Gas-flow optimization studies on brass samples using closed and open types of laser ablation cells in inductively coupled plasma mass spectrometry

The effect of the carrier gas flow rate on laser ablation inductively coupled mass spectrometry (LA-ICP-MS) signals of brass components (Cu, Zn, and Ni) were studied primarily with the use of a conventional closed ablation cell (UP-213) and with the use of an open ablation cell of our own construction. In the closed ablation cell configuration with the carrier gas flow rate in the range of 0.40-1.3 L min(-1) Ar, the MS signals increased significantly (an 8,2-fold increase for a Cu signal), and the Zn/Cu signal ratio increased 3.2 times. To identify the degree of fractionation, the conventional solution sample introduction method was selected as the reference method because it is expected free from fractionation for Cu and Zn. To obtain a theoretical value, calculations were made based on Saha's relationship of ionization, which resulted in fair agreement with the experimental results of the solution method. By comparing the Zn/Cu signal ratios obtained from both the LA and the solution method, a fractionation factor of 226 was deduced for these two components. To explain the increased signals described above, the transport efficiency as a function of the carrier gas flow rate was calculated for different particle size fractions based on existing theories. It was demonstrated that the large increase in the signals with carrier gas flow rate is predominantly due to processes taking place in the ablation cell (i.e., neither during the transportation nor in the ICP). The results of the novel fundamental works on aerosol formation under LA conditions were considered and complemented with the application of the Kelvin (Gibbs-Thomson) equation to calculate critical sizes of aerosol particles in the nucleating vapors of Cu, Zn, Ni and Pb elements. It was concluded that the noted increase of signals was due to the intensification of mixing of the expanding vapor-cloud with cold gas when applying an increasing carrier gas flow rate. It was also concluded that the fractionation factor mentioned above for Zn/Cu components (226) was the consequence of fractional vaporization. The open ablation cell and aerosol transporting pump system did not satisfy the demands of the sensitive LA-ICP-MS determinations. The primary reason for this was the low efficiency of the miniature pump used for aerosol transportation. (C) 2012 Elsevier B.V. All rights reserved.