ATOMIC ABSORPTION SPECTROMETRY

INTRODUCTION

Atomic Absorption Spectrometry (AAS) is a technique which is used for the analysis of quantities of elements present in a sample by measuring the absorbed radiation by the chemical element of interest.

This is done by measuring the spectra produced when the sample is excited by radiation. The atoms absorb ultraviolet or visible light and get excited to higher energy levels. Atomic absorption technique measures the amount of energy in the form of photons of light that are absorbed by the sample.

A detector measures the wavelengths of light transmitted by the sample, and compares them to the wavelengths which originally passed through the sample. A signal processor then integrates the changes in wavelength absorbed, which appear in the readout as peaks of energy absorption at discrete wavelengths.

The energy required for an electron to leave an atom is known as ionization energy and is specific to each and every element. When an electron moves from one energy level to another within the atom, a photon is emitted with energy E. Atoms of an element emit a characteristic spectral line. Every atom has its own distinct pattern of wavelengths at which it will absorb energy, due to the unique configuration of electrons in its outer shell.

This enables the qualitative analysis of a sample. The concentration is calculated based on the Beer-Lambert law. Absorbance is directly proportional to the concentration of the analyte absorbed for the existing set of conditions. The concentration is usually determined from a calibration curve, obtained using standards of known concentration or certified reference materials (CRMs). However, applying the Beer-Lambert law directly in AAS is difficult due to:

·       variations in atomization efficiency from the sample matrix non-uniformity of concentration and path length of analyte atoms (in graphite furnace AA).

The chemical methods used are based on matter interactions, i.e. chemical reactions. For a long period of time these methods were essentially empirical, involving, in most cases, great experimental skills. In analytical chemistry, AAS is a technique used mostly for determining the concentration of a particular metal element within a sample. AAS can be used to analyse the concentration of over 62 different metals in a solution. Typically, the technique makes use of a flame to atomize the sample, but other atomizers, such as a graphite furnace, are also used. Three steps are involved in turning a liquid sample into an atomic gas:

1. Desolvation – the liquid solvent is evaporated, and the dry sample remains;

2. Vaporization – the solid sample vaporizes to a gas; and

3. Volatilization – the compounds that compose the sample are broken into free atoms.

To measure how much of a given element is present in a sample, first of all , we must establish a basis for comparison using certified reference materials or known quantities of that element to produce a calibration curve.

To generate this curve, a specific wavelength is selected, and the detector (Usually Photomultiplier tube detectors are used) is set to measure only the energy transmitted at that wavelength. As the concentration of the target atom in the sample increases, the absorption will also increase proportionally.

A series of samples containing known concentrations of the element to be measured are analysed, and the corresponding absorbance, which is the inverse percentage of light transmitted, is recorded.

The measured absorption at each concentration is then plotted, so that a straight line can then be drawn between the resulting points. From this line, the concentration of the substance under investigation is extrapolated from the substance’s absorbance. The use of special light sources and the selection of specific wavelengths allow for the quantitative determination of individual components in a multi-element mixture.

BASIC PRINCIPLE

The selectivity in AAS is very important, since each element has a different set of energy levels and gives rise to very narrow absorption lines. Hence, the selection of the monochromator is vital to obtain a linear calibration curve (Beers' Law), the bandwidth of the absorbing species must be broader than that of the light source; which is difficult to achieve with ordinary monochromators. The monochromator is a very important part of an AA spectrometer because it is used to separate the thousands of lines generated by all of the elements in a sample.

Without a good monochromator, detection limits are severely compromised. A monochromator is used to select the specific wavelength of light that is absorbed by the sample and to exclude other wavelengths. The selection of the specific wavelength of light allows for the determination of the specific element of interest when it is in the presence of other elements. The light selected by the monochromator is directed onto a detector,typically a photomultiplier tube, whose function is to convert the light signal into an electrical signal proportional to the light intensity. The challenge of requiring the bandwidth of the absorbing species to be broader than that of the light source is solved with radiation sources with very narrow lines.

The study of trace metals in wet and dry precipitation has increased in recent decades because trace metals have adverse environmental and human health effects. Some metals, such as Pb, Cd and Hg, accumulate in the biosphere and can be toxic to living systems.

