Evaluation of various nebulizers for use in microwave induced plasma optical emission spectrometryw

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1 TECHNICAL NOTE Journal of Analytical Atomic Spectrometry Evaluation of various nebulizers for use in microwave induced plasma optical emission spectrometryw Henryk Matusiewicz,* a Mariusz Ślachciński, a Montserrat Hidalgo b and Antonio Canals* b Received 28th March 2007, Accepted 19th June 2007 First published as an Advance Article on the web 9th July 2007 DOI: /b704612j Three different micronebulizers, the micro3 (M3), the flow focusing pneumatic nebulizer (FFPN) and the microcapillary array nebulizer (NAR-1), were compared with a conventional Meinhard pneumatic concentric nebulizer (PN) working at low liquid flow rates for the elemental analysis of liquid samples by microwave induced plasma optical emission spectrometry (MIP-OES). Three nebulizers (M3, FFPN, PN) were operated in conjunction with the same cyclonic spray chamber. A critical evaluation has been carried out of the nebulization stages, such as the formation of the primary and tertiary aerosol, separation of large droplets (drop size distribution) in the spray chamber (also evaluated for the NAR-1 nebulizer) and aerosol transport to the plasma torch. Atomic emission was measured for Ba, Ca, Cd, Cu, Fe, I, Mg, Mn, Pb, Sr and Zn. Analytical performance of the nebulization systems were characterized by determination of the limits of detection (LODs), the precision (RSDs) and the memory effects (wash-out time) for these elements, and ranged between mg ml 1, 4 8%, and 5 7 minutes, respectively. Analysis of certified reference materials (TORT-1, Human Hair No. 13, Lichen IAEA-336, Soya Bean Flour INCT-SBF-4) were performed to determine the accuracy and precision available with the presented nebulization systems. These materials were microwave/nitric acid digested and analyzed by calibration with synthetic solutions of the analytes. In general, the results indicated that the FFPN nebulizer gave rise to higher emission signals and slightly lower LOD values than the other three nebulizers. The nebulizers exhibited no clogging problems. 1. Introduction A variety of emission sources, including flames, the inductively coupled plasma (ICP), the microwave induced plasma (MIP) and the direct current plasma (DCP), have been investigated for use in optical emission spectrometry (OES) for routine multielemental analysis of various materials. Among these, the microwave induced plasma (MIP) offers some attractive characteristics, such as its unique features of high excitation efficiency for metal and non-metal elements, capability of working with various gases, capabilities for simultaneous determination of elements, low cost for instrumentation and maintenance, and convenience of operation. 1 3 The most commonly used approach for introducing sample material into the flame and plasma is based on the injection of sample aerosols generated by nebulization of aqueous solutions, and one of the methods used is pneumatic nebulization. The latter is the most popular form of sample introduction as it is simple, inexpensive and robust, without being particularly prone to memory effects. 4,5 However, as already specified by Browner a Politechnika Poznan ska, Departament of Analytical Chemistry, Poznan, Poland. Henryk.Matusiewicz@put.poznan.pl b Departamento de Quimica Analitica, Nutricio n y Bromatologia, Universidad de Alicante, Apdo. 99, E Alicante, Spain. a.canals@ua.es w Electronic supplementary information (ESI) available: optimization procedure, Tables 1 10, Fig. 1 8 and video clips 1 and 2. See DOI: /b704612j and Boorn 6,7 the sample introduction is a critical step in most atomic spectroscopic techniques. Because samples differ in viscosity, salt content, and solids content, no one nebulizer is best for all sample types (biological, clinical, environmental, etc.) and all analyses. Therefore, the proper nebulizer must be chosen for a given analysis. Application of nebulizers for sample introduction into microwave induced plasma sources became possible since MIPs can be operated at atmospheric pressure. 