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.
