Calibration Studies on Dried Aerosols for Laser Ablation–Inductively Coupled Plasma Mass Spectrometry† D. GU� NTHER*a , H. COUSINb, B. MAGYARb AND I. LEOPOLDc aSwiss Institute of T echnology Zurich, Institute for Isotope Geology, Sonneggstrasse 5, CH-8092 Zu�rich, Switzerland bSwiss Institute of T echnology Zurich, Institute of Inorganic Chemistry, Universita�tsstrasse 6, CH-8092 Zu� rich, Switzerland cInstitute of Plant Biochemistry, Weinberg 3, D-06120 Halle/Saale, Germany A simultaneous dried solution aerosol (Mistral nebulizer/ EXPERIMENTAL aerosol dryer) and laser-induced aerosol introduction system Instrumentation was used to investigate the calibration capabilities of dried solutions for LA–ICP-MS.Gas flow rates for the simultaneous A VG PQII+ instrument was used in combination with a Mistral nebulizer with a conventional Meinhard nebulizer (VG systems were optimized and gave the best results with 0.5 l min-1 for the laser gas flow rate and 0.5 l min-1 for the Elemental) and a LaserLab laser ablation unit (VG Elemental) The parameters used for the experiments are listed in Table 1.solution gas flow rate, at which a sensitivity of 65–80% for LA compared with 1.05 l min-1 single gas flow LA–ICP-MS The experimental set-up used for this investigation is shown schematically in Fig. 1. The connection piece for mixing the was maintained. The optimum temperatures for the Mistral nebulizer were 143°C ( heating) and -3 °C (cooling).Oxide two aerosols, mounted directly in front of the torch, was tested with angles a of 90° and 60°. The 90° connection piece showed formation (CeO+/Ce+) under these conditions is less than 0.3%. Introduction of enriched 207Pb (as a solution) and particle deposition inside the adapter glass surface. Therefore, the experiments were carried out with a 60° adapter, to reduce natural lead (via LA) allows the optimization of both sample introduction systems separately. Analyses were performed on memory effects from deposited particles.Optimization and tuning of the Mistral nebulizer were synthetic polyethylene materials, IAEA Soil-7 and CSB-1 reference standards. The RSDs on two sample pellets with five carried out with a 20 ng g-1 standard solution of Be, Co, Rh, In, Ce, Pb, Bi and U (1000 mg g-1 tune stock solutions; Merck, replicates each were better than 10%. Quantitative analyses for all REEs were based on In as the internal standard and Rh as the reference element.Fractionation effects of the internal Table 1 Instrumentation and parameters standard relative to the REE were not observed. ICP-MS— VG PlasmaQuad II+ Keywords: L aser ablation; inductively coupled plasma mass Power 1350 W spectrometry ; calibration ; aerosols Auxiliary gas flow rate 0.8 l min-1 Cool gas flow rate 13 l min-1 Carrier gas flow rate (laser) 0.5 l min-1 The use of standard solutions for calibration in LA–ICP-AES Carrier gas flow rate (liquid) 0.5 l min-1 and LA–ICP-MS is becoming an increasingly common pro- Sample uptake (constant) 1 ml min-1 cedure because of the simplicity of solution preparation for Acquisition mode Peak jumping the different applications. Sample introduction is based on the Points per peak 1 Dwell time 10 ms simultaneous introduction of a liquid aerosol and a laser- Detector mode Pulse induced aerosol, as described elsewhere.1–4 Results for LA–ICP- Preablation 10 s AES are not influenced by the use of a wet aerosol.2 However, Acquisition time 30 s the same technique in LA–ICP-MS leads to higher back- Replicates 5 grounds and more polyatomic interferences, e.g., ArO+, and L aser— VG LaserLab higher oxide formation, which, for REEs, are caused by the Laser type Nd5YAG, pulsed solvent.Laser wavelength 1064 nm The optimization procedure for LA–ICP-MS is greatly Laser mode Q-switched improved by a simultaneous sample introduction system with Flash lamp voltage 800 V Laser energy 0.2 J per shot-1 continuous solution sample uptake.The constant signals can Aerosol path length 1.5 m be used to optimize all gas flows, lens tuning and torchbox Ablation frequency 4 s-1 alignment (x, y, z). However, the tuning