Stephen Hamilton, Andor Technology, and George Asimellis, Aristotle University of Thessaloniki
Although the first laser plasma was generated in the early years of laser development, it was not until the late 1990s that laser-induced breakdown spectroscopy escaped the borders of scientific curiosity. One can simply describe it as a technique for identifying the elemental composition of various samples, regardless of state: It can be used to interrogate solids, liquids and gases.
Its principle is simple (Figure 1). First, a high-power laser pulse is focused onto a sample. Nearly instantaneous absorption then locally ablates a thin layer of material, which subsequently gains energy from the laser beam and is heated into the ionic state, creating the laser-induced plasma, or “spark.” The excited atoms and ions relax, in part, by emitting light at characteristic atomic emission lines, and spectral analysis of the plasma can be used to determine the ablated sample’s elemental composition.
Figure 1. Results from a laser-induced breakdown spectroscopy system can be obtained in seconds. A typical system’s main components are illustrated in this diagram.
Emission from the plasma is directed to a spectrograph, and an attached detector collects data for qualitative and quantitative analysis, which is accomplished using specifically designed software. Depending on the type of spectrograph, researchers can conduct simultaneous multielement analysis.
This technique offers several advantages over others, such as atomic emission spectrometry with inductively coupled plasma (ICP-AES) and x-ray fluorescence. Laser-induced breakdown spectroscopy is a highly versatile technique requiring little or no sample preparation. The user need only bring the pulsed laser onto the sample or bring the sample under the laser pulse. There is no need to have the sample ground, as is the case for ICP-AES. Sample inhomogeneity might be either exploited (to identify different layers, for example) or avoided.
Many laser-induced breakdown spectroscopy instruments are also field-portable. In some cases, samples may not be moved from site; for example, archaeological samples sometimes must be analyzed in situ, and industrial online processing can be implemented only when the analytical instrument is on-site.
This technique also can detect all elements, even the low atomic numbers. The sampling is as extensive as the focused laser spot; that is, it makes submillimeter localized analysis possible. Finally, the technology is remarkably fast. Results can be obtained in a few seconds, or minutes in the case of a multispot analysis.
Burgeoning applications
Its range of applications is growing rapidly and becoming increasingly diverse, driven largely by requirements for instrumentation in the environmental monitoring and materials processing industries.
A key setting for this technique is the oil industry. Most oil exploration consists of multidirectional drilling from one rig site to multiple “spoke” destinations. The drills must be continuously monitored as they drive through various geological formations and/or faults because oil deposits are typically sandwiched between specific geological formations. These formations can be identified by monitoring specific elemental data that differentiate them.
A laser-induced breakdown spectroscopy instrument at the well site can monitor cutting samples from the shale shakers to help operators make more informed drilling decisions, such as picking casing and coring points; this aids them in forecasting drilling operations. The favorable cost-benefit equation makes laser-induced breakdown spectroscopy a viable competitor to conventional well-site biostratigraphy.
Another important application is in the pharmaceutical industry. Manufacturers of solid dosage treatments are taking an interest in the promising elemental analysis capabilities that this method provides, particularly in quality control; pill samples can be examined for concentration of specific elements in prescribed depths.
Successive shots and analysis can provide sample depth profiling, blend and tablet uniformity, coating thickness analysis, lubricant and disintegrant distribution, and spatial resolution and mapping information of solid dosage forms. Such operations are not possible with ICP-AES or x-ray fluorescence.
In the coal industry, laser-induced breakdown spectroscopy detection is used to monitor sulfur levels in coal. In mines, it can simultaneously detect phosphorus and silicon levels to help ore beneficiation. The recycling industry uses this technique to identify classes of recyclable materials, depending on concentration of core elements. Instruments for this technology can also be found in industrial applications such as security, archaeology, nuclear power, art restoration, environmental contamination and space exploration.
Emerging trends
Three major trends in instrumentation have emerged. One is the high-end instrument, in which resolutions and detection limits are advancing rapidly (though usually at the expense of higher costs and less portability). The most advanced systems use high-power Nd:YAG lasers in the vicinity of 200 mJ per pulse, a high-resolution Czerny-Turner spectrograph and an intensified CCD to perform accurate time-domain measurements.
Typical production instruments of this class have also been demonstrated as field-deployable; they can be transported to the site and can operate on limited infrastructure support. In most cases, an electric generator is sufficient.
The capabilities of such an instrument are dictated by its sensitivity and robustness as well as by the spectral range of the intensified CCD. Limits of detection are currently in the region of one to 10 parts per million, and precision is, at best, in the vicinity of 2 to 3 percent. To gain a competitive status with ICP-AES, sensitivity, precision and accuracy must improve. Improvements in intensified CCD technology will be necessary to obtain parts-per-billion performance at comparable levels of sensitivity.
A second trend is the configuration of cost-effective instruments with smaller spectrometers, less powerful Nd:YAG lasers and nonintensified CCDs. These systems can be successfully used in “clean” matrices for qualitative analysis only, not quantitative. However, because of their small size, they can be field- or man-portable. A spectroscopy system of this sort could be particularly effective in applications such as security and environmental monitoring, in which a mass of data must be analyzed quickly.
Simultaneous measurement
The third key trend highlights the need to simultaneously — i.e., with one shot — map a large amount of spectral information. While the spectral window in a Czerny-Turner spectrograph may be between 20 and 60 nm at a time, specific applications require a much wider range. Two approaches currently allow simultaneous measurements of the bandwidth of interest. One approach is to deploy multiple spectrographs, with simultaneous feeds from the plasma emission via a fiber split. Another is to use an echelle spectrometer, which includes a specialized grating and a cross-disperser to image a large number of spectral orders on a single CCD device.
Figure 2. With Andor’s iStar system containing an intensified CCD and echelle spectrograph, the spectral orders do not merge, allowing a greater number of lines to be resolved.
Andor Technology’s iStar has a combined spectrograph and detector that employs a high-resolution, high-dynamic-range intensified CCD and a unique patented echelle spectrograph (Figure 2). This allows a greater number of lines to be resolved because the spectral orders do not merge as a result of the dispersion-balanced order sorting system. This application is the enabling technology for simultaneous spectral mapping, promising calibration-free operation as well as spectral fingerprinting.
Contact: Stephen Hamilton, physicist, Andor Technology, Belfast, Northern Ireland; e-mail: [email protected]. George Asimellis, assistant professor, Aristotle University of Thessaloniki, Thessaloniki, Greece; e-mail: [email protected].