Spectrochimica Acta Part B 65 (2010) 927–934

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Spectrochimica Acta Part B

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Principal component analysis of the main factors of line intensity enhancementsobserved in oscillating direct current plasma

Milovan M. Stoiljković a,⁎, Igor A. Pašti a,b, Miloš D. Momčilović a, Jelena J. Savović a, Mirjana S. Pavlović a

a VINČA Institute of Nuclear Sciences, Department of Physical Chemistry, University of Belgrade, P. O. Box 522, 11001 Belgrade, Serbiab Faculty of Physical Chemistry, University of Belgrade, P. O. Box 137, 11001 Belgrade, Serbia

⁎ Corresponding author. Tel.: +381 11 64 212 73 84.E-mail address: missa@vinca.rs (M.M. Stoiljković).

0584-8547/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.sab.2010.09.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 February 2010Accepted 4 September 2010Available online 17 September 2010

Keywords:DC arc plasmaLine intensity enhancementMagnetic field effectFirst ionization energyOxide bond energyPrincipal component analysis

Enhancement of emission line intensities by induced oscillations of direct current (DC) arc plasma withcontinuous aerosol sample supply was investigated usingmultivariate statistics. Principal component analysis(PCA) was employed to evaluate enhancements of 34 atomic spectral lines belonging to 33 elements and 35ionic spectral lines belonging to 23 elements. Correlation and classification of the elements were done notonly by a single property such as the first ionization energy, but also by considering other relevant parameters.Special attention was paid to the influence of the oxide bond strength in an attempt to clarify/predict theenhancement effect. Energies of vaporization, atomization, and excitation were also considered in theanalysis. In the case of atomic lines, the best correlation between the enhancements and first ionizationenergies was obtained as a negative correlation, with weak consistency in grouping of elements in score plots.Conversely, in the case of ionic lines, the best correlation of the enhancements with the sum of the firstionization energies and oxide bond energies was obtained as a positive correlation, with four distinctivegroups of elements. The role of the gas-phase atom-oxide bond energy in the entire enhancement effect isunderlined.

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1. Introduction

In previous experiments, we measured emission intensity enhance-ments with analytes in atmospheric pressure direct current (DC) arcplasmawithoscillations inducedbyanexternalmagneticfield [1]. Theseenhancements meant better line intensity ratios in the oscillatingplasma and steady-state plasma, respectively. A mechanism thatexplains the obvious demixing effect [2], which limits analyte entryinto the high-temperature region of steady-state arc plasma, has alsobeen proposed [1,3,4]. Consequently, the appropriate conditionsestablished in steady-state arc plasma and an approach to possibleovercoming of demixing (primarily in the lateral direction) by fastlateral shifts of the hot plasma core, induced by an external oscillatingmagnetic field, were suggested [1]. We thereby supposed that only thefirst ionization energy of the analyte determines the partially depletedradiation zone of the arc column and the surrounding gas containinganalyte vapor and aerosol. However, that was not adequate to interpretthe great diversity in the intensity enhancements among the elements.We confirmed that only a few elements show an extremely highintensity enhancement. For most elements, the enhancements aresignificant, while some are poorly enhanced or even depressed. The factthat the intensity enhancements of ionic lines are generally larger in

comparison to the atomic lines also remained unclear, needing furtherinvestigation. Due to the large number of elements considered, thediversity and periodicity of their intrinsic properties which maycontribute to emission intensity, a multivariate statistical approachwas applied for better understanding of plasma-aerosol sampleinteraction.

Principal component analysis (PCA) is a useful multivariatestatistical tool that has found application in different fields for findingpatterns among the variables in multidimensional data sets. PCA is abilinear modeling method, which can render interpretation ofinformation contained in such data sets. It is a data reductiontechnique that aims to explain most of the variance in the data whilstreducing the number of variables to a few uncorrelated components.PCA provides information on whether data sets are the same ordifferent and what variables are responsible for the differencesobserved.

Multivariate methods, primarily PCA, have rarely been used in thestudy of diverse effects in analytically useful plasmas with analyteaerosol supply. They are commonly applied in analytical opticalspectrometry where it is needed to correlate concentration ofnumerous analytes in numerous samples of different origin, natureor locations [5–7].

Nevertheless, Lopez-Molinero et al. [8] have used PCA to classifyspectral lines frequently used in inductively coupled plasma-atomicemission spectrometry (ICP-AES) according to their intrinsic proper-ties only (statistical weights, transition probabilities of the upper and

928 M.M. Stoiljković et al. / Spectrochimica Acta Part B 65 (2010) 927–934

lower states, etc.). In a similar manner, Lopez-Molinero and Castilloet al. [9] have applied multivariate statistical characterization of thetolerance of argon ICP to organic solvents. As in previous work, theyhave not used any measured line intensities as variables, but only themain physical properties of solvents (surface tension, viscosity, etc.).