Anthropogenic activities have substantially increased trace metal concentrations in the atmosphere. In addition, acid precipitation promotes the dissolution of many trace metals, which enhances their bioavailability. In recent decades, heavy metal concentrations have increased not only in the atmosphere but also in pluvial precipitation. Metals, such as Pb, Cd, As, and Hg, are known to accumulate in the biosphere and to be dangerous for living organisms, even at very low levels. Many human activities play a major role in global and regional trace element budgets. Additionally, when present above certain concentration levels, trace metals are potentially toxic to marine and terrestrial life. Thus, biogeochemical

perturbations are a matter of crucial interest in science.

The atmospheric input of metals exhibits strong temporal and spatial variability due to short atmospheric residence times and meteorological factors. As in oceanic chemistry, the impact of trace metals in atmospheric deposition cannot be determined from a simple consideration of global mass balance; rather, accurate data on net air or sea fluxes for specific regions are needed.

Particles in urban areas represent one of the most significant atmospheric pollution problems, and are responsible for decreased visibility and other effects on public health, particularly when their aerodynamic diameters are smaller than 10 μm, because these small particles can penetrate deep into the human respiratory tract. There have been many studies measuring concentrations of toxic metals such as Ag, As, Cd, Cr, Cu, Hg, Ni, Pb in rainwater and their deposition into surface waters and on soils. Natural sources of aerosols include terrestrial dust, marine aerosols, volcanic emissions and forest fires. Anthropogenic particles, on the other hand, are created by industrial processes, fossil fuel combustion, automobile mufflers, worn engine parts, and corrosion of metallic parts. The presence of metals in atmospheric particles are directly associated with health risks of these metals. Anthropogenic sources have substantially increased trace metal concentrations in atmospheric deposition.

The instrument used for atomic absorption spectrometry can have either of two atomizers. One attachment is a flame burner, which uses acetylene and air fuels. The second attachment consists of a graphite furnace that is used for trace metal analysis. Figure 1 depicts a diagram of an atomic absorption spectrometer.

Fig. 1. The spectral, or wavelength, range captures the dispersion of the grating across the linear array.

           

 Flame and furnace spectroscopy has been used for years for the analysis of metals. Today these procedures are used more than ever in materials and environmental applications. This is due to the need for lower detection limits and for trace analysis in a wide range ofsamples. Because of the scientific advances of Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), have left Atomic Absorption (AA) behind. This technique, however, is excellent and has a larger specificity that ICP does not have.

Figure 2 shows a diagram of an atomic absorption spectrometer with a graphite furnace.

 AAS is a reliable chemical technique to analyze almost any type of material. This post describes the basic principles of atomic absorption spectroscopy in the analysis of trace metals, such as Ag, As, Cd, Cr, Cu, and Hg, in environmental samples. For example, the study of trace metals in wet and dry precipitation has increased in recent decades because trace metals have adverse environmental and human health effects. Anthropogenic activities have substantially increased trace metal concentrations in the atmosphere. In recent decades, heavy metal concentrations have increased not only in the atmosphere but also in pluvial precipitation.

Many human activities play a major role in global and regional trace element budgets. Additionally, when present above certain concentration levels, trace metals are potentially toxic to marine and terrestrial life. Thus, biogeochemical perturbations are a matter of crucial interest in science.

The atmospheric input of metals exhibits strong temporal and spatial variability due to short atmospheric residence times and meteorological factors. As in oceanic chemistry, the impact of trace metals in atmospheric deposition cannot be determined from a simple consideration of global mass balance; rather, accurate data on net air or sea fluxes for specific regions are needed.

Particles in urban areas represent one of the most significant atmospheric pollution

problems, and are responsible for decreased visibility and other effects on public health, particularly when their aerodynamic diameters are smaller than 10 μm, because these small particles can penetrate deep into the human respiratory tract. There have been many studies measuring concentrations of toxic metals such as Ag, As, Cd, Cr, Cu, Hg, Ni, Pb in rainwater and their deposition into surface waters and on soils. Natural sources of aerosols include terrestrial dust, marine aerosols, volcanic emissions and forest fires. Anthropogenic particles, on the other hand, are created by industrial processes, fossil fuel combustion, automobile mufflers, worn engine parts, and corrosion of metallic parts. The presence of metals in atmospheric particles and the associated health risks of these metals.