parameters are slightly different for laser-induced aerosols and have to be reoptimized on laser aerosol signals using a 4 s-1 repetition rate. This paper describes the optimization of gas flows, mixing behavior and tuning parameters for a simultaneous solution aerosol and laser-induced aerosol introduction system, in combination with a drying system (Mistral, VG Elemental, Winsford, UK) so as to achieve reduced oxide formation and associated polyatomic species formation.The applicability of this calibration technique is demonstrated for the determination of REEs in polyethylene and mixed reference materials. Fig. 1 Schematic diagram of a simultaneous ‘dried’ aerosol and laser- † Presented in part at the 1994 Winter Conference on Plasma Spectrochemistry, San Diego, CA, USA.induced aerosol introduction system (dual gas flow system). Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (165–170) 165Darmstadt, Germany). The same element solutions were mixed for Mistral nebulization were added as 0.1 M nitric acid solutions. with polyethylene, dried, homogenized in a ball-mixer/mill, pressed to a pellet (blank=2 mg g-1) and used as synthetic Experiments for the determination of the transport efficiency of individual gas flows were carried out with enriched 207Pb test samples for the optimization procedure of the laser ablation system.solution (prepared from enriched 207Pb, 90.4%; Medgenix Diagnostics, Germany). Sample Preparation Reference soil samples and synthetic test materials were pre- RESULTS pared by the following procedure. Polyethylene powder (PE) Optimization of the Dual Gas System in spectroscopy-grade Uvasol (Merck) was used as a binder to prepare the pellets.The internal standard (In) and the For the characterization of a new dual gas flow system (combining laser-induced aerosols and Mistral ‘dry’ nebuliz- reference element (Rh) were added to each of the pellets. The PE mixture was homogenized in a Mixer/Mill in polystyrene ation), each individual system was separately optimized. The optimization procedure was divided into two stages: (a) measur- vials (3 in×1 in id), containing one 1/2 in and one 3/8 in methacrylate ball (8000 Spex Mixer/Mill, Spex Industries, ement of element isotopes for the determination of the sensitivity and (b) measurements of molecular species for the Edison, NJ, USA).The vials and balls were cleaned with 20% HNO3 and rinsed with high purity water (Milli-Q system, observation of changes in the plasma conditions. The optimum intensity for the single gas flow LA–ICP-MS measured on Millipore, Bedford, MA, USA). One blank, four sample pellets (two CSB-1 and two IAEA Soil-7) and five test material pellets 115In was determined as 1.05 l min-1.The optimum intensity for single gas flow Mistral nebulization ICP-MS on 115In was were prepared as described below. A 2.5 g amount of PE was weighed into a vial, with the following additions: (a) for a in the range 0.75–0.80 l min-1, depending on the heating temperature, and for single gas flow liquid nebulization it was blank pellet, 50 mg of 1000 mg g-1 Rh stock solution; (b) for sample pellets, 50 mg of IAEA Soil-7 and CBS-1 (dried at 0.8 l min-1.The results of the optimization are summarized in Figs. 2–5. 105 °C) and 50 mg of 1000 mg g-1 Rh stock solution; (c) for standard pellets (0–500 ml of the 10-fold diluted 100 mg ml-1 The isotopes 9Be, 59Co, 103Rh, 115In, 140Ce, 208Pb, 209Bi and 238U (to represent the whole mass range) and the molecular REE; Johnson Matthey Alfa Products, Royston, Hertfordshire, UK), 0.05–2 mg g-1, and 50 mg 1000 mg g-1 Rh stock solution; species 12C16O16O+, 14N16O16O+, 40Ar12C+, 40Ar14N+, 40Ar16O+ and 40Ar12C16O+ were measured.and (d) for tune pellets (2 mg g-1), 50 mg of tune stock solution (of Be, Mg, Co, In, Ce, Pb, Bi, U, 100 mg g-1, Merck) and Fig. 2(a) shows the absolute intensities of all three sample introduction systems usigle gas flow (1.05 l min-1 50 mg of 100 mg g-1 Rh stock solution. A 1.0 ml volume of ethanol (puriss p.a. grade, Fluka, Buchs, Switzerland) was then laser, 0.8 l min-1 Mistral, 0.8 l min-1 