Guillermo
Grindlay
*,
Luis
Gras
,
Juan
Mora
and
Vicente
Hernandis
Departamento de Química Analítica, Nutrición y Bromatología, Universidad de Alicante, P.O. Box 99, 03080, Alicante, Spain. E-mail: guillermo.grindlay@ua.es
First published on 3rd August 2007
A re-designed microwave-based thermal nebulizer (MWTN) that overcomes all the drawbacks shown by previous prototypes (i.e. lack of robustness, poor heating efficiency) is presented. The new device employs an optimized TM010 cavity and nozzle design. MWTN performance was evaluated in inductively coupled plasma atomic emission spectrometry (ICP-AES) and compared to that provided by a concentric pneumatic nebulizer. Current MWTN design improves sample heating efficiency and system robustness. As a consequence, a 10 times lower matrix concentration than previous designs can be employed. The minimum inorganic (i.e. acids and salts) and organic (i.e. alcohols) concentrations required to nebulize are 0.25 and 40% w/w, respectively. The influence of microwave power (180–290 W), matrix nature (acids, salts and organics), sample uptake rate (0.9–1.8 mL min–1) and nebulizer critical dimensions, such as nozzle and PTFE capillary internal diameter (150–300 and 300–800 µm) and PTFE capillary length (50–150 cm) on the behaviour of the MWTN was studied. Optimum operating conditions for this nebulizer are obtained employing high microwave powers, matrix concentration and sample uptake rate, as well as nozzle and PTFE capillary with a narrow internal diameter. Although the MWTN behavior depends on the samples physical properties, aerosol drop size distributions and analyte transport rate for inorganic solutions above 2% w/w are independent of matrix composition. Current MWTN shows higher sensitivity and lower limits of detection (up to 12 times for acid solutions) than a conventional pneumatic nebulizer. The performance of the new prototype has been evaluated analyzing a certified bovine liver (CRM 185 R) and a spirit sample. Experimental results indicate that MWTN can be employed successfully for routine sample analysis.
The thermal nebulizer or thermospray (TN) consists of a narrow stainless steel capillary which is heated above the boiling point of the solvent.2 The aerosol is generated by the interaction between the liquid stream and the gas generated by the vaporization of a solvent fraction. The TN has been considered pneumatic in nature, since nebulization takes place by the interaction of a liquid and gas stream. Operating with the thermospray, a careful optimization of experimental parameters must be performed for improving limits of detection (LOD), since the aerosol characteristics depend, in a complex way, on these conditions (i.e. matrix nature, heating temperature, sample uptake rate, capillary internal diameter).6
The TN generates finer aerosols than a typical concentric pneumatic nebulizer improving the analyte and solvent transport rates to the plasma.7 Due to this reason, a desolvation system unit to reduce or overcome interferences is required.8 Coupling the TN to a desolvation system reduces limits of detection (LODs) one order of magnitude in respect to those obtained with a PN.9,10 Among drawbacks shown by TN it can be noted: (i) corrosion of the capillary when working with acidic solutions (a very common matrix in elemental analysis). Sometimes, a fused silica capillary has been inserted into the stainless steel capillary to avoid corrosion, improving chemical inertness and providing lower background signals. However, silica capillary heating is less efficient, making vaporization control more difficult;10,11 (ii) handling solutions with a high concentration of total dissolved solids is not recommended; and (iii) poor short and long-term stability.
The last energy sources evaluated with a TN has been the microwave radiation (MW). MW has been mainly applied in atomic spectrometry for sample preparation12 or plasma generation.13 Nonetheless, from 10 years on, it has also been successfully used as an alternative energy source for sample introduction purposes, such as desolvation14–18 and thermal nebulization (MWTN).19–21 Although MWTN and TN operate in a similar way, both systems show several differences in the sample heating process and hardware. Using a conventional TN, the sample is heated through capillary walls by means of a conductive–convective mechanism. However, using the MWTN the sample is heated directly by the interaction between MW radiation and solvent molecule dipoles or ions. As a consequence of the differences in the heating mechanism, the MWTN design shows several modifications as compared with the standard TN. Firstly, the stainless steel capillary must be replaced with a PTFE one, since a microwave transparent material is mandatory. Secondly, the capillary is placed inside of a MW cavity where it interacts with the MW radiation produced by a magnetron. Finally, due to the need of using capillaries with a larger internal diameter, a silica restriction (i.e. a nozzle) placed at the end of PTFE capillary is needed, in order to improve vapour and liquid interaction.
