Elemental Fractionation Studies in Laser Ablation Inductively Coupled Plasma Mass Spectrometry on Laser-Induced Brass Aerosols

Abstract

Previous investigations on laser-induced aerosols of brass samples showed that preferential vaporization of zinc occurs during the ablation process leading to elemental fractionation and limited possibilities for non matrix matched calibration. In a variety of experiments carried out within this study it is shown that multiple effects are complicating the quantification of brass using IA-ICPMS. It is shown that the ablated copper and zinc is not homogeneously distributed within the laser-produced aerosol. Copper was found enriched up to 100% in particles larger than 100 nm as shown from EDX measurements (electron excited) on individual particles, and zinc was enriched by over 40% in the particles smaller than the lowest measurable particle size (below 100 nm or in the vapor phase). Solution nebulization analysis on digested filter-collected aerosols results in a higher Cu/Zn ratio than the certified value for the brass sample. ESEM pictures with analysis of the electron excited X-rays measured on the filter-collected material support the results showing copper enrichment. However, online LA-ICPMS measurements carried out under the same operating conditions as for filtering show a copper depletion within the ICP, which leads to the conclusion of partial vaporization and ionization of the aerosol particles in the ICP. The larger particles containing more or exclusively copper are not completely ionized. Within this study, three sources of elemental fractionation can be distinguished and described: (A) The ablation process leads to no measurable copper enrichment at the ablation crater rim. (B) Zinc deposition between the ablation site and the aerosol collection on filters leads to an up to 37% higher Cu/Zn ratio on the filter in comparison to the certified value. (C) On-line laser ablation aerosols measured within the ICPMS lead to significantly lower Cu/Zn ratios in comparison to the certified value. (D) Combination of the various studied sources of fractionation can finally lead to an agreement between measured and certified values due to inverse overlapping of various fractionation sources.

Femtosecond laser ablation inductively coupled plasma mass spectrometry: achievements and remaining problems

“…For metals and semiconductors, however, quantification by non-matrix-matched calibration turned out to be problematic, since strong heat diffusion into the target results in material re-distribution during the ablation process. As a consequence, the total composition of aerosols formed by ns-LA can considerably deviate from the bulk value, especially if intensively fractionating matrices such as metal alloys are analyzed [15,16]. In order to restrain these effects, the laser pulse duration needs to fall below the material-specific thermal relaxation time.…”

Fundamental and methodological investigations for the improvement of elemental analysis by inductively coupled plasma mass soectrometry

“…The ablation process itself is also a source of fractionation; that is, the aerosol generated by laser ablation can have different composition than the actual solid sample 53,61 .…”

Development of an On-line Low Gas Pressure Cell for Laser Ablation-ICP-Mass Spectrometry

“…Even with a low laser energy fluence (< 1 J/cm 2 ), numerous signal spikes can be found in the measured intensity profile, suggesting the introduction of particles larger than the critical size into the ICP. 6,7 It should be noted that the signal stabilizer device, which acts as a particle filter, was used with the present system. Even with the stabilizer device, signal spikes could not be fully removed, and the number of signal spikes increased with a higher laser pulse energy, and therefore we could not apply a laser fluence of higher than 1 J/cm 2 .…”

“…The size distribution of the sample aerosol is a critical parameter for the complete evaporation, atomization or ionization of the analytes; ablated particles larger than 150 nm for glass samples 6 and 400 nm for metals 7 are not completely vaporized and ionized in the ICP. The particle size distribution is a function of various parameters such as, the laser wavelength, [8][9][10][11] laser irradiance, 12,13 laser pulse length, 13,14 depth of the crater, cell geometry [15][16][17] or the type of carrier gas.…”