We custom-built a novel UV femtosecond laser ablation system as a micro-analytical tool for elemental and isotopic analysis of solid materials.
Commercial laser ablation systems employing excimer or Nd:YAG lasers have pulse lengths > 5 nanoseconds (ns). This interval gives sufficient time for photon energy to disperse as heat in the sputtered material during the laser pulse. The ns ablation process is associated with melting, boiling, and vaporisation. These processes affect the accuracy and precision of concentration and isotope measurements.
Femtosecond (fs, 10-15 seconds) lasers, in contrast, ablate with minimal thermal heating of the volume surrounding the crater due to the short laser pulse length as compared to the phonon relaxation time; i.e., the laser energy can be deposited into the material before it can thermally equilibrate. This predominantly non-thermal ablation offers the potential to eliminate fractionation and matrix dependence. Femtosecond ablation provides less sample heating, no laser–plasma interaction and smaller aerosol particle sizes.
Our system consists of the latest generation femtosecond solid-state laser (Newport Spectra Physics Solstice) combined with a computer-controlled sample stage. The pulsed laser beam has an ultra-short pulse width of about 100 femtoseconds and operates at a wavelength of 196 nm.
The system provides full control on laser parameters, such as spot size, energy density, pulse width, repetition rate (from 1 to 1000 Hz) and beam shape. Beam diagnostic instruments such as a power meter, a beam profiling camera and an autocorrelator to measure the pulse length are implemented in the system. The spot size can be varied from 10 to 100 µm in diameter with energy densities on the sample between 0.1 and 50 J/cm2.
The samples (thin sections or polished blocks) are contained in a He-flushed sample cell.
The laser beam is focused into a spot (10 to 100 µm) on the solid sample surface, which is held within a He-flushed sample chamber. The ablated sample aerosol is transferred by the gas stream into the plasma source of our Varian ICP-OES (for elemental analyses) or our Thermo Neptune MC-ICP-MS (for isotopic analyses). The system allows fully automated isotope analyses through synchronised operation of the laser with the Neptune MC-ICP-MS.
The main application of the laser system is the analysis of stable isotope compositions of Fe, Si, and Mg at the micrometer scale on a wide range of materials.
International reference materials are routinely analysed to validate accuracy and precision of the laser ablation method. Here, pure silicon (IRMM-17) and basaltic glass (BHVO2-G) were repeatedly analysed using quartz (NBS28) as bracketing standard. The results indicate an external reproducibility of the laser ablation method of better than 0.14 ‰ (2SD, n= 27) and 0.23 ‰ (2SD, n= 27) for d29Si and d30Si, respectively. Moreover, the good agreement of our results with data obtained by other studies using solution MC-ICP-MS on purified Si solutions indicates that matrix effects caused by differences in chemical composition between samples and the quartz bracketing standard are not detectable using fs laser ablation MC-ICP-MS.