Studies of Matrix Effects for the Analysis of Zinc in Sodium Chloride Using Direct Sample Insertion-Inductively Coupled Plasma Atomic Emission Spectrometry

Abstract

Direct sample insertion (DSI) is an alternative sample introduction technique for the inductively coupled plasma (ICP).A small quantity of the sample is placed in a sample probe and inserted directly into the plasma for vaporization, atomization, excitation and/or ionization. DSI is applicable to solid, powder, as well as liquid samples with 100% sample introduction efficiency. Since the technique was first introduced by Salin and Horlick in 1979, 1 there has been continuous efforts concerning DSI development in the past two decades. 2,3 DSI-ICP is similar to graphite-furnace atomic absorption spectrometry (GF-AAS) in several aspects. Firstly, graphite sample probes are commonly used in DSI-ICP, 3 although tungsten wire loops 4 and metal sample cups 5,6 have also been reported. Secondly, because the sample is thermally evaporated from the graphite probe, the analyte and matrix vaporization behavior and pattern are similar for GF-AAS and DSI-ICP. In addition, analyte vapor is measured in atomic form in a hot and inert atmosphere above the sample probe or in the furnace. DSI-ICP has an advantage that the analyte enters a plasma of higher temperature than the sample probe, while the temperature of the gas atmosphere inside a graphite furnace is usually lower than that of the inside wall of the furnace. Yet, matrix effects in GF-AAS, e.g., vaporization interference and gas-phase reactions, are relevant to DSI-ICP. Vaporization interference is considered to be the major matrix effect for DSI-ICP. 7 The matrix effects on the excitation and ionization characteristics of the ICP are thought to be relatively minor, although suppression in the ICP emission intensity of Cu I, Cd I, Zn I and Pb II in the presence of pyrolysis products of an ion exchange resin has been attributed to changes in the ICP excitation conditions due to matrix effects. 8 It is remarkable that matrix effects on ICP excitation conditions are seldom observed for DSI. The mass flux of the matrix for DSI is much higher than that of solution nebulization. For a sample solution containing 3% NaCl and a typical sample volume of 10 µl, the average mass flux of NaCl introduced into the ICP by DSI is 6×10 -5 g/s if NaCl is completely evaporated in 5 s. The same mass flux for solution nebulization implies a 18% NaCl solution, assuming a 2% nebulization efficiency and a 1 ml/min uptake rate. The introduction of such a high concentration of easily-ionized-element (EIE) into the ICP, if possible, would cause severe matrix effects. [9][10][11][12] In this work, the matrix effects for DSI-ICP on analyte vaporization and on the ICP excitation conditions were studied. A slow insertion rate of the sample probe was used so that a partial separation in time for the vaporization of the test element and the matrix could be obtained. With such a setup, the temporal profile of the ICP emission intensity contained information about both the thermal vaporization and matrix influence of the ICP. Since the uncertainties in the intensity from run to run were eliminated, a direct...

Effects of analyte appearance time on relative atomic and ionic emission intensities of Zn in direct sample insertion inductively coupled plasma atomic emission spectrometry

“…[15][16][17] Plasma excitation conditions and background emission intensity of the ICP correlate with the position of the sample probe in the ICP. 10,11 The background emission starts to drop when a sample probe is inserted into the plasma. 2,10,11 In Fig.…”

“…The experimental set-up was similar to the one described previously. 10 Briefly, a Fassel-type ICP torch with enlarged central channel was used (Glass Workshop, Department of Chemistry, University of Alberta, Canada). The internal diameter of the central channel was enlarged to 9.0 mm to accommodate the sample probe.…”

“…A characteristic of DSI-ICP is that the background emission intensities drop significantly [7][8][9] and the plasma excitation conditions change 10,11 with the insertion of a sample probe into the plasma. The extent of reduction in the plasma excitation temperature and electron number density correlates with the diameters of the sample probes, regardless of the heat capacity and material of the probe.…”

Novel setup for investigation of the plasma conditions in direct sample insertion inductively coupled plasma atomic emission spectrometry (DSI-ICP-AES)

“…2,3 Upon sample probe insertion into the plasma, the background emission intensities reduce significantly [4][5][6] and the plasma excitation temperature (T exc ) and electron number density (n e ) reduce by thousands of degrees and orders of magnitude, respectively. 7,8 In a previous study, 8 we showed that the extent of reduction in plasma excitation conditions due to sample probe insertion mainly depended on the outer diameter of the probe, regardless of the form (solid rod, solid rod with undercut, and hollow cup with undercut) and material (graphite or molybdenum) of the probe. The probe effects (represented by reduction in Zn II/Zn I ratio) increased with probe diameter.…”

“…In DSI-ICP-AES, analyte appearance time, shape (width and height) of the analyte peak, SBR, and background intensity depend on the position of the sample probe in the ICP discharge. [1][2][3][4][5][6][7][8][9][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Generally, increasing the cup position from a lower position of the ICP (e.g., below the top of the load coil) to a higher position (e.g., above the top of the load coil) gives sharper and more symmetrical analyte emission peaks and, therefore, enhanced SBR. The effects of probe position on plasma excitation conditions, however, are rarely discussed in the literatures.…”