liquid).The introduction of two different aerosol gas flows is limited by a total carrier added and the mixture was dried in a vacuum oven at 60°C and 10 kPa for 2 h. The dry mixture was then homogenized gas flow rate of 1.3 l min-1. Higher gas flow rates lead to reduced excitation, lower ion transmission or, in the worst for 1 h in the mixer/mill. Pellets of about 1 g were pressed in a conventional pellet die for IR spectrometry at 10 t for 30 s.case, instrument shutdown. The absolute intensities of all isotopes (excluding molecular species) under all single gas flow The corresponding tune and standard solutions for Mistral and liquid nebulization were prepared from the above stock conditions (laser, Mistral, liquid) were within two orders of magnitude. solutions in 0.1 M HNO3 (Suprapur grade, Merck). The blanks Fig. 2 (a) Intensities for single gas flow optimization of laser, Mistral, and liquid aerosol.(b) Intensities for dual gas flow optimization of laser, Mistral, and liquid aerosol at 0.5 l min-1 (no added gas flow). 166 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Fig. 3 Mistral aerosol gas-flow tuning (aerosol-carrier flow 0.5 l min-1). A, U-238; B, Pb-208; C, Be-9; D, In-115; E, Co-59; F, ArN-54; G, ArO-56; H, ArCO-68; I, ArC-52. Fig. 4 Liquid nebulizer aerosol gas-flow tuning (aerosol-carrier flow 0.5 l min-1). Key as for Fig. 3. Fig. 5 Laser aerosol gas-flow tuning (aerosol-carrier flow 0.5 l min-1). Key as for Fig. 3. A lower flow rate of 0.5 l min-1 aerosol carrier gas (no (corresponding to a total argon gas flow of 0.5–1.20 l min-1). Intensities using the Mistral nebulization are improved by up added gas flow) is effectively the minimum possible total gas flow in the dual flow system [Fig. 2(b)], as carrier gas flow to a factor of 100 (above 0.3 l min-1 added gas flow, Fig. 3), whereas the liquid introduction intensities are improved by a rates below 0.5 l min-1 lead to S/B of <3 and significantly higher molecular interferences.The signal intensities for all factor of only 10 (at 0.2 l min-1 added, Fig. 4). The intensities of polyatomic species such as ArC+, ArN+, optimized dual flow systems are of the same order of magnitude and the reproducibility for five replicates was less than 5%. ArO+ and ArCO+ were measured under all experimental conditions as an indicator for (a) changes in excitation due to Because of their absolute intensities and their reproducibility, it was concluded that Mistral and laser were suitable for direct the secondary gas flow, (b) the formation of O species as a result of the remaining water content (Mistral) and (c) the comparison in the dual gas mode.The signal intensity is improved by a factor of 10 by Mistral nebulization compared influence of carbon from the ablated polyethylene matrix. The results shown in Figs. 3–5 suggest an optimum dual gas flow with liquid nebulization.The sensitivity of laser induced aerosols (test material pellets with 2 mg g-1) were three orders of rate of 0.5 l min-1 for laser and Mistral nebulization as gas flow rates above 0.5 l min-1 lead to lower isotope intensities. magnitude lower compared with the Mistral nebulization intensities [solution with 0.02 mg g-1, Fig. 2(b)]. Fig. 6 demonstrates the reduced formation of oxides (e.g., CeO+/Ce+, ArO+/In+, ArO+/Co+, CoO+/Co+) from single Figs. 3–5 illustrate the effect of a progressively measured dual flow for each dual introduction system (starting at a 0.5 liquid nebulization (A), relative to single Mistral nebulization (B–D), single laser ablation (E) and the optimized dual l min-1 aerosol flow) with added gas flows of 0–0.70 l min-1 for Mistral and laser, and 0–0.3 l min-1 for liquid nebulization Mistral–laser system (F). It was found that the excitation Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 167and 115In intensities on the dual gas flow system (with laser ablation and Mistral blank introduction). The intensities of the 103Rh and 115In signals measured with the dual gas system were normalized to their respective intensities at the optimum single gas flow laser ablation