Jesús M.
Cano
,
José L.
Todolí
,
Vicente
Hernandis
and
Juan
Mora
*
Departamento de Química Analítica, Universidad de Alicante, P.O. Box 99, 03080, Alicante, Spain. E-mail: juan.mora@ua.es
First published on 3rd December 2001
The role of the nebulizer on the sodium chloride interferent effects in ICP-AES was investigated. Three different pneumatic nebulizers coupled to a cyclonic spray chamber were investigated: a V-groove (VGN) and two new pneumatic concentric nebulizers specifically designed to work with saline solutions (the ‘Seaspray’ nebulizer, SSN, and experimental nebulizer, EN). The effect of the salt concentration on the characteristics of the aerosols generated by the nebulizers (primary aerosols) and those at the exit of the spray chamber (tertiary aerosols) on the transport parameters and on the analytical figures of merit in ICP-AES were evaluated. The characteristics of the primary aerosols were related to their critical dimensions and were independent of the solution salt concentration. Solvent and analyte transport rates were related to the characteristics of the primary aerosols. Thus, the SSN provided the highest solution transport rates followed by the EN and VGN. The Mn II emission signal correlated quite well with transport parameters. When the salt concentration increased, an interference effect on the ICP-AES emission signal was observed. This effect was related to the nebulizer employed. Thus, for the SSN, the effect of the sodium chloride on the emission signal was different for the ionic and atomic lines. For the remaining nebulizers, a depressive effect of the salt concentration on the emission signal was always observed irrespective of the line considered. The magnitude of the interference was different for each nebulizer and was related to the amount of solvent transported to the plasma. In general terms, by increasing the salt concentration, poorer signal precision and limits of detection were obtained. These results confirm the importance of sustaining robust plasma conditions to reduce the sodium chloride interferences.
The effect of EIE in ICP-AES is quite complex. In general terms, interferences can arise from: (i) changes in the amount of aerosol introduced into the plasma; and, (ii) changes in the plasma excitation conditions. The former are directly related to the sample introduction system. The latter can be related to the plasma experimental conditions (i.e., rf power, gas flows, etc).
A conventional liquid sample introduction system in ICP-AES consists of a nebulizer, usually pneumatic concentric, coupled to a spray chamber. Sometimes a desolvation system is also included to reduce the amount of solvent introduced into the atomization/excitation cell. Thus, interferences due to the presence of solvent in the plasma can be avoided or, at least, reduced. The effect of the spray chamber on the acid- and the EIE interferences in ICP-AES has been thoroughly studied.8–11 Nevertheless, little evidence about the role of the nebulizer on the EIE effects has been found in the literature. Several reasons can be found to explain this fact. Among them, the risk of clogging of the conventional pneumatic concentric nebulizers when working with high salt content solutions must be considered. To overcome this drawback, different nebulizers have been developed.12 Most of them are of a pneumatic nature, based either on the Babington principle,13–15 such as the V-groove,14 the conespray16 and the Hildebrand grid nebulizers,15 or on modifications of the conventional concentric nebulizers.15,17 Unfortunately, these nebulizers usually give rise to poorer analytical figures of merit than the conventional ones. To overcome this problem, several highly efficient nebulizers have been proposed. Among them, nebulizers such as the single-bore high-pressure pneumatic nebulizer18 or the microwave-based thermal nebulizer19 are good alternatives to the conventional pneumatic ones to operate with high salt content solutions. The main problem of these nebulizers is the requirement of a desolvation system, due to the high amount of solvent transported to the plasma.
The purpose of the present work is to provide evidence of the role of the sample introduction system, mainly the nebulizer, in the analysis of high salt (sodium chloride) content solutions by ICP-AES. To this end, two new pneumatic concentric nebulizers and a V-groove nebulizer have been used. The effect of the salt concentration on the characteristics of the aerosols generated, on the transport parameters and on the analytical figures of merit in ICP-AES has been evaluated.
