Evaluation of a direct injection high-efficiency nebulizer (DIHEN) by

comparison with a high-efficiency nebulizer (HEN) coupled to a

cyclonic spray chamber as a liquid sample introduction system for

ICP-AES

Jose-Luis Todolı*a and Jean-Michel Mermetb

aDepartamento de Quımica Analıtica, Universidad de Alicante, 03080 Alicante, SpainbLaboratoire des Sciences Analytiques (UMR 5619), Universite Claude Bernard-Lyon, F-69622 Villeurbanne Cedex, France

Received 24th November 2000, Accepted 22nd February 2001First published as an Advance Article on the web 28th March 2001

In the present work the advantages and limitations of introducing the sample directly into the plasma through

the use of a direct injection high-efficiency nebulizer (DIHEN) have been evaluated in ICP-AES. The reference

system was a high-efficiency nebulizer (HEN), precursor of the DIHEN, coupled to a cyclonic spray chamber.

It was found that the plasma can only be operated under non-robust conditions because of the high solvent

loading. The issues considered in the present work were: (i) the aerosol characteristics; (ii) the ICP-AES

analytical figures of merit; and (iii) the interferences induced by the presence of inorganic as well as organic

matrices. The results indicated that the aerosols obtained by the DIHEN are coarser than those leaving the

cyclonic spray chamber. Nonetheless, and due to the higher analyte transport efficiency, the DIHEN affords

ICP-AES sensitivities and LODs up to 10–18 times better than the cyclonic spray chamber. Also, it was found

that, under the plasma conditions tested, the optimum nebulizer gas and liquid flow rates in terms of sensitivity

for the DIHEN were very different from those for the HEN with the cyclonic spray chamber. In the first case,

the signal peaked at rather low liquid (Ql) and gas (Qg) flow rates, whereas for the cyclonic spray chamber

moderate and/or high liquid and gas flow rates were advisable. Under a given set of plasma conditions, the

DIHEN has been proven to produce problems due to deterioration in the plasma, derived from the higher

solvent load with respect to the HEN coupled to the cyclonic spray chamber. Despite these findings and the

fact that a non-robust plasma was used, the matrix effects for inorganic and organic species were significantly

minimized, for a low Ql and for some lines, in the case of the DIHEN.

Introduction

Usually, a common liquid sample introduction system for usein ICP-AES consists of a nebulizer that transforms the liquidsample into an aerosol and a spray chamber that selects themaximum drop size that will be introduced into the plasma.1,2

The use of a spray chamber is sometimes inadvisable, becausethere is a large mass of nebulized solution that is lost in thissecond component of the sample introduction system. Typicalanalyte and solvent transport efficiencies are around 5% at bestwith standard uptake rates, i.e., in the range 1–2 ml min21.When the sample volume is limited, very low liquid flow rates(i.e., several hundreds of microlitres per minute) must be usedand some commercially available nebulizers can be employed.These are the so-called micronebulizers. Among them are thehigh-efficiency nebulizer (HEN),3,4 the microconcentric nebu-lizer5 and the MicroMist. With these nebulizers the analytetransport efficiencies rise by up to 50%.

Another drawback of the spray chamber is its contributionin enhancing the matrix effects.6 Among the different spraychamber designs found in the literature, the two most widelyemployed have been the double pass spray chamber and thecyclonic type.7 Several studies have demonstrated that thecyclonic spray chambers provide higher ICP-AES sensitivitiesthan the double pass ones. Moreover, the interferencesobserved with inorganic matrices are less severe.8,9 Also inthis case, several low inner volume spray chambers have beendeveloped in order to analyze microsamples, thus making thewash-out times shorter.2,10,11 Finally, the spray chamber also

constitutes a source of signal flicker (additive) noise.12

Although, it must be indicated that the spray chamber canact as a pump pulse damping system.

The problems associated with the use of a spray chamber canbe mitigated by eliminating it and using nebulizers especiallydesigned to introduce the aerosol directly into the plasma atvery low liquid flow rates. These are the so-called directinjection nebulizers (DINs).13 So far, two different designs ofthis nebulizer have appeared: the original design calledDIN14,15 and the so-called direct injection high-efficiencynebulizer (DIHEN) recently developed by McLean et al.,16

which is commercially available.The DIHEN is basically a modification of the HEN. The

main difference between them lies in the length of thesenebulizers, the DIHEN being longer. Thus, the injector isreplaced by this nebulizer, and the aerosol is generated 4–5 mmbelow the plasma base. Because of its characteristics, thisnebulizer has proven to be suitable as an interface betweenseparation techniques and ICP-MS.17 The main advantagesincorporated by the DIHEN with respect to other conventionalliquid sample introduction systems for ICP-MS, like a cross-flow nebulizer coupled to a double pass spray chamber, arehigher sensitivities, better signal stability, lower limits ofdetection and short wash-out times. On comparing the DIHENwith the DIN, one can highlight the lower cost and thegeneration of finer and less dispersed aerosols by the former. Sofar, several studies have been conducted with this nebulizermainly with ICP-MS instrumentation for the determination oflong-lived radionuclides,18,19 quantification of chromium–

514 J. Anal. At. Spectrom., 2001, 16, 514–520 DOI: 10.1039/b009430g

This journal is # The Royal Society of Chemistry 2001

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DNA adducts20 and determination of lead chromate in humanlung cells.21

Besides the improvement in ICP-AES analytical figures ofmerit, it has been indicated that the matrix effects induced byinorganic acids are significantly reduced when using a DIN.22

This is because the use of this kind of nebulizer reduces thelikelihood of production of aerosol transport phenomena thatcan be different in the presence and absence of a matrix.

