Chapters 11 & 20: Mass Spectrometry Flashcards
Overview of Mass Spectrometry
What is the analytical technique that produces, separates, and detects ions in the gas phase?
Mass Spectrometry
Overview of Mass Spectrometry
What analytical technique is powerful and widely applicable available for both qualitative and quantitative analysis?
Mass Spectrometry
Overview of Mass Spectrometry
Mass spectrometry can determine what?
1) isotopic ratios of atoms in a sample
2) composition of mixture
3) elemental composition of samples
4) structures of inorganic, organic, and biological molecules
5) structure and composition of solid surface
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Components of a Typical Mass Spectrometer
What are the components of a typical mass spectrometer?
- The inlet system introduces a very small amount of sample (<micromole) into the
mass spectrometer. - The ion source convert the components of a sample into ions where the output is
a stream of positive or negative ions, which are accelerated into the mass analyzer. - The mass analyzer separates ions based on their mass-to-charge (m/z) ratios.
- The detector converts beams of ions into an electrical signal.
- The Vacuum system is required to create low pressures to minimize collisions of
ions with atmospheric particles (increase mean free path of the ions).
IIMDV
Components: Inlet Systems
What is the purpose of the inlet system?
The purpose of the inlet systems is to permit theintroduction of a representative sample into the ion source with minimal loss of vacuum.
Components: Inlet Systems
Most modern instruments have several types of inlets including?
- batch inlets
- direct probe inlets
- flow, chromatographic, capillary electrophoretic inlets
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Inlet Systems: Batch Inlet Systems
Give the details of batch inlet systems.
- Simplest and most conventional inlet system.
- Used for volatile liquid (up to 500°C) and gaseous samples.
- Sample is volatized externally in the batch inlet system and subsequently allowed to leak
through a metal/glass pinhole into the evacuated ionization region.
Inlet Systems: Batch Inlet Systems
What are the limitations of batch inlet systems?
Not useful for thermally unstable compounds and samples must be a gas when
entering the ionization region.
Inlet Systems: Direct Probe Inlet
Give the details of direct probe inlet.
- Solid and nonvolatile liquids are introduced into the ionization zone via a sample
holder/probe. - Lock system is designed to minimize the volume of air that must be pumped from the
system after insertion of the probe. - The sample is typically held by a glass or aluminum capillary tube, fine wire, or cap probe that is positioned within a few millimeters of the ionization source.
Inlet Systems: Direct Probe Inlet
What are the benefits of direct probe inlets?
Excels with thermally unstable compounds.
Ion Sources Classification
Ion sources are classified as being what?
Hard or soft sources
Ion Sources are Also Classified as Being HARD or SOFT Sources
Hard sources do what?
Hard sources impart enough energy to analyte molecules to leave them in highly excited energy state. Relaxation of the molecules results in rupture of the bonds,
producing fragment ions that have mass to charge ratios less than that of the molecular ion.
Ion Sources are Also Classified as Being HARD or SOFT Sources
Hard sources provide what types of information?
Hard sources provide information
about functional groups and
structural information.
Ion Sources are Also Classified as Being HARD or SOFT Sources
Limitations of hard sources.
Hard sources are not utilized for
mixtures.
Ion Sources are Also Classified as Being HARD or SOFT Sources
Provide details on soft sources.
Soft sources cause little fragmentation and often consist of the molecular ion.
Ion Sources are Also Classified as Being HARD or SOFT Sources
Soft sources provide what types of information?
Soft sources provide information
about the molecular mass of the
analyte.
Ion Sources are Also Classified as Being HARD or SOFT Sources
What are the benefits of soft sources?
Soft sources are useful for
mixture analysis.
Ion Sources: Electron-Impact Source
Provide details on electron-impact source.
- Sample is brought to a temperature to produce a molecular vapor, which is then ionized by bombarding the resulting molecules with a beam of energetic electrons.
- Electrostatic repulsions of the electron beam impacting the analyte molecule (M) produces
electrons and the molecular ion: M + e - → M *+ + 2e - - Ions produced are accelerated to the mass analyzer by applying a potential to a series of
accelerating plates.