Brenner et al. [10] have used PCA to differentiate the behaviors ofrare earth elements' (REEs) line intensity depressions when the nitricacid was sprayed into the ICP-AES. They measured emissionintensities of ionic lines of REEs and correlated changes with thefirst ionization energies and oxide bond strengths, as variables. Theyconcluded that the sum of these energies is an essential parameter forthe effect studied. In this case, three groups of the REEs weredistinguished.

The presence of metal oxides in ICP was confirmed by Furuta [11].He mapped the spatial intensity distribution of the gas-phase yttriummonoxide (YO) within a very narrow plasma zone just above the loadcoil of the ICP. He concluded that the red, visible emission above thecoil in the tail flame plasma is also due to YO. That was attributed tothe reaction of recombination of atomic yttrium with entrained air[11].

In contrast to the above, introduction of propane into ICP alters theatomic emission intensity of various analytes [12]. The proposedmechanism of emission increases is based on reduction of the gas-phase metal oxide (Y, La, Al, Ba, Cr) by elemental carbon, rather thanthe increase of plasma temperature. However, the observations arenot consistent, because some analytes (Ca, Mg, Fe, Cu) expressemission depression, instead of enhancement, which is attributed toformation of metal carbides.

The goal of the present work was to provide an extendedassessment of the enhancement effect of the intensities of atomicand ionic lines in DC arc plasmawith continuous analyte supply and tosuggest some explanations for extremely large enhancement effectson a few elements. Line emission enhancements of numerouselements produced by fast lateral oscillatory shifts of the plasmacolumnwere statistically correlated with eight intrinsic variables, fiveof which were physical properties and the other three were some oftheir linear combinations.

2. Experimental section

Experimental set-up, including the sketch of the stabilizedatmospheric pressure direct current (DC) arc plasma device with acontinuous sample aerosol supply, magnetic coil that generates theappropriate magnetic field and magnetic/optical arrangements havebeen described in detail previously [1,13].

For clarity, the set-up will be re-described briefly here.

2.1. Experimental set-up

2.1.1. Stabilized arc deviceThe atmospheric pressure DC arc plasma is generated in an axial

channel formed within 4 mm circular orifices in successive water-cooled and electrically insulated brass and copper segments. Lowercopper anode (water-cooled) and upper graphite cathode arecoaxially fixed. Analyte aerosol particles carried by argon stream(2 dm3 min−1) are introduced into the circular cavity of the centralsegment (36 mm in diameter) forming a gas vortex that spatiallystabilizes the mid of the arc current channel. The upper and loweropenings on the central segment are large enough to avoid excessiveconstriction of that portion of the arc column, sustaining aerosolintroduction into the plasma. A cylindrical, 12 mm high portion of thearc column, confined between the upper opening in the centralsegment and the coaxial orifice in the upper segment, represent theanalytical space. That space projected onto the monochromatorentrance slit enables a side-on plasma view.

2.1.2. The magnetic fieldAn 8 ampere DC arc plasma is coupled with an external alternating

magnetic field of the appropriate parameters. The electromagnet coilis made of ferrous sheets core (60 mm long, 15×35 mm in cross-section) wounded with 650 turns of insulated solid copper wire. Ahomemade power amplifier, coupled with an audio-frequencysinusoidal function generator and an adjustable 40 V, 4 A DC powersupply are used for generating an alternating magnetic field. Bylinking the appropriate capacitor in sequence with a coil, at a resonantfrequency of 350 Hz, a magnetic field of 90 mT (peak-to-peak) isgenerated. In such a way, we obtain the arc core sinusoidal lateralshifts with amplitude of 7.5 mm. At higher amplitudes the arc plasmabecomes unstable and dies out.

Magnetic coil core is positioned in the middle of the analyticalplasma section, and 18 mm away from its axis. Plasma column axis,monochromator optical axis, and the electromagnet core axis arefixed orthogonally to one another. Such alignment allows side-onintensity measurements of the steady-state plasma, as well asrecording of the longitudinal oscillations (along the optical axis) ofthe plasma column [13].

2.1.3. Monochromator and measuring equipmentA 2-meter plane grating spectrograph (Carl-Zeiss, Jena) is used as

the monochromator. Its grating has 651 grooves mm−1, andreciprocal linear dispersion of 0.73 nmmm−1 in the first spectralorder. The spectrograph has been adapted into a single channelspectrometer with photoelectric detection. A side-on photomultiplierR928 (Hamamatsu TV, 180–880 nm) is used. The photocurrent isregistered via a Burr-Brown PC interface PCI-20428Wmultifunctionalboard. The emission is optically integrated by imaging the selectedzone within the analytical plasma space onto the monochromatorcollimator. That zone is 2 mm high and 6 mm above the upperopening of the central segment. The photocurrent signal is averagedover a period of 10 s. A Tektronix TDS 1002 oscilloscope is used formeasuring the oscillations of the magnetic field.

2.1.4. NebulzerA Meinhard concentric glass nebulizer is used for aerosol

generation, in conjuction with a Scott-type cloud chamber for aerosolseparation. Water solutions of each analyte considered did not exceed50 mg dm−3. The line width was tested through self-absorptionmeasurements of the most intense lines in the given concentrationrange. No self-absorption was detected.

