Investigation of a medium power radiofrequency capacitively

coupled plasma and its application to high-temperature

superconductor analysis via atomic emission spectrometryw

Alpar Simon,*a Tiberiu Frentiu,b Sorin Dan Anghela and Simion Simona

a Faculty of Physics, Babes-Bolyai University, M Kogalniceanu 1, 400084 Cluj-Napoca,Romania. E-mail: asimon@phys.ubbcluj.ro; anghels@phys.ubbcluj.ro; simons@phys.ubbcluj.ro

b Faculty of Chemistry, Babes-Bolyai University, J Arany 11, 400028 Cluj-Napoca, Romania.E-mail: ftibi@chem.ubbcluj.ro

Received 1st March 2005, Accepted 20th April 2005First published as an Advance Article on the web 17th May 2005

A medium power radiofrequency capacitively coupled plasma (275 W, 27.12 MHz) with low argonconsumption (0.4 l min�1) was investigated to be used for the analysis of high critical temperature Bi–Sr–Ca–Cu–O and Bi(Pb)–Sr–Ca–Cu–O superconducting materials by atomic emission spectrometry. The plasmatorch was operated in two geometric configurations, coaxial and coaxial–annular, with single (SRT-rf-CCP)or double ring electrodes (DRT-rf-CCP), respectively. The optimum experimental parameters (observationheight, distance between ring electrodes and coupling geometry) and matrix interference of Ca and Sr wereestablished. Under optimum operating conditions of the DRT-rf-CCP torch detection limits of 2.5–3 ng ml�1

for Ca, 4.5–5 ng ml�1 for Sr, 11–16.6 ng ml�1 for Cu, 56–95 ng ml�1 for Pb and 195–545 ng ml�1 for Biwere found in the presence of Ca and Sr in the range of 0–500 mg ml�1. Figures of merit of the investigatedplasma torch were compared with those of ICP-AES. Each of the relevant metals from the superconductingmaterials could be determined successfully (the recovery levels were in the range from 97.7 � 3.3% for Ca to103.8 � 3.8% for Pb and the precision between 0.8 and 3.6%). Therefore, a good agreement, based on at-test and a two-tailed F-test, was found between the DRT-rf-CCP-AES results and those obtained byICP-AES and given by the supplier. Overall precision of the stoichiometric coefficients by DRT-rf-CCP-AESwas between 2.25–2.55%. The real stoichiometry of the superconductors obtained by DRT-rf-CCP-AES wasalso confirmed by the results of X-ray diffraction measurements.

Introduction

The discovery of high temperature superconducting materialshas attracted a lot of interest from two major points of view:manufacturing and applications. Both Y-based (YBa2Cu3O7�d)and Bi-based (Bi2�xPbxSr2Cay�1CuyO101d, where y ¼ 1, 2, 3and 0 o x o 1) materials are generally accepted as the mostpromising high temperature superconductors for technicalapplications.1–4 The purity of the material could criticallyinfluence the structure, the superconducting properties andthe further applications. For this purpose complex analyticalinvestigations of the precursor materials as well as the phasediagrams, phase stability, determination of the major metallicelements (stoichiometric composition), determination of oxy-gen and oxidation states for Bi, Pb and Cu and trace analysis ofcontaminants should be employed. For many years, the classi-cal chemical methods of analysis based on wet procedures,such as complexometric and iodimetric titrations with andwithout photometric, potentiometric or coulometric equiva-lence point detection, have been used for the determination ofthe chemical and stoichiometric composition of the super-conducting materials.5–10 Although they offer a sufficient pre-cision and accuracy, unfortunately the classical methodsinvolve very complicated separation schemes for analyte ormasking concomitants, and they are very time- and reagents-consuming.

As a consequence more rapid and sensitive spectrometricprocedures, especially based on inductively coupled plasma

atomic emission spectrometry (ICP-AES) using scanning spec-trometers and charge coupled device (CCD) or charge injectiondevice (CID) multichannel spectrometers are required.9–13 It isknown that the quality of the superconducting materials isdependent on the precursor’s purity, but its determination canonly be achieved by using the ICP-AES method.14 Therefore,ICP has been found to be a very robust and powerful excitationsource for many applications in the product control, but it istoo expensive to buy and maintain for some special applica-tions, where smaller and cheaper spectral sources could beused. Besides innovation in the area of the ultraviolet–visibledetectors and sample introduction systems, the development ofminiaturized plasma sources, which offer inexpensive opticalinstruments and a favourable cost-to-performance ratio, suchas low power and gas consumption, the radiofrequency capa-citively coupled plasma (rf-CCP) and the microwave plasmatorch (MPT) should be mentioned.15–21 The rf-CCPs can bedeveloped by their connection with different sample introduc-tion systems using three electrode geometries (annular, paral-lel-plate and coaxial) which enable their development atatmospheric pressure at low and medium powers (1–600 W)and low argon or helium consumption, usually below 1 lmin�1. They have been used for the analysis of liquid, con-ducting and non-conducting solid samples and as a specificdetector in gas chromatography.22–30 At the same time thefigures of merits for some elements are similar to thoseobtained with other plasma sources, such as microwave capa-citively coupled plasma atomic emission spectrometry (CMP-AES).23,31,32 In our laboratory a low power rf-CCP (60 W and185 W) has been used for the direct analysis of solid sam-ples.28,29 Also, another rf-CCP in the coaxial configuration

w Presented at the 2005 European Winter Conference on PlasmaSpectrochemistry, January 30–February 3, 2005, Budapest, Hungary.

