. Mass spectrometry is a powerful analytical technique which has gained consider
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Question
. Mass spectrometry is a powerful analytical technique which has gained considerable popularity in all fields of science. The technique is based primarily on measurements of mass to charge ratio of analyte ions. The production, processing, and analysis of the ions are the key to obtaining accurate analytical information. (a) Discuss three methods that are commonly used to produce molecular ions. In each case, indicate weaknesses and strengths of the method, (10 points) b) There are four primary mass spectrometry configurations in use today: (i) magnetic sector, (ii) double focusing, (iii) quadrupole, and (iv) ion trap. Discuss these configurations, in each case giving the strengths and weaknesses. (10 points)Explanation / Answer
a)
Electron Ionization (EI)
Also referred to as electron impact ionization, this is the oldest and best-characterized of all the
ionization methods. A beam of electrons passes through the gas-phase sample. An electron that
collides with a neutral analyte molecule can knock off another electron, resulting in a positively
charged ion. The ionization process can either produce a molecular ion which will have the same
molecular weight and elemental composition of the starting analyte, or it can produce a fragment
ion which corresponds to a smaller piece of the analyte molecule.
The ionization potential is the electron energy that will produce a molecular ion. The appearance
potential for a given fragment ion is the electron energy that will produce that fragment ion.
Most mass spectrometers use electrons with an energy of 70 electron volts (eV) for EI. Decreasing
the electron energy can reduce fragmentation, but it also reduces the number of ions formed.
Sample introduction
. heated batch inlet
. heated direct insertion probe
. gas chromatograph
. liquid chromatograph (particle-beam interface)
Benefits
. well-understood
. can be applied to virtually all volatile compounds
. reproducible mass spectra
. fragmentation provides structural information
. libraries of mass spectra can be searched for EI mass spectral "fingerprint"
Limitations
. sample must be thermally volatile and stable
. the molecular ion may be weak or absent for many compounds.
Mass range
. Low Typically less than 1,000 Da.
2 Chemical Ionization (CI)
Summary
Chemical ionization uses ion-molecule reactions to produce ions from the analyte. The chemical
ionization process begins when a reagent gas such as methane, isobutane, or ammonia is ionized
by electron impact. A high reagent gas pressure (or long reaction time) results in ion-molecule
reactions between the reagent gas ions and reagent gas neutrals. Some of the products of these
ion-molecule reactions can react with the analyte molecules to produce analyte ions.
Example (R = reagent, S = sample, e = electron, . = radical electron , H = hydrogen):
R + e ---> R+. + 2e
R+. + RH ---> RH+ + R.
RH+ + S ---> SH+ + R
(of course, other reactions can occur)
Sample introduction
. heated batch inlet
. heated direct insertion probe
. gas chromatograph
. liquid chromatograph (particle-beam interface)
Benefits
. often gives molecular weight information through molecular-like ions such as [M+H]+,
even when EI would not produce a molecular ion.
. simple mass spectra, fragmentation reduced compared to EI
Limitations
. sample must be thermally volatile and stable
. less fragmentation than EI, fragment pattern not informative or reproducible enough for
library search
. results depend on reagent gas type, reagent gas pressure or reaction time, and nature of
sample.
Mass range
. Low Typically less than 1,000 Da
Desorption Chemical Ionization (DCI)
Summary
This is a variation on chemical ionization in which the analyte is placed on a filament that is rapidly
heated in the CI plasma. The direct exposure to the CI reagent ions, combined with the rapid
heating acts to reduce fragmentation. Some samples that cannot be thermally desorbed without
decomposition can be characterized by the fragments produced by pyrolysis DCI.
Sample introduction
. sample deposited onto a filament wire
. filament rapidly heated inside the CI source.
Benefits
. reduced thermal decomposition
. rapid analysis
. relatively simple equipment
Limitations
. not particularly reproducible
. rapid heating requires fast scan speeds
. fails for large or labile compounds
Mass range
Low Typically less than 1,500 Da.
b)
Magnetic Sector Mass Analyzer
Principal of operation
The analogy between scanning mass spectrometry and scanning optical spectroscopy is most apparent
for magnetic sector mass spectrometers.
In a magnetic deflection mass spectrometer, ions leaving the ion source are accelerated to a high
velocity. The ions then pass through a magnetic sector in which the magnetic field is applied in
a direction perpendicular to the direction of ion motion. From physics, we know that when acceleration
is applied perpendicular to the direction of motion of an object, the object's velocity remains
constant, but the object travels in a circular path. Therefore, the magnetic sector follows an
arc; the radius and angle of the arc vary with different ion optical designs.
