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.

In laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the term elemental fractionation is normally used to summarize all non-stoichiometric effects occurring during sample ablation, aerosol transport and vaporization, atomisation and ionisation within the ICP. Nevertheless, there are different types of elemental fractionation occurring between different sized particles within an aerosol, as previously shown for metal alloy ablations. In the present work, laser generated aerosols from glass samples were filter collected before entering the ICP to study their total and particle size dependent composition. Furthermore, elemental concentrations in different particle size fractions on filters were compared with their response measured in the ICP. For the NIST SRM 610 glass, elemental fractionation effects between small particles (v125 nm and v340 nm), and the total aerosol containing all particle sizes up to 1 mm or even larger, were measured for 42 major and trace elements using a 266 nm Nd:YAG laser and scanning ablation conditions to produce the aerosol. Particles above 125 nm and 340 nm were separated from the aerosol by a particle separation device and the remaining particles were collected on a filter, digested and measured using solution nebulization ICP sector field MS (SFMS). Results show an enrichment of certain elements such as Cu, Zn, Ag, Tl, Pb and Bi of up to 90% relative to Ca within the small particle size fraction of an aerosol in comparison with the total composition of the aerosol. The same elements are depleted in large aerosol particles measured from deposited particles within the ablation cell. However, the total transportable aerosols produced using different laser wavelengths (193/266 nm), and gas environments (He/Ar), which were also filtered and digested show no significant deviation in their overall stoichiometry from the original sample (except Be, Fe, Cd for all lasers and gases) within the uncertainty of the measurements. Therefore, the composition of filter collected aerosol of glass samples indicates that the elemental fractionation in LA-ICP-MS, detected at the beginning of a 266 nm single hole ablation, is predominantly caused by incomplete vaporization of large particles within the ICP and is not dominated by non-stoichiometric ablation of the glass. 1 1 5 8 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 1 5 8 -1 1 6 4 T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 1 5 8 -1 1 6 4 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 1 5 8 -1 1 6 4 View Article Online J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 1 5 8 -1 1 6 4 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 1 5 8 -1 1 6 4

Ongoing discussions about the origin of elemental fractionation occurring during LA-ICP-MS analysis show that this problem is still far from being well understood. It is becoming accepted that all three possible sources (ablation, transport, excitation) contribute to elemental fractionation. However, experimental data about the vaporisation size limit of different particles in the ICP, as produced in laser ablation, have not been available until now. This information should allow one to determine the signal contributing mass within the ICP and would further clarify demands on suitable laser ablation systems and gas atmospheres in terms of their particle size distribution. The results presented here show a vaporisation size limit of laser induced particles, which was found at particle sizes between 90 nm and 150 nm using an Elan 6000 ICP-MS. Due to the fact that the ICP-MS response was used as evaluation parameter, vaporisation and ionisation limits are not distinguishable. The upper limit was determined by successively removing the larger particles from the aerosol, which was created by ablation of a NIST 610 glass standard at a wavelength of 266 nm, using a recently developed particle separation device. Various particle fractions were separated from the aerosol entering the ICP. The decrease in signal intensity is not proportional to the decrease in volume, indicating that particles above 150 nm in diameter are not completely ionised in the ICP. Due to the limited removal range of the particle separation device, which cannot remove particles smaller than 150 nm, single hole ablations were used to determine the lower vaporisation limit. This is based on measurements showing that larger particles occur dominantly during the first 100 laser pulses only. After this period, the ratio of ICP-MS counts and total particle volume was found to be constant while most of the particles are smaller than 90 nm, indicating complete vaporisation and ionisation of these particles. To describe the influence of different plasma forward powers on the vaporisation limit, the range 1000-1600 W was studied. Results indicate that optimum vaporisation and ionisation occurs at 1300 W. However, an increase of the particle ionisation limit towards larger particles was not observed within the accuracy of this study using the full range of parameters available for optimisation on commonly used ICP-MS instruments.

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Fundamental understanding of aerosol formation and particle transport are important aspects of understanding and improving laser-ablation ICP-MS. To obtain more information about particles entering the ICP, laser aerosols generated under different ablation conditions were collected on membrane filters. The particles and agglomerates were then visualised using scanning electron microscope (SEM) imaging. To determine variations between different sample matrices, opaque (USGS BCR-2G) and transparent (NIST SRM 610) glass, CaF(2), and brass (MBH B26) samples were ablated using two different laser wavelengths, 193 and 266 nm. This study showed that the condensed nano-particles ( approximately 10 nm in diameter) formed by laser ablation reach the ICP as micron-sized agglomerates; this is apparent from filters which contain only a few well-separated particles and particle agglomerates. Ablation experiments on different metals and non-metals show that the structure of the agglomerates is matrix-dependent. Laser aerosols generated from silicates and metals form linear agglomerates whereas particle-agglomerates of ablated CaF(2) have cotton-like structures. Amongst other conditions, this study shows that the absorption characteristics of the sample and the laser wavelength determine the production of micron-sized spherical particles formed by liquid droplet ejection.