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with a single annular electrode (SRT-rf-CCP) and coaxial–annular with double ring electrodes (DRT-rf-CCP), operatedat 27.12 MHz, 275 W and 0.4 l min�1, was used for the analysisof pneumatically nebulized liquid samples by AES.33 Thesubsequent results have emphasized that this spectral sourceensures a good precision, accuracy and sensitivity in thedetermination of different elements in complex matrices fromenvironmental and biological samples pneumatically nebulizedusing sequential and simultaneous atomic emission spectro-meters.34–38

It was the aim of this work to investigate whether the SRT-rf-CCP-AES and DRT-rf-CCP-AES could be used in thequality control (QC) of superconducting materials with anadequate precision and accuracy. The accuracy and the preci-sion of the method were verified through the analysis ofsuperconducting materials (Bi–Sr–Ca–Cu–O and Bi(Pb)–Sr–Ca–Cu–O) supplied by Aldrich. The average contents of majorcomponents determined with our rf-CCP were compared bymeans of the t-test and two-tailed F-test39 with those obtainedby two ICP-AES spectrometers. The results were also com-pared with the composition given by the supplier. Calculatedstoichiometric coefficients and their standard deviation ob-tained by DRT-rf-CCP-AES were statistically compared withthe theoretical stoichiometric coefficients, the coefficients cal-culated using the composition given by the supplier and withthose obtained by ICP-AES. The stoichiometric compositionof the studied materials was confirmed by the phase composi-tions found using X-ray diffraction (XRD) and by ac suscept-ibility. In order to obtain the lowest detection limits of themajor components from the superconducting materials theoperating conditions of rf-CCP were optimized in respect ofthe coupling geometry, distance between ring electrodes andobservation heights. The true detection limits were establishedby studies of the Ca and Sr influence on the emission of thecomponents from the superconducting materials, and carriedout as a function of their concentration and the distancebetween ring electrodes.

Experimental

Instrumentation

Details of the construction of the SRT-rf-CCP and DRT-rf-CCP used in this study have previously appeared in theliterature.33 The experimental setup is presented in Fig. 1,while the experimental conditions of the spectrometric system,manufactured at Research Institute for Analytical Instrumen-tation, Cluj-Napoca, Romania, are shown in Table 1.

The relevant operating conditions for the SPECTRO-FLAME ICP-AES scanning spectrometer (Spectro AnalyticalInstruments, Kleve, Germany) and the SPECTRO CIROSCCD

ICP-AES multichannel spectrometer with axial plasma viewing(Spectro) also used in this study are given in Table 2.The ac susceptibility measurements were performed with the

Oxford Instruments MagLab System 2000 (frequency: 1 kHz,magnetic field: 1 Oe), while the XRD patterns were obtainedusing a Dron2 system using the polycrystal method with Cu Ka

radiation and Ni b-filter.

Reagents and superconducting sample digestion

All the reagents used in this study were of analytical grade(Fluka). Stock solutions of 1000 mg ml�1 of Pb and Cu wereobtained by dissolving high purity metallic powders in HNO3

(1 þ 1), the Bi stock solution by dissolving Bi granules inHNO3 (1þ 1), the stock solution of Sr by dissolving Sr(NO3)2 �4H2O in 2% v/v HNO3, and the Ca stock solution wasobtained from CaCO3 digested in HNO3 (1 þ 4). All the stocksolutions were diluted to 1 l with 2% v/v HNO3 solution.For the study of the detection limits and non-spectral matrix

effects in rf-CCP-AES, monoelement solutions were preparedfor each analyte in the absence and presence of differentconcentration of Sr or Ca up to 500 mg ml�1.The superconducting materials (Aldrich) used in our study

were Bi-based: 2212 type Bi2.0Sr2.0Ca1.0Cu2.0O81x, (x ¼ 0.15–0.20, Product number: 365106, Lot number: 04803CR, CAS:114901-61-0) and 2223 type, Bi1.6Pb0.4Sr1.6Ca2Cu2.8O9.21x (x¼0.45, Product number: 378720, Lot number: 04123JY, CAS:116739-98-1).About 100 mg of each sample was weighed out, dissolved in

5 ml of 7 mol l�1 HNO3 by gentle heating and oxidized with afew drops of H2O2. After cooling, the solutions were filled upto 100 ml with 2% v/v HNO3. From these solutions thecontents of Pb, Bi, Ca, Sr and Cu were determined by usingthe standard addition method in rf-CCP-AES and ICP-AES,respectively. A solution of 5 ml of 7 mol l�1 HNO3 diluted to100 ml with 2% v/v HNO3 was used as blank.

Validation of results obtained by rf-CCP-AES

The accuracy for the determination of Bi, Pb, Ca, Sr and Cu byrf-CCP-AES was assessed by a comparison of the mean valuesof the concentrations with both those given by Aldrich andthose determined by ICP-AES, using the t-test for a 95%confidence level. The precision of the measurements by rf-CCP-AES was verified by comparison of the standard devia-tions with those obtained by ICP-AES using the two-tailedF-test.39

Using the average contents, the stoichiometric coefficientsand standard deviations were calculated for the superconduct-ing materials. The overall precision for the determination ofstoichiometric coefficients was calculated to be the ratio be-tween the square root of the sum of the determined variancesand the sum of the stoichiometric coefficients obtained.In addition, figures of merit for rf-CCP-AES such as instru-

mental and true detection limits, as well as the non-spectralmatrix effect of Ca and Sr on the emission of other elementsfrom the superconductor, were evaluated. The non-spectralmatrix effect (expressed as %) was calculated according to thewell-known formula:

100� I � I0

I0

where I and I0 are the analyte emission intensity in the presenceof the matrix and in its absence, respectively. Instrumentaldetection limits, which offer useful information about theperformance of the instrumentation, were determined usingsamples containing only analytes (Bi, Pb, Ca, Sr, and Cu) atFig. 1 The rfCCP-AES experimental set-up.

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low concentrations from the signal-to-background ratio andthe relative standard deviation of the background using the 3scriteria.

The true detection limit is a figure of merit of real sampleanalysis and includes matrix influence on the instrumentaldetection limit. Thus, in this paper, the true detection limitsof Pb, Bi and Cu were determined in the presence of differentCa or Sr concentrations. The reciprocal influence of Ca and Srwas also estimated from their true detection limits using binarysolutions of the elements in question.

The instrumental detection limits obtained viaDRT-rf-CCP-AES were compared with those obtained with ICP-AES.

