PIXE
Encyclopedia
Particle-induced X-ray emission or proton-induced X-ray emission (PIXE) is a technique used in the determining of the element
al make-up of a material or sample. When a material is exposed to an ion
beam, atomic interactions occur that give off EM radiation of wavelength
s in the x-ray
part of the electromagnetic spectrum
specific to an element. PIXE is a powerful yet non-destructive elemental analysis technique now used routinely by geologists, archaeologists, art conservators and others to help answer questions of provenance, dating and authenticity
.
The technique was first proposed in 1970 by Sven Johansson of Lund University, Sweden, and developed over the next few years with his colleagues Roland Akselsson and Thomas B Johansson.
Recent extensions of PIXE using tightly focused beams (down to 1 μm) gives the additional capability of microscopic analysis. This technique, called microPIXE, can be used to determine the distribution of trace elements in a wide range of samples. A related technique, particle-induced gamma-ray emission (PIGE) can be used to detect some light elements.
Only elements heavier than fluorine can be detected. The lower detection limit for a PIXE beam is given by the ability of the X-rays to pass through the window between the chamber and the X-ray detector. The upper limit is given by the ionisation cross section, the probability of the K electron shell
ionisation, this is maximal when the velocity of the proton matches the velocity of the electron (10% of the speed of light
), therefore 3 MeV proton beams are optimal.
analysis using microPIXE allow for the determination of the elemental composition of liquid and crystalline proteins. microPIXE can quantify the metal content of protein molecules with a relative accuracy of between 10% and 20%.
The advantage of microPIXE is that given a protein of known sequence, The X-ray emission from sulfur
can be used as an internal standard to calculate the number of metal atom per protein monomer. Because only relative concentrations are calculated there are only minimal systematic errors, and the results are totally internally consistent.
The relative concentrations of DNA
to protein (and metals) can also be measured using the phosphate
groups of the bases
as an internal calibration.
, HEPES
, MES
, MOPSO
or PIPES
compounds). Excessive amounts of chlorine
in the buffer should also be avoided, this will overlap with the sulfur peak, KBr
and NaBr
are suitable alternatives.
radiation, although there is some from the emission of Auger electron
s, there is significantly less than if the primary beam was itself an electron beam.
Because of the higher mass of protons relative to electrons, there is less lateral deflection of the beam, this is important for proton beam writing
applications.
.
) through either the hardening of a polymer
(by proton induced cross-link
ing), or through the degradation of a proton sensitive material. This may have important effects in the field of nanotechnology
.
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
al make-up of a material or sample. When a material is exposed to an ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
beam, atomic interactions occur that give off EM radiation of wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s in the x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
part of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
specific to an element. PIXE is a powerful yet non-destructive elemental analysis technique now used routinely by geologists, archaeologists, art conservators and others to help answer questions of provenance, dating and authenticity
Authenticity in art
Authenticity in art has a variety of meanings related to different ways in which a work of art or an artistic performance may be considered authentic.Denis Dutton distinguishes between nominal authenticity and expressive authenticity....
.
The technique was first proposed in 1970 by Sven Johansson of Lund University, Sweden, and developed over the next few years with his colleagues Roland Akselsson and Thomas B Johansson.
Recent extensions of PIXE using tightly focused beams (down to 1 μm) gives the additional capability of microscopic analysis. This technique, called microPIXE, can be used to determine the distribution of trace elements in a wide range of samples. A related technique, particle-induced gamma-ray emission (PIGE) can be used to detect some light elements.
Theory
Three types of spectra can be collected from a PIXE experiment:- X-rayX-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
emission spectrum. - Rutherford backscattering spectrum.
- Proton transmission spectrum.
X-ray emission
Quantum theory states that orbiting electrons of an atom must occupy discrete energy levels in order to be stable. Bombardment with ions of sufficient energy (usually MeV protons) produced by an ion accelerator, will cause inner shell ionization of atoms in a specimen. Outer shell electrons drop down to replace inner shell vacancies, however only certain transitions are allowed. X-rays of a characteristic energy of the element are emitted. An energy dispersive detector is used to record and measure these X-rays.Only elements heavier than fluorine can be detected. The lower detection limit for a PIXE beam is given by the ability of the X-rays to pass through the window between the chamber and the X-ray detector. The upper limit is given by the ionisation cross section, the probability of the K electron shell
Electron shell
An electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
ionisation, this is maximal when the velocity of the proton matches the velocity of the electron (10% of the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
), therefore 3 MeV proton beams are optimal.
