Mass spectrometry (MS) is an analytical technique that produces spectra (singular spectrum) of the masses of the molecules comprising a sample of material. The spectra are used to determine the elemental composition of a sample, the masses of particles and of molecules, and to elucidate the chemical structures of molecules, such as peptides and otherchemical compounds. Mass spectrometry works by ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios. [1] In a typical MS procedure, a sample, which may be solid, liquid, or gas, is ionized. The ions are separated according to theirmass-to-charge ratio. [1] The ions are detected by a mechanism capable of detecting charged particles. Signal processing results are displayed as spectra of the relative abundance of ions as a function of the mass-to-charge ratio. The atoms or molecules can be identified by correlating known masses to the identified masses or through a characteristic fragmentation pattern. A mass spectrometer consists of three components: an ion source, a mass analyzer, and a detector. [2] The ionizer converts a portion of the sample into ions. There is a wide variety of ionization techniques, depending on the phase (solid, liquid, gas) of the sample and the efficiency of various ionization mechanisms for the unknown species. An extraction system removes ions from the sample, which are then trajected through the mass analyzer and onto the detector. The differences in masses of the fragments allows the mass analyzer to sort the ions by their mass-to-charge ratio. The detector measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present. Some detectors also give spatial information, e.g. a multichannel plate. Mass spectrometry has both qualitative and quantitative uses. These include identifying unknown compounds, determining theisotopic composition of elements in a molecule, and determining the structure of a compound by observing its fragmentation. Other uses include quantifying the amount of a compound in a sample or studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in a vacuum). MS is now in very common use in analytical laboratories that study physical, chemical, or biological properties of a great variety of compounds. As an analytical technique it possesses distinct advantages such as: 1. Increased sensitivity over most other analytical techniques because the analyzer, as a mass- charge filter, reduces background interference 2. Excellent specificity from characteristic fragmentation patterns to identify unknowns or confirm the presence of suspected compounds. 3. Information about molecular weight. 4. Information about the isotopic abundance of elements. 5. Temporally resolved chemical data. A few of the disadvantages of the method is that often fails to distinguish between optical and geometrical isomers and the positions of substituent in o-, m- and p- positions in an aromatic ring. Also, its scope is limited in identifying hydrocarbons that produce similar fragmented ions. [3]
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Mass spectrometry (MS) is an analytical technique that produces spectra (singular spectrum) of the masses
of the molecules comprising a sample of material. The spectra are used to determine the elemental
composition of a sample, the masses of particles and of molecules, and to elucidate the chemical structures of
molecules, such as peptides and otherchemical compounds. Mass spectrometry works by ionizing chemical
compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.[1]
In a typical MS procedure, a sample, which may be solid, liquid, or gas, is ionized. The ions are separated
according to theirmass-to-charge ratio.[1] The ions are detected by a mechanism capable of detecting charged
particles. Signal processing results are displayed as spectra of the relative abundance of ions as a function of
the mass-to-charge ratio. The atoms or molecules can be identified by correlating known masses to the
identified masses or through a characteristic fragmentation pattern.
A mass spectrometer consists of three components: an ion source, a mass analyzer, and a detector.[2] The ionizer converts a portion of the sample into ions. There is a wide variety of ionization techniques,
depending on the phase (solid, liquid, gas) of the sample and the efficiency of various ionization mechanisms
for the unknown species. An extraction system removes ions from the sample, which are then trajected through
the mass analyzer and onto the detector. The differences in masses of the fragments allows the mass analyzer
to sort the ions by their mass-to-charge ratio. The detector measures the value of an indicator quantity and thus
provides data for calculating the abundances of each ion present. Some detectors also give spatial information,
e.g. a multichannel plate.
Mass spectrometry has both qualitative and quantitative uses. These include identifying unknown compounds,
determining theisotopic composition of elements in a molecule, and determining the structure of a compound
by observing its fragmentation. Other uses include quantifying the amount of a compound in a sample or
studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in a vacuum). MS is
now in very common use in analytical laboratories that study physical, chemical, or biological properties of a
great variety of compounds.
As an analytical technique it possesses distinct advantages such as: 1. Increased sensitivity over most other
analytical techniques because the analyzer, as a mass-charge filter, reduces background interference 2.
Excellent specificity from characteristic fragmentation patterns to identify unknowns or confirm the presence of
suspected compounds. 3. Information about molecular weight. 4. Information about the isotopic abundance of
elements. 5. Temporally resolved chemical data.
