Characteristics of nebulizers for microwave-induced plasma atomicemission spectrometryI. Pneumatic nebulizers

Krzysztof Jankowskia,*, Dorota Karmasza, Leszek Starskia, Andrzej Ramszab,Andrzej Waszkiewiczb

aWarsaw University of Technology, Faculty of Chemistry, Department of Analytical Chemistry, ul. Noakowskiego 3, 00-664 Warszawa, PolandbSelmar, ul. Chłodna 39, 00-867 Warszawa, Poland

Received 14 January 1997; accepted 15 May 1997

Abstract

Frit and microcapillary array nebulizers were designed and evaluated as low-flow introduction devices of liquid samples tothe low-power microwave-induced plasma. Zirconia ceramics were examined as a new frit material. A critical evaluation hasbeen carried out of the nebulization stages, such as the formation of the primary aerosol, separation of large droplets in the spraychamber and aerosol transport to the plasma torch; changes in the design of the nebulizers studied were introduced on this basis.The characteristics of nebulizers were supplemented by spectroscopic measurements of the measured signal stability, detectionlimits for selected elements and wash-out times.q 1997 Elsevier Science B.V.

Keywords:Pneumatic nebulizers; Microwave-induced plasma; Nebulizer characteristics; Flow rate effects

1. Introduction

The performance of a nebulization system isusually a major factor limiting the analyticalperformance of plasma atomic spectrometricmeasurements. A good nebulizer is expected to behighly efficient as well as able to produce a fineaerosol continuously flowing to the plasma. Besidesthis, the nebulizer should be matched to otherelements of the excitation source with respect to thegas flow and concentration of the aerosol generated.Low-power microwave plasma is a specific excitationsource characterized by a high discharge sensitivity inrelation to the amount and quality of the aerosol

introduced [1]. Moreover, in the microwave-inducedplasma (MIP) technique relatively low gas flows areapplied, often below 400 ml min−1, which are beyondthe optimal working range for some of popularpneumatic nebulizers. The maintaining of a stableaerosol stream and minimization of the memoryeffects are difficult to achieve under such conditions.

Studies on new designs of nebulizers, and also ofmethods for the evaluation of their performancehave been carried out for many years. In popularpneumatic nebulizers, such as concentric, cross-flowor Babington’s, the drop of pressure across thenebulizer is the driving force of the nebulizationprocess, and its efficiency is directly connected withthe gas flow [2–4]. Ultrasonic nebulizers produce afine aerosol of low droplet diameter and the process is

Spectrochimica Acta Part B 52 (1997) 1801–1812

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0584-8547/97/$17.00 Copyrightq 1997 Elsevier Science B.V. All rights reservedPII S0584-8547(97)00070-0

* Corresponding author.

almost independent of the gas flow. The application ofultrasonic nebulizers in spectrochemical analysisallowed improvements in detection limits for anumber of elements and the precision of the results[5]. The thermospray is the third type of nebulizer,causing distinctive interest as an interface in liquidchromatography–atomic emission spectrometrytechniques [6,7]. The direct injection nebulizerfound application especially in the ICP-MS method[8,9]. The fundamentals of pneumatic nebulizers andspray chambers were widely described by Sharp[10,11].

A majority of the studies described concernnebulizers applied in ICP and AAS techniques.Browner et al. proposed a theoretical model ofaerosol transport under such operating conditions[12]. However, many types of nebulizer mentionedare not effective for the MIP technique, in which,usually, relatively small gas and analyte flows areused. For small gas flows the memory effectsand possibility of clogging increase and thenebulization efficiency decreases. The performanceof nebulizers working below 0.8 l min−1 wasinvestigated by Michaud-Poussel and Mermet [13].A poor performance of the concentric and cross-flow nebulizers was found for these values of thecarrier gas flow. However, the frit, ultrasonic andVee nebulizers exhibited reasonable characteristics.Limitations of the early nebulizer designs (such asclogging, high liquid and gas flow rates, lowefficiency, inability to cope with high solids-containing analyte) have led to the development ofimproved nebulizers by Babington [14], to theV-groove [15] and GMK designs [16]. The maindrawbacks of the nebulizers specified above aretheir low efficiency, reaching 5% at most, highanalyte consumption, high gas flow rates and widedistribution of aerosol droplets which requires aspecial design of the aerosol chamber. Even in themost up-to-date designs the typical flow rates are1 ml min−1 of analyte and 1 l min−1 of gas.