2.2. Input data matrix

Emission intensity enhancement is expressed as the ratio of lineemission intensities in the oscillating plasma (I) and the steady-stateplasma (I0). Relevant data for the part of the measured atomic andionic lines of various elements have been presented previously [1]. Toincrease the number of elements considered, extra sets of atomic andionic line enhancements are measured and merged with previousdata sets. In this way, input data set are enlarged and finally consistedof 50 individual measurements (cases), each. The case here meanseach data (intensity enhancement) measured. Each individual linemeasurement, repeated measurement of the same line, or ameasurement of several different lines of the same element,represents an individual case.

For ionic lines, 50 cases belong to 35 lines that cover 23 elements.For atomic lines, 50 cases also, belong to 34 lines that cover 33elements. The final list of elements and corresponding wavelengths isgiven in Table 1. Consequently, a total of 69 spectral lines weremeasured, covering 45 elements. That is about half of all naturalelements available, and involves a wide range of the first ionizationenergies Eion [14,15], oxide bond energies, Ex−o [16], and excitationenergies, Eexc [14,15].

Table 1Wavelengths of all measured atomic and ionic lines of various elements. Total of 69lines belonging to 45 elements are listed.

Element Wavelength, nm Element Wavelength, nm

1. Ag 328.068 Ba 493.4092. Al 396.153 Ba 455.4033. Ar 427.217 Be 313.0424. As 228.812 Ca 396.8475. Au 267.595 Ca 393.3676. B 249.773 Cd 226.5027. Ba 553.548 Ce 413.7658. Be 234.861 Dy 338.5019. Bi 306.772 Dy 353.17010. Ca 422.673 Dy 396.83411. Cd 228.802 Er 326.47912. Co 345.350 Er 337.27613. Cr 425.435 Eu 390.71014. Cu 324.754 Fe 259.94015. Fe 371.994 Gd 310.05016. Ga 417.205 Gd 342.24717. In 410.177 Ho 345.60018. K 766.491 Ho 389.10219. Li 670.784 La 408.67220. Mg 285.213 La 394.91021. Mn 403.076 Mg 279.55322. Mo 379.825 Mn 257.61023. Ni 352.454 Mo 287.15124. P 213.618 Pr 390.80525. P 253.565 Pr 422.29326. Pb 368.346 Pr 422.23527. Pd 340.458 Sc 357.25328. Pt 306.471 Sc 361.38429. Re 346.046 Sm 359.26030 Si 251.611 Ti 334.94031. Ti 498.173 U 386.59232. Tl 377.572 V 311.83833. V 437.924 Y 324.22834. Y 410.238 Y 371.03035. Yb 328.937

Fig. 1. Correlation of the emission intensity enhancements (I/I0) of atomic and ionic lineswith: Evap [16], Eatom [16], Eion [14,15], Ex−o [16], Eion+Eexc [14], ΣE4=Evap+Eatom+Eion+Ex−o, Eion+Ex−o, and ΣE4+Eexc.

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The primary objective was to measure the effect of line intensityenhancements caused by the fast oscillating plasma column with asmany elements of diverse first ionization energies as possible.However, the selection of elements and their spectral lines waslimited, for several reasons. Some of the reasons were the absence ofionic lines in the atmospheric pressure low-current arc plasmaspectrum (alkaline elements, noble gases or other elements withextremely high first ionization energy), unavailability of the spectrum(vacuum UV) or its complexity (molecular bands overlapped).Therefore, it was not possible to represent each selected element byboth atomic and ionic lines. Some of the elements are characterizedusing atomic lines, and some using ionic lines, only. In addition, a fewelements are presented by several wavelengths. Themeasured atomicand ionic spectral lines are generally the most intense and easilydetectable in the spectrum of an element [14].

To check the uncertainty of measurements, 14 previouslymeasured atomic and ionic lines were re-measured. The repeatability,expressed as relative standard deviation, RSD, ranges from 5% to 20%for both data sets, and mostly depends on particular line intensityemission in steady-state plasma. Such variability of the data isacceptable for that type of statistical analysis. We underscore that allmeasured cases were included in the PCA without any averaging.Moreover, for a few elements, up to three different lines weremeasured, giving somewhat different enhancements. In such way, theuncertainty of the measured data was included into the PCA.

3. Results and discussion

Atomic and ionic line data sets were independently processed bythe PCA using SPSS software. In each data set, the input matrixconsists of 9 variables (columns) and 50 cases (rows). Five different

physical quantities (Evap, Eatom, Eion , Ex−o and Eexc) consideredimportant, were included as individual variables or as their linearcombinations. The intensity enhancement (I/I0) was definitelyconsidered as a dependent variable. Finally, the variables includedwere:

• I/I0 – line intensity enhancement, where I stands for line intensity inthe oscillating and I0 in the steady-state plasma,

• Evap – energy of vaporization,• Eatom – energy of atomization,• Eion – first ionization energy,• