Energy-dispersive X-ray spectroscopy

Energy-dispersive X-ray spectroscopy

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Encyclopedia
Energy-dispersive X-ray spectroscopy (EDS or EDX or EDAX) is an analytical technique used for the element
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 analysis or chemical characterization
Characterization (materials science)
Characterization, when used in materials science, refers to the use of external techniques to probe into the internal structure and properties of a material...

 of a sample. It relies on the investigation of an interaction of a some source of X-ray excitation and a sample. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

 allowing X-rays that are characteristic of an element's atomic structure to be identified uniquely from one another.

To stimulate the emission of characteristic X-rays from a specimen, a high-energy beam of charged particles such as electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s or proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s (see PIXE), or a beam of X-rays, is focused into the sample being studied. At rest, an atom within the sample contains ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...

 (or unexcited) electrons in discrete energy levels or 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,.....

s bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...

 where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The number and energy of the X-rays emitted from a specimen can be measured by an energy-dispersive spectrometer. As the energy of the X-rays are characteristic of the difference in energy between the two shells, and of the atomic structure of the element from which they were emitted, this allows the elemental composition of the specimen to be measured.

X-ray generation


The equipment measures the number of emitted X-rays. Higher accelerating voltages penetrate deeper into the sample; the beam also penetrates deeper into lighter elements, which thus generate more counts.

Equipment: the connection with SEM


There are four primary components of the EDS setup: the beam source; the X-ray detector; the pulse processor; and the analyzer.

A number of free-standing EDS systems exist. However, EDS systems are most commonly found on scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...

s (SEM-EDS) and electron microprobe
Electron microprobe
An electron microprobe , also known as an electron probe microanalyzer or electron micro probe analyzer , is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials...

s. Scanning electron microscopes are equipped with a cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 and magnetic lenses to create and focus a beam of electrons, and since the 1960s they have been equipped with elemental analysis capabilities. A detector is used to convert X-ray energy into voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 signals; this information is sent to a pulse processor, which measures the signals and passes them onto an analyzer for data display and analysis.

Technological variants



The excess energy of the electron that migrates to an inner shell to fill the newly-created hole can do more than emit an X-ray. Often, instead of X-ray emission, the excess energy is transferred to a third electron from a further outer shell, prompting its ejection. This ejected species is called an 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...

, and the method for its analysis is known as Auger electron spectroscopy
Auger electron spectroscopy
Auger electron spectroscopy is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science...

 (AES).

X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...

 (XPS) is another close relative of EDS, utilizing ejected electrons in a manner similar to that of AES. Information on the quantity and kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 of ejected electrons is used to determine the binding energy
Binding energy
Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...

 of these now-liberated electrons, which is element-specific and allows chemical characterization of a sample.

EDS is often contrasted with its spectroscopic counterpart, WDS (wavelength dispersive X-ray spectroscopy
Wavelength dispersive X-ray spectroscopy
The Wavelength dispersive X-ray spectroscopy is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging x-ray and the crystal's lattice spacings are related by Bragg's law and produce constructive interference if they fit the...

). WDS differs from EDS in that it uses the diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 patterns created by light-matter interaction as its raw data. WDS has a much finer spectral resolution than EDS. WDS also avoids the problems associated with artifacts in EDS (false peaks, noise from the amplifiers, and microphonics). In WDS, only one element can be analyzed at a time, while EDS gathers a spectrum of all elements, within limits, of a sample.

Accuracy of EDS


Accuracy of EDS spectrum can be affected by many factors. First three elements of Periodic Table (H, He, and Li) do not have enough electrons to produce characteristic X-ray, so they are not detectable by X-ray spectroscopy. Also many elements will have overlapping peaks (e.g., Ti Kβ and V Kα, Mn Kβ and Fe Kα). The accuracy of the spectrum can also be affected by the nature of the sample. X-rays can be generated by any atom in the sample that is sufficiently excited by the incoming beam. These X-rays are emitted in any direction, and so they may not all escape the sample. The likelihood of an X-ray escaping the specimen, and thus being available to detect and measure, depends on the energy of the X-ray and the amount and density of material it has to pass through. This can result in reduced accuracy in inhomogeneous and rough samples.

Emerging technology


There is a trend towards a newer EDS detector, called the silicon drift detector
Silicon drift detector
Silicon drift detectors are X-ray radiation detectors used in x-ray spectrometry and electron microscopy . Their chief characteristics compared with other X-ray detectors are:*high count rates*comparatively high energy resolution Silicon drift detectors (SDDs) are X-ray radiation detectors used...

 (SDD). The SDD consists of a high-resistivity silicon chip where electrons are driven to a small collecting anode. The advantage lies in the extremely low capacitance of this anode, thereby utilizing shorter processing times and allowing very high throughput. Benefits of the SDD include:
  1. High count rates and processing,
  2. Better resolution than traditional Si(Li) detectors at high count rates,
  3. Lower dead time (time spent on processing X-ray event),
  4. Faster analytical capabilities and more precise X-ray maps or particle data collected in seconds,
  5. Ability to be stored and operated at relatively high temperatures, eliminating the need for liquid nitrogen
    Liquid nitrogen
    Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

     cooling.


Because the capacitance of the SDD chip is independent of the active area of the detector, much larger SDD chips can be utilized (40 mm2 or more). This allows for even higher count rate collection. Further benefits of large area chips include:
  1. Minimizing SEM beam current allowing for optimization of imaging under analytical conditions,
  2. Reduced sample damage and
  3. Smaller beam interaction and improved spatial resolution for high speed maps.


In recent years, a different type of EDS detector, based upon a microcalorimeter
Calorimeter
A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal microcalorimeters, titration calorimeters and accelerated rate calorimeters are among the most common...

, has become commercially available. This new model allegedly has the simultaneous detection capabilities of EDS as well as the high spectral resolution of WDS. The EDS microcalorimeter relies highly on two components: an absorber, and a thermistor
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...

. The former absorbs X-rays emitted from the sample and converts this energy into heat; the latter measures the subsequent change in temperature due to the influx of heat (in essence, a thermometer
Thermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

).
The EDS microcalorimeter has suffered from a number of drawbacks; including low count rates, poor collection efficiencies and small detector areas. The count rate is hampered by its reliance on the time constant
Time constant
In physics and engineering, the time constant, usually denoted by the Greek letter \tau , is the risetime characterizing the response to a time-varying input of a first-order, linear time-invariant system.Concretely, a first-order LTI system is a system that can be modeled by a single first order...

 of the calorimeter’s electrical circuit. The collection efficiency is a function of the absorber material and remains to be optimized. The detector area must be small in order to keep the heat capacity
Heat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...

 as small as possible and maximize thermal sensitivity (resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...

). Innovative engineering solutions are necessary for further improvement of spectroscopic microanalysis.

See also

  • Elemental mapping
  • SEM-EDX
  • Transmission electron microscopy
    Transmission electron microscopy
    Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

  • Wavelength dispersive X-ray spectroscopy
    Wavelength dispersive X-ray spectroscopy
    The Wavelength dispersive X-ray spectroscopy is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging x-ray and the crystal's lattice spacings are related by Bragg's law and produce constructive interference if they fit the...

  • X-ray fluorescence
    X-ray fluorescence
    X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...

  • X-ray microtomography

External links

  • MICROANALYST.NET - Information portal with X-ray microanalysis and EDX contents