Laser Induced Breakdown Spectroscopy (LIBS)
LIBS stands for laser-induced breakdown spectroscopy
and is a rapid chemical analysis technique, first introduced in 1960s. Unlike
in Raman spectroscopy, it uses short laser pulses to create plasma plumes on
the surface of a sample and the spectrum is characterized of this plasma using low-cost
spectrometers. It is also categorized as an in situ operation, that’s duration
can last up to only a few seconds.
HOW DOES IT WORK
The whole process can be separated into few
distinguished parts: laser ablation, plasma formation and the cooling of the
ignited plasma. The focused laser pulse initiates a high temperature raise,
generation of free-electrons. During this process a part of the sample mass is
removed, the process is called laser ablation. Afterwards, the ablated mass
interacts with the rest of the laser pulse, and microplasma is formed. It
contains free electrons, excited atoms and ions. Such excited particles are
formed, when the laser-emitted photons are absorbed by the sample atoms and
molecules, allowing them to reach excited energy levels. As a result plasma is
formed at the surface of the sample. When
the laser pulse ends, the plasma begins to expand and cool. The energy is then lost
in the process of recombination. Later on, the emitted light is collected and analyzed
by a high-speed camera, detector or a sensor, the most popular one being Czerny-Turner
and Echelle spectrographs. The secondary light of the plasma is collected and
transported through an optical fiber to the detector. Spectroscopy principles
can be applied to this operation, since each element or material has a defined
peak intensity wavelength that it emits. A specific element has its own LIBS
spectral peak fingerprint, that can be used to identify the existing elements
in the sample. The received spectra are defined by wavelength, intensity and
the shape of the signal. This intensity information on the received spectra
peaks can define the quantity and concentration of a specific element in the
chemical composition.
LASER INDUCED PHOTOABLATION
Photoablation, or laser ablation is defined as a
process, in which a miniscule amount of the surface material (solid or liquid)
is removed off of a sample. The high amount of energy leads to the evaporation
or sublimation of a material, that is induced due to the received energy of the
absorbed photons. The energy amount is so great, that the material is converted
to plasma state.
The most important part of the whole LIBS system and
the main tool to attain the ablation effect is a pulsed laser. Both a
femtosecond and a picosecond laser can be utilized, though the nanosecond laser
is recalled as the most preferable one. Most commonly, a pulsed fundamental
harmonics (1064 nm or 1030 nm wavelength) laser is used, as well as its
harmonic wavelengths (532 (515), 355 (343), 266 (257) nm). All of these selected devices create
different effects on the sample material, but when a nanosecond laser is used,
the main driving force of the ionization process are thermal effects. Since the
duration of the course of heating is about 104 times shorter that
the laser pulse itself, for a brief moment, a part of the sample is melted and
vaporized. It is worth mentioning, that both a free-space and a fiber laser can
be used, both are a great light source for LIBS setups.
The properties of the selected laser tightly
correspond with the absorption of energy, since the received energy depends on
the emitted photon characteristics. These parameters include the lasers wavelength,
pulse duration and fluence. For the reason of energy being reversely dependent
on wavelength, a shorter wavelength induces a greater ablation rate, though a
higher wavelength can result in a more efficient plasma excitation. Another,
already mentioned, parameter is the pulse duration - it determines the effects
taking place before the plasma is created. When nanosecond pulses are selected,
the material undergoes thermal processes and changes from solid, through liquid
and gas, to plasma, while the rest of the pulse heats the plasma (the plume
itself can reach up to 15000-30000 K temperature). Pulses of nanosecond
duration are favored, since they heat up the microplasma at a slower rate, as a
result a larger and less dense plume is generated. This leads to clear and
intense emission lines, that make the data reading process much more easy and
smooth. Lastly, the intensity and fluence (energy per area) of the laser pulse
have to reach a certain threshold (ionization potential) to produce
sublimation, evaporation and the formation of the high-density plasma. A quite
similar characteristic is irradiance (power per surface area) - it must be
optimized, for the reason of most effectively ablating the smallest speck
possible. This can be achieved by combining a lens with a high irradiance laser
source. Certainly, other, non-laser related, environmental factors and
properties of the sample material play a role in the whole LIBS process, though
is it the laser source that is readjusted accordingly to the given
circumstances. As all the mentioned properties play a significant part of the
laser ablation process, it is the delicate combination of all, that produces
the desired effect.