Ion Sources: Electron-Impact Source
Limitations of electronn-impact source.
Only one molecule in a million undergoes this reaction.
Ion Sources: Electron-Impact Source
What are the types of ions peaks produced by electron impact sources?
- Isotope peaks have the same chemical formula but different isotope compositions. The relative intensities of the peaks correspond to the natural abundance of the isotopes.
- Base peaks are ion fragments of the analyte.
- Molecular ions (M + ) corresponds to the molar mass of the analyte.
Ion Sources: Electron-Impact Source
What are the advantages of electron-impact sources?
- Convenient to use
- Produce high ions currents (high
sensitivity) - Extensive fragmentation enables
identification of analytes
Ion Sources: Electron-Impact Source
What are the disadvantages of electron-impact sources?
- Disappearance of molecular ion
peak so molar mass determination
is challenging - Sample needs to be volatized
(thermal decomposition) - Only useful for molar masses
smaller than 10^3 Da
Ion Sources: Chemical Ionization Sources
Provide details of chemical ionization sources
Most modern instruments can carry out both electron
impact and chemical ionization interchangeably.
Chemical ionization is softer than electron impact.
Ion Sources: Chemical Ionization Sources
Explain how chemical ionization sources work?
Gaseous atoms of the sample are ionized by collision with ions produced by electron bombardment of an excess of a reagent gas. Reagent gas is added to the reaction region of an electron impact to produce an ion that react with
the analyte.
Ion Sources: Chemical Ionization Sources
How do chemical ionization sources compare to electron impact?
Chemical ionization results in less fragmentation and is useful for less stable compounds.
Ion Sources: Chemical Ionization Sources
Provide details on the most common reagent for chemical ionization sources.
- One of the most common reagents is methane (CH4 ), where CH4 reacts with high-energy electrons to produce several ions including CH4+ , CH3+ , and CH2+ where ~90% of the reaction products are CH4+ and CH3+ ions.
- Collisions between the analyte molecule (M) and CH5+ and C 2 H5+ are highly reactive and involve proton or hydride transfer to create ions.
Ion Sources: Matrix Assisted Laser Desorption-Ionization (MALDI)
What does MALDI enable mass spectra to do?
Enables mass spectra to obtained for
thermally delicate biochemical species and species with masses greater than 100,000 Da.
Ion Sources: Matrix Assisted Laser Desorption-Ionization (MALDI)
How does MALDI work?
Low concentration of the analyte
(micromolar) is dispersed in a solid or liquid matrix. Laser beam is focused onto the sample where the matrix absorbs the laser radiation and ionizes the matrix and the analyte producing an ion plume.
Ion Sources: Matrix Assisted Laser Desorption-Ionization (MALDI)
Provide details on MALDI.
- Time of flight mass analyzers are typically used with MALDI
- Mechanism for the formation of the ion plume is not fully understood.
Ion Sources: Matrix Assisted Laser Desorption-Ionization (MALDI)
What are the disadvantages of MALDI?
MALDI is not quantitative where ionization depends on the sample and
matrix used.
Ion Sources: Electrospray Ionization
Provide details on electrospray ionization.
- One of the most important techniques for analyzing biomolecules, inorganic species, and polymers having molecular weights of 100,000 Da or more.
- Takes place under atmospheric pressures and temperatures.
- Little fragmentation of large and thermally fragile biomolecules.
- Typically coupled to a quadrupole mass analyzers because ions are multiply charged.
- Readily adapted to direct sample introduction from HPLC and electrophoresis
- Tandem mass spectrometry enables the ions from the ionization process to be separated and subjected to fragmentation before being mass analyzed. Discussed Later!!!
Components of a Typical Mass Spectrometer: Mass Analyzer
How do mass analyzers seperate ions?
Mass analyzer separate ions based on their mass-to-charge (m/z) ratios.
Components of a Typical Mass Spectrometer: Mass Analyzer
What should mass analyzers be capable of?