The sample, as it happens with TN, must be pumped by means of an HPLC pump. Nevertheless, conventional stainless-steel head pumps are inadequate to operate with the acidic or high salts content solutions usually employed when using the MWTN. For this reason, Bordera et al.19,20 introduced the sample by means of an injection valve and a large loop (12 mL) placed between the HPLC pump and the MW cavity. However, this approach does not afford continuous signals and requires the need to refill the loop periodically. As an alternative, peristaltic pumps have also been evaluated for sample pumpìng.21 The results obtained indicate that nebulization is possible, but the aerosol generation process becomes instable.
Two different MWTN designs, based on the use of different microwave cavities, have been reported in the literature. The first one employs a domestic multimodal cavity,20 whereas the second one uses a focalized cavity.19,21 The use of focalized cavities allows a better coupling between liquid sample and the MW radiation, giving rise to a more efficient heating process. The multimodal cavities employed do not afford a homogeneous MW field distribution and show several drawbacks: (i) the magnetron must be kept at its maximum nominal power since, otherwise, MW generation becomes irreproducible; (ii) a dummy load, placed inside the cavity, must be used for avoiding magnetron damage due to reflected radiation, so that most of the MW radiation available for aerosol generation is wasted by heating the dummy load. Liang et al. evaluated different focalized cavity designs.22 Among them, the so called TM010 affords adequate control of the heating processes and, hence, a reproducible aerosol generation. Using a TM010 cavity, these authors show that it is possible to generate aerosols using only 60 watts power.21
MWTN behaviour is affected by the same variables as conventional thermospray (i.e. matrix, sample uptake rate, MW power, and capillary and nozzle dimensions). However, when using MWTN, the dielectric properties of the solution also play a significant role since they determine the sample efficiency in transforming MW radiation into heat.
MWTN generates finer aerosols than a PN.19 Thus, for instance, the median of the volume drop size distribution (D50) for a 1.6% nitric acid solution at an uptake sample rate of 2 mL min–1 is 0.88 µm and 5.48 µm for a MWTN and a PN, respectively. Similar results can be obtained when using pure alcoholic solutions. As a rule, the reduction of the aerosol drop size distribution increases the analyte transport rate and hence sensitivity in ICP. However, as it usually happens with a conventional TN, a desolvation unit is advisable to avoid plasma cooling. When the MWTN is coupled to a two step desolvation system, the sensitivity increases up to 10 times compared with a concentric pneumatic nebulizer.21 In spite of this signal improvement, the MW device shows similar LODs to a PN, due to the higher background and blank RSD values shown by the former.20,21
MWTN designs reported in literature have shown several drawbacks: (i) lack of robustness; (ii) poor, long and short-term reproducibility; (iii) poor heating efficiency; and (iv) high matrix effects.19–21 All of these factors have made it impossible to apply this type of nebulizer for routine sample analysis. The main drawbacks shown by previous MWTN prototypes arise from an inefficient sample heating control due to the characteristics of the cavity, the MW generator, the pumping system and the nozzle. Therefore, it is expected that an improvement of the analytical figures of merit can be performed by redefining all these components. The goal of this work is to design and evaluate an improved microwave-based thermal nebulizer for liquid sample introduction in inductively coupled plasma atomic emission spectrometry. To this end, all the nebulizer components (i.e. MW cavity, nozzle) have been re-designed and the experimental variables optimized in order to get the better analytical response. The performance of the new prototype has been validated using samples of different natures.