8,9 This system allowed liquid aerosols to be nebulized directly into the atmospheric discharge. Microwave plasmas are normally operated at substantially lower applied power than ICP devices. The low power levels do not produce sufficiently energetic plasma for efficient processes on the plasma (desolvation, volatilization etc.). In addition, plasma stability can be affected seriously when solutions are injected directly. For these reasons, microflow devices are ideal for this excitation source. Over the past several years, some micronebulizer designs have been developed for use with microwave induced plasma optical emission spectrometry (MIP-OES). Among them are the glass frit nebulizer, 10,11 thermospray nebulizer, 12 ultrasonic nebulizer, 13 Hildebrand grid nebulizer, 14 glass capillary array nebulizer, 11,15 direct injection nebulizer 16,17 and V-groove Babington nebulizer 18,19 which can even be used when introducing powdered suspensions using slurry nebulization. So far, no systematic comparison has been performed between some of these different pneumatic micronebulizers 1174 J. Anal. At. Spectrom., 2007, 22, This journal is c The Royal Society of Chemistry 2007

2 in MIP-OES. In comparison with other plasma sources, like ICP or DCP, the microwave plasma is more sensitive toward composition changes caused by introducing the sample in the form of an aerosol. Therefore, the aim of the present study was to evaluate the behavior of several pneumatic micronebulizers in MIP-OES with aqueous sample solutions, by reference to a commercially available conventional nebulizer. Four different reference samples were analyzed against pure aqueous standards so as to assess the results obtained on interferences. Analytical potentialities are discussed. 2. Experimental 2.1. MIP-OES instrumentation and operating conditions A Carl Zeiss Echelle spectrometer (Model PLASMAQUANT 100) using fibre-optical light-guides and photomultiplier tubes (PMT) and a TE101 microwave plasma cavity assembly was used, and was essentially the same as previously described Instrument settings and operational parameters used for the experimental MIP-OES system are summarized in Table 1 (see ESIw) Sample introduction systems Three different pneumatic micronebulizers were used: a micro3 (M3) (Burgener Research Inc., Mississauga, Canada), a flow focusing pneumatic nebulizer (FFPN) (Ingeniatrics, Sevilla, Spain) and microcapillary array nebulizer (NAR-1). A commercial pneumatic concentric nebulizer TR-30-A3 (PN) (Meinhard Glass Products, Golden, Colorado, USA) was used for comparison, since this nebulizer is the standard nebulizer on many plasma-based instruments. Fig. 1 (see ESIw) shows pictures of the nebulizers. In order to compare the behaviors of the nebulizers evaluated, the same glass cyclonic spray chamber Cinnabar (Glass Expansion, West Melbourne, Australia) (ca ml inner volume) was used with M3, FFPN and PN nebulizers to transport the aerosol towards the microwave plasma torch. However, a different glass spray chamber (5 ml inner volume) was used with the NAR-1 (Fig. 2, see ESIw). Together with the pneumatic micronebulizers mentioned above, one other has been proposed based on a different principle, such as microcapillary array nebulizer (NAR-1). 11 A schematic diagram of the NAR-1 is shown in Fig. 2(a) and an illustration of this design is shown in Fig. 2(b) (see ESIw). Since the NAR-1 system has been described in detail in a previous paper, 11 this will not be discussed again here, but briefly summarized only. The NAR-1 nebulizer consists of orifices of o10 mm diameter located in the V-groove. The glass capillary arrays used were 4.1 mm in diameter and 2.5 mm thick. Table 2 gives their optimum working parameters (see ESIw). The liquid samples were introduced through all nebulizers by means of a Gilson Minipuls 3 peristaltic pump (Villiers Le Bel, France). The gas flow rate was controlled by means of a mass flow controller (DHN, Warsaw, Poland). Argon was used as the plasma and nebulizing-carrier gas Aerosol characterization and transport variables Drop size and velocity distributions of the aerosols were determined using a two-dimensional Phase Doppler Particle Analyzer (2D-PDPA, TSI Inc., USA). 26,27 The primary aerosol was sampled 5.0 mm from the nebulizer tip along the centerline of the aerosol. The tertiary aerosol was measured 1.0 mm from the end of the spray chambers used, and at the centerline of the chamber exit. To follow the conditions used in MIP, the nebulizers were horizontally positioned for primary aerosol diameter velocity measurements, whereas the tertiary aerosol was vertically sampled. In each PDPA acquisition, approximately droplets were measured to determine particle size and velocity distributions. Analyte and solvent transport rates were measured by means of direct collection methods. 