The aim of the present work was to evaluate the suitability ofthe DIHEN as a liquid sample introduction system for ICP-AES. Therefore, an investigation of the behavior of thisnebulizer in terms of aerosol characteristics and ICP-AESanalytical figures of merit was performed. In the second part ofthe present work, the matrix effects induced by the presence ofinorganic species (nitric acid and sodium chloride) as well asorganic compounds (acetic acid and ethanol) was alsoevaluated. Because the DIHEN was initially created to workat very low liquid flow rates, the results were compared to thoseobtained for a HEN linked to a cyclonic spray chamber.

Experimental

Two different nebulizers were employed: a HEN and aDIHEN, both from J. E. Meinhard Associates Inc., SantaAna, CA. According to the manufacturer, both nebulizersrequire a 170 psig (i.e., 11.9 bar) argon pressure to reach1 l min21 gas flow rate. Refs. 16 and 23 provide further detailsabout the critical dimensions of the DIHEN and HEN. Asrecommended in ref. 16, the DIHEN was placed inside theplasma torch, 26 mm from the end and 2 mm from the end ofthe torch inner tube. A cyclonic spray chamber (GlassExpansion, Pty, Australia) with a 47 ml inner volume wasused with the HEN. From now on, this combination will bereferred to as HEN-Cy.

The sample was supplied to the nebulizer by means of aGilson Minipuls 3 peristaltic pump (Villiers Le Bel, France).The liquid flow rate (Ql) was varied between 30 and110 ml min21 by using 0.25 mm id Tygon capillaries. Due tothe high gas pressure required by the DIHEN, in order to setthe nebulizer gas flow rate, an additional Ar cylinder was usedinstead of using the mass flow controller of the ICP-AESsystem. The DIHEN gas flow was then controlled by keepingthe back pressure at a given level.

The drop size distributions (DSDs) of the aerosols generatedby the nebulizer (primary aerosols) and those leaving the spraychamber (tertiary aerosols) were characterized by means of alaser Fraunhofer diffraction system (Model 2600c, MalvernInstruments, Malvern, Worcestershire, UK). The primaryaerosols were measured 5 mm from the nebulizer tip, whereas,in the case of the tertiary ones, the exit of the spray chamberwas placed 5 mm away from the laser beam.

The emission signal was measured with a Perkin-ElmerOptima 3000 radial viewing-based ICP-AES instrument (Nor-walk, USA). Table 1 summarizes the plasma instrumentalconditions. Samples containing 1 mg ml21 of eight elementswere prepared in plain water from an ICP multielementstandard solution (IV; Merck, Darmstadt, Germany). Table 2lists the wavelengths and ionic line energy sum values, Esum

(i.e., sum of ionization and excitation energies) as well as theexcitation energy values for the atomic lines employed.

In order to investigate the interferences produced by severalmatrices, solutions containing 1 mg ml21 of each element wereprepared in nitric acid (1 and 3.6 mol l21), sodium chloride(5000 mg ml21), acetic acid (10% v/v) and ethanol (10% v/v). Toavoid the risk of the nebulizer tip being blocked by solidparticles, every solution was passed through PTFE filters (1 mmpore size, Millipore).

Results and discussion

Aerosol DSDs

Two of the most critical variables that affect the aerosolcharacteristics of a pneumatic concentric nebulizer are the gasand liquid flow rates.24,25 As indicated in previous work, thehigher the gas flow rate and the lower the liquid flow rate, thefiner the aerosols. Moreover, these two variables also imposeon the aerosol size distribution as a function of the position onthe aerosol cone.16 This behavior has also been found for theHEN.26 Fig. 1 shows the variation of the percentage of liquidvolume in band versus the drop diameter in a semi-logarithmicrepresentation for the aerosols generated by the DIHEN andthose leaving the spray chamber. According to this representa-tion, a shift of the curves towards the left would indicate fineraerosols. From Fig. 1 it can be seen that the liquid volume ofthe primary aerosol for the DIHEN is almost all contained indroplets with diameters lower than 10 mm. The maximum dropsize found for the DIHEN under the conditions tested in Fig. 1was 12 mm, which is consistent with that reported in the workof Montaser and co-workers.16,23 This result is also similar tothat found for the primary aerosols generated by the HEN.26

As regards the shape of the curve A, it can be seen that there aresome maxima located at around 8, 5 and 2 mm. These resultsare consistent with those found by McLean et al.16 whoindicated that the maxima can be explained by the samplecapillary oscillation, and that their position depends on the gasflow rate tested.

On comparing the primary aerosols for DIHEN with thetertiary for the HEN-Cy, it seems that the latter device gave rise

Table 1 Plasma instrumental conditions

Rf power/kW 1.3Integration time/ms 100Sampling time/ms 1000Outer gas flow rate/l min21 15Intermediate gas flow rate/l min21 0.5Central gas flow rate/l min21 VariableViewing height above load coil/mm 15Torch Fassel typeInjector id/mm 2

Table 2 Elements, wavelengths and energy sum values of the lines used

Element Wavelength/nm Esum/eVa

Ba II 455.403 7.93Cd II 214.438 14.77Cr II 205.560 12.80Mg II 280.270 12.25Mg I 285.213 4.35Mn II 257.610 12.25Ni II 221.647 14.27Sr II 407.771 8.73Zn I 213.856 5.80Zn II 202.548 15.51aEsum (only for ionic lines)~ionization energyzexcitation energy.