Anthropogenic sources have substantially increased trace metal concentrations in

atmospheric deposition. In addition, acid precipitation favors the dissolution of many trace metals, which enhances their bioavailability. Trace metals from the atmosphere are deposited by rain, snow and dry fallout. The predominant processes of deposition by rain are rainout and washout (scavenging). Generally, in over 80 % of wet precipitation, heavy metals are dissolved in rainwater and can thus reach and be taken up by the vegetation blanket and soils. Light of a specific wavelength, selected appropriately for the element being analyzed, is given off when the metal is ionized in the flame; the absorption of this light by the element of interest is proportional to the concentration of that element.

Quantification is achieved by preparing standards of the element.

• AAS intrinsically more sensitive than Atomic Emission Spectrometry (AES)
• Similar atomization techniques to AES
• Addition of radiation source
• High temperature for atomization necessary
• Flame and electrothermal atomization
• Very high temperature for excitation not necessary; generally no plasma/arc/spark in AAS

We will discuss the Flame AAS technique and AAS with Graphite Furnace (GFAA) in the upcoming posts.

ENERGY DISPERSIVE X-RAY FLUORESCENCE (ED-XRF)

INTRODUCTION

In Energy Dispersive X-Ray Fluorescence spectrometry (ED-XRF), the identification of characteristic lines is performed using detectors that directly measure the energy of the photons. In energy dispersive X-ray fluorescence analysis (EDXRF), a solid-state detector is used to count the photons, simultaneously sorting them according to energy and storing the result in a multichannel memory. The result is an X-ray energy vs. intensity spectrum. The range of detectable elements ranges from Be (Z = 4) for the light elements and goes up to U (Z = 92) on the high atomic number Z side. In principle, XRF analysis is a multielement analytical technique and in particular, the simultaneous determination of all the detectable elements present in the sample is inherently possible with EDXRF. In WDXRF both the sequential and the simultaneous detection modes are possible. Although energy dispersive detectors generally exhibit poorer energy resolution than wavelength dispersive analyzers, they are capable of detecting simultaneously a wide range of energies. The most frequently used detector in EDXRF is the silicon semiconductor detector, which nowadays can have excellent energy resolution.

INSTRUMNTATION

An ED-XRF system consists of several basic functional components, as shown in

Figure The major components are as follows :

1. X- Ray excitation source
2. Sample Chamber
3. Si (Li) detector
4. Preamplifier
5. Main Amplifier
6. Multichannel Pulse Height Analyzer

The properties and performances of an EDXRF system differ upon the electronics and the enhancements from the computer software.   

Typical ED-XRF detection arrangement.

We will discuss in detail for every component :

1. Excitation Mode

A) Direct Tube Excitation .

Because of the simplicity of the instrument and the availability of a high photon output flux by using direct tube excitation, the X-ray fluorescence spectrometer equipped with an Xray tube as direct excitation source is gaining more and more attention from manufactures. The spectrometer is more compact and cheaper compared to secondary target systems. Of course, the drawback is still the less flexible selection of excitation energy. However, by using an appropriate filter between tube and sample, one can obtain an optimal excitation.

The most popular X-ray tube used in direct excitation ED spectrometer is the side window tube for reasons of simplicity and safety. With direct tube excitation, low powered X-ray tubes (< 100 W) can be used. These air cooled tubes are very compact, less expensive, and only require compact, light, inexpensive, highly regulated solid state power supplies. In a WD spectrometer, on the other hand, high-power tubes (3-4 kW) are essential to compensate for the losses in the crystal and collimator. With the low-power tubes used in ED spectrometer, better excitation of light elements (i.e. low-Z element), analysis of smaller samples, small spot analysis, and compact systems can be obtained.

B) Secondary Target Excitation.

The principle of secondary target excitation was developed to avoid the intense

Bremsstrahlung continuum from the X-ray tube by using a target between tube and sample. 