Results and discussion

Optimization of the instrumental parameters for the

rf-CCP-AES

In order to establish the operating conditions for rf-CCP-AESwe determined the optimum plasma observation heights andthe optimum distance between electrodes for which the lowestdetection limits are achieved in both presence and absence ofCa and Sr matrices. Therefore, the background and the emis-sion signals of elements were measured for concentrations of 10mg ml�1 Cu, Pb and Bi in the presence and absence of 50 mg

ml�1 Ca or Sr as concomitants. In the same conditions theemission signals of 10 mg ml�1 Ca in the absence and presenceof 50 mg ml�1 Sr and the converse were also measured. Thus,the evaluation of non-spectral matrix effects of alkaline-earthelements as a function of observation height, coupling geome-try and distance between ring electrodes were studied.All the determinations were carried out at 275 W rf power

level and 0.4 l min�1 Ar flow-rate, established during previousresearch.33

Observation height and the plasma torch geometry

A typical dependence of the instrumental detection limits forBi, Pb, Sr, Ca and Cu as a function of observation height forSRT-rf-CCP-AES and DRT-rf-CCP-AES (distance betweenring electrodes H ¼ 6 cm) is presented in Fig. 2(a) and 2(b).The dependence of instrumental detection limits for the two

rfCCP torches as a function of the observation height isinfluenced by two major factors: the stability and the magni-tude of the emission signal. The detection limits reach minimaat heights where both parameters in question are maxima.According to Fig. 2(a) and 2(b) detection limits are improvedin the presence of the upper ring electrode in the case of DRT-rf-CCP-AES as compared with SRT-rf-CCP-AES. This fact is

Table 1 Experimental conditions and constructive details of the rf-CCP-AES spectrometric system

Plasma rf power supply Free running Colpitts type radiofrequency oscillator 27.12 MHz, 275 W

Plasma torch and support gas Capacitively coupled, developed inside a quartz tube (16 mm id, 16 cm length) and radial viewing at

different heights above the tubular electrode, operated in two configurations:

� coaxial with tubular Mo electrode (3.5 mm id) and brass ring electrode at 5 mm above the tubular

electrode (SRT-rf-CCP)

� coaxial–annular with Mo tubular electrode and double brass ring electrodes (DRT-rf-CCP) distanced in

the range of 4–12 cm

5.0 quality Ar (Gas SRL Cluj-Napoca, Romania), flow-rate 0.4 l min�1

Tesla coil for plasma ignition

Sample introduction system Concentric pneumatic glass nebulizer equipped with two channel peristaltic pump and double passing Scott

spray chamber (120 cm3)

Aerosol intake into the core of the plasma through the tubular electrode (1 ml min�1, 5% nebulization

efficiency)

Optics 190–800 nm scanning monochromator (0.001 nm increment), 1 m Czerny–Turner mounting, grating

constant

2400 grooves mm�1, blazed at 330 nm, slit width 20 mm, 32 � 1 1C thermostated, f ¼ 110 mm focusing lens,

range for wavelengths 0.2 nm

Detector EMI 9781R photomultiplier supplied at 950 V (Thorn EMI, Ruislip, Middlesex, UK)

Driving, data acquisition and

processing

PC, laboratory constructed interface, data acquisition time 64 ms, house soft, three successive measurements

for each parallel sample

Table 2 ICP-AES operating conditions

SPECTROFLAME SPECTRO CIROSCCD

rf generator rf power: 1200 W rf power: 1400 W

Frequency: 27.12 MHz Frequency: 27.12 MHz

Plasma torch Radial viewing at 15 mm height,

through optical fibres

Axial viewing, torch position: x ¼ �3.9 mm,

y ¼ þ3.6 mm, z ¼ þ2.5 mm

Outer gas flow 12 l min�1 12 l min�1

Intermediate gas flow 0.6 l min�1 0.6 l min�1

Nebulizer gas flow 1 l min�1 1 l min�1

Sample introduction Concentric glass nebulizer, Meinhardt type K

(TR-30-K3), double pass Scott type

spray chamber

K2 cross flow nebulizer, double pass Scott type

spray chamber

Sample uptake rate 2 ml min�1 2 ml min�1

Flushing time and time delay 40 s and 20 s, respectively 40 s and 20 s, respectively

Optics 160–800 nm double scanning monochromator 160–800 nm, double grating, Paschen–Runge

multichannel mount

Detector EMI 9781R (Thorn EMI, Ruislip,

Middlesex, UK) supplied at 1000 V

22 CCD detectors

Data processing Smart Analyzer software, integration

time 10 s, three successive measurements

for each parallel sample

Smart Analyzer software, best SNR strategy, integration

time 45 s, three successive measurements for

each parallel sample

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due mainly to the increase of emission signal caused by theincrease in sample species atomization and excitation.

Similar dependences of the detection limits were observedfor Bi, Pb, Sr and Cu in the presence of 50 mg ml�1 Ca and forBi, Pb, Ca and Cu in the presence of 50 mg ml�1 Sr, respec-tively. In the presence of these two matrices, the lowestdetection limits are obtained at lower observation heights thenthose obtained in their absence. This experimental observationsuggests the idea that matrices are influencing the atomization,ionization equilibrium and excitation of the analyte atoms inplasma.

In Fig. 3(a) the effect of 50 mg ml�1 Ca on the emission of 10mg ml�1 Bi, Pb, Sr and Cu is illustrated, while in Fig. 3(b) theeffect of 50 mg ml�1 Sr on the emission of the same concentra-tions of Bi, Pb, Ca and Cu, as a function of distance betweenring electrodes for DRT-rf-CCP, is shown.