NaCl concentration (%) | Density/g mL−1 | Viscosity/cP | Surface tension/dyn cm−1 |
---|---|---|---|
0 | 1.000 | 1.00 | 70.43 |
2 | 1.014 | 1.00 | 71.24 |
4 | 1.028 | 1.02 | 71.94 |
6 | 1.042 | 1.01 | 72.42 |
8 | 1.056 | 1.05 | 72.96 |
10 | 1.070 | 1.08 | 73.60 |
![]() | ||
Fig. 1 Scheme of the pneumatic concentric nebulizers used: a, liquid capillary inner diameter; b, liquid capillary wall thickness; c, distance between the outlets of liquid and gas streams. |
Nebulizer | a a/mm | b b/mm | c c/mm | A g d/×102 mm2 | Aerosol yield (%) |
---|---|---|---|---|---|
a Liquid capillary inner diameter. b Liquid capillary wall thickness. c Distance between liquid capillary and gas outlets. d Effective gas outlet section area. | |||||
SSN | 0.22 | 0.05 | 0.80 | 1.45 | 100 |
EN | 0.40 | 0.04 | 1.20 | 1.04 | 43 |
CN | 0.20 | 0.10 | 0.92 | 1.50 | 100 |
AN | 0.40 | 0.06 | 0 | 2.8 | 100 |
SN | 0.52 | 0.03 | 1.32 | 1.41 | 32 |
VGN | — | — | — | 1.16 | 100 |
A polypropylene cyclonic-type spray chamber (Glass Expansion, Victoria, Australia) with a 62 mL inner volume was used in all cases. This spray chamber was selected since it shows lower matrix effects than the conventional double pass one.8 Thus, the role of the nebulizer on the analytical response will be highlighted.
Sample uptake rate (Ql) to the nebulizer was controlled by means of a peristaltic pump (Model Minipulse 3, Gilson, Villiers-Le-Bel, France). The nebulizer gas flow rate (Qg) was controlled by means of a mass flow controller (Model FC-260, Tylan, Torrance, CA, USA). Argon was always used as the nebulizer gas.
Several atomic lines covering a range of excitation energies, Eexc, from 3.14 eV (Al I 396.152) to 5.80 eV (Zn I 213.856) were used. Ionic lines were selected to cover a range of energy sum values, Esum, (i.e., sum of the ionization, Eion, and excitation energies) from 7.93 eV (Ba II 455.403) to 14.79 eV (Pb II 220.353). Table 4 shows the wavelengths and energies of the lines tested.
Element | Wavelength/nm | E exc/eV | E ion/eV | E sum a/eV |
---|---|---|---|---|
a E sum = ionization energy (Eion) + excitation energy (Eexc). | ||||
Al I | 396.152 | 3.14 | 3.14 | |
Cr I | 357.869 | 3.46 | 3.46 | |
Cu I | 324.754 | 3.82 | 3.82 | |
Mg I | 285.213 | 4.35 | 4.35 | |
Zn I | 213.856 | 5.80 | 5.80 | |
Ba II | 455.403 | 5.21 | 2.72 | 7.93 |
Sr II | 407.771 | 3.04 | 5.69 | 8.73 |
Mg II | 280.270 | 7.65 | 4.42 | 12.07 |
Mn II | 257.610 | 7.44 | 4.81 | 12.25 |
Cr II | 205.560 | 6.77 | 6.03 | 12.80 |
Cr II | 267.716 | 6.77 | 6.16 | 12.92 |
Fe II | 238.204 | 5.20 | 7.87 | 13.07 |
Co II | 228.616 | 7.86 | 5.84 | 13.70 |
Ni II | 221.647 | 7.64 | 6.39 | 14.03 |
Cd II | 214.438 | 5.78 | 8.99 | 14.77 |
Pb II | 220.353 | 7.42 | 7.37 | 14.79 |
![]() | ||
Fig. 2 Effect of the sodium chloride concentration on the Sauter mean diameter of the primary aerosol, pD3,2, for all the nebulizers tested. |
Both the SSN and the EN are capable of continuous operation with concentrated sodium chloride solutions without any of the clogging problems typically observed with the conventional pneumatic concentric nebulizers. It has been claimed that the distance between the liquid capillary and the nebulizer tip (c, in Fig. 1 and Table 2) enables the concentric nebulizers to operate with high salt content solutions.22
Regarding the relative behaviour of the nebulizers evaluated, Fig. 2 shows that, for all the solutions tested, the finest primary aerosols are obtained with the SSN and the coarsest with the EN. This behaviour can be explained in relation to the main dimensions of the nebulizers.16,23 Firstly, the gas outlet section area (Ag) determines the amount of kinetic energy available for the aerosol generation. Thus, for a given Qg, the energy of the gas stream increases as Ag decreases (increased pressure required). Therefore, the capability of generating a new surface is also increased with the result of finer aerosols. Secondly, the liquid capillary inner diameter and its wall thickness (a and b in Fig. 1, respectively) influence the effectiveness of the energy transference process to the liquid bulk. For a given amount of energy, the smaller the dimensions of these parameters, the higher is the effectiveness of the process and, therefore, the finer the aerosols generated. Among the concentric nebulizers tested, the SSN shows the highest Ag (Table 2). Therefore, by just considering this parameter, the SSN should produce the coarsest primary aerosols. Nevertheless, the lowest value of the capillary inner diameter (Table 2) counterbalances this factor and, as a result, the SSN is the nebulizer that generates the aerosols with the lowest pD3,2 values (Fig. 2).