Schematic illustration of secondary target excitation

The ratio of the intensity of the characteristic lines to that the continuum in secondary target excitation is much higher than that in direct tube excitation because the continuum part of the excitation spectrum of the secondary target is generated only by scattering. One can excite various elements efficiently by selecting a secondary target that has characteristic lines just above the absorption edges of the elements of interest in the sample. Therefore, secondary target excitation has some obvious advantages over direct tube excitation: its flexibility for getting an optimized and near monochromatic excitation providing a better selectivity and an improved sensitivity. However, to compensate for the intensity losses that occur at the secondary scatterer, a high-powered tube as used in WD spectrometers is required; making the whole system more sophisticated and expensive compared to direct tube excitation setups.

C) Radio Isotopic Excitation.

A variety of about 30 commercially available radio-isotopic materials can be chosen for an optimal excitation. The X-rays and/or γ-rays emitted from these radio-isotopic sources cover a wide range (10 – 60 keV) of excitation energies. With a high energy source like 241 Am, K lines instead L lines can be used for quantification in the case of analyzing high-Z rare earth elements, with considerably less matrix effects and spectrum overlaps. Sometimes the same idea as in the secondary target excitation is used to avoid non-photon radiation. A proper design of excitation-detection geometry can improve greatly the sensitivity and accuracy of the XRF analysis with such excitation source. The disadvantages of using radioisotopic sources however lie in their low photon output, intensity decay and storage problems.

2. Detectors

Energy dispersive X-ray spectrometry is based upon the ability of the detector to create signals proportional to the X-ray photon energy, therefore, mechanical devices, such as analyzing crystals, are not required as in wdxrf . Several types of detectors have been employed, including silicon, germanium and mercuric iodide .

Cross section of an Si(Li) detector crystal with p-i-n structure and the

production of electron-hole pair.

The solid state, lithium-drifted silicon detector, Si(Li), was developed and applied to Xray detection in the 1960’s. Early 1970’s, this detector was firmly established in the field of X-ray spectrometry, and was applied as an X-ray detection system for scanning Electron Microscopy (SEM) as well as X-ray spectrometry. The principal advantage of the Si(Li) detector is its excellent resolution.

Si(Li) detector can be considered as a layered structure in which a lithium-drifted active region separates a p-type entry side from an 

n-type side. Under reversed bias of approximately 600 V, the active region acts as an insulator with an electric field gradient throughout its volume. When an X-ray photon enters the active region of the detector, photoionization occurs with an electron-hole pair created for each 3.8 eV of photon energy. Ideally, the detector should completely collect the charge created by each photon entry, and result in a response for only that energy. In reality, some background counts appear because of the energy loss in the detector. Although these are kept to a minimum by engineering, incomplete charge collection in the detector is a contributor to background counts. In the X-ray spectrometric, important region of 1 – 20 keV, silicon detectors have excellent efficiency for conversion of X-ray photon energy into charge. Some of the photon energy may be lost by photoelectric absorption of the incident X-ray, creating an excited Si atom which relaxes to yield an Si Kα X-ray. This X-ray may escape from the detector, resulting in an energy loss equivalent to the photon energy; in the case of Si Kα, this is 1.74 keV. Therefore, an escape peak 1.74 keV lower than the true photon energy of the detected X-ray may be observed for intense peaks. For Si(Li) detectors, these are usually a few tenths of one percent, and never more than 2%, of the intensity of the main peak.

 The Si(Li) detector schematic

Resolution of an energy dispersive X-ray spectrometer is normally expressed as the Full Width at Half Maximum amplitude (FWHM) of the Mn X-ray at 5.9 keV. The resolution will be somewhat count rate dependent. Commercial spectrometers are supplied routinely with detectors which display approximately 145 eV (FWHM @ 5.9 keV). The resolution of the system is a result of both electronic noise and statistical variations in conversion of the photon energy. Electronic noise is minimized by cooling the detector, and the associated preamplifier with liquid nitrogen (Figure). In many cases, half of the peak width is a result of electronic noise.

3. Pulse Height Analysis

The X-ray spectrum of the sample is obtained by processing the energy distribution of X-ray photons which enter the detector. A single event of one X-ray photon entering the detector causes photoionization and produces a charge proportional to the photon energy. Numerous electrical sequences must take place before this charge can be converted to a data point in the spectrum.