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Fig. 1 Experimental setup of the MWTN. (1) HPLC pump; (2) PTFE capillary; (3) TM010 cavity; (4) tuning screw; (5) nebulizer nozzle, (5a) PEEK-SIL capillary; (5b) PEEK adapter; (5c) PEEK nut; (5d) Tefzel® ferrules; (6) magnetron; (7) circulator; (8) dummy load. |
MWTN design | ||||
---|---|---|---|---|
Characteristics | #1 (ref. 19) | #2 (ref. 20) | #3 (ref. 21) | Current |
a Inorganic matrices. b Organic matrices. c Spiked with acid 5%. | ||||
Cavity | Focalized | Multimodal | Focalized | Focalized |
MW power/W | 300 | 800 | 300 | 300 |
Power control | Yes | No | Unknown | Yes |
Pumping system | HPLC pump | HPLC pump | Peristaltic pump | HPLC pump |
Q l /mL min–1 | >1.5 | >1.0 | >0.5 | >0.8a/>0.5b |
Matrix | Inorganic | Inorganic/organic | Inorganic | Inorganic/organic |
Minimum concentration (%) | ||||
Inorganic matrices | >4.5 | >0.8 | >3.0 | >0.25 |
Organic matrices | 100c | 100 | — | >40 |
Nozzle i.d. (dN)/µm | 75–100 | 75–100 | 75 | 150–300 |
PTFE capillary length (lC)/cm | 20–30 | 1800 | 30 | 50–150 |
PTFE capillary i.d. (dC)/µm | 500 | 500 | 500 | 300–800 |
Nozzle robustness/sample leaks | Poor/yes | Poor/yes | Poor/yes | High/no |
The TM010 cavity employed was previously designed for liquid aerosol heating purposes, so that their dimensions were calculated for obtaining a maximum microwave field in the volume occupied by a cylindrical spray chamber placed at the cavity axis.17 For this reason, the PTFE capillary of the nebulizer must be placed vertically at the centre of the cylindrical chamber. The magnetron used allows precise microwave power control, since the supplied power can be smoothly modified in steps of 1 W and the reflected power measured by a power-meter.
Up to now, the nozzles employed in previous designs frequently showed problems of leakage, making necessary a skilled operator for the replacing of nozzles with different internal diameter. This drawback, together with those arising from the sample pumping, caused poor reproducibility and similar LODs than those with a pneumatic nebulizer. An ideal nozzle for the MWTN should fulfill the following requirements: (i) low dead volume; (ii) withstand corrosive solutions, high temperatures and pressures without deterioration; (iii) handle high solid contents solutions without clogging; and (iv) low thermal conductivity to keep sample evaporate from being condense. The new nozzle design presented in this work fulfils all the requirements above mentioned and has been fully made with commercially available components (Upchurch, Oak Harbor, Washington, USA). The nozzle restriction (5 in Fig. 1) consists of a 5 cm PEEK-SIL® tube (5a) (i.e.quartz capillary coated with PEEK polymer) (ref: 615005, 62005 and 63005), which can be easily coupled to a PEEK adapter (5b) (ref: 702), by means of PEEK nuts (5c) (ref: P-235) and Tefzel® ferrules (5d) (ref: P-200). The PTFE capillary at the exit of the TM010 cavity is also coupled to the PEEK adapter by means of PEEK nuts and Tefzel® ferrules. This way, the dead volume inside the nozzle is negligible, leakage is eliminated and all components can be replaced in just a few seconds without any tools.
A peristaltic pump and an HPLC pump have been tested for sample introduction with the MWTN design. Peristaltic pumps are less expensive and easier to handle than HPLC pumps. It has been observed that the peristaltic pump was not able to provide a reproducible sample uptake rate as the system pressure increases (>50 psi), as a consequence of the sample vaporization by MW heating. The HPLC pump employed in this work was designed to handle all kinds of concentrated inorganic matrices (i.e. acids and salts), since all pump components in contact with the sample are made of PEEK instead of stainless-steel. For this reason, an injection valve is unnecessary and a stationary signal is obtained. The pump also incorporates a self-flush system for cleaning the pistons from any possible matrix deposit. The pressure inside the PTFE capillary can be easily monitored and employed as a nebulizer performance control.