Fig. 1 Volume DSD curves for the aerosols generated by the DIHEN(A) and the HEN-Cy (B). Solvent, water; Ql~90 ml min21; andQg~0.7 l min21.

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to finer aerosols than the former. Under the conditions,summarized in Fig. 1, the median of the volume DSD (D50) was3.3 and 1.5 mm for the DIHEN and HEN-Cy, respectively.These data indicate that the aerosols introduced into theplasma are finer when using the spray chamber than theDIHEN. Obviously, the reason for this behavior is the spraychamber filtering action. According to the results shown inFig. 1, droplets with diameters higher than 9 mm are eliminatedfrom the aerosol stream when using the cyclonic spraychamber. On considering the smallest droplets, it was observedthat, at 90 ml min21 and 0.7 l min21, the percentage of aerosolliquid volume contained in droplets with diameters lower than1.2 mm (V1.2) was around 42 and 25% for the HEN-Cy andDIHEN, respectively. These data make it possible to expecthigher analyte excitation efficiencies, and lower noise andinterferences due to plasma effects when using the HEN-Cythan the DIHEN. In contrast, it should be borne in mind thatthe analyte mass transported towards the plasma was lower forthe cyclonic spray chamber and that the aerosol transportphenomena were minimized for the DIHEN.

Because of the fact that the tip of the DIHEN was placedclose to the plasma base, it was interesting to investigate thevariation in the aerosol characteristics as a function of theposition. The D50 values were slightly higher at 15 mm from thenebulizer tip. Thus, under the conditions tested, the D50 valueswere 3.0 and 3.4 mm at 5 and 15 mm, respectively. These resultsare explained by taking into account that the finest aerosoldroplets were evaporated and the drop coalescence wasproduced in a more significant way as the sampling distanceincreased.3,27 Therefore, it is expected that the nebulizerposition inside the torch would affect the signal finally obtainedin ICP-AES. Note that the D50 value obtained 15 mm from thenebulizer tip in these experiments is very similar to thatmentioned before (Fig. 1) at 5 mm. In this case, it is interestingto note that the instrument employed to carry out thesemeasurements affords very good short term precision but, ashas been indicated previously, the long term reproducibility is,in some cases, poor.28

Plasma solvent load

Fig. 2 shows the emission spectra obtained in the region around309 nm because of the presence of some O–H emission bands.29

The results are plotted for the HEN-Cy in Fig. 2(a) and theDIHEN in Fig. 2(b). First, it can be observed that the netemission signals obtained for the bands corresponding to theDIHEN were higher than those for the HEN-Cy. This facthighlights the higher amount of solvent that was introducedwith the DIHEN. Note that, for the HEN-Cy at 80 ml min21

and 0.7 l min21, the solvent transport rate was around30 mg min21 instead of 80 mg min21 for theDIHEN. Secondly, as is represented in these figures, anincrease in the liquid flow rate resulted in an increase in thepeak height for the four bands in the case of theDIHEN. Meanwhile, the background became slightly loweras the liquid flow rate increased. The results obtained for theHEN on increasing Ql [Fig. 2(a)] did not reveal any remarkablevariations in the net emission intensity for the lines considered

in this case. This was because, for this system, the transport rateof the total mass of solvent leaving the spray chamber varied toa lesser extent with Ql than when using the DIHEN. Fromthese results, it could be anticipated that, under the conditionstested in the present work, the plasma when using the DIHENwould be more sensitive to changes in Ql than when using theHEN-Cy.

Mg ionic-to-atomic (Mg II/Mg I) net emission intensity ratios

A parameter that has been widely used to test the plasmathermal capability, in particular, the sensitivity to plasmasolvent load, is the magnesium ionic-to-atomic net emissionintensity ratio.30 In the present study, this ratio was calculatedby taking into account the detector response as a function ofthe wavelength. Thus, the net emission signal obtained for MgII was divided by that for Mg I. This ratio was multiplied by theMg II to Mg I intensity ratio for the blank (i.e., from 0.8 to 1.1,depending on the system employed) in the ICP-AES spectro-meter employed in the present work.

Usually, a combination of 1.2–1.4 kW with aerosol flowrates below 0.6 l min21 leads to the so-called robust conditionswhen a conventional nebulizer and radial viewing are used (MgII/Mg I ratios higher than 8).31 However, as will be shown later,because of the increase in nebulizer efficiency, an increase in theplasma solvent load produced Mg II/Mg I values lower than 6.As a consequence, it is necessary to use 1.5 kW rf power.Nevertheless, in our case, it was observed that operating at rfpower values w1.4 kW led to generator stability problems. Inaddition, the use of high rf power damaged the torch walls,which could also affect the DIHEN performance leading to tipmelting. Therefore, in this work, the variations in the Mg II/MgI ratio have been followed rather than their absolute values.

In the case of the DIHEN, the higher the liquid flow rate, thelower the Mg II/Mg I ratio, thus indicating that the plasmaconditions deteriorated. At 0.14 l min21, this ratio had valuesranging from 3 to 0.7 as Ql increased from 30 to 90 ml min21.Nevertheless, this relative parameter was independent of theliquid flow rate in the case of the HEN-Cy. At 0.38 l min21

these values were around 2.4 irrespective of the liquid flow rateused.