Frit nebulizers have been claimed to producevery fine aerosols, resulting in analyte transportefficiencies up to 95% [17]. The extremely longwashout times (. 60 s) associated with thesenebulizers limit their use for routine analysis. Memoryeffects were lowered considerably in the glasscapillary array design [18].

Various parameters describing the nebulizerperformance are discussed. Physical parameters,such as the effectiveness of primary and tertiaryaerosol generation, aerosol density or droplets sizedistribution, are related directly to the work of thenebulizer. Spectroscopic parameters, such as theintensity of the signal measured, signal stability anddetection limits, are more useful for an analyst, butthey also depend on the performance of otherelements of the spectrometer (discharge tube, powergenerator, analyte pump, monochromator).

The nebulizer effectiveness depends on a numberof working parameters, such as gas and analyte flow,kind of gas, physical properties of the nebulizedliquid, shape of the spray chamber and others. Theseparameters should be optimized and are specific to aparticular nebulizer, even of the same generic type.The optimal values of parameters can changedepending on the element or sample determined.When comparing the efficiency of nebulizers oneshould remember that each of them should work, ifpossible, at their own optimal conditions.

The design and characteristics of three types ofnebulizer used for the introduction of liquid samplesto MIP are described in this work. Pneumaticnebulization with the frit and microcapillary arraynebulizers is described in part I. The design andstudy of an ultrasonic nebulizer are reported in partII of this work. In comparison with other plasmasources like ICP or DCP, the microwave plasma ismore sensitive toward composition changes causedby introducing the sample in the form of an aerosol.The capacity of an MIP discharge for high gas flowsand high vapor loads was limited due to the lowdischarge power.

2. Experimental

2.1. Instrumentation

Physical parameter measurements were carried outusing mass-flow controllers by DHN (Poland) andperistaltic pump type N 306 by Unipan (Poland).Spectrometric characteristics were established usingan MIP atomic emission spectrometer MIP 750 MVby Analab (Poland) with a horizontal plasma torch.To study vertical plasma configuration, the MIP

1802 K. Jankowski et al./Spectrochimica Acta Part B 52 (1997) 1801–1812

excitation source by Plazmatronika (Poland) was used.The operating conditions are presented in Table 1.

2.1.1. Frit nebulizerLaboratory-made, with a 50 cm3 glass spray

chamber (see Fig. 1). The 20 mm diameter porousfrit disc made with zirconia ceramics of controllablemedium porosity of 1–6mm was glued to the ceramictube. The tube was connected to an argon supply andplaced in the PTFE block. The analyte was applied withPTFE tubing. An argon flow of up to 1000 ml min−1

with a gas back pressure of 80 psi was achieved.

2.1.2. Microcapillary array (MCA)The laboratory-made nebulizer design is shown in

Fig. 2. It is of the same construction as the frit one, butthe frit plate is replaced by a glass MCA of 480orifices of 5mm diameter located in the V-groove. Aplasma gas flow up to 600 ml min−1 with a backpressure of 80 psi was achieved.

2.2. Nebulizer optimization

The effectiveness of the formation of primaryaerosol (the nebulizer without spray chamber) wasevaluated by weighing the uptake of feed water and

that dribbled from the frit or MCA. The tertiary aerosolgeneration efficiency was measured by nebulizing aknown volume of water for a specified time at varyingflow rates and measuring the solution recovered in thedrain and remaining in the spray chamber. Then theaerosol concentration was calculated.

2.3. Spectrometric measurements

The analytical utilities of nebulizers were testedusing the MIP system. The nebulizer flow ratesoptimization for spectroscopic measurements wasevaluated for selected elements. A short-term signaland background stability, as well as wash-in andwash-out times for the several elements, weremeasured. All detection limits were calculated forraw, unsmoothed data based on a 3j criterion.

3. Results and discussion

3.1. Design considerations

The high nebulization effectiveness and smalldiameter of the generated aerosol droplets are goodparameters of an efficient nebulizer. The high

Table 1Experimental instrumentation for MIP-AES measurements

Component MIP 750 MV Plazmatronika

Generator 2450 MHz, applied power 0–300 W 2450 MHz, applied power 0–300 WResonator TM010 Beenakker type, water cooled TE101 rectangular type, water cooledPlasma torch Horizontal position, quartz tube i.d. 2 mm, air cooled Vertical position, quartz tube i.d. 2 mm, air cooledPlasma gas Argon, 50–500 ml min−1 Argon, 50–500 ml min−1

Analyte pump 10–500ml min−1 10–500ml min−1

Monochromator 0.75 m Czerny–Turner

Fig. 1. Frit nebulizer design for horizontal plasma configuration.