When an X-ray photons enters the Si(Li) detector, it is converted into an electrical charge which is coupled to a Field Effect Transistor (FET). The FET, and the rest of the associated electronics which make up the preamplifier, produce an output proportional to the energy of the X-ray photon. Using a pulsed optical preamplifier, this output is in the form of a step signal. Because photons vary in both energy and number per unit time, the output signal, due to successive photons being emitted by a multielement sample, resembles a staircase with various step heights and time spacing. When the output reaches a predetermined level, the detector and the FET circuitry is reset to its starting level, and the process repeated.

The preamplifier stage integrates each detector charge signal to generate a voltage step proportional to the charge. This is then amplified and shaped in a series of integrating and differentiating stages. Owing to the finite pulse-shaping time, in the range of microseconds, the system will not accept any other incoming signals in the meanwhile (dead time), but extend its measuring time instead. In a further step the height of these signals is digitized as a channel number (analog-to-digital converter, ADC), stored to a memory (multichannel analyzed, MCA) and finally displayed as a spectrum, where the number of counts reflects the respective intensity. In a more modern approach, the output signals of the preamplifier are digitized directly, which can increase the throughput of the system significantly.

4. Energy Resolution

Mn-Kα spectrum and calibrated pulser

The energy resolution of the EDXRF spectrometer determines the ability of a given system to resolve characteristic X-rays from multiple-element samples and is normally defined as the full width at half maximum (FWHM) of the pulse-height distribution measured for a monoenergetic X-ray. A conventional choise of X-ray energy is 5.9 keV, corresponding to the Kα energy of Mn. Figure II.6 shows a typical pulse-height spectrum of Mn-Kα X-rays simultaneously with a calibrated pulser. The purpose of the pulser measurement is to monitor the resolution of the electronic system independent of any peak broadening due to the detector itself. Typical state-of the art detectors Si(Li) and Ge(HP) achieve 130 to 170 eV, but depends strongly on the size of the crystal. The smaller the crystal, the better is the resolution.

X RAY FLUORESCENCE

INTRODUCTION

X-ray fluorescence (XRF) analysis is one of the most common non-destructive methods for  qualitative as well as quantitative determination of elemental composition of materials. It is suitable for solids, liquids as well as powders. There are two main methodological techniques that are wavelength dispersive analysis (WD-XRF) and energy dispersive analysis (ED-XRF) (In the next post we will briefly discuss about WDXRF & EDXRF ,this post will only explain the basics of x-ray fluorescence which is required to understand the upcoming posts about WDXRF & EDXRF ). The spectra are collected simultaneously in a wide energy range. The range of detectable materials covers all elements from Sodium (Na) to Uranium (U) and the concentration can range from 100% down to ppm. Detection limit depends upon the specific element and the sample matrix but in general heavier elements have higher detection limit.

X-ray Fluorescence (XRF) Spectroscopy involves measuring the intensity of X-rays emitted from a specimen as a function of energy or wavelength. The energies of large intensity lines are characteristic of atoms of the specimen. The intensities of observed lines for a given atom vary as the amount of that atom present in the specimen. Qualitative analysis involves identifying atoms present in a specimen by associating observed characteristic lines with their atoms. Quantitative analysis involves determining the amount of each atom present in the specimen from the intensity of measured characteristic X-ray lines. The emission of characteristic atomic X-ray photons occurs when a vacancy in an inner electron state is formed, and an outer orbit electron makes a transition to that vacant state. The  energy of the emitted photon is equal to the difference in electron energy levels of the transition. As the electron energy levels are characteristic of the atom, the energy of the emitted photon is characteristic of the atom. Molecular bonds generally occur between outer electrons of a molecule leaving inner electron states unperturbed. As X-ray fluorescence involves transitions to inner electron states, the energy of characteristic X-ray radiation is usually unaffected by molecular chemistry. This makes XRF a powerful tool of chemical analysis in all kinds of materials. In a liquid, fluoresced X-rays are usually little affected by other atoms in the liquid and line intensities are usually directly proportional to the amount of that atom present in the liquid. In a solid, atoms of the specimen both absorb and enhance characteristic X-ray radiation. These interactions are termed 'matrix effects' and much of quantitative analysis with XRF spectroscopy is concerned with correcting for these effects.