A rise in Qg led to decreases in the Mg II/Mg I ratio for theDIHEN. This variation was more pronounced at low Ql values.Thus, at 30 ml min21, the ratio was around 3 and 2 at Qg~0.14and 0.63 l min21, respectively. Meanwhile, at 68 ml min21

the ratio was 1.4 and 1.2 at Qg~0.14 and 0.63 l min21,respectively. For the cyclonic spray chamber this parametervaried slightly with Qg.

Signal-to-background (SBR) ratio optimization and ICP-AESanalytical figures of merit

Besides the aerosol characteristics, Qg and Ql affect the analyteand solvent transport rates and the analyte residence time.Fig. 3 shows that, for the HEN coupled to the cyclonic spraychamber, the SBR increased with Ql, irrespective of the lineconsidered. This fact is explained by the concomitant increasein the analyte transport rate.

Fig. 2 Emission spectra obtained in the region of 309 nm for (a) HEN-Cy and (b) DIHEN. Dotted line, Ql~90 ml min21; dashed line,Ql~40 ml min21; solid line, Ql~30 ml min21. Qg~0.38 l min21.

Fig. 3 Signal-to-background ratio (SBR) versus liquid flow rate (Ql) for(a) Ba II and (b) Mn II. Nebulizer, HEN-Cy. A, Qg~0.38 l min21; B,Qg~0.50 l min21; C, Qg~0.63 l min21.

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For the DIHEN, the relative SBR variation with Ql

depended on the line considered. Thus, for Ba II 455 nm,i.e., an ionic line with a low Esum value (Table 2), an increase inthe liquid flow rate led to a maximum in SBR at Ql valueswithin the 45 to 65 ml min21 range [Fig. 4(a)]. For another ionicline, Mg II 280.2 nm with a higher Esum value (Table 2), anincrease in this variable led to a sharp decrease in the SBR[Fig. 4(b)]. The signal decreased within the range 4–10 as Ql

increased from 30 to 90 ml min21. Concerning atomic lines, twomain trends were found: for Mg, a more or less pronouncedmaximum in SBR located at Ql between 40–50 ml min21 wasobserved [Fig. 4(c)] and, for Zn, the signal increased steeplywith the liquid flow rate [Fig. 4(d)]. The differences in SBRbehavior with Ql for the lines tested, which, in some cases, didnot correspond to the variation in analyte mass injected intothe plasma, indicated that, under the conditions tested (i.e., theplasma was operated under non-robust conditions), there was aplasma effect probably produced by the increase in the solventload and/or modification in the aerosol characteristics.

It appears that some of the trends concerning the effect of theliquid flow rate do not agree with those found in ICP-MS byother authors.16,18 In those cases, the higher the liquid flowrate, the higher was the sensitivity for a wide range of elementalmasses. It is interesting to note that, for the reasons givenbefore, the rf power employed in the present work was 1.3 kW(lower than that used in other studies, i.e., 1.5–1.6 kW). Thusthe ICP-MS sensitivity variations with Ql found by Montaserand co-workers,16,23 are affected by higher Ql values than thosein the present study, for ionic lines havingEsumw9 eV. Moreover, it is worth mentioning that, as pointedout by Becker et al.,18 the Ql optimum can be different for twodifferent DIHENs.

The gas flow rate was also varied for both nebulizers. Whenconsidering the HEN-Cy (Fig. 3), two main trends were found:for atomic lines and Ba II, an increase in the gas flow rateproduced increases in the sensitivity; and, for ionic lines withEsum values higher than 9 eV, the signal peaked at approxi-mately 0.5 l min21.

From Fig. 4 it can be seen that, as reported in previouslypublished work for the DIHEN,16,18,23 at a given liquid flowrate, a decrease in Qg results in an increase in the SBR. Adecrease in the gas flow rate leads to the generation of coarseaerosols, an increase in the analyte residence time and adecrease in the amount of cold gas that is being introduced intothe plasma. The former effect resulted in a less efficientexcitation of the analyte, whereas the two latter factorscontributed to an increase in the sensitivity. This behaviorwas observed for all the lines tested except for the Zn I line[Fig. 4(d)].

Summarizing all the studies concerning the optimization ofthe nebulization conditions, the gas and liquid optimum

conditions in terms of ICP-AES sensitivity depend on thesystem and the line tested. Table 3 summarizes the optimumconditions for both systems and the different lines tested. Forthe DIHEN, the sensitivity was higher when working at lowliquid and gas flow rates. This was true for ionic lines having anEsum value higher than around 9 eV. In contrast, if eitheratomic lines or ionic lines with Esum values lower than 9 eVwere measured, the optimum operating conditions for theDIHEN were moderate values of Ql and Qg.

The optimum conditions for the HEN-Cy were differentfrom those for the DIHEN and also depended on the linemeasured. Hence, in the case of ionic lines with Esumw9 eV, themaximum signal was obtained at high liquid flow rates andmoderate gas flow rates (Table 3). Meanwhile, for atomic linesor ionic lines with Esumv9 eV, the SBR peaked at high Ql andQg values.

As mentioned before, the use of a spray chamber eliminates avery important fraction of the solution supplied to thenebulizer. Under appropriate conditions, this should meanhigher sensitivities for the DIHEN. Table 4 shows the SBRvalues obtained for the DIHEN in comparison with those forthe HEN-Cy for the different elements. Two groups ofnebulization conditions were considered. In the first group,both systems were operated under low Ql and Qg, i.e.,conditions favorable for the DIHEN. In the second group,both systems were operated under their own optimumconditions (Table 3). It can be observed that the relativesensitivity depended strongly on the line studied. Thus, forconditions suitable for the DIHEN, this increase was between1.4 (for Zn II) and 18 (for Ba II). When these two systems wereused under their optimum conditions, the SBR increase waswithin the range 1.3–6.7. Note that, under the liquid and gasflow rates tested, the DIHEN gave rise to an analyte transportefficiency improvement factor of up to about 5 times.