Ideally, mass analyzers should be capable of:
- distinguishing minute mass differences
- allow passage of sufficient number of ions to yield readily measurable on currents
But these two properties are not entirely compatible!!!
Resolution of Mass Spectrometers
What is resolution?
The capability of a mass spectrometer to differentiate between masses
𝐑 = 𝐦/∆𝐦
where, Δm is the mass difference between two adjacent peaks and m is the nominal mass of the first peak (the mean mass of the two peaks can be used instead). The two peaks are considered to be separated if the height of the valley between them is no more than a given fraction of their height (usually 10%)
Mass Analyzers: Magnetic Sector Analyzers
How do magnetic sector analyzers work?
- Magnetic sector analyzers employ a permanent magnet or electromagnet to cause the beam from an ion source to travel in a circular path (typically 180°, 90°, 60°)
- Ions of different mass can be scanned across the exit slit by varying the field strength of the magnet or the accelerating potential between slits A and B.
(Resolution (R) of magnetic
sector analyzers is 2000)
Mass Analyzers: Magnetic Sector Analyzers
The kinetic energy (KE) of an ion of a mass (m, kg) bearing a charge, z, is equal to:
𝐾𝐸 = z𝑒𝑉 = (1/2) 𝑚v^2
where V is the voltage difference between electrodes A and B, ν is the velocity (m/s) of the ion after acceleration, and e is the electronic charge ( 1.60 × 10-19 C)
Learn Slides 23 - 25 of PowerPoint on your own
Learn Slides 23 - 25 of PowerPoint on your own
Mass Analyzers: Single-Focusing Analyzer Versus
Double-Focusing Analyzer
Magnetic sector analyzers are sometimes called what?
single-focusing analyzers
Mass Analyzers: Single-Focusing Analyzer Versus
How does a single-focusing analyzers work?
- When ions exit an ionization source that have the same mass-to-charge ratios they will a diverging directional distribution. In a magnetic sector analyzer, the magnetic field brings the ions with different direction orientations to focus as they leave the magnetic sector analyzer.
- These distribution of translation energies, Boltzman’s distribution imperfections in the magnetic filed, limits the resolution of a magnetic section analyzer (R ≤ 2000)
Mass Analyzers: Single-Focusing Analyzer Versus
What does a double-focusing analyzers account for and why does it do that?
- To achieve higher resolution spectra, one must correct for the directional distribution and the energy distribution of ions leaving the source.
- Double-focusing analyzers account for both the directional and energy distributions.
Mass Analyzers: Double-Focusing Analyzers
Double-focusing analyzers have two components an electrostatic analyzer (ESA) and a magnetic sector analyzer. What do each account for?
- The ESA accounts for energy distribution where ions with energies greater than the average strike the upper side of the ESA and ions with energies less
than average strike the lower side of the ESA. - The magnetic sector accounts for the directional distribution.
Resolution is increased to 10^5
Mass Analyzers: Quadrupole Mass Filter
Provide details about quadrupole mass filters.
- The most common type of mass analyzer because of its size, cost ($10,000), ruggedness, real-time scan rates (100 ms) compared to other type of mass
spectrometers. - The resolution (R) of a quadrupole mass analyzer is 3000 - 4000.
- Apply both a AC and DC potential to select a specific mass-to-charge ratio.
Mass Analyzers: Quadropole Mass Filter
Learns Slides 29 & 30 from PowerPoint on your own.
Learns Slides 29 & 30 from PowerPoint on your own.
Mass Analyzers: Quadropole Mass Filter
Provide extra details about quadrupole mass filters.
- Combining the two poles produces a notch mass-to-charge filter
- Scanning the quadrupole for the various mass to charge rations involves simultaneously increasing the AC and DC voltage.
Mass Analyzers: Ion Trap
Explain and provide details on ion traps?
- Ions can be formed and confined for extended period of times (15 min) by having ions with the appropriate mass-to-charge ratio orbit within the cavity.