conditions (1.05 l min-1). The intensities of 65% for 115In and 80% for 103Rh indicate an increased loading of the carrier gas due to the lower sample velocity and increased residence time of the aerosol in the ablation cell.Calibration Method The first stage in the calibration procedure consisted of blank Fig. 6 Description of the dryness of the aerosol by comparing the nebulization and analyte and reference element calibration ratio of CeO+/Ce+ and ArO+/In+. (Mistral). Subsequently the samples were ablated and transported into the plasma together with the blank Mistral aerosol (nitric acid, 20 pg g-1 In). The calibration was based on the conditions for laser aerosol and Mistral aerosol are of the following equation: same order of magnitude. The efficiency of the Mistral nebulizer depends upon the heating and condensation temperature CA,solid CRh,solid = CA,laser CRh,laser (1) (B–D) and has to be optimized for each application.The slightly higher oxide formation in the dual gas flow system is probably due to air entrainment and the summation of oxide where CA,solid=unknown concentration, CRh,solid=reference forming species from both individual gas flows.element concentration, CA,laser= concentration determined The evaluation of the gas flow mixing behavior was studied using Mistral calibration and CRh,laser= concentration deterusing enriched 207Pb (99.7%) for Mistral nebulization and mined using Mistral calibration. With constant working parnatural Pb (207Pb, 22.08%) for laser ablation. Based on two ameters (e.g., excitation, gas flow rates, transportation volume), extreme points (100% Mistral flow, Pb207/Pb208=10.99±0.25; the Mistral calibration functions are 100% laser flow, Pb207/Pb208=0.426±0.045; Table 2, first two rows), the actual introduction rate was calculated assuming a IA Iln =f (CA , Cln=constant) (2) linear dependence of gas mixing (from the measured isotopic ratio Pb207/Pb208). The corresponding values are shown in Table 2 (last three rows).IRh Iln =f (CRh , C1n=constant) (3) The sample introduction efficiency for the dual system is always dominated by the Mistral nebulization (59.6–66%).A The concentrations of the laser-induced aerosols [required possible explanation of this may be the particle size distribution for eqn. (1)] can be directly determined using eqns. (2) and and the particle shape of the two different aerosols, such that (3). Indium (the internal standard) was always introduced as more particles might pass through the plasma without being Mistral dried aerosol and used for corrections of the dual completely dissociated and ionized.flow system. The drawback of the dual gas flow system is the theoretical The results of this calibration after subtracting blanks are decrease in sensitivity by a factor of 2 (0.5 l min-1 Mistral and given in Table 3. Where the concentrations are below 0.5 0.5 l min-1 laser). Fig. 7 shows the dependence of the 103Rh mg g-1 they show high deviations from the theoretical values, which depend on the amount of laser ablated material introduced into the plasma; where the concentrations are higher Table 2 Sample introduction rate of single and dual gas flow systems than 0.5 mg g-1, the intensities are equivalent to concentrations Response of about 1 ng g-1 on the Mistral calibration curves.The Ar flow/ ratio, M accuracy and reproducibility can be improved by higher sensi- Mode l min-1 Tuned* Pb207/Pb208 (%) tivity. The results in Table 3 also show that Rh has the same Mistral 0.79 M 10.99 100 ablation behavior as the REEs; fractionation as described by Laser 1.05 L 0.426 0 Longerich et al.5 and Fryer et al.6 was not observed.The Mistral/Laser 0.5/0.5 M 6.3 59.6 difference in behavior could be related to dilution of the sample Mistral/Laser 0.5/0.5 L 6.7 63.5 and/or the matrix. Mistral/Laser 0.5/0.5 M/L 6.91 66 The calibration technique presented in this paper was compared with the external calibration technique using pressed * M=tuned on Mistral, L=tuned on laser sample introduction.Fig. 7 Signal reduction using a dual gas flow system (total gas flow 0.5–1.25 l min-1). 