INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

4. LASER ABLATION ICP-MS

WHAT IS IT ?

Laser ablation ICP-MS (LA ICP-MS) is a microanalytical technique for the determination of trace elements in solid materials with a growing number of applications in fields such as geochemistry, materials science, forensics and environmental studies. LA ICP-MS combines the micrometer-scale resolution of a laser probe with the speed, sensitivity and multi-element capability of ICP-MS, and rivals other microbeam techniques such as the proton microprobe and secondary ion mass spectrometry (i.e. the "ion probe").

HOW DOES IT WORK ?

A laser ablation module connected to an ICP-MS instrument represents just one of a number of different sample introduction systems used in ICP-MS analysis. A pulsed laser beam is used to ablate a small quantity of sample material which is transported into the Ar plasma of the ICP-MS instrument by a stream of Ar carrier gas. The laser beam can be thought of as a "light chisel" which interacts with the solid sample material by a physical process (ablation) that generates very fine solid particles leaving behind a minute ablation crater (in the order of tens of æm in diameter).

INSTRUMENTATION

The word laser is an acronym for "light amplification by the stimulated emission of radiation". A laser is a device that produces a narrow, pulsed or continuous beam of monochromatic, coherent light in the visible, infrared (IR) or ultraviolet (UV) regions of the electromagnetic spectrum. The power of laser beams generated by commercial and military lasers ranges from a fraction of a milliwatt to more than a megawatt.

The key element of any laser is the active laser medium that contains a component which can be excited to an elevated, unstable energy level. Sufficient excitation of the laser medium results in "population inversion", during which a greater number of active species are in the excited state than in the lower energy ground level. A transition from the excited state to the ground level results in the emission of a photon. This photon may interact with other active species in the excited state and stimulate the emission of further photons. The laser medium is housed in an optical cavity with mirrored surfaces on either end (the mirror on one end is totally reflective, the other mirror is only partially reflective). The photons produced in the laser medium are reflected off these mirrors and oscillate repeatedly through the laser medium, causing yet further photons to be emitted by active species in the exited state and hence amplifying the intensity of light produced. Some of the photons oscillating in the cavity pass through the partially reflective mirror and emerge as a tightly collimated beam of laser light.

A wide variety of different lasers have been invented, differing both in laser medium (gases, liquids, solids, semi-conductors) as well as in design and structure. The first lasers to be used in LA ICP-MS were ruby lasers producing IR laser light and recently some laboratories have started to use eximer lasers operating in the UV part of the electromagnetic spectrum, but the type of laser most commonly used is the Nd-YAG laser. The active medium in a Nd-YAG laser is an artificially grown Y-Al garnet doped with a small quantity of Nd. The Nd atoms in this garnet are excited to an elevated energy level by light from a flash lamp. Nd-YAG lasers produce pulsed IR laser light with a fundamental wavelength of 1064 nm. Since UV laser light interacts more efficiently with most solids than IR laser light, the frequency of the light produced by Nd-YAG lasers is commonly optically quadrupled to generate UV laser light with a wavelength of 266 nm. The interaction between UV laser light and most solids tends to involve mostly a physical ablation process of mechanical break-up, whereas IR laser light may often cause a greater degree of sample heating and melting which is generally undesirable for quantitative analysis as it may lead to trace element fractionation.

As a laser beam's concentrated light delivers energy only where it is focused, the laser ablation instrument requires an accurate optical system of lenses, prisms and mirrors that conducts and focuses the laser beam onto the sample. Some of the components of this optical system may be transparent or reflective only to light with a wavelength of exactly 266 nm, thereby helping to "clean" the laser beam which may include a limited range of different wavelengths. The optical system may also include a system of apertures of different diameters that can be used to vary the beam diameter.

The sample to be analysed is placed in a sample chamber with a quartz glass lid transparent to UV light. A stream of Ar carrier gas enters the sample chamber through a small opening in the floor, swirls through the interior of the chamber in a cyclonic pattern and exits it at the top, in the process picking up fine sample particles produced by the ablation process and transporting them into the Ar plasma of the ICP-MS instrument. The sample chamber is mounted on a stage that allows the sample to be moved relative to the laser beam in the forward-backward, left-right and up-down (i.e. focus) directions. The laser is focused and the ablation process can be observed on a colour monitor showing the images captured by a colour video camera.