A few of the disadvantages of the method is that often fails to distinguish between optical and geometrical
isomers and the positions of substituent in o-, m- and p- positions in an aromatic ring. Also, its scope is limited
in identifying hydrocarbons that produce similar fragmented ions.[3]
Mass spectrometry (MS) is an analytical technique that produces spectra (singular spectrum) of the masses
of the molecules comprising a sample of material. The spectra are used to determine the elemental
composition of a sample, the masses of particles and of molecules, and to elucidate the chemical structures of
molecules, such as peptides and otherchemical compounds. Mass spectrometry works by ionizing chemical
compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.[1]
In a typical MS procedure, a sample, which may be solid, liquid, or gas, is ionized. The ions are separated
according to theirmass-to-charge ratio.[1] The ions are detected by a mechanism capable of detecting charged
particles. Signal processing results are displayed as spectra of the relative abundance of ions as a function of
the mass-to-charge ratio. The atoms or molecules can be identified by correlating known masses to the
identified masses or through a characteristic fragmentation pattern.
A mass spectrometer consists of three components: an ion source, a mass analyzer, and a detector.[2] The ionizer converts a portion of the sample into ions. There is a wide variety of ionization techniques,
depending on the phase (solid, liquid, gas) of the sample and the efficiency of various ionization mechanisms
for the unknown species. An extraction system removes ions from the sample, which are then trajected through
the mass analyzer and onto the detector. The differences in masses of the fragments allows the mass analyzer
to sort the ions by their mass-to-charge ratio. The detector measures the value of an indicator quantity and thus
provides data for calculating the abundances of each ion present. Some detectors also give spatial information,
e.g. a multichannel plate.
Mass spectrometry has both qualitative and quantitative uses. These include identifying unknown compounds,
determining theisotopic composition of elements in a molecule, and determining the structure of a compound
by observing its fragmentation. Other uses include quantifying the amount of a compound in a sample or
studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in a vacuum). MS is
now in very common use in analytical laboratories that study physical, chemical, or biological properties of a
great variety of compounds.
As an analytical technique it possesses distinct advantages such as: 1. Increased sensitivity over most other
analytical techniques because the analyzer, as a mass-charge filter, reduces background interference 2.
Excellent specificity from characteristic fragmentation patterns to identify unknowns or confirm the presence of
suspected compounds. 3. Information about molecular weight. 4. Information about the isotopic abundance of
elements. 5. Temporally resolved chemical data.
A few of the disadvantages of the method is that often fails to distinguish between optical and geometrical
isomers and the positions of substituent in o-, m- and p- positions in an aromatic ring. Also, its scope is limited
in identifying hydrocarbons that produce similar fragmented ions.[3]
Figure: Components of a Mass Spectrometer
With all the above components, a mass spectrometer should always perform the following processes:
1. Produce ions from the sample in the ionization source.
2. Separate these ions according to their mass-to-charge ratio in the mass analyzer.
3. Eventually, fragment the selected ions and analyze the fragments in a second analyzer.
4. Detect the ions emerging from the last analyzer and measure their abundance with the detector that
converts the ions into electrical signals.
5. Process the signals from the detector that are transmitted to the computer and control the
instrument using feedback.
Analysis of Biomolecules using Mass Spectrometry
Mass spectrometry is fast becoming an indispensable field for analyzing biomolecules. Till the1970s, the only
analytical techniques which provided similar information were electrophoretic, chromatographic or
ultracentrifugation methods. The results were not absolute as they were based on characteristics other than
the molecular weight. Thus the only possibility of knowing the exact molecular weight of a macromolecule
remained its calculation based on its chemical structure.
The development of desorption ionization methods based on the emission of pre-existing ions such as
plasma desorption (PD), fast atom bombardment (FAB) or laser desorption (LD), allowed the application of
mass spectrometry for analyzing complex biomolecules.
Analysis of Glycans
Oligosaccharides are molecules formed by the association of several monosaccharides
linked through glycosidic bonds. The determination of the complete structure of oligosaccharides is more
complex than that of proteins or oligonucleotides. It involves the determination of additional components as
a consequence of the isomeric nature of monosaccharides and their capacity to form linear or branched
oligosaccharides. Knowing the structure of an oligosaccharide requires not only the determination of its
monosaccharide sequence and its branching pattern, but also the isomer position and the anomeric
configuration of each of its glycosidic bonds.
Advances in glycobiology involves a comprehensive study of structure, bio-synthesis, and biology of sugars
and saccharides. Mass spectrometry (MS) is emerging as an enabling technology in the field of glycomics
and glycobiology.
