Analyte- and matrix-dependent elemental response variations in laser ablation inductively coupled plasma mass spectrometry

Abstract

Elemental response variations as a function of carrier gas flow rate in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were studied for a wide range of analytes. The effects of rf power, focus lens settings, thermodynamic properties of analytes and sample matrix were thoroughly examined. It was found that, with the experimental set-up used for this work, processes occurring in the ICP, rather than during ablation and transport, play the decisive role in determining the shapes of flow rate plots observed with LA. Responses for analytes of lower nominal masses and vaporization enthalpies peak at consistently higher flow rates (1.15 l min 21 ) than other elements, independent of matrix. On the other hand, the magnitude of the maximum sensitivity is matrix dependent, even for these elements. Involatile elements display much broader maxima at considerably lower flow rates; the more refractory the matrix, the lower the optimum flow rate. This behaviour is consistent with the residence time in the ICP necessary to maximize the efficiency of analyte ion production. The existence of inter-elemental differences in the locations of zones of maximum ion densities formed in the ICP can thus be related to the times required for vaporization of any given analyte from the particles produced by LA. Such differences may be responsible for numerous fractionation effects mentioned in the LA literature. It is also demonstrated that the ion sampling process can affect the shapes of the flow rate plots, potentially shifting the apparent position of the optimum flow rate and confounding the interpretation of inter-element response differences.

On-line additions of aqueous standards for calibration of laser ablation inductively coupled plasma mass spectrometry: theory and comparison of wet and dry plasma conditions

“…[4][5][6] Consequently, these particles are vaporised along an extended region of the ICP, leading to wider ion density distributions along the central channel for LA in comparison to solution nebulisation. 7,8 The dual sample/standard approach, using a wet or a dry plasma, requires internal standardisation to compensate for the different mass transport rates of the two sample introduction sources. Consequently, an element of known concentration and homogenous distribution must be present in the sample.…”

Laser assisted plasma spectrochemistry: laser ablation

“…[1][2][3][4][5][6][7][8][9][10] Many of these current issues were addressed when laser ablation initially was studied in the early 1980s for chemical analysis. There was a tremendous body of literature on laser ablation using other excitation sources before ICP-MS. High-voltage sparks, 11 microwave plasmas, 12 direct current plasmas, 13 graphite furnaces 14 and ICP-AES have been used as excitation sources with laser ablation.…”

“…The research studying laser properties is based on the need to ablate samples without matrix dependence and without fractionation (ablated mass vapor is not chemically equal to the original sample). [8][9][10][25][26][27][28][29][30] Understanding and eliminating elemental fractionation has been one of the most significant research agendas in laser ablation sampling. [31][32][33][34][35][36][37][38][39][40] It is important to point out that the occurrence of fractionation does not preclude the use of laserablation sampling for accurate chemical analysis: numerous successful applications have been demonstrated.…”

Dried-droplet laser ablation ICP-MS of HPLC fractions for the determination of selenomethionine in yeast

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] However, elemental fractionation, 15 leading to non-stoichiometric response, frequently occurs, although the sources of this problem are still debated. 2,3,16,17 This makes it difficult to achieve quantitative analytical results using this technique, especially if no suitable standard reference materials of similar matrix composition are available for calibration. Isotope dilution approaches have been applied by several research groups [10][11][12][13][14] for the direct determination of trace elements in various solid samples using LA-ICP-MS in attempts to achieve enhanced accuracy and precision of results.…”