Table 1 compares the characteristics of previous and current MWTN designs. The first MWTN prototype (#1) employed a focalized cavity from a commercial MW digestor.19 Although this type of cavities allows a good interaction between sample and microwave radiation, the results obtained were poor, since the MW irradiation zone was too narrow (i.e. 1 cm). For this reason, a high sample uptake rate (>1.5 mL min–1) and acid matrix concentration (>5%) were required to generate a useful aerosol for ICP-AES purposes. Softer nebulization conditions (i.e. lower Ql and matrix concentration) were achieved using the multimodal cavity from a domestic oven with a more powerful MW generator (#2).20 However, sample heating was irreproducible since it was necessary to introduce a water dummy load inside the cavity to protect the magnetron from reflected radiation. Ding et al.21 reported an improved MWTN design using a focalized TM010 cavity (#3). Nevertheless, the lack of robustness of the pumping system (a peristaltic pump) and the nebulizer nozzle caused the analytical figures of merit not to be better than those obtained with a conventional pneumatic concentric nebulizer. The MWTN presented in this work has been designed for improving, simultaneously, both the system robustness and heating efficiency. When using this new MWTN it is possible to nebulize 0.25% w/w inorganic solutions (concentrations ten times lower than those required by previous prototypes) using capillaries and nozzles of larger internal diameters. As regards organic solutions, the current design can nebulize continuously alcoholic solutions above 40% w/w, whereas previous prototypes were only able to nebulize pure alcoholic solutions. It is important to note that unlike previous prototypes, matrix concentration and sample uptake rate employed with current MWTN are close to standard experimental conditions usually employed for routine sample analysis.
The MWTN was coupled to a home-made thermostated cyclonic spray chamber (inner volume 33 cm3) for ICP-AES measurements. Aerosol carrier gas flow was introduced inside the spray chamber by means of a T-joint placed at the nebulizer entrance and was controlled by means of a rotameter (Cole-Palmer Ins. Co., Chicago, Illinois, USA). The spray chamber was cooled at –5 °C using a thermostated bath (Haake F3-K, Haake Mess-Technik GmBH. U. Co., Karlsruhe, Germany) to avoid plasma cooling. When nebulizing organic samples, an extra desolvation step made up of a home-made Peltier23 cooler (30 cm length and 8 mm internal diameter) at –30 °C was employed.
For the sake of comparison, a concentric pneumatic nebulizer (Model TR-30-K2, Meinhard, Santa Ana, California, USA) coupled to the previously described cyclone spray chamber was used. Due to the low backpressure of concentric pneumatic nebulizers, a peristaltic pump (Model Minipulse 3, Gilson, Villiers-Le-Bel, France) and Tygon tubes (SC0007, Ismatec SA, Labortechnik-Analytik, Glattbrugg, Switzerland) were used to deliver the sample. Nebulizer gas flow and spray chamber temperature were controlled by the same rotameter and thermostated bath employed with the MWTN.
Perkin-Elmer Optima 3000 ICP-AES | |
---|---|
a Inorganic matrix. b Organic matrix. c PN d MWTN | |
Plasma forward power/W | 1450 |
Argon flow rate/L min–1: | |
Plasma | 16 |
Auxiliary | 0.7a–1.5b |
Nebulizer | 0.6c–0.5d |
Sample uptake rate/mL min–1 | Variable |
Observation height/mm ALC | 12a–10d |
Injector inner tube diameter/mm | 0.8 |
Integration time/ms | 100 |
Readings/replicates | 10/3 |
Element | Wavelength/nm | E sum /eV |
---|---|---|
Li I | 670.781 | 1.85 |
Sr I | 460.673 | 2.69 |
Ca I | 422.673 | 2.93 |
Al I | 396.152 | 3.14 |
Cr I | 357.869 | 3.46 |
Ag I | 328.068 | 3.78 |
Cu I | 324.754 | 3.82 |
Al I | 309.271 | 4.02 |
Mg I | 285.213 | 4.35 |
Ni I | 232.003 | 5.34 |
Cd I | 361.061 | 7.38 |
Ba I | 455.403 | 7.93 |
Sr II | 421.552 | 8.64 |
Sr II | 407.771 | 8.73 |
Ba II | 233.527 | 11.22 |
Mg II | 280.270 | 12.07 |
Mn II | 257.610 | 12.29 |
Cr II | 267.710 | 12.92 |
Fe II | 238.204 | 13.07 |
Ca II | 317.933 | 13.16 |
Co II | 238.892 | 13.46 |