X rays are electromagnetic radiation. All X-rays represent a very energetic portion of

the electromagnetic spectrum (Table 1) and have short wavelengths of about 0.1 to 100 angstroms (Å). They are bounded by ultraviolet light at long wavelengths and gamma rays at short wavelengths X-rays in the range from 50 to 100 Å are termed soft X-rays because they have lower energies and are easily absorbed.The range of interest for X-ray is approximately from 0.1 to 100 Å. Although,angstroms are used throughout these notes, they are not accepted as SI unit. Wavelengths should be expressed in nanometers (nm), which are 10-9 meters (1 Å = 10-10 m), but most texts and articles on micro probe analysis retain the use of the angstroms. Another commonly used unit is the micron, which more correctly should be termed  micrometer  (μm), a micrometer is 104 Å. The relationship between the wavelength of electromagnetic radiation and its corpuscular energy (E) is derived as follows. 

For all electromagnetic radiation:

E = h ν ;

where:

h is the Planck constant (6.62 10-24 J.s);

ν is the frequency expressed in Hertz.

For all wavelengths,

ν = c / λ ;

where:

c = speed of light (2.99782 108 m/s);

λ= wavelength (Å).

Thus:

E = hc / λ = 1.9863610−24 /λ ;

where E is in Joule and λ in meters.

The conversion to angstroms and electron volts (1 eV = 1.6021 10-19 Joule) yields the

Duane-Hunt equation:

E(eV) 12.396/ (A)

= λ . 

Note the inversion relationship. Short wavelengths correspond to high energies and long wavelengths to low energies. Energies for the range of X-ray wavelengths are 124 keV (0.1 Å) to 124 eV (100 Å). The magnitudes of X-ray energies suggested to early workers that Xrays are produced from within an atom. Those produced from a material consist of two distinct superimposed components: continuum (or white) radiation, which has a continuous distribution of intensities over all wavelengths, and characteristic radiation, which occurs as a peak of variable intensity at discrete wavelengths.

PROPERTIES OF X-RAYS

A general summary of the properties of X-rays is presented below:

• Invisible.
• Propagate with velocity of light (3.10^8 m/s).
• Unaffected by electrical and magnetic fields.
• Differentially absorbed in passing through matter of varying composition, density and thickness.
• Reflected, diffracted, refracted and polarized.
• Capable of ionizing gases.
• Capable of affecting electrical properties of solids and liquids.
• Capable of blackening a photographic plate.
• Able to liberate photo electron. And recoils electrons.
• Emitted in a continuous spectrum.
• Emitted also with a line spectrum characteristic of the chemical element.
• Found to have absorption spectra characteristic of the chemical element.

THE ORIGIN OF X-RAYS

An electron can be ejected from its atomic orbital by the absorption of a light wave

(photon) of sufficient energy. The energy of the photon (hν) must be greater than the energy with which the electron is bound to the nucleus of the atom. When an inner orbital electron is ejected from an atom, an electron from a higher energy level orbital will transfer into the vacant lower energy orbital (Figure). During this transition a photon may be emitted from the atom. To understand the processes in the atomic shell, we must take a look at the Bohr’s atomic model. The energy of the emitted photon will be equal to the difference in energies between the two orbitals occupied by the electron making the transition. Due to the fact that the energy difference between two specific orbital shells, in a given element, is always the same (i.e., characteristic of a particular element), the photon emitted when an electron moves between these two levels will always have the same energy. Therefore, by determining the energy (wavelength) of the X-ray light (photons) emitted by a particular element, it is possible to determine the identity of that element.

PRINCIPLE OF THE X-RAY FLUORESCENCE PROCESS

If the primary energy of X-rays is equal to or is larger than the binding energy of an inner shell electron it is likely that electrons will be ejected and consequently vacancies are created. The hole state has certain life time and becomes refilled again. The transition of the excited atom into a state with lower energy occurs via two competitive processes, the above mentioned photoelectric and Auger effects. In the photoelectric effect, the recombination is accompanied by a transfer of electrons from the outer shells with energy Em into the inner shells with energy En filling the vacancies. This process induces the emission of a characteristic X-ray (fluorescence) photon with energy

                                                                   hV = Em - En

Therefore the energy of these secondary X-rays is the difference between the binding energies of the corresponding shells in the figure below. The excited atom can also recombine by emission of Auger electrons, instead of characteristic X-rays, via the Auger effect.