Analysis of Lipids
Lipids are made up of many classes of different molecules which are soluble in organic solvents. Lipidomics,
a major part of metabolomics, constitutes the detailed analysis and global characterization, both spatial and
temporal, of the structure and function of lipids (the lipidome) within a living system.
Many new strategies for mass-spectrometry-based analyses of lipids have been developed. The most popular
lipidomics methodologies involve electrospray ionization (ESI) sources and triple quadrupole analyzers. Using
mass spectrometry, it is possible to determine the molecular weight, elemental composition, the position of
branching and nature of substituents in the lipid structure.
Analysis of Proteins and Peptides
Proteins and peptides are linear polymers made up of combinations of the 20 amino acids linked by peptide
bonds. Proteins undergo several post translational modifications, extending the range of their function via
such modifications.
The term Proteomics refers to the analysis of complete protein content in a living system, including co- and
post-translationally modified proteins and alternatively spliced variants. Mass Spectrometry has now become
a crucial technique for almost all proteomics experiments. It allows precise determination of the molecular
mass of peptides as well as their sequences. This information can very well be used for protein identification,
de novo sequencing, and identification of post-translational modifications.
Analysis of Oligonucleotides
Oligonucleotides (DNA or RNA), are linear polymers of nucleotides. These are composed of a nitrogenous
base, a ribose sugar and a phosphate group. Oligonucleotides may undergo several natural covalent
modifications which are commonly present in tRNA and rRNA, or unnatural ones resulting from reactions with
exogenous compounds. Mass spectrometry plays an important role in identifying these modifications and
determining their structure as well as their position in the oligonucleotide. It not only allows determination of
the molecular weight of oligonucleotides, but also in a direct or indirect manner, the determination of their
sequences.
Software for Mass Spectrometric Data Analysis
SimGlycan® predicts the structure of glycans and glycopeptides from the MS/MS data acquired by mass
spectrometry, facilitating glycosylation and post translational modification studies. SimGlycan® accepts the
experimental MS profiles, of both glycopeptides and released glycans, matches them with its own database
and generates a list of probable structures. The software also supports multi stage mass spectrometry data
analysis which enables structural elucidation and identification of fragmentation pathways.
SimLipid is an innovative lipid characterization tool which enables structural elucidation of unknown lipids
using MS/MS data. The software analyzes lipid mass spectrometric data for characterizing and profiling lipids.
SimLipid can also annotate mass spectra with the lipid structures identified using abbreviations.
THE MASS SPECTROMETER
This page describes how a mass spectrum is produced using a mass spectrometer.
How a mass spectrometer works
The basic principle
If something is moving and you subject it to a sideways force, instead of moving in a straight line, it will move in a curve - deflected out of its original path by the sideways force.
Suppose you had a cannonball travelling past you and you wanted to deflect it as it went by you. All you've got is a jet of water from a hose-pipe that you can squirt at it. Frankly, its not going to make a lot of difference! Because the cannonball is so heavy, it will hardly be deflected at all from its original course.
But suppose instead, you tried to deflect a table tennis ball travelling at the same speed as the cannonball using the same jet of water. Because this ball is so light, you will get a huge deflection.
The amount of deflection you will get for a given sideways force depends on the mass of the ball. If you knew the speed of the ball and the size of the force, you could calculate the mass of the ball if you knew what sort of curved path it was deflected through. The less the deflection, the heavier the ball.
Note: I'm not suggesting that you personally would have to do the calculation, although the maths isn't actually very difficult - certainly no more than A'level standard!
You can apply exactly the same principle to atomic sized particles.
An outline of what happens in a mass spectrometer
Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't.
The sequence is :
Stage 1: Ionisation
The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form
ions at all (argon, for example). Mass spectrometers always work with positive ions.
Stage 2: Acceleration
The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection
The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.
The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.
Stage 4: Detection
The beam of ions passing through the machine is detected electrically.
A full diagram of a mass spectrometer
Understanding what's going on
The need for a vacuum
It's important that the ions produced in the ionisation chamber have a free run through the machine without hitting air molecules.
Ionisation
The vaporised sample passes into the ionisation chamber. The electrically heated metal coil gives off electrons which are attracted
to the electron trap which is a positively charged plate.
The particles in the sample (atoms or molecules) are therefore bombarded with a stream of electrons, and some of the collisions are energetic enough to knock one or more electrons out of the sample particles to make positive ions.
Most of the positive ions formed will carry a charge of +1 because it is much more difficult to remove further electrons from an already positive ion.