Table 2 shows the aerosol yields obtained for all the nebulizers tested. It can be observed that, among the three nebulizers studied (SSN, EN and VGN), only the EN gives rise to nebulization yields lower than 100%. Under the nebulization conditions listed in Table 3, the EN transforms only 43% of the solution into an aerosol. The rest of the solution is lost at the nebulizer tip (see Fig. 3). This behaviour must be related, for given experimental conditions, to the main dimensions of the concentric nebulizer, i.e., gas outlet section area (Ag), liquid capillary inner diameter (a) and wall thickness (b), and the recessed area of the liquid capillary with respect to the nebulizer tip (c). To provide evidence for this fact, five pneumatic concentric nebulizers with different critical dimensions were used and the aerosol yields measured (Table 2). As can be seen in Table 2, the EN and SN give rise to aerosol yields lower than 100%. A cluster analysis of the critical dimensions of the nebulizers was performed and results clearly establishe two groups of nebulizers. The first group includes the SSN, CN and AN (i.e., nebulizers with aerosol yields of 100%) and the second group is formed by the EN and SN (i.e, those with aerosol yields of lower than 100%). Trying to understand which of these dimensions determine the aerosol yield, a detailed examination of the data shown in Table 2 is required. Nevertheless, a few conclusions can be directly drawn: the EN provides a lower aerosol yield than the AN (43% and 100%, respectively) in spite of the fact that these are the nebulizers that have the lowest and highest Ag value, respectively. Therefore, the aerosol yield does not seem to be related to Ag. However, with regard to the recessed area of the liquid capillary (dimension c), EN and SN show the highest value of this parameter and this seems to be the controlling factor in the aerosol yield. To get more information, a detailed cluster analysis of the variables was performed. Results indicate that the recessed area and inner diameter of the liquid capillary are the most influential parameters, which simultaneously affect the amount of solution that is converted into an aerosol.
![]() | ||
Fig. 3 Illustrations of the pneumatic concentric nebulizers operating under the same experimental conditions. |
Fig. 3 shows the differences in cone angles of the aerosols generated by the different nebulizers. Fig. 4 shows a section of the cone of aerosols generated by the nebulizers evaluated. As can be seen, the VGN is the nebulizer that generates the widest aerosol cones. The EN and SSN generate aerosols with similar cone angles, with those of the SSN being slightly sharper.
![]() | ||
Fig. 4 Aerosol cones generated by the nebulizers tested. |
![]() | ||
Fig. 5 Effect of the sodium chloride concentration on the solvent, Stot, (a) and analyte, transport rate, Wtot, (b) for all the nebulizers tested. |
Comparing the relative behaviour of the different nebulizers, Fig 5 reveals that the SSN is the nebulizer that gives rise to the highest transport parameters. This is a direct consequence of the finest aerosols generated with the SSN. The EN gives rise to higher Stot and Wtot values than the VGN, in spite of the fact that the latter nebulizer produces finer primary aerosols and a higher nebulization yield than the former. This apparently anomalous behaviour can be explained by considering the aerosol cone widths and the dimensions and position of the nebulizer inside the cyclonic spray chamber. Fig. 6 shows an outline of the nebulizer position inside the spray chamber. It can be observed that the VGN tip is recessed in the spray chamber with respect to the EN tip. This fact, together with the widest aerosol cone generated by the VGN, makes the fraction of the aerosol generated by the VGN that impacts against the inner walls of the spray chamber higher than that generated by the EN. As a consequence, and contrary to what is expected from their respective pDSD, the transport parameters obtained with the VGN are lower than those with the EN.