Proton backscattering
Protons can also interact with the nucleus of the atoms in the sample through elastic collisions, Rutherford backscattering, often repelling the proton at angles close to 180 degrees. The backscatter give information on the sample thickness and composition. The bulk sample properties allow for the correction of X-ray photon loss within the sample.Proton transmission
The transmission of protons through a sample can also be used to get information about the sample.Protein analysis
ProteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
analysis using microPIXE allow for the determination of the elemental composition of liquid and crystalline proteins. microPIXE can quantify the metal content of protein molecules with a relative accuracy of between 10% and 20%.
The advantage of microPIXE is that given a protein of known sequence, The X-ray emission from sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
can be used as an internal standard to calculate the number of metal atom per protein monomer. Because only relative concentrations are calculated there are only minimal systematic errors, and the results are totally internally consistent.
The relative concentrations of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
to protein (and metals) can also be measured using the phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
groups of the bases
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
as an internal calibration.
Data analysis
Analysis of the data collected can be performed by the programs Dan32, the front end to gupix.Limitations
In order to get a meaningful sulfur signal from the analysis, the buffer should not contain sulfur, (i.e. no BES, DDTDDT
DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
, HEPES
HEPES
HEPES is a zwitterionic organic chemical buffering agent; one of the twelve Good's buffers...
, MES
MES (buffer)
MES is the common name for the compound 2-ethanesulfonic acid. Its chemical structure contains a morpholine ring. It has a molecular weight of 195.2 and the chemical formula is C6H13NO4S...
, MOPSO
MOPS
MOPS is the common name for the compound 3-propanesulfonic acid, a buffer introduced by Good et al. in the 1960s. It is a structural analog to MES. Its chemical structure contains a morpholine ring. HEPES is a similar pH buffering compound that contains a piperazine ring...
or PIPES
PIPES
PIPES is the common name for piperazine-N,N′-bis, and frequently used buffering agent in biochemistry. It is an ethanesulfonic acid buffer developed by Good et al. in the 1960s.-Applications:...
compounds). Excessive amounts of chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
in the buffer should also be avoided, this will overlap with the sulfur peak, KBr
Potassium bromide
Potassium bromide is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the United States. Its action is due to the bromide ion...
and NaBr
Sodium bromide
Sodium bromide is an inorganic compound with the formula NaBr. It is a high-melting white, crystalline solid that resembles sodium chloride. It is a widely used source of the bromide ion and has many applications.-Synthesis, structure, reactions:...
are suitable alternatives.
Advantages
There are many advantages to using a proton beam over an electron beam, there is less crystal charging from BremsstrahlungBremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
radiation, although there is some from the emission of Auger electron
Auger electron
The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in...
s, there is significantly less than if the primary beam was itself an electron beam.
Because of the higher mass of protons relative to electrons, there is less lateral deflection of the beam, this is important for proton beam writing
Proton beam writing
Proton beam writing is a new direct-write lithography process developed by Frank Watt and colleagues at the Centre for Ion Beam Applications , Department of Physics, National University of Singapore....
applications.
Scanning
Two-dimensional maps of elemental compositions can be generated by scanning the microPIXE beam across the target.Cell and tissue analysis
Whole cell and tissue analysis is possible using a microPIXE beam, this method is also referred to as nuclear microscopyNuclear microscopy
Nuclear microscopy uses a device called a microprobe. A microprobe is a device that uses electromagnetic or electrostatic lenses to focus an ion beam. In this way a microprobe is very similar to a scanning electron microscope. One difference is that the nuclear microprobe's beam is usually composed...
.
Artifact analysis
MicroPIXE is a useful technique for the non-destructive analysis of paintings and antiques. Although it provides only an elemental analysis, it can be used to distinguish and measure layers within the thickness of an artifact.Proton beam writing
Proton beams can be used for writing (proton beam writingProton beam writing
Proton beam writing is a new direct-write lithography process developed by Frank Watt and colleagues at the Centre for Ion Beam Applications , Department of Physics, National University of Singapore....
) through either the hardening of a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
(by proton induced cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
ing), or through the degradation of a proton sensitive material. This may have important effects in the field of nanotechnology
Nanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
.