LIBS Basics, Part II: Hardware - - Spectroscopy
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LIBS Basics, Part II: Hardware


Spectroscopy
Volume 29, Issue 4, pp. 26-31

Laser-induced breakdown spectroscopy (LIBS) is an emerging analytical method that has been the focus of substantial research over the last 25 years. The recent emergence of commercially available LIBS systems from major manufacturers is a sign of the maturing of the technology. This column installment is the second of a three-part series focusing on major aspects of LIBS. The first installment set the stage with the basics of the LIBS measurement physics and standard applications. This installment discusses the choices for LIBS hardware in detail, particularly lasers and spectrometers, and illustrates the trade-offs between cost, size, and performance. The third installment will discuss LIBS analysis in some depth, exploring the various ways to go from a LIBS spectrum to a solution. Overall, this series is intended to provide an overview for those considering implementation of LIBS to solve a particular analytical problem, and an introduction for those interested in learning more about LIBS.

The growth of interest in laser-induced breakdown spectroscopy (LIBS) analysis of materials has been rapid during the past 10 years, similar to the rise of Raman spectroscopy or atomic emission spectroscopy in earlier decades. Part I of this series (1) documented the nearly exponential rise in LIBS papers from 1995 through 2010. We are now at the point at which many, if not most, people in the analytical community have heard of LIBS, and there are several companies offering commercial LIBS systems that are both laboratory- and industrially focused. The capabilities of the LIBS systems are defined first by their hardware, which defines the laser-sample interaction, the plasma, and the collected spectra, and second by the analytics that interpret the spectra. This installment of "Lasers and Optics Interface" focuses on the hardware portions of LIBS systems, particularly on the laser and spectrometer choices.

Lasers

Three basic parameters are at play in selecting and integrating a laser into a LIBS system: pulse duration, laser wavelength, and laser fluence on the sample. Secondarily, one may consider a double-pulse arrangement in which the ablation laser pulse and analytical pulse are separated by time on the order of microseconds. However, the double-pulse arrangement is typically deployed only in laboratory experiments or very specialized applications because of the added complexity and element- and matrix-specific enhancement observed in the LIBS signal. Because the double-pulse setup is not yet a feature in mainstream systems, we will leave out a detailed discussion of double-pulse interactions and refer readers to one of several excellent reviews available (2).

Nanosecond Versus Femtosecond Lasers

Two basic choices are available for laser pulse duration, nanosecond-class lasers and femtosecond-class lasers. Characteristic lattice vibrations (phonon frequencies) are 1012 –1013 Hz, corresponding at the highest rates to wavelengths on the order of atomic spacing in solids. Laser pulses on the order of a nanosecond (10-9 s) duration are thus very long with respect to characteristic lattice vibration periods in solids, so there is significant heat transfer in the solid during the laser pulse. Picosecond lasers, with pulses on the order of tens or hundreds of picoseconds, are typically in a similar mode of interaction to nanosecond lasers. It is only femtosecond lasers, with pulses typically on the order of 10-13 s, that can interact with solid materials without significant heat transfer.


Figure 1: Craters obtained by (a) nanosecond and (b) femtosecond laser ablation. Adapted with permission from reference 3.
The implications of the nanosecond versus femtosecond laser choice can be observed pictorially in Figure 1, which shows a laser ablation crater from each class of laser. The nanosecond laser crater (Figure 1a) looks like an impact crater on the moon, with a ridge around the laser impact point and debris scattered around the crater. The crater itself shows evidence of significant melting, and the debris are from direct physical removal and melting as well as nucleation and condensation of small particles from melted vapor in the laser plume. The femtosecond crater (Figure 1b), on the other hand, looks very smooth and appears to have no melting. Femtosecond ablation is much more of a mechanical process, with Coulomb explosion and direct photochemical bond breaking being a few of the major removal processes. Unlike nanosecond ablation spots, there is a minimal heat-affected zone because the process is not thermally driven. As a result, femtosecond ablation produces a more stoichiometric and representative mixture in the plume than does nanosecond laser ablation, because there are no thermal effects.

For laser ablation inductively coupled plasma–mass spectrometry (ICP-MS) and laser ablation optical emission spectroscopy (OES), femtosecond lasers have been shown to provide more precision and accuracy than nanosecond lasers (4). This is thought to result largely because of the particle distribution from femtosecond ablation, which is markedly smaller (in average size) than the distribution from nanosecond ablation, and thus evaporates better in the plasma torch. However, for LIBS there are countervailing factors in favor of nanosecond lasers for LIBS. The plasma from a typical nanosecond laser is much more robust than a femtosecond pulse. As a result, the integrated emission may be greater, and studies have shown that early in the plasma electron densities and temperatures are higher (5). Overall, the detection limits are comparable for nanosecond and femtosecond LIBS, in most cases, and additional variables such as laser fluence and interactions with the particular solid material being analyzed play a role in the precision and accuracy of the measurement.

Given the considerations in choosing between a nanosecond and a femtosecond laser, cost and reliability typically become the driving factors. Although great strides have been made in femtosecond laser technology in the past decade, femtosecond lasers are still roughly 8–10 times more expensive than comparable nanosecond-class lasers such as Nd:YAG. These lasers cannot be used outside of a laboratory because they are generally more fragile and subject to alignment and reliability issues than nanosecond lasers are. For these reasons, except for high-end laboratory systems that are primarily used in research and development, LIBS systems are typically outfitted with nanosecond-class lasers.


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