PLASMA LIFETIME
The life time of the plasma plume has been observed in
depth, its evolution and effects occurring during it have been described
properly. Several states in the evolution of plasma can be separated, such
including formation, shielding, cooling and emission radiation.
The plasma itself is created after the photoablation
process. After the plasma creation, during a following time period called
plasma shielding, the plasma becomes opaque to the laser, and the photon pulses
begin to interact only with the surface of the created plasma plume. The latter
reflects the laser energy away from the rest of the sample, reducing the overall
ablation rate. During this time period, the microplasma is heated up and
expands, thus sending shock waves through the environment, as the surrounding
particles are compressed. The remaining time is dominated by the process of
cooling and spectrum emission. This happens through recombination of electrons
and positive ions and de-excitation of atoms, ions and molecules. The excited
particles begin to lose their kinetic energy in the form of light, with their
characteristics, relevant to the chemical structure of the sample.
During the lifetime of the plasma, few types of
radiation can be observed, including continuum, atomic, ionic and molecular
emissions, all of them revealing different plasma components. The first moments
of the ignited plasma plume are dominated by continuum radiation. The emission
that shows the relevant information is the atomic radiation, which happens
right after the plasma has cooled. The received spectra are described by their
wavelength, shape and intensity. The wavelength of the received signal helps to
deduce the energy levels of the sample material, while the different intensity
of atomic lines indicate the quantity of the specific element. The collected
data greatly depends on the mass that was ablated, the received energy from the
laser and environmental conditions.
In the process of LIBS, it is highly important to
carefully choose the gate time interval, the exact moment when it begins and
ends, due to the fact that plasma creates delays the atomic lines emission
window. The delay time and integration time are both defined by the wavelength
of the laser utilized. Both of these time periods are analyzed in relation to
one another – by experimentally comparing the delay and integration time, a
signal to noise ratio is observed. Often the integration time is referred to as
a gate window, this exact time period provides the needed information for trace
spectroscopic analysis.
APPLICATIONS
The described technique is a valuable tool for
research of delicate or fragile objects or materials. Numerous examples can be
listed, one of them being the field of archeology, where LIBS proves itself to
be a practical way to date certain samples, such as dyed artifacts, sculptures,
ceramics. The method is utilized in biomedicine – it can perform as a tool to
analyze and classify various tissues, detect and treat tumors. In the
industrial field, it is a common technique used on different products, to specify
their chemical composition, like determine dopants in dielectrics, sort
recyclable metals or plastics. LIBS has also been used for measurements
including dangerous materials, such as radioactive and toxic substances or
explosives. It is possible to detect the same toxic materials or waste in
geological samples, as well as monitor selected substances in a specific
environment (e. g. pollution in water or air). Such environmental analysis can be
carried out by adapting LIBS, even so, that these devices are being used for
space exploration, like in the Martian mission – the rover Curriosity contains
a LIBS instrument ChemCam, which is used for chemical composition analysis of
rocks and soil of Mars. Indisputably, laser induced breakdown spectroscopy is a
widely applicable analysis technique.
Some advantages that LIBS carries with itself are worth mentioning - no preparation is needed for the sample, the short measurement time (a few seconds for a specific spot of the specimen), the technique can cover a broad variety of elements, can include surface of depth profiling by using different sampling protocols. In all cases, LIBS has been described as a beneficial chemical analysis technique, with a gradually growing development and usage.