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Laser Ablation

Laser Ablation is a relatively new technique for introducing solid samples into an ICPMS for elemental analysis.  This avoids the dissolution step normally required for preparing the sample for ICPMS analysis.

The laser is part of a microscope system, so small spots or large areas of solids can be sampled.  Bodycote-WCAS has installed a New Wave Research UP266MACRO system capable of sampling 20-780 µm spots.  The Nd:YAG laser is typically pulsed at 10 Hz with up to 15 J/cm², enough energy to sample metals, plastics, ceramics, or glass.  Large areas can be sampled by mapping the pulses.

New Wave Research
UP266MACRO
Laser Ablation System

Applications of LA-ICPMS

Trace analysis of solids (to sub ppm) and tiny particles (to ~10 ppm) are now possible.  With the microscope, small spots with particles of 20 µm can be focused, photographed, and analyzed for elemental composition.  (For comparison, the naked eye can usually see spots of ~100 µm.)

Very thin layers (sub-micron) can be analyzed by scanning the surface at low power.  Using this technique, layers of paint or surface contamination can be analyzed.  This was used to examine toys from China where lead paint may have been used.  Multiple layers can be sampled using depth profiling.

There have been several recent reports in the literature where LA-ICPMS has been used to generate chromatograms from thin layer chromatography plates.

Bulk Analysis

For the analysis of bulk materials, the laser can scan a large area of the surface, typically 5 mm² or more.  The surface is usually cleaned with a pre-ablation scan to remove surface contamination, then data is acquired in an ablation scan.  As an example, the calibration graph below was obtained from a set of high purity nickel standards.  The calibration range was from 0.1 to 10 ug/g.  The detection limit was found to be 0.01 µg/g. 

If matrix matched standards are not available, good estimates (generally within 30% of the true value) can be obtained using relative responses from solution standards.

Depth Profiling

While X-ray Fluorescence and mechanical cross sectioning are useful in determining layer thickness, they are not useful in measuring the composition of gradients. 

In this example a material was coated using a diffusion technique to form a gradient.  The element that was diffused into the surface was measured by ICPMS  while ablating a 100 micron spot over a period of 100 sec to a depth of 20 microns.  There is a 10 sec delay between the start of ablation and the ICPMS signal.  The element is present

While the elements and materials in this study must be kept confidential, a similar study was recently published (G. Sarah, et al., JAAS 2007, 1163-1167).  Depth profiles were used to characterize ancient silver coins where the copper has been lost from the first few microns of the surface.  They concluded that LA-ICPMS is currently the only technique capable of measuring these gradients in a nearly non-destructive way.

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