Co II | 228.616 | 13.70 |
Ni II | 231.064 | 14.03 |
Cd II | 214.438 | 14.77 |
Pb II | 220.353 | 14.79 |
Zn II | 202.548 | 15.51 |
Cu II | 224.700 | 15.96 |
Limits of detection (LODs) were calculated according to the approach described by Boumans:25
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Fig. 2 Effect of nominal incident microwave power on the Mn net emission intensity for different nitric acid concentrations. (●) 2% w/w; (■) 1% w/w; (▲) 0.5% w/w; (×) 0.25% w/w. Ql 1.8 mL min–1; dN 200 µm; lC 1 m; dC 500 µm. |
In order to explain this behaviour, it must be considered that the ICP emission signal depends mainly on the characteristics of the aerosols generated by the nebulizer and on the spray chamber design.1,26 Finer aerosols improve solution (i.e.analyte and solvent) transport to the plasma, and hence the emission signal. Nonetheless, excessive solvent (or matrix) loading can deteriorate excitation conditions, giving rise to a decrease in the emission intensity.27Wtot can be used to evaluate the analyte transport rate, and Mg ratio28,29 or Ar intensity to measure the plasma excitation conditions.30,31 Results indicate that there is no influence of MW power on Mg ratio and Ar intensity for all conditions studied in Fig. 2. Therefore, the behaviour shown in Fig. 2 is likely to be related to aerosol characteristics.
When the incident microwave power is increased, more energy can be absorbed by the sample leading to a better heating process. Therefore, the amount of solvent evaporated in the capillary increases and higher energy is available to generate the aerosol (i.e. higher pressure inside the capillary). The higher pressure provided by the HPLC pump evidences this fact. As a consequence, primary aerosols become finer as MW power is increased. In fact, when MW power increases from 180 to 290 W for 1% w/w nitric acid solution, pressure raises by 90%, whereas D50 (i.e., the median of the volume drop size distribution) decreases from 26.42 µm to 12.89 µm. Lower DSD means increased analyte transport rate to the plasma. Table 4 shows the Wtot values obtained for different matrices and experimental conditions. As it can be observed in this Table, for nitric acid solutions, when increasing MW power, higher Wtot values are obtained. On comparing the results shown in Fig. 2 and Table 4, it can be derived that Wtot and intensity values are correlated. Thus, for instance, when MW power is increased from 180 W to 290 W for a 2% w/w acid nitric solution, both Wtot and emission signals increase 1.9 times.
This behaviour can be explained by taking in mind that, when using the MWTN, the aerosol generation process depends on the sample’s physical properties. The effectiveness of the sample solution in transforming MW radiation into heat is usually evaluated by means of the loss factor (tanδ). Once the sample is heated and partially vaporized, other sample properties such as expansion factor (i.e., the gas volume produced by the vaporization of a single mass unit of solvent at a given boiling temperature),32 surface tension and viscosity determine the characteristics of the aerosols generated with a thermal-based nebulizer.
Inorganic solutions are mainly heated by ionic conductance, since ions migrate under the influence of the electrical field and produce heat following the Joule effect. The higher the ionic concentration (i.e. higher tanδ) the higher the amount of heat released,33 and then, the greater amount of sample evaporated inside the PTFE capillary. As a consequence, finer aerosols are generated. Thus, for instance, D50 decreases from 21 to 12 µm when nitric acid ionic concentration is increased from 0.25% to 2% w/w. In addition, due to this drop size reduction, more analyte can be transported to the plasma (see Table 4), increasing the analytical signal (Fig. 2).
As it has been pointed out previously, emission signal is more influenced by matrix concentration at low MW powers. This fact can be explained taking into account that when MW power is increased, more heat can be produced independent of sample dielectric properties (i.e. tanδ).
