What is LIBS?

Laser-Induced Breakdown Spectroscopy (LIBS) is a spectroscopic technique using a laser-generated plasma to ablate and excite a sample, which can initially be in solid, liquid, or gaseous form. Emission generated from the plasma is used to identify material constituents and can be used to identify, sort, and classify materials.

What can LIBS be used for?

LIBS can be used both in-process and in the laboratory for material identification. A very versatile method, it has the primary advantages of

	Rapid analysis
	No sample preparation for most samples
	Sensitive to a wide variety of elements
	Simultaneous reporting of elements

Laser-Induced Breakdown Spectroscopy utilizes a focused pulse from a high-powered laser to create a plasma in or on a solid, liquid, or gaseous media. Some of the energy in the plasma is used to ablate solid or liquid material (if present), and the plasma rapidly expands to form a gas plasma which is used to analyze the ablated particles.

In the core of the plasma, effective temperatures can easily exceed 50,000 K or several eV. During this stage, material in the core of the plasma is vaporized and atomized, and the plasma is typically highly ionized. Depending on conditions, but typically after 0.5 – 1 microsecond, the neutral states of the plasma typically reach local thermodynamic equilibrium. From this time onward, upper electronic states of atoms are thermally populated in Boltzmann equilibrium, such that the emission intensity of the atomic fluorescence I ~ exp(E/kT), where E is the upper state energy level of the fluorescing species.

As the plasma cools, continuum emission from the plasma (Bremstraalung emission, which we see as bright white emission) fades, typically much faster than emission lines from neutral and singly-ionized atomic lines, such that each elemental emission line has a particular optimum in a particular plasma. This optimum depends on the time / temperature history of the plasma, which in turn is dependent on the laser pulse energy and pulse length. For typical 50 – 400 mJ, ~10 ns pulses, the sequence of events in the plasma is shown below; in higher-energy, longer-pulse plasmas, events are shifted to longer times, while for lower energy, shorter-pulse plasmas events are at shifted to shorter times

The time-line of the plasma looks something like:

The emission from the spectra can be then quantified – calibration curves can be obtained by standard peak integration or by use of chemometrics, and/or pattern-matching routines can fingerprint material to determine its type.

Broadband LIBS spectra have a wealth of information:

Calibrations can be used to quantify concentrations, or chemometrics can be used to sort material:

Seminar at Caltech: Discussion of LIBS plasma evolution with time

Last week I had the opportunity to give a LIBS seminar at Caltech to a group of astute engineers and scientists focused on analytical geochemistry. One major vein of questions and discussion that arose was surrounding the transferability of calibrations and the importance of experimental conditions. This is a deep topic indeed and all we can do here is touch on it, but I do believe that there are some easy ways to capture the main issues.

First, realize that LIBS analysis proceeds as the result of two coupled but distinct processes.  The laser ablation process removes material from the sample for introduction to the analytical plasma.  Laser ablation is in and of itself a well-studied process, both by LIBS researchers and those who work on laser ablation ICP mass spec and laser ablation OES.  We know, for example, that the laser may penetrate into some materials, and that in general shorter wavelengths and shorter pulses contribute to better ablation and less fractionation.

The formation of the analytical LIBS plasma, typically in the gas phase above the sample in the ablation plume, occurs as the electron and material density in the plasma exceeds a certain threshold – the plume becomes increasingly optically thick as the developing plasma frequency (determined by the electron density) exceeds the laser frequency.  For this reason, the analytical plasma forms earlier – and ablation stops sooner – with longer wavelength lasers such as 1064 nm YAGs (lower frequencies), compared with shorter 266 nm quadupled YAGs (higher frequencies).

Hence there is a trade-off between ablation on one hand and the analytical plasma on the other, and a balance of energy between the two.  Roughly speaking, if more energy goes into the material ablation, less will be available for the plasma, resulting in a cooler and/or smaller plasma.  The converse is also true.

To wrap up this train of thought, the crux is that varying conditions – in particular the fluence (typically measured in J/cm2) on the sample – change the time-temperature history of the plasma.  The time-temperature history is also influenced by the matrix, laser frequency, etc.

