance) into
an
electrical signal.
The ion abundance plotted against mass-to-charge ratio or just mass for singly
charged ions is called a mass spectrum.
Atomic and molecular masses are usually expressed in terms of the atomic
mass
scale, based on a specific isotope of carbon.
One unified atomic mass (u) unit on this scale is equal to 1/12 the mass of a
neutral 126C atom.
One unified mass unit is commonly termed one dalton (Da).
The chemical atomic mass, or the average atomic mass, of an element in
nature is
given by summing the exact masses of each isotope weighted by its
fractional abundance in nature.
The mass number is the atomic or molecular mass expressed without units.
The mass-to-charge ratio of an ion is the unitless ratio of its mass number to
the number of fundamental charges z on the ion.
29 B Mass spectrometers
The mass spectrometer is an instrument that produces ions, separates them
according to their m/z values, detects them, and plots the mass spectrum.
Resolution of Mass Spectrometers
The capability of a mass spectrometer to differentiate between masses is usually state
in terms of its resolution, R, which is defined as
R = m/m
where Δm is the mass difference between two adjacent peaks that are just resolved
and m is the nominal mass of the first peak.
In the magnetic sector analyzer, separation is based on the deflection of ions in
a magnetic field.
The trajectories that ions take depend on their m/z values.
In the double-focusing mass spectrometer, an electric sector precedes the
magnetic sector.
The electrostatic field serves to focus a beam of ions having only a narrow range of
kinetic energies onto a slit that leads to the magnetic sector.
Such instruments are capable of very high resolution.
Quadrupole analyzers are mass filters that only allow ions of a certain mass-tocharge ratio to pass.
Ion motion in electric fields is the basis of separation.
Time-of-Flight Mass Analyzers
The time-of-flight (TOF) mass spectrometer represents another approach
to mass analysis.
In a TOF analyzer, a packet of ions with nearly identical kinetic energies is
rapidly sampled, and the ions enter a field-free region.
v
2 KE
m
The time required for the ions to travel a fixed distance to the detector is
inversely related to the ion mass.
Transducers for mass spectrometry
In the discrete-dynode electron multiplier, when energetic ions or electrons
strike a Cu-Be cathode, secondary electrons are emitted.
These electrons are attracted to dynodes that are each held at a successively higher
positive voltage.
29 C Atomic mass spectrometry
Inductively coupled plasma mass spectrometry (ICPMS) is a widely
used technique for the simultaneous determination of over 70
elements.
Sources for Atomic mass Spectrometry
Other Ionization Sources for Atomic Mass Spectrometry
Spark source mass spectrometry is applied to solid samples that are not
easily dissolved and analyzed by ICP.
They are used in conjunction with ICP sources to volatilize and atomize solid
samples before introduction to the plasma.
Atomic mass Spectra and Interferences
Interference effects in atomic mass spectroscopy can be categorized as:
spectroscopic interferences and matrix interferences
Spectroscopic interferences occur when an ionic species in the plasma
has the same m/z value as an analyte ion.
Matrix effects become noticeable when the concentrations of matrix
species exceed about 500 to 1000 µg/mL.
These effects cause a reduction in the analyte signal, although
enhancements are sometimes observed.
Applications of Atomic mass Spectrometry
ICPMS is well suited for multielement analysis and for determinations such as
isotope ratios.
It is used in the semiconductor and electronics industry, in geochemistry, in
environmental analyses, in biological and medical research, and in many other
areas.
29 D Molecular mass spectrometry
Mass spectrometry is applied to the determination of the structure of
polypeptides, proteins, and other high-molecular-mass biopolymers. Its
applications include:
Molecular mass spectra
Collisions between energetic electrons and analyte molecules leaves them in an
excited state.
The positive ions produced are attracted through the slit of a mass spectrometer,
where they are sorted according to their mass-to-charge ratios.
Ion Sources
The appearance of mass spectra for a given molecular species is highly
dependent on the method used for ion formation.
These methods fall into two major categories:
With a gas-phase source, the sample is first vaporized and then ionized.
With a desorption source, the sample in a solid or liquid state is converted
directly
into gaseous ions. Desorption sources are applicable to nonvolatile and
thermally unstable samples.
Molecular mass spectrometric instrumentation
Components of molecular mass spectrometers include:
The inlet system is used to introduce a representative sample to the ion
source with minimal loss of vacuum. They are classified as batch inlets,
direct probe inlets, chromatographic inlets, and electrophoretic inlets.
The conventional (and simplest) inlet system is the batch type in which the
sample (liquid or gas) is volatilized externally and then allowed to leak into
the evacuated ionization region.
Gas chromatography (is an ideal way to introduce mixtures because the
components are separated from the mixture by the chromatograph prior to
introduction to the mass spectrometer.
The combination of gas chromatography and mass spectrometry is often
called GC/MS.
Tandem mass spectrometry or mass spectrometry-mass spectrometry
(MS/MS) is a technique that allows the mass spectrum of a preselected or
fragmented ion to be obtained.
It can produce a variety of spectra.
Product-ion spectra are obtained by scanning mass analyzer 2 while
mass analyzer 1 is held constant to act as a mass selector for the precursor
ion.
A precursor-ion spectrum can be obtained by scanning mass analyzer 1
and selecting a given product ion with mass analyzer 2.
If both mass analyzers are scanned with a small offset in mass
between them, a neutral loss spectrum can be obtained, which can
be used to identify the m/z values of all ions losing a common
molecule, such as water.
A complete three-dimensional MS/MS spectrum can be acquired
by recording a product ion spectrum for each selected precursor ion,
that is, by scanning mass analyzer 2 for various settings of mass
analyzer 1.
Analysis of Mixtures
When two or more analytical techniques or instruments are combined to form a new,
more efficient device, the resulting methodology is often termed a hyphenated metho
Examples, Gas chromatography/mass spectrometry (GC/MS),
Mass spectrometry/liquid chromatography (LC/MS)
Tandem mass spectrometry is potentially more sensitive than either of the technique
The chemical noise associated with its use is generally smaller.
A disadvantage is the greater cost of the required equipment.