The probability that characteristic X-rays will be emitted - and not an Auger electron- varies from one element to another and is described as the fluorescence yield. For elements of low atomic numbers, the Auger effect dominates, whereas emission of characteristic X-rays is more likely for heavy elements.

Each element has its unique characteristic energy spectrum (Fluorescence spectrum) composed by the allowed transitions of the specific atom in the result of X-ray excitation. XRF technique consists on the study of the produced characteristic spectrum. The XRF emission induced by photoelectron effect is shown in figure below for an atom of titanium (Z=22), whose K-shell electron acquires sufficient energy to escape from the atom.

Photoelectric effect on the K-shell

An electron in the K-shell absorbs a

photon of the primary x-ray beam and

becomes free, while the atom gains a

vacancy in the K-shell.

The K lines production

An electron from the L or M shell “jumps in” to fill the vacancy and in  turn, produces a vacancy in the L or M shell. In the process, the atom emits a characteristic photon from the x-ray  range of electromagnetic spectrum, unique to this chemical element

The L lines production

After a vacancy is created in the L shell by either the primary beam photon or by the previous event, an electron from the M or N shell “jumps in” to occupy the vacancy. In this process, the atom emits a characteristic photon, unique to this chemical element, and a vacancy in the M or N shell is produced

•  Ionization of the K-shell electron in the atom of Ti by photoelectric effect and emission of characteristic photons of different spectral series as a result of electron transitions in the atom.

              Electron transitions and emitted spectral lines in the atom after the K-shell ionization

X-ray fluorescence provides a rapid non-destructive means for both qualitative and quantitative analysis. A wide range of materials varying in size and shape can be studied with minimal requirements for sample preparation. Detection sensitivities as low as one part in a million can be obtained with this technique.The two types of X-ray fluorescence i.e EDXRF and WDXRF will be discussed and explained in the next post .

Some basic terms and definitions related to X- ray fluorescence, which can be useful in the upcoming post .

• Attenuation coefficient – a natural logarithm of the ratio of the emergent and incident radiation intensities I / I0 divided by either the depth of the radiation penetration (linear attenuation coefficient) or the surface density (mass absorption coefficient).
• Bremsstrahlung – a continuous spectrum produced by a charged particle moving with deceleration.
• Continuous spectrum – a spectrum formed by photons with non-quantized energies in a wide range.
• Detection limit – a lowest amount of chemical element that can be found with probability of 99%.
• Detector resolution – possibility to distinguish two overlapping peaks in the spectrum; depends on the ratio of the distance between the two peaks and FWHM; usually accepted as a value of FWHM.
• Efficiency of a detector – the ratio of the number of photons participated in creation of a useful signal in the detector to the total number of photons incident on the detector surface.
• Energy-dispersive technique – the technique used to simultaneously detect the photons of the line spectrum in a wide range of energies.
• Fluorescence – emission of photons by a substance that has absorbed photons with higher energy.
• FWHM – full width at half maximum of the peak usually measured in electronvolts.
• Ionizing radiation – the particles or electromagnetic waves whose energy is sufficient to ionize a neutral atom or a molecule.
• Line spectrum – a spectrum formed by photons with specific quantized energies only.
• Matrix effects – The combined effect of all components of the sample other than the analyte on the measurement of the quantity of the analyte. The two main matrix effects are::

                           -(a) The attenuation of characteristic peak intensity due to inelastic scattering of photons, emitted by atoms of one chemical element, on atoms and electrons of other components

                            -(b)The enhancement of characteristic peak intensity due to additional excitation of atoms of one element by photons, emitted by other components.

• Peak intensity – the value proportional to the total number of photons with same energy registered by a spectrometer and exposed as a bell-shaped curve called the peak.
• Quantitative analysis – determination of amount of each component (chemical element) of a sample.
• Spectral series – series of spectral peaks produced by electron transitions from different energy levels to one specific energy level; K-series corresponds to all transitions to the lowest possible energy level.
• Spectrum – a function of a number of photons versus their energy, or versus their wavelength.
• Spectrum background – A component of a spectrum which does not belong to the peak of interest, may be formed by bremsstrahlung radiation or by the tails of adjacent peaks.
• X-ray tube – A kind of a vacuum tube with a filament as a cathode, emitting electrons, and a pure metal plate as an anode, producing radiation in the x-ray range of electromagnetic spectrum.