28, Gases and reagents Compressed, ultrahigh-purity argon gas (N-50 purity, %) obtained from BOC GAZY (Poznan, Poland) was employed as the plasma and nebulizing-carrier gas. Standard solutions were prepared from a 1000 mg l 1 stock solution (ICP Multi-element Standard Solution IV CertiPURs, Merck, Darmstadt, Germany). Working standard solutions were freshly prepared daily by diluting appropriate aliquots of the stock solution in 1 M HNO 3 prepared from 69% high purity acid (Merck) in distilled pure water. Potassium iodide was Suprapur grade (Merck, Darmstadt, Germany). HNO 3 (69%, v/v) acid used was of the highest quality grade (Trace pure, Merck, Germany). Hydrogen peroxide 30% (v/v) was obtained from POCh (Gliwice, Poland). Water was initially deionized (Model DEMIWA 5 ROSA, Watek, Czech Republic) and then doubly distilled in a quartz apparatus (Heraeus Bi18, Hanau, Germany) Reference materials and real samples Validation of the method described in this work was performed using four reference materials, which were chosen to represent solid sample matrices: TORT-1 (Lobster hepatopancreas) supplied by the National Research Council of Canada (NRCC, Ottawa, Canada), Human Hair No. 13 from National Institute for Environmental Studies (NIES, Japan), Lichen IAEA-336 from International Atomic Energy Agency (Vienna, Austria) and Soya Bean Flour INCT-SBF-4 supplied by the Institute of Nuclear Chemistry and Technology (Warsaw, Poland). The following real samples were used in this study: feminatal tablets and iodide tablets (Merck, Darmstadt, Germany) Microwave digestion system A laboratory-built prototype of a high pressure temperature focused microwave heating digestion system, equipped with a closed TFM-PTFM vessel based on a design outlined in detail by Matusiewicz, 30 was employed for wet-pressure sample digestion Analytical procedures Microwave-assisted sample digestion at high-pressure in PTFE vessels. For the reference materials, approximately This journal is c The Royal Society of Chemistry 2007 J. Anal. At. Spectrom., 2007, 22,

3 mg of samples was placed in the 30 ml TFM-PTFE vessel of the microwave digestion system, moistened with 1 ml of 30% H 2 O 2 and 4 ml of concentrated HNO 3 was added. The sample was heated for 10 min at 150 W. The digested solution was transferred into a 10 ml volumetric quartz flask and diluted up to the mark with water Simplex optimization procedure. A simplex optimization approach was undertaken to establish the best conditions for liquid nebulization, transport and excitation. The parameters optimized are listed in Table 2 (see ESIw), along with the ranges over which optimization experiments were possible and conducted (Table 3, ESIw). The optimum conditions obtained from this procedure were then used to run standard element solutions and quantify the elements present in the dissolved samples MIP-OES analysis. The plasma was ignited by momentarily inserting an isolated high purity tantalum wire into the quartz discharge tube. Aerosols, produced from liquids by the nebulizers, were immediately carried by the argon plasma carrier gas through the spray chamber and into the MIP for excitation. Net analyte emissions were calculated by taking the simultaneous difference of measured emission intensities on the top of the peak and background near the peak. Instrumental settings and operational parameters are shown in Tables 1 and 2 (see ESIw). Analytical blanks were also carried out through the entire procedure outlined above, to correct for possible contaminants in the reagents used for sample preparation. Quantification of Ba, Ca, Cd, Cu, Fe, Mg, Mn, Pb, Sr and Zn was made from linear calibration curves. All limits of detection (LOD) given by MIP-OES software were calculated for raw, unsmoothed data based on a 3s criterion of the background (blank) counts. 3. Results and discussion 3.1. Optimization of instrumental parameters Preliminary analytical performance of the Ar- and He-MIP was examined by measuring the S/B ratio of selected elements. The Ar-MIP was found to be superior to the He-MIP, so was selected for all the subsequent experiments Simplex optimization of operational variables Three different types of experimental variables affect the method. These are as follows: first, variables controlling the emission response in the microwave plasma, that is, the microwave forward power of the microwave generator. Second, variables such as the argon carrier flow and sample uptake rate that regulate transport. Followed by univariate searches for the optimum values of applied power, nebulizingcarrier gas flow rate and sample uptake rate, a multivariate simplex optimization was used to establish the optimum plasma parameters for low detection limits of selected elements. Table 4 (see ESIw) lists the optimum values that the simplex and univariate experiments indicated