The signal stability is another important issue that has beenevaluated. The results indicated that, in relative terms, thesignals obtained with the DIHEN were more stable than thosefor the cyclonic spray chamber. Hence, for the HEN-Cy, theRSD calculated from a total of 20 replicates was between 1 and6%, depending on the element and on the conditions tested,whereas for the DIHEN, this parameter ranged from 0.3 toaround 2%. These results agree with previously publishedfigures in ICP-MS16 and are probably due to the improvementin sensitivity provided by the DIHEN and the removal of thespray chamber.

As a result of the increased sensitivities and similar values ofthe background noise, the DIHEN afforded lower LODs thanthe HEN-Cy (Table 4). The LODs were calculated according tothe 3sb criterion, with sb the standard deviation obtained from10 replicates of the blank. In this case the LODs were reducedby a factor of between 1.5 and 20, depending on the line andnebulization conditions.

Fig. 4 SBR versus liquid flow rate (Ql) for (a) Ba II, (b) Mg II, (c) Mg Iand (d) Zn I. Nebulizer, DIHEN. A, Qg~0.14 l min21; B,Qg~0.38 l min21; C, Qg~0.50 l min21; D, Qg~0.63 l min21.

Table 3 Optimum Qg and Ql values for the different lines and liquidsample introduction systems

Line

DIHEN HEN

Qgopt/l min21 Ql

opt/ml min21 Qgopt/l min21 Ql

opt/ml min21

Ba II 0.14 68 0.63 90Cd II 0.14 30 0.38 90Cr II 0.14 30 0.50 90Mg II 0.14 30 0.50 90Mg I 0.14 45 0.63 90Mn II 0.14 30 0.50 90Ni II 0.14 30 0.50 90Sr II 0.14 68 0.50 90Zn I 0.63 90 0.63 30Zn II 0.14 30 0.38 90

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Matrix effects for inorganic matrices

Fig. 5 shows the complete drop size distribution curves for thetertiary aerosols obtained with the HEN-Cy [Fig. 5(a)] and theprimary aerosols for the DIHEN [Fig. 5(b)] for three differentsolutions: water and nitric acid at two different concentrations(1.0 and 3.6 mol l21).

In the case of the spray chamber [Fig. 5(a)] the aerosols werequite different depending on the matrix tested. Tertiaryaerosols were finer when nitric acid was employed. Thisassessment was confirmed by the statistical diameters, with D50

lower for nitric acid than for water. Thus D50 values were 1.7and 0.8 mm for water and the two acid solutions, respectively.Also V1.2 sharply increased when nitric acid was present (i.e., 41and 80% for water and the two acid solutions, respectively). Allthese results confirm that there was a significant matrix effecton the aerosol transport through the spray chamber, whichconfirms previously published work.32 While the reasons forthis behavior are not clear yet, it seems that the origin of thiseffect could be in droplet charging effects.33

However, when comparing the curves for the primaryaerosols obtained with the DIHEN for water and nitric acidsolutions [Fig. 5(b)] there were no noticeable differencesbetween them. The D50 and V1.2 values for water and nitricacid solutions were virtually the same (i.e., 3.3 mm and 24%,respectively), although it seemed that, for the concentratednitric acid solution, the D50 value was slightly higher (3.5 mm)and the V1.2 lower (21%) indicating that slightly coarseraerosols were obtained for this solution than for water. Thereason for this behavior can be found in the higher viscosity ofthe nitric solution than water.28 These results indicate that, forthe DIHEN, the analyte was introduced into the plasma underalmost the same conditions as for water and nitric acid.Therefore, the matrix effect produced in terms of aerosolgeneration and transport towards the plasma was negligible forthis device.

Another matrix studied was a 5000 mg ml21 sodium solution.As for nitric acid, tertiary aerosols were different depending on

the solution considered, being finer when sodium was presentas the predominant element. However, for the DIHEN, verysimilar primary aerosols were generated irrespective of thesolution considered. The statistical parameters confirm thisassessment. For the HEN-Cy, the D50 values were 1.7 and1.1 mm for water and sodium, respectively. This parameter tookvalues of 3.3 and 3.5 mm for water and sodium when theDIHEN was used.

As indicated in the previous section, the DIHEN led tosatisfactory ICP-AES analytical figures of merit at very lowliquid and gas flow rates. It has been shown that theinterferences produced by inorganic matrices in ICP-AES arereduced at low Qg and high rf power values.34 Therefore, lesssevere matrix effects for the DIHEN than for HEN-Cy areexpected. On the other hand, according to the work of Cliffordet al.,35 the interferences will be less pronounced for fineaerosols. As has been previously mentioned (Fig. 1) theaerosols injected into the plasma are coarser for theDIHEN. This fact was detrimental in terms of matrix effects.By taking into account these two factors, in the present work, acompromise situation between plasma thermal characteristicsand fineness of the aerosols generated by the DIHEN wasadopted and Qg was kept constant at 0.5 l min21.