The non-spectral matrix effects were calculated using themaximum emission signals of the analyte in both the presenceand absence of the interfering matrix. Data presented in Fig.3(a) and 3(b) show the possibility of the elimination or thedecrease of the depressive non-spectral matrix effects of 50 mgml�1 Ca or Sr on the emission signals in DRT-rf-CCP by theapproach of the ring electrodes. Thus, the effect of Sr on Ca(Fig. 3(b), curve D) becomes an increasing one for distancesbetween ring electrodes smaller than 9 cm. The emissionmaximum is reached for Ca at an observation height of14 mm in the absence of the Sr matrix and of 12 mm whenthe matrix is present. Similarly, for the Sr emission in the

presence of Ca matrix (Fig. 3(a) curve B), the effect starts toincrease for ring distances smaller than 9 cm. The Sr emissionmaximum is at 12 mm in both the absence and presence of theCa interferent. For Cu (curves E), which has an emissionmaximum at 14 mm above the Mo tube, in the absence ofthe Ca and Sr matrices and at 12 mm in their presence, the non-spectral matrix effects become increasingly large for ring dis-tances smaller than 9–10 cm. For Pb (curves A) with itsemission maximum at 10 mm, the effects increase around 6–7cm distance between rings. For Bi (curves C) which hasmaximum emissions at the lowest heights (4 mm with nomatrices and 2 mm in the presence of them) the depressiveeffect of 50 mg ml�1 Ca or Sr is diminished by the approach ofthe ring electrodes at a distance lower than 6 cm.The depressive effect of the Ca and Sr matrices on the

studied emission signals at higher ring distances could beexplained by the simultaneous vaporization, atomization, io-nization and excitation of the matrix and the analyte. If it isassumed that the dissipated rf power is constant for a parti-cular distance between the rings, there is a loss of power inorder to induce the dissociation and excitation of the matrix tothe detriment of the analyte. The experimental data are show-ing that the approach of the ring electrodes caused an increasein the analyte emission signal and its maximum values areachieved at lower observation heights, both in the absence and

Fig. 2 The dependence of the instrumental detection limits for Pb I405.781 nm (A), Sr I 460.733 nm (B), Bi I 472.252 nm (C), Ca I 422.673nm (D) and Cu I 324.754 nm (E) as a function of observation height forSRT-rf-CCP-AES (a) and DRT-rf-CCP-AES (b) forH¼ 6 cm distancebetween ring electrodes. For Bi detection limits has to be multiplied bya factor of 10.

Fig. 3 The dependence of the non-spectral matrix effect of 50 mg ml�1

Ca I 422.673 nm, (a) on the emission of Pb I 405.781 nm (A), Sr I460.733 nm (B), Bi I 472.252 nm (C) and Cu I 324.754 nm (E) and 50 mgml�1 Sr I 460.733 nm, and (b) on the emission of Pb I 405.781 nm (A),Bi I 472.252 nm (C), Ca I 422.673 nm (D) and Cu I 324.754 nm (E) as afunction of the distance between rings for DRT-rf-CCP-AES.

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presence of the interfering matrix, as compared with higherring distances.

In conclusion, one can say that the presence of the upper ringelectrode closer to lower one will enhance the atomization andexcitation capabilities of the plasma due to the increaseddissipated plasma power. Thus, due to the increase of usefulenergy in the plasma, the matrix volatilization will not nega-tively affect the atomization and excitation of the analyte. Fora given element, the minimum distance between ring electrodesfor which the non-spectral matrix effect becomes an increasingone is as low as the height where the maximum emissionintensity is reached, is decreasing. Also, as the excitationenergy of the element becmes higher, the distance betweenthe ring electrodes for which the Ca and Sr non-spectral matrixeffect becomes an increasing one must to be lower.

For the DRT-rf-CCP-AES the detection limits were studiedas a function of the distance between the ring electrodes in boththe presence and absence of 50 mg ml�1 Ca and Sr. Fig. 4illustrates as examples the case of Cu (a) and Pb (b).As is shown in Fig. 4(a) and 4(b) the improvement of the

detection limits and the elimination of depressive non-spectralmatrix effects of Ca and Sr on the detection power of Cu andPb in DRT-rf-CCP-AES, by the approach of the ring electro-des (lower than 7 cm), could be observed. Similarly, thedetection limits are substantially improved if the distancebetween the ring electrodes is lower than 9 cm for Ca, 8 cmfor Sr and 7 cm for Bi. Taking into consideration theseexperimental findings, the optimum operating geometry, forwhich the lowest detection limits are obtained for all thestudied elements, is the DRT-rf-CCP with 6 cm between thering electrodes.

Line selection and instrumental detection limits

The instrumental detection limits obtained for the SRT-rf-CCP-AES and DRT-rf-CCP-AES in optimum operating con-ditions, according to the 3s criterion, as compared with thoseobtained with ICP-AES analysis using SPECTROFLAMEand SPECTRO CIROSCCD systems, are presented in Tables3 and 4.Spectral lines for the rf-CCP-AES and SPECTROFLAME

systems were selected on the basis of highest sensitivity andabsence of spectral interference. In the case of the rf-CCP-AESthe resonance lines of the elements yield the highest sensitivityand stability for this spectral source. For the analysis with theSPECTRO CIROSCCD system the possibility of simultaneousdetermination was the major selection criterion, in addition tothe high sensitivity.One of the first things that can be seen in Table 3 is the

improvement of the emission signal and detection limits by1.5–6 times and 1.4–3.2 times, respectively, when the DRT-rf-CCP is used instead of SRT-rf-CCP. Also, the enhancement ofthe emission signals and detection limits are more notable forthe elements with the highest dissociation energy of oxides(Edissoc) and with the lowest excitation energy (Eexc). As anexample, in the case of Ca, which has a dissociation energy ofthe refractory oxides of 5.00 eV and an excitation energy of2.93 eV, the increase in the emission signal is by a factor of 6.5much higher than that of Bi (by 1.9) and Pb (by 1.5): these formless refractory oxides but have a higher excitation energy. If wecompare elements with similar dissociation energies, but dif-ferent excitation energies (Sr–Cu and Bi–Pb, respectively) itcan be observed that there is a higher increase for the elementwith lower excitation energy (Bi and Sr) as compared with theelements with higher excitation energy (Pb and Cu). Also, thedata presented emphasize the fact that the maximum emissionintensity appears at lower heights in DRT-rf-CCP as comparedwith SRT-rf-CCP. For the elements forming refractory oxides(Ca, Sr and Cu) the emission signal has a maximum at higherobservation heights for both SRT-rf-CCP and DRT-rf-CCP ascompared with the volatile elements (Bi and Pb). In the case of

Fig. 4 The dependence of detection limits for Cu I 324.754 nm (a) andPb 405.781 nm (b) in the absence (A) and presence of 50 mg ml�1 Ca I422.673 nm (B) and Sr I 460.733 nm (C) as function of distancebetween rings for DRT-rf-CCP-AES.

