Quantitative analysis of small metal fragments by LA-ICP-MS

Small, irregularly shaped samples can present difficulties for the established methods of metal analysis. Laser-ablation ICP-MS offers accuracy and precision independent of sample form.

A number of well-established techniques, including spark source optical emission spectrometry (SSOES) and x-ray fluorescence (XRF), deliver accurate, precise and relatively fast analysis of steels, typically providing analysis of an area of several cm2. Both techniques require samples of a certain shape, a minimum size and particular surface preparation qualities.

In most cases, analyses at the cm scale are perfectly acceptable and the provision of the required sample geometry presents no problem. There are, however, a number of situations in which these requirements cannot be met:

• Analysis of small features or inclusions;
• Analysis of fragments/millings/turnings;
• Forensic applications;
• Failure analysis of specific sample zones;
• Composition changes - small sample areas.

While LA-ICP-MS cannot compete with SSOES and XRF in terms of accuracy and precision, it is able to generate analyses from any size or shape of sample. This study shows the power of the technique to analyse very small and irregularly shaped sample sizes, using certified reference materials in the form of millings/turnings, as 'unknown' samples.

Calibration

Instrument calibration was achieved using eight flat geometry steel setting-up samples (BAS UK), typically 'certified' to two or three significant figures. Pieces cut from all eight samples were mounted together in the ablation cell.

Ablation positions were defined on each piece and calibration data were acquired from all pieces in an unattended procedure. The samples included low-alloy and highly-alloyed steels. Some elements varied from ppm levels to high % levels (eg, Cr). The iron content varied from 61% to 99%. In this challenging application, reference was made to an internal standard element (Fe) during calibration, but not during the analysis of samples. For this reason, close attention was paid to the use of identical laser ablation conditions for all standards and all samples, to ensure that instrument response was as uniform as possible throughout.

Bulk sample analysis

Having calibrated the instrument, the setting-up samples were re-analysed as unknowns. As expected, analytical values were close to the reference values.

Analysis of small metal fragments

Small pieces of a certified reference material (NBS) were used to assess the ability of this technique to analyse very small samples. The largest dimension of each sample turning was of the order of a few millimetres and the thickness was usually of the order of a few hundred microns.

The laser tends to penetrate very thin samples and so the pre-ablation period was limited to a few seconds to preserve the sample. Small samples such as these require to be mounted to prevent them moving under the thermal and acoustic shock of each laser pulse and so they were presented to the laser chamber on pressed boric acid mounts.

Preparation of the mounts was very simple. The turnings were placed in the bottom of a compression die and boric acid powder was added on top. This was compressed, resulting in a firm boric acid pellet with the steel turnings exposed for ablation at one of its circular surfaces.

The fragments of reference materials were sampled in exactly the same way as the bulk samples, but excluding the pre-ablation period. This was to ensure that the laser did not penetrate through the specimen before the end of the analysis. Three pieces of each sample type were analysed to give triplicate results, from which a standard deviation was calculated to indicate the reproducibility of the method applied to such small sample fragments.

Conclusion

Many small sample geometries may be analysed by LA-ICP-MS with accuracy and precision independent of sample form. Sample preparation is minimal and any shape of sample may be simply mounted for presentation to the laser sampling system.