In the present study, the extent to which the signal wasmodified by the action of the matrix was assessed by obtainingthe so-called relative SBR (SBRrel) value. Thus, the SBR wasmeasured for water and for a given matrix under the sameconditions. By dividing the former by the latter for a givenelement, the value of SBRrel was obtained. When this parameterreaches a value of unity, it can be said that the matrix effects arenegligible. Fig. 6 shows the variation in SBRrel with Ql for fivedifferent ionic lines and the two sample introduction systemstested. It can be observed that, for the HEN-Cy [Fig. 6(a)], thehigher the liquid flow rate, the higher the SBRrel. In otherwords, for all the lines tested, the matrix effect induced by a1 mol l21 nitric acid solution was more pronounced as theliquid flow rate decreased.36 At liquid flow rates above80 ml min21, the matrix effect was virtually eliminated.

Table 4 Relative SBR for the different lines and systems tested

Line

Relative SBRa LOD/ng ml21

Qg~0.14 l min21;Ql~30 ml min21

Optimum conditions(see Table 3)

Qg~0.14 l min21; Ql~30 ml min21 Optimum conditions (see Table 3)

DIHEN HEN-Cy DIHEN HEN-Cy

Ba II 18 6.7 0.14 2.4 0.12 3.4Cd II 1.8 1.6 1.5 3.5 1.3 2.1Cr II 2.5 2.1 2.7 8.6 1.2 6.0Mg II 4.2 2.5 0.50 2.2 0.44 1.2Mg I 7.5 3.9 1.02 8.4 0.73 11Mn II 3.5 2.4 1.9 8.8 2.9 10Ni II 3.4 2.3 1.7 8.3 2.3 16Zn I — 1.6 — —Zn II 1.4 1.3 4.2 6.4 2.5 8.2aRelative SBR~SBRDIHEN/SBRHEN-Cy.

Fig. 5 Volume DSD curves in band for the aerosols generated by theHEN-Cy (a) and the DIHEN (b). A, water; B, 3.6 mol l21 nitric acid; C,1.0 mol l21 nitric acid. Ql~90 ml min21; Qg~0.7 l min21.

Fig. 6 Effect of the liquid flow rate on the relative SBR for (a) the HENcoupled to the cyclonic spray chamber and (b) the DIHEN. Y, Ba(455.403 nm); 6, Mg (280.270 nm); $, Mn (257.610 nm); z, Sr(407.771 nm); &, Zn (202,548 nm). Matrix, nitric acid 1 mol l21.Qg~0.5 l min21.

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For the DIHEN the opposite trend was observed. Hence, theSBRrel values went down as the liquid flow rate increased. Thistrend has been previously observed for the DIN22 and severalreasons could be argued in order to explain this result, amongthem, the deterioration in the plasma thermal characteristicsand the increase in aerosol mean drop diameter observed as Ql

increased.Similar results were encountered for a concentrated nitric

acid solution. In this case, the matrix effects were almosteliminated by working at high liquid flow rates (w80 ml min21)with the cyclonic spray chamber and low Ql values with theDIHEN (30 ml min21). As expected, under the remainingconditions, the signal depression induced by 3.6 mol l21

nitric acid was more severe than that for dilute nitric acid.22,23

For a 5000 mg ml21 Na solution, the effect of Ql on SBRrel

also depended on the system. Fig. 8 shows the variation inSBRrel values with Ql for the two systems evaluated and fivedifferent ionic lines. For the HEN-Cy [Fig. 7(a)] the behaviorwas the same as that explained for nitric acid (i.e., the higherthe liquid flow rate the less severe the matrix effect).37 At liquidflow rates above 40–50 ml min21 the effect of sodium wasnegligible. This result is consistent with some previous studiesthat indicated that the Na matrix effects can be solved by usinga cyclonic spray chamber.38 The DIHEN seemed to lead to amore element- and liquid flow rate-dependent matrix effect[Fig. 7(b)]. Hence, at Qlv80 ml min21, sodium lowered thesignal more than water for lines having an Esum value higherthan 9 eV (e.g., Zn, Mg, Mn). In contrast, an increase in Ql

above this value produced a change in the matrix effect, thesignal being higher for sodium than for water. These effectswere more noticeable for lines with Esum values higher than9 eV. As can be observed in Fig. 7, although the matrix effectcan be eliminated for some elements, the DIHEN made theplasma more sensitive to the matrix effects produced by Na.This behavior corresponds to that of a non-robust plasma.39

Matrix effects for organic matrices

When using organic matrices, for the HEN-Cy, aerosols werefound to be finer for both 10% acetic acid and ethanol solutions[Fig. 8(a)] than for water. This fact can be explained by acombination of having primary aerosols finer than for waterand having an enhancement in the solvent evaporation insidethe spray chamber. For the DIHEN, the primary aerosols were

finer for ethanol than for water [Fig. 8(b)]. This expected resultwas mainly due to the reduction in the surface tension for the10% ethanol solution.40 The modification in the aerosolcharacteristics produced by ethanol with respect to waterwas more significant than that produced by acetic acid.

Fig. 9 shows the variation in SBRrel versus Ql for fivedifferent lines for a 10% (v/v) acetic acid solution as matrix. Asseveral studies have shown, when using a spray chamber, theamount of solution transported to the plasma is higher foracetic acid than for water.41 This fact, and the finer tertiaryaerosols generated in the presence of acetic acid (Fig. 8), shouldgive rise to an increase in the ICP-AES sensitivities.42 As can beseen in Fig. 9(a), the increase in the SBRrel value induced byacetic acid was more pronounced for lines with low Esum

values. This fact seemed to indicate that, under the plasmaconditions tested, the additional solvent load transported in thepresence of acetic acid produced a deterioration in the plasmaconditions that mainly affected the lines with high Esum values.