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nebulization effectiveness permits one to decrease theflow rate of the sample solution. At small gas flowsthe effectiveness often decreases. A small diameter ofthe aerosol droplet leads to fast and effectivevaporization and excitation of the sample in theplasma torch. The vaporization of an aerosol ofsmall droplet diameter requires a lesser amount ofenergy and, therefore, a larger amount of energyintroduced to the aerosol may be used for theexcitation process. This is especially advantageousin the case of an MIP source, which works at a limitedenergy level compared with ICP or DCP. Smallsample consumption and large nebulization efficiencyare especially advantageous in multielement sequenceanalysis.

The use of small gas flows leads to an increase inthe gravitation separation and an increase in thememory effects. In order to solve this problem thespray chamber volume can be minimized andthe analyte to plasma path shortened, providing thatthe aerosol obtained is sufficiently fine. The outletleading to the torch should be placed at an angle to

Fig. 2. MCA nebulizer design for vertical plasma configuration.

Fig. 3. The effect of liquid flow rate on efficiency of aerosol generation for MCA and frit nebulizers (MCA 1—at argon flow rate of100 ml min−1; MCA 2—at 200 ml min−1; frit—at 250 ml min−1).

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the gas flow direction to remove the eventuallyformed large drops.

3.1.1. FritFrit material selection is a crucial design element.

Effective nebulization proceeds only on a materialwell moistened by the aqueous sample. A uniformaerosol stream may be obtained on a frit of largeparticipation of open pores. Glass is the easiestavailable frit material with a variety of sizes. However,a disadvantage of such a type of frit is the possibility ofsample contamination by a number of commonelements contained in glass. Aqueous samples andsolutions used for washing are usually good eluents ofthe glass components. Moreover, the non-homogeneityof the pore sizes in the glass frit worsens the distributionof the generated aerosol droplets size.

The availability of highly porous materials for fritsis offered by ceramics. The zirconia ceramicsproduction process has proved to be very flexible incontrolling the mechanical properties of the product,pore size distribution and active porosity. A materialconsisting of ZrO2 and 20 mol% of Y2O3 wasselected. A typical product had a porosity of about50%, with 80–100% open pores, and a controllablemedian pore size in the range 1–6mm. Pore sizedistribution was approximately log–normal and verysteep, 90% of pores between the grains had a diameterin the 0.45 to 1.8mm range (median 1mm), better thanthat for glass frits. The very well developed externalsurface assured very good distribution of theliquid sample within the frit. However, from thetechnological point of view, the manufacturing ofthese types of nebulizer appeared to be troublesome,since it is difficult to obtain a reproducible structure ofthe frit.

Orifices of very low diameters and very narrowrange of diameters may be produced, thus increasingthe orifice surface density and limiting the aerosoldroplet size distribution. Pressure ratio, the drivingforce of nebulization, increases, making gas velocitiesthrough the micro-orifices a little below subsonicvalues while the overall gas flow rate remainscomparatively low. As a result, the frit-type nebulizerpermits one to achieve an efficiency of up to 94% withaerosol concentrations in the plasma gas stream atleast equal to those attained in other types ofnebulizer.

The frit diameter was stated as 10 mm, so as toachieve in the gas flow range studied (between 50 to500 ml min−1) an appropriately large linear velocityof the gas output from the pores. Samples are appliedto the low pressure side of the frit to avoid cloggingand minimizing memory effects connected withretention of the analyte in the frit pores. Severaldesigns have been tested; acceptable results areobtained by placing the frit vertically, with thesharp-edge capillary tip placed about 0.1 mm awayfrom the frit and keeping the capillary internaldiameter below 0.5 mm. The frit was placed on thetube inside the spray chamber of a larger diameter toreduce the amount of liquid condensing on thechamber walls and to maintain stability of the aerosolstream flow.