for each of the three factors studied. In addition, the maximum and minimum levels of the variables to be investigated are shown in Table 4 (see ESIw). The effectiveness of the simplex procedure was confirmed with univariate searches, which assisted in verifying that the optimum lay near the simplex value Microwave forward power. The MIP is normally operated at low power levels in the range of W. In this work, the stable Ar plasma could be maintained at a level of greater than 100 W forward power. In general, for all analytical lines of studied elements, S/B ratios usually tend to level off after the microwave power approached 160, 160, 160 and 155 W for PN, M3, FFPN and NAR-1, respectively, see Table 2 and Fig. 3(a) 6(a) (see ESIw) Carrier argon flow rate. The effect of plasma argon gas flow rate was not optimized in our experiments but was selected based upon previous experience and maintaining the plasma stability and plasma shape. Stable operation of the plasma was obtained at gas flow rates of 400 ml min 1. It was also observed that the carrier gas stream flow rate has a more significant influence on the emission intensities than the plasma gas flow rate. The carrier gas affects the formation of the plasma channel, the residence time of the analyte in the plasma, and the aerosol generation and transport efficiency. 31 The effect of carrier argon flow rate on the relative emission intensities is illustrated in Fig. 3(b) 6(b) (see ESIw). The emission intensities reached maximums at 1000, 1200, 1000 and 800 ml min 1 for PN, M3, FFPN and NAR-1, respectively. The maxima are the result of opposite effects of nebulizing gas flow on aerosol characteristics and interaction of aerosol with the plasma Sample uptake rate. When the sample pumping rate was greater than approximately 2.5, 0.6, 0.8 and 0.15 ml min 1 for PN, M3, FFPN and NAR-1 nebulizers, respectively, it was found that the signal intensities would not increase further and began to decrease (Fig. 3(c) 6(c), ESIw). For pneumatic nebulization, the higher the liquid flow, the more the primary drop size distribution is shifted to bigger drop sizes. 23 Nevertheless, the absolute amount of aerosol volume contained in smaller drop sizes is increased. Therefore, the higher the liquid flow, the higher the analyte and solvent transport rates, which explains the maximum seen on the signal vs. liquid flow graph Memory effect and washout time The wash-out time required for the net analyte signal to decay to 1% of its steady-state signal value was determined. Fig. 3(d) 6(d) (see ESIw) show the memory effects of selected micronebulizers in their spray chambers to that of a conventional PN nebulizer. The PN, M3 and FFPN nebulizers have washout times of 5 min, at least a 1 2 min less than that of the NAR-1 nebulizer Analytical figures of merit Detection limits obtained on a simultaneous multi-element basis for the various sample introduction techniques employed in this work are compared to the results obtained by PN. A comparison between detection limits obtained by conventional nebulization (PN) and micro nebulization (M3, FFPN, NAR-1) for the set of lines tested is shown in Table 4 (see ESIw). The limits of detection (LOD) calculated using 1176 J. Anal. At. Spectrom., 2007, 22, This journal is c The Royal Society of Chemistry 2007

4 the IUPAC recommendation (based on a 3s blank criterion) and obtained by use of the optimized operating conditions are summarized in Table 2 (see ESIw) based on the raw, unsmoothed data. The FFPN provides lower limits of detection than the other nebulizers for all the elements evaluated in axially viewed microwave plasma with a 1% nitric acid matrix. The background equivalent concentrations values (BEC) followed a similar trend. The precision of replicate determinations was calculated from the RSD (%) of the mean of six replicate measurements of element standard using a mass 50-fold above the LOD. The precision of the FFPN is similar or slightly better than that of the other nebulizers. The precision of the elements determination ranges from 4 to 8% (relative standard deviation of 6 replicates for sample introduction) for the liquid original samples. Iodine was the only non-metallic element evaluated. The % RSD values of iodine emission intensities in aqueous standards are about 7% Drop size and velocity distributions Primary aerosols. The quality of the primary aerosols produced by the four nebulizers studied at optimized conditions of gas and liquid flow rates were quantitatively examined (Fig. 7a, ESIw). Fig. 7a shows that, under the conditions studied, practically all the primary