In the case of the DIHEN, the amount of analyte and solventinjected into the plasma for water and acetic acid was the same.Nevertheless, there was a decrease in the signal induced byacetic acid that was more remarkable for lines with high Esum

values [Fig. 9(b)]. This effect could be related to both the non-robust plasma used in the present study and the nature of thesolvent.

For ethanol, similar results as for acetic acid were found.These results are summarized in Fig. 10 for the HEN-Cy[Fig. 10(a)] and DIHEN [Fig. 10(b)]. Despite the fact that non-robust plasma conditions were selected, for the DIHEN, thematrix effects induced by organic species were only eliminatedfor lines with low Esum values and at low liquid flow rates.

Conclusions

It may be concluded that the DIHEN is an appropriate systemto analyze micro- and nano-samples by ICP-AES. Thisassessment is confirmed by the superior analytical figures ofmerit that are obtained when the DIHEN is used compared toanother system designed to work at very low liquid flow rates,such as the HEN-Cy. In addition, even for a plasma with a lowexcitation power, the interferences produced by nitric acid canbe mitigated when using the DIHEN at liquid flow rates below30–40 ml min21. Similar results have been previously reportedfor the DIN. For sodium, acetic acid and ethanol solutions, the

Fig. 7 Effect of the liquid flow rate on the relative SBR for (a) the HENcoupled to the cyclonic spray chamber and (b) the DIHEN. Y, Ba(455.403 nm); 6, Mg (280.270 nm); $, Mn (257.610 nm); z, Sr(407.771 nm); &, Zn (202.548 nm). Matrix, Na 5000 mg ml21.Qg~0.5 l min21.

Fig. 8 Volume DSD curves in band for the aerosols generated by theHEN-Cy (a) and the DIHEN (b). A, water; B, 10% acetic acid; C, 10%ethanol. Ql~90 ml min21; Qg~0.7 l min21.

Fig. 9 Effect of the liquid flow rate on the relative SBR for (a) theHEN-Cy and (b) the DIHEN. Y, Ba (455.403 nm); 6, Mg(280.270 nm); $, Mn (257.610 nm); z, Sr (407.771 nm); &, Zn(202.548 nm). Matrix, acetic acid 10% (v/v). Qg~0.5 l min21.

Fig. 10 Effect of the liquid flow rate on the relative SBR for (a) theHEN-Cy and (b) the DIHEN. Y, Ba (455.403 nm); 6, Mg(280.270 nm); $, Mn (257.610 nm); z, Sr (407.771 nm); &, Zn(202.548 nm). Matrix, ethanol 10% (v/v). Qg~0.5 l min21.

J. Anal. At. Spectrom., 2001, 16, 514–520 519

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matrix effects are eliminated only for lines with low Esum

values. In these cases, it would be advisable to selectinstrumental conditions leading to a robust plasma.

Because the matrix effects are element dependent, there isclear evidence that the phenomenon is related to a change inplasma conditions. Hence, when working with a non-robustplasma, a change in the source of matrix effects was observed(i.e., an aerosol transport problem appeared when using aspray chamber and a plasma deterioration problem occurredwhen using the DIHEN).

Nonetheless, the use of a DIHEN still has some limitations,because it is prone to tip blocking, although a new version ofthe DIHEN for use with high salt content solutions and slurrieshas been recently evaluated by Montaser and co-workers,23,43

(i.e., large bore direct injection high efficiency nebulizer, LB-DIHEN). With the DIHEN, it is advisable to work at very lowgas flow rates (v0.3 l min21), however the DIHEN is theneasily deteriorated by the plasma, because there is a reductionin the nebulizer cooling effect. On the other hand, whenworking with a plasma under non-robust conditions, thesolvent load produces serious plasma deterioration problems.Direct injection nebulization is clearly the way forward,provided that the plasma operating conditions are carefullyselected in order to get the best results when the DIHEN isemployed. Optimum conditions would require high power(w1.3 kW), which is not always possible to obtain on a routinebasis. Note that high power can also produce torch andnebulizer damage.

A suitable alternative would be the design of a directinjection nebulizer with a total aerosol production in the range20–40 mg min21, i.e., in the range obtained by conventionalpneumatic nebulizers. The total aerosol consumption wouldminimize the matrix effects arising from the spray chamber,while robust conditions could be obtained with a reasonablepower, thus minimizing changes in the plasma conditions. Asrecent results have indicated, the change in the analyte spatialdistribution is certainly a parameter to be considered,44

although recently it has been shown that the aerosol beamproduced by a DIHEN exhibits a lower divergence than aconventional concentric nebulizer.45

Acknowledgements

The authors are grateful to Professor Montaser (Department ofChemistry, George Washington University) and MeinhardAssociates for providing the DIHEN employed in the presentstudy.

References

1 J. Sneddon, in Sample Introduction in Atomic Spectroscopy,Elsevier, New York, 1990.

2 A. Montaser, M. G. Minnich, J. A. McLean, H. Liu, J. A. Carusoand C. W. McLeod, in Inductively Coupled Plasma MassSpectrometry, ed. A. Montaser, Wiley-VCH, New York, 1998.