For this nebulizer design a fine aerosol wasgenerated with very little contribution of largedroplets. In fact, it seems impossible to producedroplets large in size with frit orifices unless theanalyte is introduced in great excess. This affectssignificantly the solution of the system for aerosoldelivery to the plasma torch. A long aerosol chamber,a Scott chamber for instance, becomes detrimental tooperation of the whole system, since wash-out timesmay reach several minutes at low gas flow rates andthere is no need to remove large droplets. Thus, theroad of aerosol to the plasma may be reduced to below10 cm and chamber volumes should be kept as low as40–50 cm3.

3.1.2. MCAThe frit nebulizer has been described as highly

efficient, but characterized by considerable memoryeffects [17]. It has been shown that twisted orificesfavor clogging and increase the nebulizer memory.The Babington nebulizer, contrary to the frit one,exhibits a low efficiency, but is easy to wash. TheMCA nebulizer is a combination of these twoconcepts. The glass capillary array nebulizer wasfirst described by Babbis et al. [18]. A glass platewith densely packed mutually parallel orifices of adiameter of several micrometers is the working ele-ment of this nebulizer. In our design, adapted forsmall gas flows, we took advantage of the designsdescribed above for the frit nebulizer regarding theglass plate dimensions and spray chamberdimensions. Furthermore, the MCA was placed in

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the V-groove to prevent relatively low gas flow ratioand obtain a large gas output linear velocity and alsoto direct the analyte flow. This led to an increase in thenebulization efficiency, improvement of the washingof capillaries, and also a decrease in the carry-over ofthe analyte.

The technology of obtaining an MCA enables thediameter selection, as well as the number of orifices. Ifthe orifices are small a fine aerosol can be produced,but gas flow resistance and possibility of cloggingincrease. The nebulization effects for two capillarydiameters, 5 and 10mm, were studied and 5mm waschosen because a more stable aerosol stream wasobtained. The number of capillaries in an array affectsthe nebulization efficiency and gas flow resistance. Itwas observed that, for the gas and analyte flowsapplied, the maximum nebulization efficiency isalready obtained below 200 orifices, but a stableaerosol stream (spectroscopic emission short-termstability below 2%) was obtained only for 480capillaries. This ‘excess’ of capillaries is probablyrequired to stabilize the flow resistance changescaused by the instability of the analyte flow.

For MCA nebulizers two types of spray chamberhave been prepared to provide the nebulizer coopera-tion with both horizontal and vertical plasma config-urations. The volume and dimensions of the chambersremain comparable.

3.2. Nebulizer operation (optimization)

In classic pneumatic nebulizers the aerosol streamis highly turbulent (Re= ca. 15 000), but already at asmall distance from the capillary outlet it becomeslaminar [12,19]. During deceleration the aerosoldroplets of largest diameters are removed from thestream. This effect is observed visually in the formof a rim of condensing liquid on all the chamber wallsperpendicular to the aerosol generation surface. Theimpact of the aerosol droplets on the chamber walls isanother important process causing losses. Thus, in thehitherto used nebulizers the aerosol chambers are of alarge volume (ca. 100 ml) and elongated shape,e.g. the Scott chamber [20]. They can also beequipped with elements for homogenizing the aerosoldistribution in the gas stream (e.g. propeller in thePerkin–Elmer cross-nebulizer [12] or changing thestream flow direction in the Scott chamber. The

application of a conical chamber of small volume(ca. 45 ml) with a ‘high flow’ concentric nebulizerleads to considerable losses in the aerosol mass flow[21].

For ‘low-flow’ pneumatic nebulizers used in ourwork as sample introduction devices to MIP, a numberof differences in the aerosol generation and transporthave been observed. With a decrease in the gas flowthe aerosol stream becomes more laminar. This isprobably connected with the frit structure as well aswith the MCA. This means that the microturbulenceresulting from the gas output from the orifices outletsare parallel, which causes the output on a macro-scaleto already be laminar from the frit’s surface. Theimpact losses caused by impact of droplets onto thechamber walls are considerably lowered due to thelow gas velocity. The importance of gravitationseparation, however, increases, especially for thevertical system.

