Laser induced breakdown spectroscopy

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Schematic of a LIBS system - Courtesy of US Army Research Laboratory
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Schematic of a LIBS system - Courtesy of US Army Research Laboratory

Laser Induced Breakdown Spectroscopy (LIBS) is a type of atomic emission spectroscopy which utilises a highly energetic laser pulse as the excitation source. LIBS can analyse any matter regardless of its physical state, be it solid, liquid or gas. Even slurries, aerosols, gels, and more can be readily investigated. Because all elements emit light when excited to sufficiently high temperatures, LIBS can detect all elements, limited only by the power of the laser as well as the sensitivity and wavelength range of the spectrograph & detector. Operationally, LIBS is very similar to arc/spark emission spectroscopy.

A typical LIBS system consists of a neodymium doped yttrium aluminium garnet (Nd:YAG) solid-state laser and a spectrometer with a wide spectral range and a high sensitivity, fast response rate, time gated detector. This is coupled to a computer which can rapidly process and interpret the acquired data. As such LIBS is one of the most experimentally simple spectroscopic analytical techniques, making it one of the cheapest to purchase and to operate.

The Nd:YAG laser generates energy in the near infrared region of the electromagnetic spectrum, with a wavelength of 1064 nm. The pulse duration is in the region of 10 ns generating a power density which can exceed 1 GW·cm-2 at the focal point. Other lasers have been used for LIBS mainly Excimer (Excited dimer) type generating energy in the visible and ultraviolet regions.

The spectrometer consists of either a monochromator (scanning) or a polychromator (non-scanning) and a photomultiplier or CCD detector respectively. The most common monochromator is the Czerny-Turner type whilst the most common polychromator is the Echelle type, even so the Czerny-Turner type can be (and is often) used to disperse the radiation onto CCD effectively making it a polychromator. The polychromator spectrometer is the type most commonly used in LIBS as it allows simultaneous acquisition of the entire wavelength range of interest.

The spectrometer collects electromagnetic radiation over the widest wavelength range possible, maximising the number of emission lines detected for each particular element. Spectrometer response is typically from 1100 nm (near infrared) to 170 nm (deep ultraviolet), the approximate response range of a CCD detector. All elements have emission lines within this wavelength range. The energy resolution of the spectrometer can also affect the quality of the LIBS measurement, since high resolution systems can separate spectral emission lines in close juxtaposition, reducing interference and increasing selectivity. This feature is particularly important in specimens which have a complex matrix, containing a large number of different elements. Accompanying the spectrometer and detector is a delay generator which accurately gates the detectors response time, allowing temporal resolution of the spectrum.

LIBS operates by focusing the laser onto a small area at the surface of the specimen, when the laser is discharged it ablates a very small amount of material, in the range of nanogram to picogram which instantaneously superheats generating a plasma plume with temperatures of about 10,000-20,000 °C. At these temperatures the ablated material dissociates (breaks down) into excited ionic and atomic species. During this time the plasma emits a continuum of radiation which does not contain any useful information about the species present, but within a very small timeframe the plasma expands at supersonic velocities and cools. At this point the characteristic atomic emission lines of the elements can be observed. The delay between the emission of continuum radiation and characteristic radiation is in the order of 10 µs, this is why it is necessary to temporally gate the detector.

Because such a small amount of material is consumed during the LIBS process the technique is considered essentially non-destructive or minimally-destructive, and with an average power density of less than one watt radiated onto the specimen there is almost no specimen heating surrounding the ablation site. Due to the nature of this technique sample preparation is typically minimised to homogenisation or is often unnecessary where heterogeneity is to be investigated or where a specimen is known to be sufficiently homogeneous, this reduces the possibility of contamination during chemical preparation steps. One of the major advantages of the LIBS technique is its ability to depth profile a specimen by repeatedly discharging the laser in the same position, effectively going deeper into the specimen with each shot. This can also be applied to the removal of surface contamination, where the laser is discharged a number of times prior to the analysing shot. LIBS is also a very rapid technique giving results within seconds, making it particularly useful for high volume analyses or on-line industrial monitoring.

LIBS is an entirely optical technique, therefore it requires only optical access to the specimen. This is of major significance as fibre optics can be employed for remote analyses. And being an optical technique it is non-invasive, non-contact and can even be used as a stand-off analytical technique when coupled to appropriate telescopic apparatus. These attributes have significance for use in areas from hazardous environments to space exploration. Additionally LIBS systems can easily be coupled to an optical microscope for micro-sampling adding a new dimension of analytical flexibility.

The use of specialised optics or a mechanically positioned specimen stage can be used raster the laser over the surface of the specimen allowing spatially resolved chemical analysis and the creation of 'elemental maps'. This is very significant as chemical imaging is becoming more important in all branches of science and technology.

Recent interest in LIBS has focused on the miniaturisation of the components and the development of compact, low power, portable systems. This direction has been pushed along by interest from groups such as NASA, ESA as well as the military. The ExoMars mission plans to bring a combined Raman/LIBS onto mars. Portable LIBS systems are more sensitive, faster and can detect a wider range of elements (particularly the light elements) than competing techniques such as portable x-ray fluorescence. And LIBS does not utilise ionizing radiation to excite the sample, which is both penetrating and potentially carcinogenic.

LIBS, like all other analytical techniques is not without limitations. LIBS is subject to the matrix effect which can be minimised by good specimen preparation and the use of accurate calibration standards, it is also subject to variation in the laser spark and resultant plasma which often limits reproducibility. The accuracy of LIBS measurements is typically better than 10% and precision is often better than 5%. The detection limits for LIBS vary from one element to the next depending on the specimen type and the experimental apparatus used. Even so detection limits of 1 to 30 ppm by mass are not uncommon, but can range from >100 ppm to <1 ppm.

Recent developments in LIBS have seen the introduction of double-pulsed laser systems. This operates by discharging the laser twice in the same position on the specimen. The pulse separation is typically in the order of a couple of hundred microseconds, and the spectral analysis is conducted after the second pulse. This process increases the sensitivity of LIBS and reduces errors caused by the differential volatility of elements (such as that of Zinc compared to Copper in brasses), it also significantly reduces the matrix effects. Double-pulsed systems are also proving useful in conducting analysis underwater, as the initial laser pulse forms a cavity bubble in which the second pulse acts on the evaporated material.

LIBS is technically very similar to a number of other laser-based analytical techniques, sharing much of the same hardware. These techniques are the vibrational spectroscopic technique of Raman spectroscopy, and the Fluorescence spectroscopic technique of Laser-induced fluorescence (LIF). In fact devices are now being manufactured which combine these techniques in a single instrument, allowing the atomic, molecular and structural characterisation of a specimen as well as giving a deeper insight into physical properties.

LIBS can often be referred to as its alternative name—laser induced plasma spectroscopy (LIPS). Unfortunately the term LIPS has alternative meanings that are outside the field of analytical spectroscopy, therefore the term LIBS is preferred.

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[edit] Reference

  • Lee W. B., Wu J. Y., Lee Y. I., Sneddon J. (2004). "Recent applications of laser-induced breakdown spectrometry: A review of material approaches". Applied Spectroscopy Review 39: 27-97. DOI:10.1081/ASR-120028868.
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