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CAER Seminars

Applications of LA (laser ablation)-ICP (inductively coupled plasma)-MS (mass spectrometry) for the Analysis of Solids

Melody Bi
Chemistry Department
University of Florida

Wednesday, August 2, 2000 2:30pm
Ben Bandy Conference Center
Center for Applied Energy Research

Laser-sampling techniques, such as LA-ICP-MS, have gained popularity for analyzing solid materials because little or no sample preparation is required. Practical concerns such as the difficulty of obtaining or making matrix-matched standards prevent such techniques from becoming analytical tools. This presentation will give an overview of LA-ICP-MS as a technique for solids analysis.

Calibration strategies for trace analysis along with surface profiling will be discussed as applications for LA-ICP-MS [1-5]. By analyzing NIST soil and glass samples, a method for the determination of trace element concentrations by laser ablation ICP-MS using solution calibration and an internal standard has been studied and evaluated [1]. In most cases, the measured element concentrations were within ±15% of the certified values. The internal standard was chosen based on investigations of the proper signal intensity of certain isotopes and the homogeneity of their distribution in the sample. For soil samples, a matrix element, Mg was chosen to be the internal standard. For glass samples, a trace element, Sr was used as the internal standard.

The results indicated that in both cases, the internal standard was effective. Ni and Cu in soil gave poor results while good results were obtained for Ni and Cu in glass samples. Time resolved studies show that Ni gave many more signal spikes than other elements when the ablating laser moved across the surface of the soil sample. This indicates that one possible reason for the poor results was caused by the heterogeneity of the Ni distribution in the matrix. NIST archival leaf standards were used for reliable quantitative elemental analysis of Spanish moss samples by LA-ICP-MS [3]. Mixed standards were used to produce at least 3 data points for each calibration curve. The results were compared with that obtained from microwave digestion ICP-AES (atomic emission spectrometry) analysis. For most of the elements studied, the results for the two techniques agreed.

Standard addition method was also studied and the results showed that it is an effective method when matrix-matched standards are not available. LA-ICP-MS is a viable technique for profiling of patterned thin metal layers on a polymer/silicon substrate [2]. The parameters of the laser and ICP-MS operating conditions have been studied and optimized for this purpose. A new laser ablation chamber was designed and built to achieve the best spatial resolution. The results of profiling by LA-ICP-MS were compared to those obtained from a laser ablation-optical emission spectrometry (LA-OES) instrument, which measured the emission of the plasma at the sample surface and thus eliminated the time delay caused by sample transport into the ICP-MS system. Emission spectra gave better spatial resolution than mass spectra. However, LA-ICP-MS provided much better sensitivity and was able to profile thin metal layers (on the order of a few nm) on the silicon surface. A lateral spatial resolution of 45 mm was achieved.