aerosol is contained in droplets smaller than 50 mm. Typical cut-off diameter values (d c ) for conventional spray chambers range between 15 mm and 20 mm. 24 At optimized gas and liquid flow rates used for each nebulizer, the number percentage of primary aerosol contained in droplets having sizes smaller than 20 mm is 78%, 82%, 97% and 95% for PN, M3, NAR-1 and FFPN, respectively. Fig. 7a also shows that droplet size distribution of the aerosols from NAR-1 and FFPN are the finest and narrowest. For the optimized conditions used, the mean velocities are 24, 47, 62 and 84 m s 1 for the NAR-1, M3, PN and FFPN, respectively (Fig. 8a, ESIw) Tertiary aerosols Fig. 7b (see ESIw) shows the drop size distributions of aqueous tertiary aerosols obtained with all the nebulizers studied. For all the nebulizers tested, the majority of the tertiary aerosol is contained in droplets smaller than 20 mm, hence the spray chambers used in this study show a d c of approximately 20 mm and 15 mm for the single-pass spray chamber and cyclonic one, respectively. Large droplets (48 mm) decrease sensitivity and contribute negatively to noise. 32,33 At the optimized working conditions for each nebulizer, the number percentage of tertiary aerosol contained in droplets smaller than 8 mm are 98% for FFPN, 86% for NAR-1 and near 100% for the other two nebulizers (M3 and PN). From the data shown in Fig. 7a and 7b (see ESIw), a change of the relative position between primary and tertiary drop size distributions obtained with NAR-1, FFPN and PN and M3 can be observed. Finer primary aerosols are obtained with NAR-1 and FFPN, whereas PN and M3 generate the finer tertiary aerosols. This inversion in relative position of drop size distributions is a consequence of the spray chamber action to remove bigger droplets more significantly with PN and M3 than with FFPN and NAR-1. NAR-1 shows bigger drop sizes on the tertiary aerosols due to two main reasons: (i) the spray chamber used with this nebulizer is different (different d c ) and; (ii) the low nebulizing gas flow generates the finest and slowest primary aerosols (Fig. 7a and 7a). These conditions accumulate aerosol into the spray chamber, increasing coalescence that generates bigger droplets (see video clip 1, ESIw). The same cyclonic spray chamber was used with FFPN, M3 and PN, and a finer primary aerosol is obtained with FFPN, whereas M3 and PN generate the finest tertiary aerosols. The spray chamber reduces both size and velocity of aerosols by removing large and fast droplets. The most important aerosol losses come from inertial impact against the frontal wall of the spray chamber (see video clip 2, ESIw) and these types of losses are drop size and velocity dependent. Until the aerosol strike against the frontal wall of the spray chamber, at approximately 35 mm from the nebulizer tip, it has been experimentally observed that the PN and M3 generates aerosols with a higher number percentage of droplets contained in larger sizes (6.5%, 24.2% and 23.9% of the aerosol contained in droplets larger than 10 mm for FFPN, PN and M3, respectively). In addition, mean velocity values are 11 m s 1,15ms 1 and 19 m s 1 for the FFPN, PN and M3, respectively. These new velocities are the result of the tendency of the aerosol droplets to match their velocity to the surrounding velocity (i.e., terminal velocity), and the time to reach terminal velocity increases with droplet diameter. In short, we can conclude that the spray chamber acts as a filter of momentum and it has no fixed cut-off diameter. The mean velocities of tertiary aerosols were 0.30, 0.44, 0.53 and 0.72 m s 1 for the M3, FFPN, PN and NAR-1, respectively (Fig. 8b) (see ESIw). Comparing Fig. 7a and 7b (see ESIw) it can be concluded that the filtering action of the spray chamber has been more intense for the PN nebulizer, followed by the M3 nebulizer and finally by the NAR-1 and FFPN. At the optimized conditions, the analyte transport rate (W tot ) values are approximately 20 mg min 1,10mg min 1,8mg min 1 and 7 mg min 1 for FFPN, PN, NAR-1 and M3, respectively, whereas the solvent transport rate (S tot ) values are approximately 900 mg s 1, 750 mg s 1, 500 mg s 1 and 400 mg s 1 for FFPN, PN, M3 and NAR-1, respectively. The ideal liquid sample introduction system must supply the highest W tot and the smallest S tot values. The ratio values between these transport variables (10 4 )(W tot /S tot, both in mg s 1 ) are: 2.2, 2.3, 3.3 and 3.7 for PN, M3, NAR-1 and FFPN, respectively. The same trend is shown for the sensitivity (i.e, limits of detection) Analysis of reference materials To evaluate the accuracy and precision of the sample introduction systems tested on the determination of the elements in real samples, four certified reference materials (CRMs) were chosen. The results obtained for the analysis of CRMs by nebulization MIP-OES method using calibration with synthetic solutions of the analytes, are summarized in Tables 5 8 This journal is c The Royal Society of Chemistry 2007 J. Anal. At. Spectrom., 2007, 22,