Analyte flow should be matched with the fritparameters to prevent flooding, which leads toinstabilities. The efficiency of aerosol generationand droplet size change depending on the gas andliquid flow, as was observed earlier for both types ofnebulizer. In Fig. 3 is presented as an example thedependence of the efficiency of primary aerosolgeneration on the analyte flow for the MCA nebulizerunder selected conditions (100 ml min−1). For smallflows a complete transformation into aerosol of theliquid introduced occurs on part of the array surface.During liquid flow of about 20ml min−1 all orifices arecovered with water. Above this value, efficiency doesnot change and excessive analyte feed results inhigher droplet diameter. Finally, for a flow of about40ml min−1 saturation of the array with water occurs,flooding begins and the efficiency decreases. At a gasflow of 200 ml min−1 the array flooding begins atabout 200ml min−1, and for the frit only at about250ml min−1. This is connected with a larger workingsurface of the frit.

3.3. Analyte transport measurements

A number of methods for calculating the nebulizerefficiency by means of direct weighing measurementsor after moisture absorption on silica gel have beenproposed in the literature [13,21–23]. A balance of theelement content on the basis of emission intensity

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measurement is another evaluation of performance ofthe whole nebulizer–chamber–torch system. How-ever, the results obtained by both methods are notalways comparable.

In Fig. 4 is presented the influence of argon flow onthe transport efficiency and corresponding aerosolconcentration. The results were obtained by weighingthe mass of water introduced to the nebulizer andremaining on the chamber wall and in the drain duringa defined time. For a frit, as well as an MCA nebulizer,at a given liquid flow flooding was observed at low gasflows resulting in a decrease of the aerosol transporteffectiveness. For both nebulizers studied the curveshave a similar course, but for the frit one a higherefficiency was obtained, reaching 70%. This resultsfrom the fact that the frit nebulizer generates dropletsof a smaller size, which are more efficiently trans-ported beyond the chamber. Nevertheless, the fritand MCA nebulization efficiencies are considerablyhigher than the 5–15% obtained for cross-flow orBabington nebulizers.

The operating parameters of both nebulizers wereestablished. Analyte consumption was in the range 10to 100ml min−1. Argon flows varied from 50 to

300 ml min−1 for the MCA nebulizer and from 50 to450 ml min−1 for the frit nebulizer.

Nebulization efficiency of the frit nebulizer wasevaluated by means of signal intensity measurements.A 1000 ppm Cu solution was applied to the frit andthe amount of copper remaining in the drain wasdetermined at argon flow 100 ml min−1. The resultobtained (about 60% aerosol transport efficiency) issimilar to that determined by the weighing procedure(see Fig. 4).

Since the gas and particle velocities in these studieswere generally relatively low, three types of lossprocess have been observed. For argon flow in therange 50 to 150 ml min−1, the aerosol droplets wereseparated primarily by gravitational settling. Forhigher flows the turbulent or impact losses increased.Comparative studies were carried out for the MCAnebulizer mounted in a horizontal and verticalexcitation source configuration. In both cases achamber of the same volume and shape was used.At gas flow below 100 ml min−1 the gross nebuliza-tion efficiency for a vertical system was even up to10% lower than that for the horizontal system. Athigher gas flows the difference was ca. 5%. This can

Fig. 4. The effect of argon flow rate on aerosol transport efficiency for MCA and frit nebulizers.

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be explained by the presence of gravitational separa-tion, which is of greater importance in the case ofvertically upward aerosol transfer.

On the other hand, when comparing the efficiencyof an MCA nebulizer using a 50 cm3 chamber (Fig. 2)and another chamber of 100 cm3 capacity andelongated shape, it was found that for a biggerchamber the optimal gas flow is higher than thatfor a small one and is equal to ca. 400 ml min−1.For a small chamber, at the same gas flow,considerable impact losses were noticed on the frontchamber wall.

Aerosol production rates and flows must match thecharacteristics of the plasma torch. The primaryaerosol generation was found to be highly effectiveover a wide range of flows for both nebulizers studied.The experiment described above indicates that themain factors to optimize the nebulizer–torch systemare the volume and shape of the spray chamber. Forour system, the 50 cm3 spherical spray chamberseems to be the optimum configuration.

For both nebulizers the gas and analyte flows werefound to be interdependent and should be optimizedsimultaneously. The summary effect of those

operating parameters on aerosol concentrationobtained with the MCA nebulizer is shown in Fig. 5.