Table 3 Instrumental detection limits (LOD) for Bi, Pb, Sr, Ca and Cu for SRT-rf-CCP-AES and DRT-rf-CCP-AES (H ¼ 6 cm) torches for

optimum observation heights (h)

SRT DRT

Element

line Wavelength/nm Edissoc/eV Eexc/eV Eioniz/eV h/mm LOD/ng ml�1 h/mm LOD/ng ml�1Signal ratio

DRT/SRT

LOD improvement

DRT/SRT

Ca I 422.673 5.00 2.93 6.11 22 9 14 3 6.5 3.0

Sr I 460.733 4.85 2.69 5.69 18 16 12 5 4.7 3.2

Cu I 324.754 4.90 3.82 7.72 18 25 14 14 2.2 1.8

Pb I 405.781 4.10 4.38 7.42 14 110 10 80 1.5 1.4

Bi I 472.252 4.00 4.04 7.29 8 870 2 470 1.9 1.8

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Bi, which is a highly volatile element, the emission signalmaximum is reached at the lowest observation heights.

According to the detection limits in both SRT-rf-CCP andDRT-rf-CCP (Table 3), these plasmas would be suitable forprecursor and superconducting materials analysis, but in ouropinion the detection limit for Bi is not sufficiently low in thecase of SRT-rf-CCP-AES for such analyses. Therefore, theDRT-rf-CCP was selected for comparison with ICP-AES inthe analysis of superconducting materials.

According to Table 4 the detection limits obtained with theSPECTROFLAME system operated at 1200 W and for theSPECTRO CIROSCCD operated at 1400 W are between 3 ngml�1 for Ca and 100 ng ml�1 for Bi. Some differences betweenthe detection limits obtained with the two ICP systems are dueto the intensities of the investigated spectral lines on the onehand, and on the other hand to the difference between thesensitivities of the photomultiplier tube and the CCD detec-tors. Generally, the performance of the two devices is similar,even if for some elements the SPECTRO CIROSCCD systemyields higher detection limits as compared with the SPECTRO-FLAME, but due to the real simultaneous character of thespectrometer it allows a full recording of the spectrum to bemeasured in 3 s, hence a higher speed sample throughput canbe performed and a decrease in cost per analysis can beachieved.

In the case of the DRT-rf-CCP-AES, detection limits arebetween 3 ng ml�1 for Ca and 470 ng ml�1 for Bi. As compared

with the ICP systems, the detection limits obtained are 1–4.7times higher than those obtained with SPECTROFLAME andSPECTRO CIROSCCD. One can observe that the detectionlimits obtained for alkaline earths with the DRT-rf-CCP-AESare similar to those obtained with ICP-AES. The higherdetection limits obtained for the elements with higher excita-tion energy levels could be caused by the almost 4 times higherdissipated rf plasma power in the case of the ICP (1200–1400W instead of 275 W for rf-CCP). However, the approach usingour rf-CCP is attractive due to the favourable cost of the rfpower generator, low Ar consumption, low spectral back-ground, high stability of discharge, versatility of geometryoperation and good analytical capacity in complex matriceswithout any restriction for a broad range of metals. Thenegative influence of the solvents on the stability of low andmedium power plasmas is well known. In contrast with this,our plasma accepts a reasonable amount of sample introducedby pneumatic nebulization without any drying and facilitatesthe obtaining of a stable discharge in these conditions.

Non-spectral matrix effects as function of concentration in

DRT-rf-CCP-AES. True detection limits

The non-spectral matrix effects of Ca and Sr for concentrationsup to 500 mg ml�1 in the DRT-rf-CCP-AES, for a distancebetween ring electrodes of H ¼ 6 cm and optimum observationheights, are presented in Fig. 5(a)–(d). It is quite difficult to

Table 4 Instrumental detection limits obtained by ICP-AES in comparison with DRT-rf-CCP-AES

Detection limit ratio

Element

line Wavelength/nm SPECTROFLAME/ng ml�1SPECTRO

CIROSCCD/ng ml�1DRT-rf-CCP-AES to

SPECTROFLAME

DRT-rf-CCP-AES to

SPECTRO CIROSCCD

Ca II 396.847 3 3 1.0 1.0

Sr II 338.071 4 1.2

Sr II 421.552 5 1.0

Cu I 324.754 3 5 4.6 2.8

Pb I 283.305 40 2.0

Pb I 220.353 50 1.6

Bi I 306.772 100 4.7

Bi I 190.241 100 4.7

Fig. 5 Non-spectral matrix effects of Ca I 422.673 nm (A) and Sr I 460.733 nm (B) up to 500 mg ml�1 on the emission of Sr I 460.733 nm and Ca I422.673 nm (a), Cu I 324.754 nm (b), Pb I 405.781 nm (c) and Bi 472.252 nm (d) in DRT-rf-CCP-AES for H ¼ 6 cm.

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explain the dependence of the non-spectral matrix effect con-sidering only the thermodynamic characteristics of the matrixor of the analyte.

The shape of the curves for non-spectral matrix effectssuggests a complex dependence on the nature and concentra-tion of the matrix as well as the characteristics of the analyte.Usually, the non-spectral matrix effects are increasing with amaximum at 200 mg ml�1 of interferent for Cu (Fig. 5(b)) andPb (Fig. 5(c)) and with a maximum at 400 mg ml�1 in the caseof Bi (Fig. 5(d)). The enhancing effect of Sr on Ca has amaximum at 300 mg ml�1 (Fig. 5(a), curve B) while the effect ofCa on Sr at 400 mg ml�1 (Fig. 5a, curve A) is less marked.

The results show that Sr (curves B) has a more intense effectas compared to Ca (curves A). In other words, the non-spectralmatrix effects are as smaller as the matrix is less volatile and theionization energy of the interferent is higher.