Table 5 shows Mn net intensity for different inorganic matrices i.e.hydrochloric acid and sodium chloride. As it has been shown previously with nitric acid (Fig. 2), when hydrochloric acid and sodium chloride concentration is increased, a higher signal is obtained. The signal values obtained for hydrochloric acid solutions are very similar to those shown for nitric acid (Fig. 2). However, sodium chloride solutions provide lower emission intensity values than acid solutions for all the concentrations tested. Therefore, in this case, signal differences can be related to matrix effects on aerosol transport phenomena, since sodium chloride solutions show lower Wtot values (Table 4).34 Thus, for instance, Wtot values for 2% w/w nitric and hydrochloric acid solution are 6.7 and 6.5 µg min–1, respectively, whereas 2% w/w sodium chloride shows 5.7 µg min–1. For diluted solutions (i.e. matrix concentration lower 1% w/w), small differences in ionic conductance can affect nebulization. Ions with higher specificic conductance values (e.g.H+) are able to absorb MW radiation more efficiently than others with lower specific conductance (e.g.Na+). As a consequence of lower sodium heating efficiency, coarser aerosols are generated with diluted saline solutions. Fig. 3 shows the volume drop size distribution curves plotted in an accumulated semi-logarithmic mode for several inorganic (Fig. 3A) and organic matrices (Fig. 3B). According to the plot of Fig. 3, the curves are shifted to the left as the aerosol becomes finer. As it can be seen in Fig. 3A, primary aerosol for inorganic acid solutions are similar and finer than for sodium chloride. Finer aerosol drop size distribution for inorganic acids improve aerosol transport and, hence, signal with regard to a salt solution. Other physical properties, such as expansion factor or surface tension, have no noticeable effect on aerosol generation for inorganic matrices, since they remain almost constant for all solutions tested.
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Fig. 3 Volume drop size distribution of the primary aerosol obtained for different matrices with the MWTN. (a) Inorganic matrices: (●) nitric acid 0.5% w/w; (■) hydrochloric acid 0.5% w/w; (▲) sodium chloride 0.5% w/w. (b) Organic matrices: (●) methanol 100% w/w; (■) ethanol 100% w/w; (▲) propan-2-ol 100% w/w; (○) methanol 60% w/w; (□) ethanol 60% w/w; (△) propan-2-ol 60% w/w. Inorganic: PMW 290 W; Ql 1.8 mL min–1; dN 150 µm; lC 1 m; dC 500 µm. Organic solutions: PMW 290 W; Ql 1.4 mL min–1; dN 150 µm; lC 1 m; dC 500 µm. |
Matrix | Intensity/counts s–1 |
---|---|
HCl 0.25% | 213![]() |
HCl 2.00% | 308![]() |
NaCl 0.25% | 171![]() |
NaCl 2.00% | 226![]() |
Methanol 100% | 151![]() |
Methanol 60% | 50![]() |
Ethanol 100% | 130![]() |
Ethanol 60% | 72![]() |
Propan-2-ol 100% | 75![]() |
Propan-2-ol 60% | 100![]() |
The MWTN design evaluated in the present work shows a noticeable difference when compared with previous ones, since in this case, and for a given concentration, there is no significant effect of the acid nature on the signal (for all concentration and elements tested).19,21 This effect is a direct consequence of the improved MW heating capability afforded by the new design. Fig. 4 shows the effect of nitric acid concentration, covering a larger concentration range on Mn emission signal and on the D50 of the primary DSD with the MWTN. Similar results are obtained employing hydrochloric acid or sodium chloride. When the matrix concentration is increased the Mn signal peaks at around 2% w/w. At lowers concentrations, the signal is mainly related with the aerosol characteristics. Increasing concentration tends to reduce aerosol drop size distribution (i.e. lower D50), thus improving the analyte transport to the plasma. However, at higher concentrations, the aerosol characteristics remain almost constant (D50 ≈ 12 µm), whereas the signal can decrease up to 30%. Several authors reported that a high acid concentration may affect analyte transport or reduce plasma atomization/excitation capabilities.35 Measurements at 5% and 2% w/w nitric acid solutions show that there is no significant difference between the Wtot values obtained for both solutions. Nevertheless, Ar intensity for a 5% w/w nitric acid solution is about 30% lower than for a 2% one. From theses results, it can be concluded that signal reduction for nitric acid solutions above 2% w/w are related to plasma energetic conditions since DSD and Wtot are not influenced by matrix concentration.