![]() | ||
Fig. 6 Scheme of the (A) cyclonic spray chamber and position of the nebulizer tips; (B) SSN and EN; and (C) VGN. |
![]() | ||
Fig. 7 Effect of the sodium chloride concentration on the Sauter mean diameter of the tertiary aerosol, tD3,2, for all the nebulizers tested. |
Another parameter that can be used to characterise the tDSD is the volume of tertiary aerosol contained in droplets with diameters below the lower limit of measurement of the instrument, i.e., 1.2 µm (tV1.2). It has been observed that, for all the nebulizers tested, tV1.2 increases when switching from water to 2% sodium chloride. Further increases in the salt concentration do not affect the value of this parameter.
The above results demonstrate that the presence of salt clearly modifies the aerosol transport process, giving rise to a finer aerosol at the exit of the spray chamber, while the transport parameters remain unaffected (Fig. 5). This behaviour is not easy to explain and has been related to an ionic redistribution in the droplets.27–29
Attempting to explain the effect of the sodium chloride on the tDSD, the dependence of the characteristics of the tertiary aerosols on the pDSD and on the design of the spray chamber must be considered. Both factors, together with the physical properties of the solutions, such as density and volatility, determine the processes affecting the droplets during their transport to the atomization cell.30–32 Since there is no effect of the salt concentration on the characteristics of the primary aerosols (Fig. 2), no changes in droplet collision and coagulation are expected. Moreover, solution density does not significantly change due to the presence of sodium chloride (Table 1). Therefore, modification of the droplet inertial deposition on walls and gravitational settlings should not be produced. Nonetheless, two differences appear when comparing
water with sodium chloride aerosols that may explain the interference effect of sodium on the tDSD. Firstly, the presence of charged species in an aerosol droplet produces repulsion forces that can result in a droplet break-up, thus giving rise to finer droplets.33,34 This fact could explain the lower tD3,2 provided by the sodium chloride solutions. Secondly, it must be considered that evaporation of pure water aerosols is more severe than sodium chloride aerosols. Cresser and Browner demonstrated that, when nebulizing sodium chloride solutions of concentrations over 1%, evaporation effects on droplets of diameters greater than 0.5 µm are negligible.35 For a monodisperse aerosol, evaporation will result in a decrease in the droplet diameter. For a polydisperse aerosol, this relationship is not so evident, since the solvent evaporation rate is higher for the smallest droplets. This
effect can cause the smallest droplets to reduce in size to a value below the lower limit of the instrument (i.e., 1.2 µm), and thus to be undetected, giving rise to an apparent increase in tD3,236 and a decrease in tV1.2.
Fig. 7 shows that, irrespective of the nebulizer, the tD3,2 values for the sodium chloride solutions are around 30% lower than those obtained for pure water. These results suggest that the effect of sodium chloride on the characteristics of tDSD is independent of the nebulizer used. To verify this statement, the tDSD curves obtained for all three nebulizers with both water and 2% sodium chloride solutions must be considered. These results, shown in Fig. 8, confirm the above-mentioned conclusion, since a clear parallel relationship between the tDSD curves obtained with pure water and 2% sodium chloride can be observed.
![]() | ||
Fig. 8 Volume drop size distribution of the tertiary aerosol obtained for pure water (dotted lines) and 2% sodium (continuous lines) with the nebulizers evaluated: (○) SSN; (X) EN; and (■) VGN. |
As regards the relative behaviour of the different nebulizers tested, Figs. 7 and 8 reveal that, as expected from the pDSD, the SSN gives rise to aerosols with the lowest tD3,2. The EN, however, gives rise to finer tertiary aerosols than those obtained with the VGN. Again, this behaviour can be explained by taking into account both the dimensions and relative position of the nebulizers inside the spray chamber and the primary aerosol cone widths. In all cases, the differences in tD3,2 for the different nebulizers are lower than those for the pD3,2. This is due to the filtering effect of the spray chamber.