Among the studied elements Bi exhibits the highest non-spectral matrix effects, followed by Cu and Pb and finally bythe mutual interference in the Ca–Sr system.

From the point of view of the analyte, the shape of the non-spectral matrix effect curves could be correlated with itsthermodynamic characteristics (oxide dissociation energy andatomic excitation energy). The results show that for hardlyvolatile but easily excitable elements (Ca and Sr) the non-

spectral matrix effects are lower than for volatile but lessexcitable elements (Bi and Pb). This finding suggests the ideathat the electrons generated via ionization of the matrix areinvolved in the excitation mechanism of the analyte. Also, theincreasing non-spectral matrix effects could be explained by thedecreasing degree of ionization of the analyte in the presence ofthe concomitants.Table 5 depicts the true detection limits and the ratio to the

instrumental ones in the DRT-rf-CCP in the presence of10–500 mg ml�1 Ca or Sr matrices.The results presented in Table 5 emphasize the dependence

of the detection limits for the elements on the concentrationand nature of the matrix. An improvement of the detectionlimits could be observed by 20–25% for Bi, Pb and Cu in thepresence of Ca matrix and by 30–60% in the presence of Srmatrix, respectively. In contrast with these improvements, thedetection power decreases by a maximum 14–18% for Bi andby 10–18% for Pb, as a function of matrix concentration. Inthe case of Cu the decrease of the detection power in thepresence of matrices is insignificant. Thus, the detection limitcorrelates with the non-spectral matrix effects presentedpreviously.One may assume that the DRT-rf-CCP studied accepts

relatively high concentrations of Ca and Sr, being suitable

Table 5 True detection limits in DRT-rf-CCP-AES in the presence of 10–500 mg ml�1 Ca or Sr as compared with the instrumental LODs obtained

without matrix (water)

Element line Wavelength/nm Eexc/eV Matrix

Observation

height/mm Real LOD/ng ml�1LOD ratio

true-to-instrumental

Ca I 422.673 2.93 Water 14 3.0 —

Sr 12 2.5–2.6 0.83–0.86

Sr I 460.733 2.69 Water 12 5.0 —

Ca 12 4.5–5.0 0.90–1.00

Cu I 324.754 3.82 Water 14 16.0 —

Ca 12 12.0–16.6 0.75–1.04

Sr 12 11.0–16.5 0.69–1.03

Pb I 405.781 4.38 Water 10 80 —

Ca 10 65–88 0.81–1.10

Sr 10 56–95 0.70–1.18

Bi I 472.252 4.04 Water 2 470 —

Ca 4 360–525 0.77–1.12

Sr 4 195–545 0.41–1.16

Fig. 6 Temperature dependence of the real component of the ac susceptibility for the superconducting samples (a) and the XRD patterns for the2212 (b) and 2223 (c) samples.

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for high temperature superconductor analysis via the AEStechnique.

Analysis of the superconducting materials

Fig. 6(a) and 6(b) shows the dependence of the real componentof the ac susceptibility on temperature for the two super-conducting samples and their X-ray diffraction plot. It canbe seen that the critical temperatures for the samples are B81K for 2212 and B105 K for 2223, respectively. The shape ofthese dependences suggests the possible presence of othersecondary superconducting phases characteristic to the Bi-based family.

As observed by XRD for the 2223 sample the powderconsists primarily of Bi1.6Pb0.4Sr1.6Ca2.0Cu2.8Ox, with tracesof Bi2Sr2CaCu2Ox, (Ca, Sr)2CuO3 and CuO. For the 2212sample the powder consists primarily of Bi2Sr2CaCu2Ox, withtraces of Bi2Sr2CuOx (2201) and Ca2CuO3. The XRD dataconfirm the results obtained by ac susceptibility measurements.

The chemical compositions for the Bi-based 2212 and Pbsubstituted 2223 systems obtained with DRT-rf-CCP-AES andICP-AES for n ¼ 3 parallel samples, as compared with theconcentrations given by the supplier (Aldrich), are presented inTables 6 and 7.

Using the t-test it was found that there are no systematicerrors for a probability of 95% (tcalculated o ttabulated ¼ 4.3 forP ¼ 95% and n ¼ 2) between the average concentrationsdetermined with the plasmas and the concentrations given byAldrich. Similarly, for a confidence level of 95% there are nosystematic errors between the results obtained via DRT-rf-CCP-AES and those obtained via the two ICP-AES spectro-metric systems.

Thus, the analysis method based on the DRT-rf-CCP-AESsystem ensures a good accuracy in the analysis of supercon-ducting materials, similarly to the standard ICP-AES method(recovery degrees obtained for the two samples are: 100.0 � 1.8Bi, 100.3 � 4.0 Sr, 97.7 � 3.3 Ca, 100.9 � 2.7 Cu and 103.8 �3.8 Pb). The precision as relative standard deviation (%) byDRT-rf-CCP-AES is: 0.7 Bi, 1.5 Sr, 2.4 Ca and 3.0 Cu in the2212 system and 0.9 Bi, 3.6 Pb, 2.1 Sr, 2.3 Ca and 1.5 Cu in the2223 system, respectively.

One of the parameters used to rate the quality of a super-conductor is its stoichiometry, which determines the probabil-ity of forming or not a desired phase during the last stages ofthe manufacturing process. Thus, the stoichiometric coeffi-cients calculated, taking as reference the concentration of eachelement, are shown in Tables 8 and 9. Both the theoreticalstoichiometry as well as the calculated one based on Aldrichconcentrations are also given.There are no systematic errors for a P ¼ 95% confidence

level between the stoichiometric coefficients determined byDRT-rf-CCP-AES and ICP-AES as compared with the theo-retical coefficients and those calculated based on Aldrichresults.The accuracy and the precision of the determination of

stoichiometric coefficients depends on two major factors: onthe one hand the occurrence of secondary phases and somelosses due to selective volatilization of some componentsduring the heat treatment of the precursor powder, and onthe other on the accuracy and precision of the analysis method.For the 2223 system the pooled standard deviation obtained

by DRT-rf-CCP-AES is 0.219 and the sum of the stoichio-metric coefficients is 8.570 (instead of 8.400 theoretical value),yielding a 2.55% overall precision. For the 2212 system theoverall precision of the stoichiometric coefficients was found tobe 2.25%, with the pooled standard deviation of 0.157 and asum of coefficients of 7.010 (instead of 7.000). Although theseseem to be relatively high, similar precision was obtained byICP-AES and by data calculated based on Aldrich results,respectively. This emphasises that the accuracy and precisionof stoichiometric coefficients determination are influencedprimarily by the occurrence of secondary phases, a findingsustained by the XRD pattern of the powder. As an example,the average coefficient for Cu in the 2223 type sample wasfound to be 2.982 instead of the theoretical value of 2.8. Thedifference could be explained by using an excess of oxide in theprecursor, an idea sustained by the presence of a CuO phasenot included in the superconducting phases (Fig. 6(b)).Low power radiofrequency plasmas, such as the DRT-rf-