OPERATING PARAMETERS

The analyst has control over several operating parameters in LA ICP-MS. The particular combination of parameters must be customised for the particular application under consideration. These parameters include the laser power (for a Nd-YAG laser the beam energy typically ranges from < 0.5 to ~ 5 mJ per pulse), the pulse repetition rate (e.g. 1, 2, 4, 5, 10 or 20 Hz or pulses per second) and the number of laser pulses (or "shots") fired in succession. A choice of apertures of different diameter provide some control over the diameter of the laser beam and hence the diameter of the ablation crater generated (e.g. 10, 25, 50, 100, 200, 300 micron). Although the size of an ablation crater can be made very small, its usefulness for quantitative analysis is entirely dependent on the sensitivity of the ICP-MS instrument to the elements of interest. The smaller the ablation crater, the smaller the amount of sample material introduced into the plasma and the lower the analyte signal intensities. For good quantitative LA ICP-MS analysis at UCT, the smallest crater diameter that can be used is ~ 50 micron for most applications. The laser beam may be used in a sharply focussed or a slightly defocussed mode, the latter providing a somewhat more diffuse energy distribution. LA ICP-MS analysis may be carried out on single spots, over a raster of several spots, or by continuously scanning the laser beam along a straight line. The Ar carrier gas flow rate is also under operator control.

SAMPLE PREPARATION

In most instances, sample preparation for LA ICP-MS analysis is very simple. As long as the sample fits into the sample chamber, it may not even need a perfectly flat or polished surface. Most commonly, samples are in the form of epoxy mounts (e.g. rock fragments or mineral grains), pressed powder briquettes (for bulk rock analysis) similar to those used in XRFS, or petrographic thin-sections similar to those used in electron microprobe analysis.

CALIBRATION

For most applications, analytical calibration is achieved by external standardisation using one or more artificial or natural standard reference materials of known composition. Example of standard materials include a set of artificial silicate glasses produced by the US National Institute of Standards and Technology (e.g. NIST-610 and NIST-612) that contain various trace elements in a range of concentrations, three glasses made from natural rock standards by the US Geological Survey (BIR-1G, BCR-2G and BHVO-2G), or in-house glasses or mineral separates of known composition.

Internal standardisation is generally necessary in LA ICP-MS to correct raw counts-per-second data for differences in the ablation characteristics between standards and samples and between different elements, as well as for general instrument drift correction. In the analysis of silicate materials, Ca and Si are frequently used as internal standards. Depending largely on their volatility, different groups of elements may need normalisation to different internal standard elements to achieve a high quality of data. One of the complications of LA ICP-MS analysis is that the concentration of the internal standard elements in all standards and samples has to be determined by an alternative analytical technique (e.g. the electron microprobe or XRFS) prior to analysis.

INTERFERENCES

Like solution ICP-MS, LA ICP-MS is subject to isobaric, molecular and doubly-charged ion interferences. Due to the absence of acids and water in the sample aerosol (i.e. a "dry plasma"), however, interferences by oxide ions are greatly reduced in severity (typically < 0.5 %) in comparison with solution ICP-MS. Most interferences may be minimised or avoided altogether by prudent choice of analyte isotopes and careful optimisation of instrument operating conditions.

DETECTION LIMITS, ACCURACY AND PRECISION

LA ICP-MS detection limits vary with laser power and volume of material analysed. Theoretical detection limits for most elements are typically in the ppb to low ppm range. Accuracies and precisions in the order of ~ 1 to 10 % may be expected for most elements.

ADVANTAGES OF LA ICP-MS

simple sample preparation
speed and ease of use
high sample throughput
high sensitivity
good precision and accuracy
multi-element capability.

DISADVANTAGES OF LA ICP-MS

low spatial resolution compared to some other microbeam techniques (e.g. electron microprobe, ion probe, proton probe)
need for well-characterised, homogeneous standards
need for prior knowledge of internal standard concentrations in samples and standards.

APPLICATIONS

A growing list of materials have been analysed for various trace elements by LA ICP-MS. Some examples include:

trace elements in minerals, rocks and volcanic glasses
trace element zonations in minerals
mineral-melt partition coefficients
fluid and melt inclusions in minerals
isotope ratios
petrographic thin-sections
bulk analysis of pressed powder briquettes (e.g. rocks, soils, sediments)
archaeological materials (e.g. glass beads, ceramics)
plastics
metals
semi-conductor materials
catalysts (e.g. zeolite)
biological materials (e.g. tree rings, shells, teeth, bones)
forensic finger printing.

A GOOD GENERAL REFERENCE

Perkins, W. T. & Pearce, N. J. G., 1995. Mineral microanalysis by laserprobe inductively coupled plasma mass spectrometry. In: Potts, P. J., Bowles, J. F. W., Reed, S. J. B. & Cave, M. R. (eds.) Microprobe Techniques in the Earth Sciences. Chapman & Hall, London, pp. 291-325.


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