LA-ICP-MS: Methods

Inductively coupled plasma mass spectrometry (ICP-MS) has developed rapidly and is now a routine method for multi-elemental analysis of many types of samples. As the use of aqueous solutions is not very convenient when working with solid samples (dissolution, contamination, etc), alternative solid sample introduction techniques, such as laser ablation, are used with ICP-MS. Ablation of solid samples, using a laser with a flow of carrier gas to carry the ablated material from a suitable cell to the ICP, is a very powerful technique. The development of new laser systems (Quadrupled ND:YAG [Lit. 10], excimer [Lit. 11]), homogenization optics, increased mass spectrometer sensitivity, accompanied by an increased linear dynamic range, has led to a large number of new applications (Lit. 08).

In this work, an ArF excimer laser (193nm; Compex 110I, Lamda Physik, Goettingen, Germany), coupled with a quadrupole mass spectrometer (Elan 6100 DRC Perkin Elmer/Siex, Toronto, Canada), was used. A homogenized laser beam with a flat top beam profile was imaged onto the sample surface using mirror optics, homogenizer arrays, and a petrographic microscope. This system allows the laser beam onto the sample, while allowing the operator to observe the sample during ablation. An in-house fabricated ablation cell, which can hold five samples, including a reference material for calibration, had a total volume of 21cm³. Helium was used as the sample carrier gas (1.2 l min^-1), introduced into the cell using a nozzle with an inside diameter of 100 micrometers.

Fig. 36 Special Analyses were carried out at the Swiss Federal Institute of Technology (SFIT), Department of Chemistry, Laboratory of Inorganic Chemistry, Zurich (Switzerland).

Fig. 37 LASER ABLATION ICP-MS Analyses in the Laboratory of Inorganic Chemistry. Prof. D. Günther and Dr. A. Peretti discuss measuring strategies.

An argon gas flow (0.85 l min^-1) was mixed with the helium gas flow at .5cm after the cell, and this mixture was transported though 1m length PVC-tubing (inside diameter 0.4cm) to the ICP.

For data collection, the LA-ICP/MS system was operated in time resolved mode. Every 520 ms, the intensities of 40 selected isotopes are measured and stored. At this data acquisition repetition frequency (approximately 2 Hz), no significant fluctuation of the time resolved signal was observed.

The integration intervals (background and gross analyte) were selected manually, and then the gross signal was background corrected using the preablation signal.

The concentrations in the samples were calculated using SRM NIST 612 glass as external standard, with Al being used as internal standard. Using AI normalized sensitivity, the changes in the volume (mass) of the sample, which was ablated, was corrected. The data for samples slightly oxceeding 100% have not been corrected, because the correction results in insignificant concentrations changes (smaller than the detection limit). A small drift of the normalized sensitivity over 20 analyses (run)