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Fig. 4 Effect of nitric acid concentration on the Mn net emission intensity and on the median of the primary volume drop size distribution. (●) Mn signal; (○) D50. PMW 290 W; Ql 1.8 mL min–1; dN 200 µm; lC 1 m; dC 500 µm. |
Table 5 also shows Mn net emission for pure and 60% w/w alcohol solutions (i.e.methanol, ethanol and propan-2-ol). With the exception of propan-2-ol, the higher the alcohol concentration the higher the signal obtained. When comparing pure alcoholic solutions, the Mn sensitivity follows the following order: methanol > ethanol > propan-2-ol. However, for 60% w/w solutions, the relative behaviour among alcohols is the opposite: methanol < ethanol < propan-2-ol. In order to explain these behaviours, it must be taken into account that aerosol generation with organic matrices shows some differences with respect to inorganic ones. In one hand, organics are heated by dielectric polarization (i.e. dipole rotation motion) instead of ionic conductance. On the other hand, when nebulizing organic solutions, it is necessary to take into account that the ratio organic : water modifies both the MW heating efficiency and other important physical solution properties, such as expansion factor, surface tension, etc.
As it has been pointed out previously, thermal aerosol generation depends on the expansion factor. The higher the expansion factor, the higher the energy available for aerosol generation, and hence the finer the aerosols generated.2,32 For pure alcoholic solutions, methanol shows the highest expansion factor followed by ethanol and propan-2-ol. For this reason, methanol is expected to provide the finest aerosols. Fig. 3B shows primary aerosol volume drop size distribution curves for pure and diluted organic solutions. As it can be seen in this Figure that pure methanol solution affords finer aerosols than ethanol and propan-2-ol. So, higher Wtot and signal must be expected for methanol (Table 5). As regards dielectric properties, it seems that they are not too critical as with inorganic matrices, since methanol is the alcohol with the lowest tanδ.36
When the alcohol percentage is reduced the solution expansion factor increases whereas tanδ decreases. From the experimental results, it seems that the increase for all the alcohols tested in the expansion factor does not fully counterbalance the reduction of the heating efficiency, thus giving rise to coarser aerosols (Fig. 3B), lower Wtot, and therefore a lower emission signal. In addition, the surface tension of diluted alcohol solutions is higher than for the undiluted ones, a higher amount of energy being necessary to generate the same liquid surface. Finally, changes in the viscosity (of the solutions) may also account for the differences found between undiluted and diluted alcohol. Higher viscosity helps to dampen instabilities of the liquid surface, more energy being necessary to generate new surfaces (i.e. coarser aerosols). When the alcohol content is reduced from 100% to 60% w/w, solution viscosity increases. The enhancement factor of the viscosity is more significant for methanol (2.71) than for ethanol (2.16) and propan-2-ol (1.5).37 Hence, coarser aerosols are expected for methanol solutions than for propan-2-ol solutions (Fig. 3B) and again, lower analyte transport and signals values.
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Fig. 5 Effect of sample uptake rate on the Mn net emission intensity for different nitric acid solutions (w/w). (●) 2% w/w; (■) 1% w/w; (▲) 0.5% w/w; (×) 0.25% w/w. PMW 290 W; dN 200 µm; lC 1 m; dC 500 µm. |
Signal enhancement when Ql increases is more important for solutions under 1% w/w (4 times) than for solutions above this value (3 times). The reason for these differences may lie in the fact that the increase of the amount of sample evaporated has a higher effect on those solutions that are heated less efficiently (i.e. the more diluted ones). Similar results were obtained using the remaining aqueous and organic solutions evaluated in the present work.
d N/µm | l c /cm | d C/µm | Net intensity/counts s–1 | W tot/µg min–1 |
---|---|---|---|---|
150 | 100 | 500 | 395![]() |
8.0 |
200 | 100 | 500 | 289![]() |
6.4 |
300 | 100 | 500 | 121![]() |
2.3 |
200 | 50 | 500 | 266![]() |
5.5 |
200 | 150 | 500 | 305![]() |
7.2 |
200 | 100 | 300 | 325![]() |
7.0 |
200 | 100 | 800 | 216![]() |
5.0 |
Similar behaviour to those shown in Table 6 have been reported with conventional thermosprays32,38 or with previous MWTN19,20 designs. For the given experimental conditions, the reduction of the nozzle or PTFE capillary diameter increases the system pressure, since more energy is required for pumping the sample through a narrower tube. This fact leads to a higher velocity for gas and liquid streams and as consequence, more energy is available for aerosol generation (i.e. finer aerosol drop size distribution). Thus, for instance, when a 1% w/w nitric acid solution at 1.8 mL min–1 and 290 W is nebulized, the D50 afforded by nozzles with an internal diameter of 200 and 150 µm are 13 and 8 µm, respectively. As regards the capillary length, longer capillaries increase the amount of sample heated by MW, leading to a higher amount of gas. The higher the gas flow, the higher amount of energy available for nebulization. A reduction in aerosol drop size distributions causes the analyte transport to increase and the same happens for the emission signal (Table 6). Similar results were obtained for all the matrices studied.