168 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Table 3 Determination of REEs (mg g-1) in synthetic test samples and 0.5 l min-1 laser) lead to better sensitivities and lower using Mistral calibration molecular interferences (e.g., oxide formation) compared with a dual system with liquid nebulization.3 The decrease in Element REE-1 REE-2 REE-3 REE-4 REE-5 intensity compared with single gas flow systems is less than Theoretical values 0.051 0.1 0.51 1.02 2.01 35%.The tuning procedure and the sample introduction Yttrium 0.056 0.1 0.5 0.99 1.8 optimization are greatly improved by a dual system owing to Lanthanum 0.042 0.09 0.5 0.95 1.8 the constant sample uptake of the Mistral nebulizer. The use Cerium 0.069 0.12 0.49 0.93 1.85 of dried aerosols for tuning the plasma is one of the most Neodymium 0.089 0.17 0.53 0.98 1.82 suitable procedures so far investigated to set up laser ablation Samarium 0.081 0.12 0.54 0.95 1.83 conditions.Europium 0.071 0.12 0.5 0.94 1.79 The higher RSDs for Mistral nebulization are mainly the Gadolinium 0.075 0.13 0.52 1.02 1.87 Terbium 0.078 0.14 0.52 1.04 1.93 result of poor stability of the nebulizer. Improvement of the Dysprosium 0.069 0.11 0.52 0.99 1.87 Mistral unit, currently being implemented by the manufacturer, Holmium 0.081 0.14 0.53 0.98 1.91 especially to the drain, should lead to increased signal stability Erbium 0.053 0.09 0.5 0.99 1.96 and, therefore, to better SBRs and, consequently, higher Thulium 0.067 0.13 0.54 1.01 2.04 sensitivities.Ytterbium 0.084 0.15 0.57 1.03 2.08 The dual gas flow technique can be extended to other Lutetium 0.079 0.14 0.61 1.12 2.2 samples, e.g., minerals with stoichiometric composition, trace element determinations in raw materials and synthetic ceramic materials. However, such applications would need more pellets with an REE standard solution as described elsewere7 detailed investigations of the fractionation process and of using the reference materials IAEA Soil-7 and CSB-1, prepared suitable reference elements.More effective desolvation units as PE pellets. The results are summarized in Tables 4 and detailed studies of air entrainment from the plasma and 5. environment (bonnet around the torch9) could lead to more sensitive dual gas flow systems. CONCLUSIONS The dual gas flow system with Mistral nebulization (dry The authors are grateful to VG Elemental (Mainz-Kastel, aerosols) is an alternative calibration method for LA–ICP-MS Germany) for the loan of the Mistral nebulization unit for solid analyses.The dual gas flow optimization carried out in this study shows that equivalent flow rates (0.5 l min-1 Mistral this study. Table 4 Determination of REEs (mg g-1) in a reference soil (IAEA Soil-7) IAEA Soil-7 Mistral calibration External calibration Element Mean CI Mean±CI95% Mean±CI95% Yttrium 21 15–27 25.7±5.3 23.8±0.4 Lanthanum 28 27–29 32.3±9.6 32.5±2.3 Cerium 61 50–63 68.9±16.8 69.2±3.4 Praseodymium 15.3±4.0 13.4±0.6 Neodymium 30 22–34 34.4±10.6 31.0±1.0 Samarium 5.1 4.8–5.5 6.6±1.5 5.5±0.5 Europium 1.0 0.9–1.3 1.39±0.28 1.02±0.07 Gadolinium 5.6±0.97 4.3±0.3 Terbium 0.6 0.5–0.9 0.92±0.17 0.66±0.01 Dysprosium 3.9 3.2–5.3 5.2±0.98 4.4±0.2 Holmium 1.1 0.8–1.5 1.09±0.24 0.87±0.04 Erbium 3.06±0.57 2.5±0.1 Thulium 0.48±0.08 0.4±0.1 Ytterbium 2.4 1.9–2.6 2.71±0.56 2.3±0.2 Lutetium (0.3) (0.1–0.4) 0.48±0.08 0.28±0.02 Table 5 Determination of REEs (mg g-1) in a reference bentonite (CBS-1)8 Bentonite CSB-1 Mistral calibration External calibration Element mean mean±CI95% mean±CI95% Yttrium 34 33±12 37±3 Lanthanum 58 52±11 63±5 Cerium 122 127±27 174±20 Praseodymium 28±6 19±4 Neodymium 53.3 52±13 61±6 Samarium 13.1 11±3 14.5±1.3 Europium 0.73 0.73±0.11 1.64±0.25 Gadolinium 11.3 8.5±2.2 10.3±0.9 Terbium 1.34 1.4±0.3 1.55±0.17 Dysprosium 8.0±2.4 9.9±0.7 Holmium 1.5±0.4 2.30±0.23 Erbium 4.3±1.5 5.4±0.9 Thulium 0.74 0.73±0.20 1.13±0.13 Ytterbium 4.1 4.0±1.6 5.6±0.8 Lutetium 0.59 0.64±0.17 0.52±0.11 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 1697 Magyar, B., and Cousin, H., Mikrochim. 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