![]() | ||
Fig. 9 Effect of the sodium chloride concentration on the manganese emission intensity for all the nebulizers tested. [Mn] = 1 µg mL−1. |
Regarding the relative behaviour of the nebulizers, Fig. 9 shows that the highest Mn II emission signals are obtained with the SSN followed by the EN and the VGN. This behaviour is to be expected taking into account their respective transport parameters (Fig. 5).
The effect of sodium chloride on the emission signal was also studied for different elements and lines. Fig. 10 gathers the results obtained for all the nebulizers evaluated. In this figure two different behaviours can be observed as a function of the nebulizer considered. For the SSN, results in Fig. 10a reveal that the effect of the sodium concentration on the emission signal is line-dependent. Thus, for the atomic lines studied, Irel steadily decreases as the salt concentration is increased. The magnitude of this detrimental effect ranges between 10% (for Cd 214.438) and 25% (for Ni 221.647 and Ba 455.403). For the ionic lines, Irel increases (Zn 213.856) or reaches a minimum and then increases. When switching from water to 10% sodium chloride, the emission signal changes from 110% for Zn 213.856 to up to about 70% for Cr 357.869. The behaviour of the SSN clearly indicates that non-robust conditions are used.41 Thus, the values of Mg II/I ratios for this nebulizer range between 2.9 and 2.7 for water and 10% NaCl, respectively. For the remaining nebulizers, this ratio takes values of around 5.0. These results can be explained by taking into account that the SSN is the nebulizer that gives rise to the highest Stot (Fig. 5a). For the EN (Fig. 10b) and VGN (Fig. 10c), irrespective of the line tested, Irel steadily decreases as the salt concentration of the solution is increased. In these cases, when switching from water to 10% sodium chloride, the signal drops by 20% to 30% and by 5% to 15% for the EN and VGN, respectively. These results agree with the higher Stot values afforded by the EN. Finally, from Fig. 10, the behaviour of Irel does not seem to be related to Esum for any of the nebulizers.
![]() | ||
Fig. 10 Effect of the sodium chloride concentration on the net emission intensity relative to water for the different lines and nebulizers tested: SSN (a); EN (b) and VGN (c). |
![]() | ||
Fig. 11 Effect of the sodium chloride concentration on the LOD obtained for some elements. Nebulizer: SSN. |
As regards the relative behaviour of the nebulizers tested, Table 5 shows the LOD obtained with the SSN and EN relative to those with the VGN for different elements and sodium chloride concentrations (2% and 10%). From these results it can be concluded that, in general terms, the SSN gives rise to the lowest LOD followed by the EN and the VGN. In this table it can also be observed that the highest LODrel values are obtained at the highest sodium concentration (i.e., 10% sodium chloride). These results agree with the respective signals (Fig. 9) and stabilities.
Element | LODVGN/LODSSN | LODVGN/LODEN | ||
---|---|---|---|---|
2% NaCl | 10% NaCl | 2% NaCl | 10% NaCl | |
Al | 0.9 | 1.9 | 0.8 | 3.2 |
Ba | 0.8 | 2.2 | 0.8 | 3.1 |
Cd | 1.6 | 2.5 | 1.0 | 2.1 |
Cr | 2.0 | 2.5 | 1.3 | 1.9 |
Cu | 1.0 | 2.8 | 1.0 | 2.5 |
Mg | 1.7 | 3.2 | 1.7 | 2.3 |
Mn | 2.2 | 3.4 | 1.5 | 1.9 |
Ni | 2.0 | 2.9 | 1.6 | 1.8 |
Results shown in the present work clearly demonstrate that the interference effect of the sodium chloride in ICP-AES strongly depends on the nebulizer used. Among the concentric nebulizers tested, the SSN generates the highest interferences since it gives rise to the highest amount of solution transported to the plasma.
This journal is © The Royal Society of Chemistry 2002 |