It is important to point out that the same nozzle has been used for more than one year without any damage and solutions up to 10% of sodium chloride were nebulized without clogging.
The comparison between the primary DSD of both nebulizers reveals that only at acid concentrations over 0.5% w/w and Ql values over 1.2 mL min–1 are aerosols generated by the MWTN that are finer than those generated by a PN. Nonetheless, the aerosols generated by the MWTN leave the nebulizer at high temperatures, thus promoting evaporation along the aerosol pathway and reducing its DSD. Tertiary DSD analysis reveals that aerosols generated by the MWTN are finer than those obtained with a PN. The reduction of the tertiary aerosol size and the fact that the analyte is concentrated in the remaining liquid during the aerosol generation give rise to higher transport values. Thus, at 1.8 mL min–1 the transport ratio (Wtot MWTNWtot PN–1) for 2% and 0.25% w/w nitric acid solution are 3.8 and 2.3 times, similar to the signal ratio observed.
RSD values obtained with the new MWTN are very similar to those reported in the literature for the previous prototypes.21 However, it must be pointed out that lower matrix concentration and a nozzle of larger internal diameter has been employed, clearly indicating that the new prototype shows an improved heating efficiency in comparison with the previous designs.
Element | (LOD)PN (LOD)MWTN–1 |
---|---|
Mn | 8.4 |
Mg | 2.9 |
Al | 8.6 |
Ba | 3.1 |
Pb | 5.9 |
Li | 12.7 |
Cr | 5.1 |
Fe | 5.5 |
Co | 5.8 |
Cu | 3.7 |
Cd | 8.6 |
Ca | 7.6 |
Following Boumans’ approach,25 a LOD improvement can be obtained by increasing the signal to background ratio and/or by decreasing the blank RSD. As it has been discussed before, the MWTN shows higher sensitivity than the PN. No significant differences for MWTN and PN in blank intensity and RSD have been observed. The combination of both effects causes the LOD improvements observed in Table 7.
LODs obtained with the present MWTN design are lower than previous ones. Thus, Bordera et al.19,20 and Ding et al.,21 using a MWTN, obtained sensitivities up to 10 times higher than with a PN. However, this enhancement was counterbalanced by higher blank intensities and RSD blank values, leading to LODs 2 to 4 times lower that when using a PN. It is also important to note that previous MWTN designs employed a 2 step desolvation system so as to improve aerosol transport,19,21 whereas the current design just makes use of a thermostatized spray chamber. Differences against this new design can be attributed to the more robust design and higher MW heating efficiency afforded by the new prototype. It is expected that results with the new design could be significantly improved using an additional aerosol heating step.
Table 8 shows the concentration values obtained for several elements in the bovine liver, as well as recovery values for the spirit sample with the MWTN. Concentration values for the bovine liver sample are not statistically different from the certified values for a confidence level of 95%. As regards the spirit sample, the recovery values have been close to 100%, with a confidence interval up to 3% at a confidence level of 95%. These results point out that MWTN can be employed as an efficient sample introduction system to perform routine analysis with samples of a different nature.
Element | Bovine liver | Spirit | |
---|---|---|---|
C certified/mg kg–1 | C experimental/mg kg–1 | Recovery (%) | |
Zn | 139 ± 2 | 138 ± 3 | 98 ± 3 |
Cu | 277 ± 5 | 262 ± 11 | 96 ± 3 |
Mn | 11.1 ± 0.3 | 11.3 ± 0.6 | 98 ± 2 |
MWTN can be successfully employed for the analysis of samples of different natures.
This journal is © The Royal Society of Chemistry 2008 |