3.4. Signal response and precision

When optimizing the gas flow it should beconsidered that at a certain flow value the aerosolconcentration starts to decrease, despite the fact thatefficiency still increases. The aerosol transportefficiency decreases with an increase in the analyteflow due to the increase in size of the dropletsgenerated. On the other hand, the nebulizer optimalworking parameters, when evaluating spectroscopicparameters (e.g. maximum emission intensity), arestill different since the sample residence time in theplasma should be considered, and this is inverselyproportionally dependent on the flow. Moreover, thevaporization and analyte excitation effectivenessdecrease with an increase in the aerosol concentrationand size of droplets, especially for low-power MIP.

The influence of analyte flow rate on emission bythe MIP plasma compared with aerosol concentrationis presented in Table 2. The spectroscopic measure-ment is expressed as the signal-to-background ratio

Fig. 5. Interdependence of aerosol concentration from gas and liquid flow rates for MCA nebulizer.

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(SBR), which well describes the sensitivity of theanalytical performance of the system. A lowering ofthe zinc signal was observed at a 80ml min−1 flow foran MCA nebulizer, while the aerosol concentrationslightly increased. This is caused by an increase inthe droplet size. An approximate twofold SBR

decrease for cadmium occurred for the frit nebulizerat a nearly twofold increase in the aerosol concen-tration. The fact that at a 80 l min−1 flow the highaerosol concentration dramatically changes theexcitation conditions in the MIP plasma can be anexplanation for this. For both nebulizers the optimumintensity of emission is obtained at a median aerosolconcentration.

The effect of gas flow on the signal to noise ratio isshown in Figs 6 and 7. Here also, differences areobserved between the optimum mechanical andspectroscopic performance of nebulizers. For anMCA nebulizer the highest zinc signal is obtained ata 150 ml min−1 flow. The efficiency of the frit rises toabout 70% at 100 ml min−1 argon flow, while the SBRpeaks at 200 ml min−1.

The signal stability and noises appeared to be worsethan those described for the ICP plasma. In the case ofthe MIP plasma of low power the emissionintensity measurement precision is connected withdisturbances in the aerosol generation stability andworsening in the discharge stability after introducingthe liquid sample. The short-term stability appeared to

Table 2Effect of analyte flow rate on tertiary aerosol formation and relatedemission intensity (argon flow: 200 ml min−1; analyte: 10 mg l−1 ofzinc for MCA, 10 mg l−1 of Cd for frit)

Analyte Efficiency Aerosol conc. SBR(ml min−1) (%) (ml l−1)

MCA20 41 0.041 13.540 35 0.070 23.860 25 0.075 26.880 20 0.080 15.6

Frit10 70 0.040 20.020 69 0.078 50.040 65 0.147 47.080 54 0.244 26.2

Fig. 6. Comparison of aerosol concentration (AC) and SBR as a function of argon flow rate for frit nebulizer.

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be dependent on the gas and analyte flow only to asmall degree over the whole working range. Theprecision was worse only for small flows due to thepump pulsation effect. The background noises andsignal stability changed for the frit nebulizer from 2to 3% and from 2 to 4% respectively and for the MCAnebulizer from 1 to 2% and from 1 to 3% respectively.The worse stability of measurements for the fritnebulizer may result from the use of larger aerosolconcentrations, which worsen the plasma dischargestability.

3.5. Detection limits

The detection limits of selected elements obtainedwith the MIP-AES method were determined accord-ing to Winge et al. [24]. The single element solutionsof 10 mg l−1 concentration, as well as blank solution,were introduced to the plasma using frit or MCAnebulizers. The results obtained and spectral linesused are collected in Table 3. The detection limitsobtained by means of a frit nebulizer are slightlylower, probably due to the smaller dimensions of theaerosol droplets obtained.

3.6. Memory effects

Memory effects have often been considered as afunction of spray chamber design. For the popularpneumatic nebulizers the wash-out characteristicsare, in reality, just chamber flushing times. The useof small gas and analyte flows involves longer wash-in and wash-out times. The adaptation of a nebulizerfor working in a routine analysis mode can beachieved by simplifying the chamber design andshortening of the aerosol to plasma path. However,in the case of a frit nebulizer the extremely longwash-out times are reported as a main drawback ofthis nebulization system due to the frit disorderedstructure, where the analyte is retained. Replacementof the frit for an array of parallel capillaries has beenproposed in order to solve this problem.

A number of experiments have been carried out todetermine wash-in and wash-out times and tocompare both nebulizers (Table 4). In all cases the50 ml spray chamber was used. For the frit nebulizeractual wash-out times to signal intensity of 0.1%were 2–6 min, whereas for the MCA they wereusually below 1 min. The wash-out time proved to

Fig. 7. Comparison of aersol concentration and SBR as a function of argon flow rate for MCA nebulizer.