5 (see ESIw). The results obtained by calibration with synthetic aqueous solutions of the analytes do agree with certified values for any reference material Analysis of selected real samples The evaluated nebulizers were applied to the determination of elements in some real solid samples using the experimental conditions previously optimized. The results for the samples analyzed using the evaluated nebulizers are given in Tables 9 and 10 (see ESIw). In all cases the calibration was done against aqueous solutions of the analytes. 4. Conclusions The evaluation of commercially available and prototype micronebulizers shows that the Achilles heel of spectrometric techniques is now less vulnerable due to significant improvements observed in recent sample introduction systems. The detection limits achieved with the FFPN nebulizer were superior to those obtained with the pneumatic nebulizer (PN) and other micronebulizers. Analysis of very small samples and particular applications become possible using efficient micronebulizers and MIP-OES technique. Acknowledgements Financial support by Committee of Scientific Research, Poland (Grant No. COST/48/2006) is gratefully acknowledged. In addition, MH and AC would like to thank the Spanish Ministry of Education and Science (projects n. DPI C02-01, PTR OP and CTQ C03-01/BQU) for the financial support of this work. MH thanks the financial support of the Spanish Ministry of Science and Technology, Generalitat Valenciana and University of Alicante (Ramo n y Cajal Program). The authors specially thank Flow Focusing (USA) and Ingeniatrics S.L. (Spain) for the loan of the FFPN nebulizer. This work has been undertaken as part of the EU sponsored COST programme (Action D32, working group D32/005/04, Microwaves and Ultrasound Activation in Chemical Analysis ). References 1 R. K. Skogerboe and G. N. Coleman, Anal. Chem., 1976, 48, 611A. 2 Q. Jin, Y. Duan and J. A. Olivares, Spectrochim. Acta, Part B, 1997, 52, J. A. C. Broekaert and V. Siemens, Spectrochim. Acta, Part B, 2004, 59, J. Sneddon, Sample introduction in atomic spectroscopy, in Advances in Atomic Spectroscopy, JAI Press, Greenwich, 1992, vol. 1, p J. Mora, S. Maestre, V. Hernandis and J. L. Todolí, Trends Anal. Chem., 2003, 22, R. F. Browner and A. W. Boorn, Anal. Chem., 1984, 56, 786A. 7 R. F. Browner and A. W. Boorn, Anal. Chem., 1984, 56, 875A. 8 C. J. M. Beenakker, Spectrochim. Acta, Part B, 1976, 31, A. Bollo-Kamara and E. G. Codding, Spectrochim. Acta, Part B, 1981, 36, R. G. Stahl and K. J. Timmins, J. Anal. At. Spectrom., 1987, 2, K. Jankowski, D. Karmasz, L. Starski, A. Ramsza and A. Waszkiewicz, Spectrochim. Acta, Part B, 1997, 52, C. Yang, Z. Zhuang, Y. Tu, P. Yang and X. Wang, Spectrochim. Acta, Part B, 1998, 53, K. Jankowski, D. Karmasz, A. Ramsza and E. Reszke, Spectrochim. Acta, Part B, 1997, 52, H. Matusiewicz, J. Anal. At. Spectrom., 1993, 8, J. S. Babis, J. M. Kacsir and M. B. Denton, Appl. Spectrosc., 1989, 43, K. C. Ng and R. C. Culp, Appl. Spectrosc., 1997, 51, H. Matusiewicz, Spectrochim. Acta, Part B, 2002, 57, H. Matusiewicz and R. E. Sturgeon, Spectrochim. Acta, Part B, 1993, 48, H. Matusiewicza and B. Golik, Spectrochim. Acta, Part B, 2004, 59, H. Matusiewicz, Spectrochim. Acta, Part B, 1992, 47, H. Matusiewicz, Chem. Anal. (Warsaw), 1995, 40, W. Quillfeldt, Fresenius J. Anal. Chem., 1991, 340, B. Almagro, A. M. Gan a n-calvo and A. Canals, J. Anal. At. Spectrom., 2004, 19, B. Almagro, A. M. Gan án-calvo, M. Hidalgo and A. Canals, J. Anal. At. Spectrom., 2006, 21, B. Almagro, A. M. Gan án-calvo, M. Hidalgo and A. Canals, J. Anal. At. Spectrom., 2006, 21, J. A. McLean, H. Zhang and A. Montaser, Anal. Chem., 1998, 70, J. A. McLean, R. A. Huff and A. Montaser, Appl. Spectrosc., 1999, 53, D. D. Smith and R. F. Browner, Anal. Chem., 1982, 54, J. Mora, J. L. Todoli, I. Rico and A. Canals, Analyst, 1998, 123, H. Matusiewicz, Anal. Chem., 1994, 66, R. F. Browner, A. Canals and V. Hernandis, Spectrochim. Acta, Part B, 1992, 47, S. E. Hobbs and J. W. Olesik, Anal. Chem., 1992, 64, R. S. Houk, R. K. Winge and X. Chen, J. Anal. At. Spectrom., 1997, 12, J. Anal. At. Spectrom., 2007, 22, This journal is c The Royal Society of Chemistry 2007