- Increasing in the AC potential, heavier ions become more stable and small ions destabilize.
- Scan AC potential to eject ions of increasing mass-to-charge ratios for detection
- Can be coupled with other techniques (MS-MS)
- The resolution (R) of an ion trap mass analyzer is 500 to 1000
Mass Analyzers: Time of Flight (TOF)
How does time of flight work?
- Ions pulsed into the mass analyzer and accelerated down the field-free drift tube by a set of electrode at 10 3 to 10 4 V.
- Separation of the ions is based how long it takes for ions with the same kinetic energy to travel down the tube to the detector. The ion’s velocity is inversely related to their mass. Lighter particles arrive earlier than the heavier ones.
Mass Analyzers: Time of Flight (TOF)
What are the advantages of TOF?
simplicity, unlimited mass range, all ions are detected (high sensitivity).
- The resolution (R) of a time of flight mass analyzer is 10 4
Mass Analyzers: Fourier Transform Spectrometers
Explain how Fourier Transform Spectrometers work and provide additional details as well.
- Use an ion trap within which ions circulate
in a well-defined orbits for extended periods. - A short radio pulse that increases in
frequency (0.070 to 3.6 MHz) and record the
response of the ions. Convert between
frequency domain to mass domain - High resolution (R): 10 6
- Expensive instrumentations >$400,000
Components of a Typical Mass Spectrometer: Electron Multipliers
How common are electron multipliers in routine experiments
They are the most common.
Components of a Typical Mass Spectrometer: Electron Multipliers
What are the two types of electron multipliers?
Discrete-dynode and continuous dynode
Components of a Typical Mass Spectrometer: Electron Multipliers
Electron multipliers are:
capable of providing high-current gains
rugged
nanosecond response times
reliable
compatible with a variety of mass analyzer
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Detector: Electron Multipliers, Discrete-Dynode
Provide details on Discrete-Dynodes.
- Used to collect and convert positive ions into an electrical signals
- Analogous to a UV/Vis PMT where each dynode is held at successively higher voltages
Detector: Electron Multipliers, Discrete-Dynode
How do Discrete-Dynodes work?
When ions strike the Cu-Be surfaces of the cathode, electrons are emitted and attracted the next cathode down the chain until the last dynode where a huge number of electrons appear for every ion that strikes the initial cathode. Up to 20 dynodes can be used.
- 1 ion can produce 10^7 electrons at the last dynode
Detector: Electron Multipliers, Continuous Dynode
Provide details on Continuous Dynodes?
- Used to collect and convert positive ions into an electrical signals
- Shaped like a cornucopia and is made of glass doped with lead to give the material a small conductivity.
Detector: Electron Multipliers, Continuous Dynode
How do Continuous Dynodes work?
A voltage (1.8 to 2 kV) is applied across the length of transducer producing a voltage
gradient from one end to the other.
Ion that strike the surface near the entrance eject electrons which are attracted to higher-
voltage points further in the cornucopia.
- 1 ion can produce 10^5 to 10^7 electrons.
Detector: Array Transduces, Microchannel Plates
What are Microchannel Plates?
Tiny tubes of lead glass with metal electrodes deposited on both side. Voltage of 1500 V is applied between the ends of the channels.
Detector: Array Transduces, Microchannel Plates
How do Microchannel Plates work?
- Each tube acts an electron multiplier, where 1 ion can produce 10^3 electrons
- A phosphorescent screen is placed behind the microchannel plate so that the cascade of the electrons produces a flash of light which is collected by an optical array.
- Provides 2D resolution of ions at the focal plane.
Tandem Mass Spectrometry
What is tandem mass spectrometry?
Mass spectrometry of preselected and fragmented ions.
Tandem Mass Spectrometry
How does tandem mass spectrometry work?
A soft ionization source produces ions and some fragments and are inputted into the first mass analyzer, which selects the precursor ions. In the interaction cell, the precursor ion can decompose, interact with a laser or gas molecule to produce fragments called product ions. These ions are detected by the mass analyzer.