These positive ions are persuaded out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge.
Note: As you will see in a moment, the whole ionisation chamber is held at a positive voltage of about 10,000 volts. Where we are talking about the two plates having positive charges, these charges are in addition to that 10,000 volts.
Acceleration
The positive ions are repelled away from the very positive ionisation chamber and pass through three slits, the final one of which is at 0 volts. The middle slit carries some intermediate voltage. All the ions are accelerated into a finely focused beam.
Deflection
Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on:
the mass of the ion. Lighter ions are deflected more than heavier ones.
the charge on the ion. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.
These two factors are combined into the mass/charge ratio.Mass/charge ratio is given the symbol m/z (or sometimes m/e).
For example, if an ion had a mass of 28 and a charge of 1+, its mass/charge ratio would be 28. An ion with a mass of 56 and a charge of 2+ would also have a mass/charge ratio of 28.
In the last diagram, ion stream A is most deflected - it will contain ions with the smallest mass/charge ratio. Ion stream C is the least deflected - it contains ions with the greatest mass/charge ratio.
It makes it simpler to talk about this if we assume that the charge on all the ions is 1+. Most of the ions passing through the mass spectrometer will have a charge of 1+, so that the mass/charge ratio will be the same as the mass of the ion.
Note: You must be aware of the possibility of 2+ (etc) ions, but the vast majority of A'level questions will give you mass spectra which only involve 1+ ions. Unless there is some hint in the question, you can reasonably assume that the ions you are talking about will have a charge of 1+.
Assuming 1+ ions, stream A has the lightest ions, stream B the
next lightest and stream C the heaviest. Lighter ions are going to be more deflected than heavy ones.
Detection
Only ion stream B makes it right through the machine to the ion detector. The other ions collide with the walls where they will pick up electrons and be neutralised. Eventually, they get removed from the mass spectrometer by the vacuum pump.
When an ion hits the metal box, its charge is neutralised by an electron jumping from the metal on to the ion (right hand diagram). That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it.
A flow of electrons in the wire is detected as an electric current which can be amplified and recorded. The more ions arriving, the greater the current.
Detecting the other ions
How might the other ions be detected - those in streams A and C which have been lost in the machine?
Remember that stream A was most deflected - it has the smallest value of m/z (the lightest ions if the charge is 1+). To bring them on to the detector, you would need to deflect them less - by using a smaller magnetic field (a smaller sideways force).
To bring those with a larger m/z value (the heavier ions if the charge is +1) on to the detector you would have to deflect them more by using a larger magnetic field.
If you vary the magnetic field, you can bring each ion stream in turn
on to the detector to produce a current which is proportional to the number of ions arriving. The mass of each ion being detected is related to the size of the magnetic field used to bring it on to the detector. The machine can be calibrated to record current (which is a measure of the number of ions) against m/z directly. The mass is measured on the 12C scale.
Note: The 12C scale is a scale on which the 12C isotope weighs exactly 12 units.
What the mass spectrometer output looks like
The output from the chart recorder is usually simplified into a "stick diagram". This shows the relative current produced by ions of varying mass/charge ratio.
The stick diagram for molybdenum looks lilke this:
You may find diagrams in which the vertical axis is labelled as either "relative abundance" or "relative intensity". Whichever is used, it means the same thing. The vertical scale is related to the current received by the chart recorder - and so to the number of ions arriving at the detector: the greater the current, the more abundant the ion.
As you will see from the diagram, the commonest ion has a mass/charge ratio of 98. Other ions have mass/charge ratios of 92, 94, 95, 96, 97 and 100.
That means that molybdenum consists of 7 different isotopes. Assuming that the ions all have a charge of 1+, that means that the masses of the 7 isotopes on the carbon-12 scale are 92, 94, 95,
96, 97, 98 and 100.
Note: If there were also 2+ ions present, you would know because every one of the lines in the stick diagram would have another line at exactly half its m/z value (because, for example, 98/2 = 49). Those lines would be much less tall than the 1+ ion lines because the chances of forming 2+ ions are much less than forming 1+ ions.
If you want to go straight on to how you use these mass spectra to calculate relative atomic masses you can jump straight to that page by following this link rather than going via the menus below.
Mass Spectrometer
The mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules. It makes use of the basicmagnetic force on a moving charged particle.
Derive radius expression How does a velocity selector work?
Magnetic interactions with charge Applications of mass spectrometers
If a charge moves into a magnetic field with direction perpendicular to the field, it will follow a circular path. The magnetic force, being perpendicular to the velocity, provides the centripetal force.