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be gas-flow dependent. For the frit at an argon flow of100 ml min−1 the wash-out time of 100 mg l−1 ofcadmium was 6 min; however, it decreased to 3 minat 300 ml min−1, whereas the wash-in time was only70 s. Nevertheless, the wash-out of 100 mg l−1 ofcalcium at 100 ml min−1 gas flow was about 2 min.For the MCA nebulizer the wash-in and wash-outtimes of 100 mg l−1 of palladium were 25 s at a gasflow of 150 ml min−1, and at 200 ml min−1 the silveror calcium solutions of 100 mg l−1 were washed-outwithin 20 s.

Wash-out experiments confirm that water or a 2%nitric acid solution efficiently remove any samplematerial itself; however, particularly high salt contentsamples are removed faster with the nitric acidsolutions. Since the MIP discharge can withstand alimited amount of solvent it is impossible to use a‘rabbit switch’ mode of peristaltic pump to rinse thenebulizer in the course of analysis. However, the useof a ‘stop flow’ mode after a 30 s period of washinggives satisfactory results when removing high saltcontent samples. The liquid flow was stopped forabout 20 s and then the wash cycle was completedwith the additional portion of rinsing solution.

The MCA nebulizer was utilized in routine analysiswith the MIP system for several hours per day over5 months. The long-term stability of aerosol transportefficiency was enough to obtain reproducible results.To avoid clogging the nebulizer was rinsed with nitricacid solution and water for about 10 min and then driedwith argon flow at the end of the measurement cycle.

4. Conclusions

The frit and MCA nebulizers have proved tobe useful as sample introduction devices for theMIP-AES system. At gas flows in the range50–300 ml min−1 and liquid flows below 50ml min−1

they produce effectively a fine aerosol of concen-tration up to 0.2 ml l−1. Since droplets of a largediameter are generally not formed, and linearresidence times are greater than in other nebulizers,Scott or conical chamber design solutions proveunnecessary and chamber volume should be kept toa reasonable minimum.

Both of the nebulizers studied need improvements.The zirconia ceramics seem to be a prospective frit

Table 3Comparison of detection limits for frit and MCA nebulizers obtained with MIP-AES

Element Analysis wavelength (nm) Detection limit (mg l−1)

Frit MCA

Ag (I) 338.289 11 18Ca (II) 393.366 3 7Cd (I) 228.802 3 8P (I) 213.618 120 120Pd (I) 340.458 35 40Zn (I) 213.856 15 20

Table 4Comparison of wash-in and wash-out times obtained for MCA and frit nebulizers (analyte concentration 100 mg l−1)

Analyte Wash-in time [argon flow] Wash-out time [argon flow] MCA(s) [l min−1] (s) [l min−1]

MCAPd 25 [0.15] 25 [0.15] —Ca, Ag 25 [0.2] — 20 [0.2]

FritCd 70 [0.3] 360 [0.1] 180 [0.3]Ca 85 [0.1] 120 [0.1] —

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material if the technological problems of obtaining areproducible frit structure can be overcome. The useof a new frit material did not solve the problem of longwash-out times, which are unacceptable in routineanalysis.

Change of the frit for an MCA seems to be a goodsolution. The latter operates over a wide range of gasflow with aerosol transport efficiency up to 50%. Theapplication of an MCA V-groove design permits theuse of small sample output, which is favorable forsequential multielement analysis. The MCA nebulizerdesign proved to be very flexible to match it with theplasma torch operating conditions. By changing thenumber of capillaries in the array and the length andcapacity of the spray chamber, the optimal workingparameters of the nebulizer can be easily fitted to theneeds of the measuring system used.

It seems possible to use an MCA not only for MIPbut also for other spectroscopic techniques. Since con-siderably great aerosol concentration is obtained, theaddition of a desolvation unit, as in the case of theultrasonic nebulizer, seems to be a requirement. Theuse of a more inert material than glass for the prepara-tion of the MCA and the carrying out of certaintechnological and design improvements may lead tothe obtaining of a more effective nebulization system.

Acknowledgements

This work was supported by the State CommitteeFor Scientific Research (project no. 3 T09A 103 10sponsored in 1996–1998).

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