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).
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
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.
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.
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.
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.
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
good precision and accuracy
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
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
bulk analysis of pressed powder briquettes (e.g. rocks, soils, sediments)
archaeological materials (e.g. glass beads, ceramics)
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.)
Techniques in the Earth Sciences. Chapman & Hall, London, pp. 291-325.