As the plasma cools from incredible temperatures and conditions at the moment of formation, a cascade of events occurs.  Early in time, the plasma emission is dominated by Brehmsstralung emission as decelerating charged particles conserve energy and emit photons.  Highly charged ions recombine and relax, emitting photons.  At relatively early times (0.3 – 2 microseconds in typical plasmas) emission from charged states dominates, while atomic emission is strong from approximately 1 to 10 microseconds (depending upon the atom), and at later times, emission from recombining molecular fragments (CN and C2, for example) is strong.  These numbers are all for atmospheric-pressure LIBS plasmas of moderate energy (50-150 mJ) … your mileage may vary.

An analytical plasma should be one that is in local thermodynamic equilibrium (LTE), which corresponds to conditions in which the electron and gas temperatures are the same, or nearly so, in a local region.  This may be 0.5 to 1 microsecond, commonly.  In a typical plasma, this time to LTE is a function of initial energy and the cooling rate of the plasma – clearly affected by the matrix and by the fluence.  In fact, as an excellent paper has recently argued, the idea of LTE is really a misnomer in a fluidly changing plasma, with steep temperature and diffusion gradients (Christoforetti et al., Spectrochimica Acta B, 65 (2010) 86-95).

At the very least, the considerations thus far lead us to the conclusion that differing initial energies coupled into the analytical plasma lead to a different time-temperature progression – based on the initial size and cooling rate of the plasma, for example.  Hence the optimum detector timing – the delay after the plasma onset to avoid the continuum emission and the gate width when the camera is open – changes with plasma conditions.  For this reason, standard LIBS calibrations are both fluence-dependent and relatively closely matrix-dependent.

Nevertheless, despite these considerations, I would argue (and indeed we and several others have shown) that the Taylor-Sedov blast wave theory does allow useful scaling and similarity solutions for a nondimensional time in laser plasmas.  Changing the energy deposited in the plasma changes the scaling of the plasma decay.  However, knowing the energy in the plasma could allow recovery of the nondimensional time and the implementation of a more universal calibration scheme.

There are myriad ways to approach quantification of LIBS emission, from standard peak integration methods to “calibration-free” methodologies and chemometric methods.  Look for more comments soon on these and other calibration methods right here.

‘Til then, yours in LIBS

Steve

Photon Machines LIBS Blog, Volume 1, issue 1!

What this isn’t: 1) This is not a liberal political blog.  2) This has nothing to do with Nicola Tesla and his Photon Machines.  3) The prefixes Mad- and Womens- have nothing to do with this kind of LIBS.

Welcome to the Laser-Induced Breakdown Spectroscopy (LIBS) blog hosted by Photon Machines.  Here we will delve, wade, and otherwise likely blunder in to all kinds of issues regarding the implementation of LIBS, both for laboratory and industrial uses.  Our biases will be evident and our opinions carry all disclaimers – although they are informed by years of academic and industrial experience.  Hopefully they will be helpful to you, our readers, as you explore the uses of LIBS for analysis of your particular measurement needs.

The benefits of LIBS are many and will be periodically reviewed here.  They include:

• Rapid analysis with minimal sample preparation (no digestion for those of you who practice ICP-MS!)
• Broadband spectra with many simultaneous elemental lines – allowing fingerprinting as well as quantification
• The ability to measure light elements that are difficult or impossible with XRF
• Measurements can be made at a distance (stand-off LIBS)
• Detection limits between 1 and 100 ppm for most elements

All of these mean that there are some pretty interesting applications out there for LIBS.  At the same time, LIBS is NOT for every analysis situation – it does not have the lowest detection limits and the best measurements have an uncertainty between 1 and 4 or 5%.  However, for many situations LIBS has some unique features and new measurement capabilities that add significantly to the analyst’s repertoire.  Not only that, but LIBS is unusually easy, and, we think, fun!

Things that we will talk about in the coming weeks and months include:

• Metallurgical analysis – a nice example of what can be done with LIBS
• Typical approaches to LIBS quantification
• Chemometrics and LIBS
• Pulse energy and laser wavelength considerations
• Spectrometer selection for LIBS

We will do our best to keep the discussion lively and grounded in literature and data.  Please stay tuned, join in often, and stick us on your RSS feeds!  We look forward to an engaging interchange!

Yours in LIBS, Steve Buckley