A magnetic sector alone will separate ions according to their mass-to-charge ratio. However, the
resolution will be limited by the fact that ions leaving the ion source do not all have exactly the
same energy and therefore do not have exactly the same velocity. This is analogous to the chromatic
aberration in optical spectroscopy. To achieve better resolution, it is necessary to add an
electric sector that focuses ions according to their kinetic energy. Like the magnetic sector, the
electric sector applies a force perpendicular to the direction of ion motion, and therefore has the
form of an arc.
Benefits
Double focusing magnetic sector mass analyzers are the "classical" model against which other
mass analyzers are compared.
. Classical mass spectra
. Very high reproducibility
. Best quantitative performance of all mass spectrometer analyzers
. High resolution
. High sensitivity
. High dynamic range
. Linked scan MS/MS does not require another analyzer
. High-energy CID MS/MS spectra are very reproducible
Limitations
. Not well-suited for pulsed ionization methods (e.g. MALDI)
. Usually larger and higher cost than other mass analyzers
. Linked scan MS/MS gives either limited precursor selectivity with unit product-ion
resolution, or unit precursor selection with poor product-ion resolution
Applications
. All organic MS analysis methods
. Accurate mass measurements
. Quantitation
. Isotope ratio measurements
double-focusing mass spectrometer
A mass spectrometer which has magnetic and electric analysers combined in a specified geometrical configuration in order to ac-complish both direction and velocity focusing of an ion beam from an ion source. DFMS provides a higher resolution
and more accurate mass measurements of ions.
Electrostatic sector analyzers are energy focusers, where an ion beam is focused for energy. Electrostatic and magnetic sector analyzers when employed individually are single focusing instruments. However when both techniques are used together, it is called a double focusing instrument., because in this instrument both the energies and the angular dispersions are focused.
Quadrupole Ion Traps
Principal of Operation
Ions are dynamically stored in a three-dimensional quadrupole ion storage device. The RF and
DC potentials can be scanned to eject successive mass-to-charge ratios from the trap into the detector
(mass-selective ejection)
Benefits
. High sensitivity
. Multi-stage mass spectrometry (analogous to FTICR experiments)
. Compact mass analyzer
Limitations
. Poor quantitation
. Very poor dynamic range (can sometimes be compensated for by using auto-ranging)
. Subject to space charge effects and ion molecule reactions
. Collision energy not well-defined in CID MS/MS
. Many parameters (excitation, trapping, detection conditions) comprise the experiment
sequence that defines the quality of the mass spectrum
Applications
. Benchtop GC/MS, LC/MS and MS/MS systems
. Target compound screening
. Ion chemistry
Trapped-Ion Mass Analyzers
There are two principal trapped-ion mass analyzers: three-dimensional quadrupole ion traps
("dynamic" traps), and ion cyclotron resonance mass spectrometers ("static" traps). Both operate
by storing ions in the trap and manipulating the ions by using DC and RF electric fields in a series
of carefully timed events. This provides some unique capabilities, such as extended MS/MS
experiments, very high resolution, and high sensitivity. The tradeoff is that trapping the ions for
long periods of time (milliseconds to days) provides plenty of time for the ions fall apart spontaneously
(unimolecular decomposition), to experience undesirable interactions with other ions
(space charge effects), neutral molecules (ion-molecule reactions), or perturbations in the ion
motion due to imperfect electric fields. This can lead to artifacts and unexpected changes in the
mass spectrum (so called "non-classical mass spectra").
Ion Cyclotron Resonance
Principal of Operation
Ions move in a circular path in a magnetic field. The cyclotron frequency of the ion's circular motion
is mass-dependent. By measuring the cyclotron frequency, one can determine an ion's mass.
The working equation for ICR can be quickly derived by equating the centripetal force (mv**2/r)
and the Lorentz force evB experienced by an ion in a magnetic field:
Benefits
. The highest recorded mass resolution of all mass spectrometers
. Powerful capabilities for ion chemistry and MS/MS experiments
. Well-suited for use with pulsed ionization methods such as MALDI
. Non-destructive ion detection; ion remeasurement
. Stable mass calibration in superconducting magnet FTICR systems
Limitations
. Limited dynamic range
. Strict low-pressure requirements mandate an external source for most analytical applications
. Subject to space charge effects and ion molecule reactions
. Artifacts such as harmonics and sidebands are present in the mass spectra
. Many parameters (excitation, trapping, detection conditions) comprise the experiment
sequence that defines the quality of the mass spectrum
. Generally low-energy CID, spectrum depends on collision energy, collision gas, and
other parameters
Applications
. Ion chemistry
. High-resolution MALDI and electrospray experiments for high-mass analytes
. Laser desorption for materials and surface characterization
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