3 J. W. Olesik, J. A. Kinzer and B. Harkleroad, Anal. Chem., 1994,66, 2022.

4 H. Liu and A. Montaser, Anal. Chem., 1994, 66, 3233.5 F. Vanhaecke, M. Van Holderbeke, L. Moens and R. Dams,

J. Anal. At. Spectrom., 1996, 11, 543.6 J. M. Mermet, J. Anal. At. Spectrom., 1998, 13, 419.7 L. Ebdon and R. Collier, Spectrochim. Acta, Part B, 1988, 43, 355.8 M. Hoenig, H. Baeten, S. Vanhentenrijk, G. Ploegaerts and

T. Bertholet, Analusis, 1997, 25, 13.

9 S. Maestre, J. Mora, J. L. Todolı and A. Canals, J. Anal. At.Spectrom., 1999, 14, 61.

10 J. L. Todolı, S. E. Maestre, J. Mora, A. Canals and V. Hernandis,Fresenius’ J. Anal. Chem., 2000, 368, 773.

11 G. A. Meyer, Intra Micro-spray ICP Torch, US Pat.,No. 4 990 740, Feb. 5, 1991.

12 D. R. Wiederin, F. G. Smith and R. S. Houk, Anal. Chem., 1991,63, 219.

13 K. E. Lawrence, G. W. Rice and V. A. Fassel, Anal. Chem., 1984,56, 289.

14 D. R. Wiederin and R. S. Houk, Appl. Spectrosc., 1991, 45, 1408.15 S. C. K. Shum, S. K. Johnson, H. Pang and R. S. Houk, Appl.

Spectrosc., 1993, 47, 575.16 J. A. McLean, H. Zhang and A. Montaser, Anal. Chem., 1998, 70,

1012.17 V. Majidi, J. Qvarnstrom, Q. Tu, W. Frech and Y. Thomassen,

J. Anal. At. Spectrom., 1999, 14, 1933.18 J. S. Becker, H.-J. Dietze, J. A. McLean and A. Montaser, Anal.

Chem, 1999, 71, 3077.19 J. S. Becker and H.-J. Dietze, J. Anal. At. Spectrom., 1999, 14,

1493.20 J. Singh, J. A. McLean, D. E. Pritchard, A. Montaser and

S. R. Patierno, Toxicol. Sci., 1998, 46, 260.21 J. Singh, D. E. Pritchard, D. L. Carlisle, J. A. McLean,

A. Montaser, J. M. Orenstein and S. R. Patierno, Toxicol. Appl.Pharmacol., 1999, 161, 240.

22 J. L. Todolı and J. M. Mermet, J. Anal. At. Spectrom., 1998, 13,727.

23 B. W. Acon, J. A. McLean and A. Montaser, Anal. Chem., 2000, 8,1885.

24 A. Canals, V. Hernandis and R. F. Browner, J. Anal. At.Spectrom., 1990, 5, 61.

25 J. L. Todolı, A. Canals and V. Hernandis, Spectrochim. Acta, PartB, 1993, 48, 373.

26 J. L. Todolı, V. Hernandis, A. Canals and J. M. Mermet, J. Anal.At. Spectrom., 1999, 14, 1289.

27 B. L. Sharp, J. Anal. At. Spectrom., 1988, 3, 939.28 A. Canals, V. Hernandis, J. L. Todolı and R. F. Browner,

Spectrochim. Acta, Part B, 1995, 50, 305.29 J. M. Mermet, in Inductively Coupled Plasma Emission Spectro-

scopy, ed. P. W. J. M. Boumans, John Wiley and Sons, New York,1987, ch. 10.

30 J. M. Mermet, Spectrochim. Acta, Part B, 1989, 44, 1109.31 A. Fernandez, M. Murillo, N. Carrion and J. M. Mermet, J. Anal.

At. Spectrom., 1994, 9, 217.32 J. L. Todolı and J. M. Mermet, J. Anal. At. Spectrom., 2000, 15,

863.33 Q. Xu, G. Mattu and G. R. Agnes, Appl. Spectrosc., 1999, 8, 965.34 J. L. Todolı and J. M. Mermet, Spectrochim. Acta, Part B, 1999,

54, 895.35 R. H. Clifford, P. Sohal, H. Liu and A. Montaser, Spectrochim.

Acta, Part B, 1992, 47, 1107.36 J. L. Todolı, J. M. Mermet, A. Canals and V. Hernandis, J. Anal.

At. Spectrom., 1998, 13, 55.37 M. Catasus, L. Gras, J. L. Todolı and V. Hernandis, J. Anal. At.

Spectrom., 2000, 15, 1203.38 C. Dubuisson, E. Poussel, J. L. Todolı and J. M. Mermet,

Spectrochim. Acta, Part B, 1998, 53, 593.39 C. Dubuisson, E. Poussel and J. M. Mermet, J. Anal. At.

Spectrom., 1997, 12, 281.40 J. Mora, V. Hernandis and A. Canals, J. Anal. At. Spectrom.,

1991, 6, 573.41 C. Dubuisson, E. Poussel, J. M. Mermet and J. L. Todolı, J. Anal.

At. Spectrom., 1998, 13, 63.42 S. Greenfield, M-McD. McGeachin and P. B. Smith, Anal. Chim.

Acta, 1976, 84, 67.43 J. A. McLean, B. W. Acon, A. Montaser, J. Singh, D. E. Pritchard

and S. R. Patierno, Appl. Spectrosc., 2000, 54, 659.44 E. Bjorn and W. Frech, J. Anal. At. Spectrom., 2001, 16, 4.45 A. Montaser, Lecture O-23, Presented at European Winter

Conference on Plasma Spectrochemistry, Lillehammer, Norway,2001.

520 J. Anal. At. Spectrom., 2001, 16, 514–520

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