Application in mass spectrometer Application in cyclotron
Magnetic confinement
Index
Magnetic force
Magnetic field
concepts
HyperPhysics***** Electricity and Magnetism R NaveGo Back
A velocity selector is used with mass spectrometers to select only charged particles with a specific velocity for analysis. It makes use of a geometry where opposingelectric and magnetic forces match for a specific particle speed. It therefore lets through undeflected only those particles with the selected velocity.
Application in mass spectrometer
Index
Magnetic field
concepts
HyperPhysics***** Electricity and Magnetism R NaveGo Back
Mass spectrometers are sensitive detectors of isotopes based on their masses. They are used in carbon dating and other radioactive dating processes. The combination of a mass spectrometer and a gas chromatograph makes a powerful tool for the detection of trace quantities of contaminants or toxins. A number of satellites and spacecraft have mass spectrometers for the identification of the small numbers of particles intercepted in space. For example, the SOHO satellite uses a mass spectrometer to analyze the solar wind.
Mass spectrometers are used for the analysis of residual gases in high vacuum systems.
Display from residual gas analyzer
Isotopic abundances of krypton
Index
Magnetic field
concepts
HyperPhysics***** Electricity and Magnetism R NaveGo Back
A magnetohydrodynamic generator has been described as a magnet on the tail of a jet engine. A super-hot plasma is created, ionizing the atoms of the fuel mixture. The magnetic field deflects positive and negative charges in different directions. Collecting plates for the charges provide a DC voltage.
Magnetohydrodynamics as an electricity generation process holds the possibility of very efficient fuel utilization because the extremely high temperatures at which it operates correlate to a high Carnot efficiency. Its practical application has been slow in coming because of a number of problems, including a high rate of damage to the combustion chamber by the high velocity particles.
Magnetic interactions with charge
Index
Magnetic field
concepts
HyperPhysics***** Electricity and Magnetism R NaveGo Back
The word spectrograph had become part of the international scientific vocabulary by 1884.[4][5] The linguistic
roots are a combination and removal of bound morphemes and free morphemes which relate to the
terms spectr-um and phot-ograph-ic plate.[6] Early spectrometry devices that measured the mass-to-charge
ratio of ions were called mass spectrographs which consisted of instruments that recorded a spectrum of mass
values on a photographic plate.[7][8] A mass spectroscope is similar to a mass spectrograph except that the
beam of ions is directed onto a phosphor screen.[9] A mass spectroscope configuration was used in early
instruments when it was desired that the effects of adjustments be quickly observed. Once the instrument was
properly adjusted, a photographic plate was inserted and exposed. The term mass spectroscope continued to
be used even though the direct illumination of a phosphor screen was replaced by indirect measurements with
anoscilloscope.[10] The use of the term mass spectroscopy is now discouraged due to the possibility of
confusion with lightspectroscopy.[1][11] Mass spectrometry is often abbreviated as mass-spec or simply as MS.[1]
History[edit]
For more details on this topic, see History of mass spectrometry.
Replica of an early mass spectrometer
In 1886, Eugen Goldstein observed rays in gas discharges under low pressure that traveled away from the
anode and through channels in a perforated cathode, opposite to the direction of negatively chargedcathode
rays (which travel from cathode to anode). Goldstein called these positively charged anode
rays "Kanalstrahlen"; the standard translation of this term into English is "canal rays". Wilhelm Wienfound that
strong electric or magnetic fields deflected the canal rays and, in 1899, constructed a device with parallel
electric and magnetic fields that separated the positive rays according to their charge-to-mass ratio (Q/m).
Wien found that the charge-to-mass ratio depended on the nature of the gas in the discharge tube. English
scientist J.J. Thomson later improved on the work of Wien by reducing the pressure to create the mass
spectrograph.
The first application of mass spectrometry to the analysis of amino acids and peptides was reported in 1958.[12] Carl-Ove Andersson highlighted the main fragment ions observed in the ionization of methyl esters.[13]
Some of the modern techniques of mass spectrometry were devised by Arthur Jeffrey Dempster and F.W.
Aston in 1918 and 1919 respectively. In 1989, half of the Nobel Prize in Physics was awarded to Hans
Dehmelt and Wolfgang Paul for the development of the ion trap technique in the 1950s and 1960s. In 2002,
the Nobel Prize in Chemistry was awarded to John Bennett Fenn for the development of electrospray
ionization (ESI) and Koichi Tanaka for the development of soft laser desorption (SLD) and their application to
the ionization of biological macromolecules, especially proteins.[14]