CCP, could be operated at lower cost than the ICP, but theyare disadvantaged by the presence of more emphasized non-spectral matrix effects and therefore need a more careful and

Table 6 Composition of 2212 system obtained by DRTrfCCP-AES

and ICP-AES as compared to ALDRICH composition

Bi Sr Ca Cu

Aldrich composition 41 20 4.3 13

DRT-rf-CCPa 41.6 � 0.3 19.4 � 0.3 4.2 � 0.2 13.3 � 0.4

SPECTROFLAMEa 41.8 � 0.4 19.5 � 0.3 4.2 � 0.2 13.4 � 0.3

SPECTRO CIROSCCDa 41.7 � 0.3 19.4 � 0.4 4.3 � 0.2 13.3 � 0.4

c � sb 41.7 � 0.3 19.4 � 0.3 4.2 � 0.2 13.3 � 0.4

a Average concentration obtained using 3 parallel samples and three

successive measurements, verified with Q-test. b c ¼ average concen-

tration obtained with the three plasma sources, s ¼ spooled (n1 ¼ n2 ¼n3 ¼ 3 and nt ¼ 3).

Table 7 Composition of 2223 system obtained by DRT-rf-CCP-AES and ICP-AES as compared with Aldrich composition

Bi Pb Sr Ca Cu

Aldrich composition 34 8 14 9 20

DRT-rf-CCPa 33.5 � 0.3 8.3 � 0.3 14.5 � 0.3 8.8 � 0.2 19.9 � 0.3

SPECTROFLAMEa 33.3 � 0.3 8.5 � 0.3 14.4 � 0.2 8.9 � 0.2 19.9 � 0.3

SPECTRO CIROSCCDa 33.4 � 0.4 8.3 � 0.2 14.3 � 0.3 8.8 � 0.2 19.8 � 0.3

c � sb 33.4 � 0.3 8.4 � 0.3 14.4 � 0.3 8.8 � 0.2 19.9 � 0.3

a Average concentration obtained using 3 parallel samples and three successive measurements, verified with Q-test. b c ¼ average concentration

obtained with the three plasma sources, s ¼ spooled (n1 ¼ n2 ¼ n3 ¼ 3 and nt ¼ 3).

Table 8 Stoichiometric composition of the 2212 type superconducting

material

Theoretical Bi2.0Sr2.0Ca1.0Cu2.0Based on Aldricha Bi1.866�0.121Sr2.172�0.140Ca1.021�0.066Cu1.946�0.1263DRT-rf-CCPa Bi1.899�0.082Sr2.113�0.092Ca1.000�0.043Cu1.998�0.087SPECTROFLAMEa Bi1.900�0.083Sr2.115�0.092Ca0.996�0.043Cu2.004�0.087SPECTRO

CIROSCCDaBi1.892�0.084Sr2.099�0.093Ca1.017�0.045Cu1.984�0.087

c � sb Bi1.897�0.083Sr2.109�0.092Ca1.004�0.044Cu1.995�0.087a Elementx�s, where: x ¼ stoichiometric coefficient, s ¼ standard

deviation calculated using all the 4 elements as reference. b Ele

menty�s, where: y ¼ average coefficient obtained using the results of

the three plasma sources; s ¼ spooled (n1 ¼ n2 ¼ n3 ¼ 4 and nt ¼ 3).

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laborious optimization, especially when large amounts of con-comitant metals are present. The introduction of samples asdry aerosols could be a possible solution for overcoming thisinconvenience and to enhance the detection limits of thisparticular type of plasma. This technique is under investigationin our laboratory.

Due to an accuracy and precision similar to ICP-AES, and afairly sensitivity, the DRT-rf-CCP-AES studied and presentedin this work could be successfully used in the quality control ofthe manufacturing process of superconducting materials.

Acknowledgements

The authors wish to thank the BCUM Laboratory (RegionalLaboratory for Spectrochemical Analysis, Babes-Bolyai Uni-versity, Cluj-Napoca, Romania) for its support and to DrGheorghe Borodi (Institute of Molecular and Isotopic Tech-nologies, Cluj-Napoca, Romania) and Dr Iosif Grigore Deac(Faculty of Physics, Babes-Bolyai University, Cluj-Napoca,Romania) for their help in the physical characterization ofthe superconducting samples and for the helpful discussions.

References

1 M. K. Wu, J. R. Aushbrun, C. J. Torrig, P. H. Hor, R. L. Meng,L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett.,1987, 58, 908–910.

2 J. M. Tarascon, L. H. Greene, W. R. McKinnon and G. Hull,Phys. Rev., 1987, 35, 7115–7118.

3 H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Jpn. J. Appl.Phys., 1988, 27, L209–L210.

4 M. Takano, J. Takada, K. Oda, H. Kitaguchi, Y. Miura, Y. Ikeda,Y. Tomii and H. Mazaki, Jpn. J. Appl. Phys., 1988, 27, L1041–L1043.

5 W. Gruner and A. Drescher, Fresenius’ J. Anal. Chem., 1989, 335,304–308.

6 S. Scheurell, E. Kemnitz, G. N. Maso, S. Y. Kasin, S. W.Naumow and Y. N. Badun, Fresenius’ J. Anal. Chem., 1991,340, 353–356.

