LIBS, is a spectroscopy technique in which a short laser pulse beam is focused on a target sample. Laser energy ionizes the sample material by heating it, creating small area of plasma. Excited ions in the plasma state emits light waves which are collected and the spectrum is resolved by a spectrometer and analyzed by suitably calibrated photon or light detector. Each chemical element has
a unique wavelength or signature which can be optically resolved from
the obtained spectrum. As result, the composition of the elements which constitutes in the target sample can be determined. Below provided some of the general information about the technique :
i Advantages ii Considerations
- Spectral coverage vs.
resolution
- Light sensitivity
iii. General Applications
I. Advantages
LIBS is considered one of the most efficient and user friendly analytical techniques for trace elemental analysis in gases,
solids, and liquids. Some of its major advantages include:
- Real-time
measurements: online monitoring and quality control of industrial
processes
- Noninvasive,
nondestructive technique: valuable samples can be reused, sensitive materials
can be analyzed, suitable for in-situ biological analysis
- Remote
measurements can be done from up to 50 meters distance: can be used in
hazardous environments and for space exploration missions on other
planets
- Compact
and inexpensive equipment: can be widely used in industrial
environments, perfect for field measurements
- High-spatial
resolution: can obtain 2D chemical and mechanical profiles of virtually
any solid material with up to 1 µm precision
- Non
or very little sample preparation is required: reduced measurement time,
greater convenience, less opportunity for sample contamination
- Samples
can be in virtually any form: gas, liquid, or solids
- Analysis
can be performed with a very small amount of sample (nanograms): very
useful in chemistry for characterization of new chemicals and in
material science for characterization of new composite materials or
nanostructures
- Virtually
any chemical element can be analyzed, such as heavier elements
unavailable for X-ray fluorescence
- Analysis
can be done on extremely hard materials like ceramics and
superconductors; these materials are difficult to dissolve or sample to
perform other types of analysis
- In
aerosols both particle size and chemical composition can be analyzed
simultaneously
- Spectral
Coverage vs. Resolution
Compact echelle spectrometers designed for LIBS
applications are offered by several manufacturers.
In the rare occasion that an application requires
even higher resolution, the Acton Series of spectrometers with
their long focal lengths are extremely useful. The latest models use
toroid mirrors with improved spectral quality.
For a detector with 1024 horizontal pixels, each of which is 26 m wide, the
theoretical field of view is 26.6 mm. But since a standard 25 mm intensifier
is used, the field of view is 25 mm. For example, if you decided to utilize a 2400 groove/mm grating in the Acton
Series 2500 in order to enhance resolution, the linear dispersion will
be 0.6 nm/mm while the spectral coverage will be 0.6*25 = 15 nm. To cover a
spectral range between 300 and 600 nm (for example), you will need to
perform at least 20 laser shots each time, moving the spectrometer grating to
a new position and "gluing" all 20 spectra together. This is a very
standard procedure which can be done painlessly and automatically.
The only disadvantage to this is that
acquisition of one spectrum could take up to a few dozen seconds or longer,
which is why the echelle spectrometer has become extremely popular,
especially in industrial and field applications where real-time measurements
such as online quality control is a must.
Typically, the laser pulse in LIBS applications
lasts for femto- to nanoseconds (10-15 to 10-9 s).
Especially in applications where non-invasive and non-destructive analysis is
required, a relatively small amount of laser energy is transferred to the
sample. Therefore, one laser pulse produces a weak emission signal which is
hard or impossible to collect with conventional CCD detectors. That is why
intensified CCDs (ICCDs) are widely used in LIBS.
To improve the emitting signal on the order of
10-30 times, a scheme with two orthogonal lasers beams is often used. In this
dual-scheme, the first and usually more powerful laser pulse ablates and
atomizes sample material while the second one heats the ablated material even
further, allowing it to improve the intensity of atomic or ionic lines.
Factors such as the level of laser excitation energy for both pulses and the
time delay between the pulses play a crucial role in achieving signal
intensity enhancement. This technique increases the sensitivity of LIBS
by at least one order of magnitude and allows for a
greater possible number of applications.
If measurement time duration is not an issue, a
regular CCD, (1024x1024 pixels, 13 µm pixel size), can
be used together with the an spectrometer for LIBS applications. To
obtain the reasonable light level required for a non-intensified CCD, long
exposure time measurements should be performed. In this case, plasma emission
signal is accumulated on the CCD from a multiple laser pulse. However, one
should be careful about excessive accumulation of background noise and low
signal-to-noise ratio. It is especially important when performing
measurements in the open air without an enclosed sample chamber. Since the
CCD stays open for a long period of time, all sources of stray light in the
room should be eliminated and measurements should be conducted in
darkness. CCD usually proves a more sophisticated
system than the ICCD because intensified CCDs are prone to
permanent damage by excessive light levels. Extra care should be
taken so as not to expose ICCDs to the bright sources of light like
laser reflections. In the case of a regular CCD, it is difficult to damage
with excessive light.
III. General Applications
The fact that LIBS generally requires
little-to-no sample preparation, simple instrumentation, and can easily be
performed on-the-field in hazardous industrial environments in
real-time, it is a very attractive analytical tool. The following
are a few examples of real life applications, where LIBS is successfully
used:
- Express-analysis
of soils and minerals (geology, mining, construction)
- Exploration
of planets (such as projects using LIBS for analyzing specific
conditions on Mars and Venus to understand their elemental
composition)
- Environmental
monitoring (Real-time analysis of air and water quality, control of
industrial sewage and exhaust gas emissions)
- Biological
samples (non-invasive analysis of human hair and teeth for metal
poisoning, cancer tissue diagnosis, bacteria type detection, detection
of bio-aerosols and bio-hazards, anthrax, airborne infectious disease,
viruses, sources of allergy, fungal spores, pollen). Replacing antibody,
cultural, and DNA types of analysis
- Archeology
(analysis of artifacts restoration quality)
- Architecture
(quality control of stone buildings and glasses restoration)
- Army
and Defense (detection of biological weapons, explosives, backpack-based
detection systems for homeland security)
- Forensic
(gun shooter detection)
- Combustion
processes (analysis of intermediate combustion agents, combustion products,
furnace gases control, control of unburned ashes)
- Metal
industry (in-situ metal melting control, control of steel sheets
quality, 2D mapping of Al alloys)
- Nuclear
industry (detection of cerium in U-matrix, radioactive waste disposal)
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