LASER INDUCED BREAKDOWN SPECTROSCOPY

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 II. Considerations  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.  Light sensitivity 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)

COMPARISON BETWEEN ED-XRF AND WD-XRF

We have discussed about  ED-XRF & WD-XRF in the earlier posts . Now I would like to highlight the major differences between the two X-ray techniques . The most important point of comparison are listed below ::

1. RESOLUTION :

It describes the width of the spectra peaks. The lower the resolution number the more easily an elemental line is distinguished from the nearby X-ray line intensities.

a) The resolution of the WD-XRF system is dependent on the crystal and optics design,particularly collimation, spacing and positional reproducibility. The effective resolution of a WD-XRF system may vary from 20 eV in an inexpensive bench top to 5 eV or less in a laboratory instrument. The resolution is not detector dependant.

Advantage of WD-XRF: High resolution means fewer spectral overlaps and lower

background intensities

b)  The resolution of ED-XRF system is dependent on the resolution of the detector. This can vary from 150 V or less for a liquid nitrogen cooled Si(Li) detector, 150 – 220 eV for various solid state detectors, or 600 eV or more for gas filled proportional counter.

Advantage of ED-WRF: WD-XRF crystal and optics are expensive, and are one more failure mode.

2. SPECTRAL OVERLAPS:

Spectral deconvolutions are necessary for determining net intensities when two spectral lines overlap because the resolution is too high for them to be measured independently.

a) With a WD-XRF instrument with very high resolution (low number of eV) spectral

overlap corrections are not required for a vast majority of elements and applications.

The gross intensities for each element can be determined in a single acquisition.

Advantage WD-XRF: Spectral deconvolutions routines introduce error due to counting statistics for every overlap correction onto every other element being corrected for. This can double or triple the error

b) The ED-XRF analyzer is designed to detect a group of elements all at once. The some type of deconvolutions method must b used to correct for spectral overlaps. Overlaps are less of a problem with 150 eV resolution systems, but are significant when compared to WD-XRF. Spectral overlaps become more problematic at lower resolutions.

3. BACKGROUND: 

The background radiation is one limiting factor for determining detection limits, repeatability, and reproducibility.

a) Since a WD-XRF instrument usually uses direct radiation flux the background in the region of interest is directly related to the amount of continuum radiation within the region of interest the width is determined by the resolution.

b) The ED-XRF instrument uses filters and/or targets to reduce the amount of continuum radiation in the region of interest which is also resolution dependant, while producing a higher intensity X-ray peak to excite the element of interest.

Even, WD-XRF has the advantage due to the resolution. If a peak is one tenth as wide it has one tenth the background. ED-XRF counters with filters and targets that can reduce the background intensities by a factor of ten or more.

4. EXCITATION EFFICIENCY:

Usually expressed in PPM per count-per-second (cps) or similar units, this is the other main factor for determining detection limits, repeatability, and reproducibility. The relative excitation efficiency is improved by having more source x-rays closer to but above the absorption edge energy for the element of interest.

a. WDXRF generally uses direct unaltered x-ray excitation, which contains a continuum of energies with most of them not optimal for exciting the element of interest.

b. EDXRF analyzers may use filter to reduce the continuum energies at the elemental

lines, and effectively increasing the percentage of X-rays above the element absorption edge. Filters may also be used to give a filter fluorescence line immediately above the absorption edge, to further improve excitation efficiency. Secondary targets provide an almost monochromatic line source that can be optimized for the element of interest to achieve optimal excitation efficiency.