7 I. Vogel, R. Kucharkowski and G. Krabbes, Fresenius’ J. Anal.Chem., 1997, 357, 69–73.

8 O. L. Kabanova, V. I. Shirova, I. V. Markova, A. M. Demkin andA. E. Denisova, Zavod. Lab. Diagn. Mater., 2001, 67, 13–17.

9 L. Paama, P. Peramaki and L. H. J. Lajunen, Anal. Chim. Acta,1996, 330, 259–263.

10 R. Kucharkowski, C. Vogt and D. Marquardt, Fresenius’ J. Anal.Chem., 2000, 366, 146–151.

11 R. Kucharkowski and C. Vogt, J. Anal. At. Spectrom., 2002, 17,263–269.

12 M. J. Kulkarni, A. A. Argekar, S. K. Thulasidas, A. G. Page andM. D. Sastry, Fresenius’ J. Anal. Chem., 1992, 342, 367–369.

13 D. Geilenberg, M. Gerards and J. A. C. Broekaert, Mikrochim.Acta, 2000, 133, 319–323.

14 L. Paama, E. Parnoja and P. Peramaki, J. Anal. At. Spectrom.,2000, 15, 571–572.

15 M. W. Blades, P. Banks, C. Gill, D. Huang, C. Le Blanc andD. Liang, IEEE Trans. Plasma Sci., 1991, 19, 1090–1113.

16 J. A. C. Broekaert, Anal. Bioanal. Chem., 2002, 374, 182–187.17 S. D. Anghel, IEEE Trans. Plasma Sci., 2002, 30, 660–664.18 A. Bas, C. Chevalier and M. W. Blades, J. Anal. At. Spectrom.,

2001, 16, 919–921.19 R. Stomies, S. Schermer, E. Voges and J. A. C. Broekaert, Plasma

Sources Sci. Technol., 2004, 13, 604–611.20 R. Guchardi and P. C. Hauser, J. Anal. At. Spectrom., 2003, 18,

1056–1059.21 J. Franzke, K. Kunze, M. Miclea and K. Niemax, J. Anal. At.

Spectrom., 2003, 18, 802–807.22 M. W. Blades, Spectrochim. Acta, 1994, 49B, 47–57.23 E. A. Cordos, S. D. Anghel, T. Frentiu and A. Popescu, J. Anal.

At. Spectrom., 1994, 9, 635–641.24 F. Herwig and J. A. C. Broekaert, Microchim. Acta, 2000, 134,

51–56.25 S. Luge and J. A. C. Broekaert, Anal. Chim. Acta, 1996, 332,

193–199.26 S. Y. Lu, C. W. LeBlanc andM.W. Blades, J. Anal. At. Spectrom.,

2001, 16, 256–262.27 M. Rahmann and M. W. Blades, Spectrochim. Acta, 2000, 55B,

327–338.28 S. D. Anghel, T. Frentiu, E. A. Cordos, A. Simon and A. Popescu,

J. Anal. At. Spectrom., 1999, 14, 541–545.29 S. D. Anghel, T. Frentiu, A. M. Rusu, L. Bese and E. A. Cordos,

Fresenius’ J. Anal. Chem., 1996, 355, 252–253.30 T. Frentiu, A. M. Rusu, M. Ponta, S. D. Anghel and E. A.

Cordos, Fresenius’ J. Anal. Chem., 1996, 355, 254–255.31 B. M. Patel, J. P. Deavon and J. D. Winefordner, Talanta, 1988,

35, 641–645.32 M. Seelig, N. H. Bings and J. A. C. Broekaert, Fresenius’ J. Chem.

Anal., 1998, 360, 161–166.33 E. A. Cordos, T. Frentiu, A. M. Rusu, S. D. Anghel, A. Fodor

and M. Ponta, Talanta, 1999, 48, 827–837.34 T. Frentiu, M. Ponta, A. M. Rusu, S. D. Anghel, A. Simon and E.

A. Cordos, Anal. Lett., 2000, 33, 323–335.35 T. Frentiu, M. Ponta, S. D. Anghel, A. Simon, I. Marginean and

E. A. Cordos, Mikrochim. Acta, 2003, 143, 245–254.36 T. Frentiu, E. Darvasi, S. D. Anghel, A. Simon, M. Ponta and E.

A. Cordos, Chem. Anal. (Warsaw), 2002, 47, 725–736.37 T. Frentiu, S. D. Anghel, M. Nicola, E. Darvasi, A. Simon and E.

A. Cordos, Croat. Chem. Acta, 1999, 72, 763–778.38 T. Frentiu, M. Ponta, S. D. Anghel, A. Simon, A. M. Incze and

Cordos, Microchim. Acta, 2004, 147, 93–103.39 J. C. Miller and J. N. Miller, Statistics for Analytical Chemistry,

Ellis Horwood Ltd., Chichester, 2nd edn., 1988, pp. 53–62.

Table 9 Stoichiometric composition of the 2223 type superconducting material

Theoretical Bi1.6Pb0.4Sr1.6Ca2.0Cu2.8Based on Aldricha Bi1.562�0.109Pb0.371�0.026Sr1.534�0.107Ca2.156�0.150Cu3.223�0.210DRT-rf-CCPa Bi1.529�0.078Pb0.382�0.019Sr1.578�0.081Ca2.094�0.107Cu2.987�0.153SPECTROFLAMEa Bi1.513�0.078Pb0.389�0.020Sr1.560�0.081Ca2.108�0.109Cu2.973�0.154SPECTRO CIROSCCDa Bi1.531�0.079Pb0.384�0.019Sr1.563�0.080Ca2.103�0.108Cu2.985�0.154c � sb Bi1.524�0.078Pb0.385�0.019Sr1.567�0.081Ca2.101�0.108Cu2.982�0.154a Elementx�s, where: x ¼ stoichiometric coefficient, s ¼ standard deviation calculated using all the 5 elements as reference. b Elementy�s, where:

y ¼ average coefficient obtained using the results of the three plasma sources; s ¼ spooled (n1 ¼ n2 ¼ n3 ¼ 5 and nt ¼ 3).

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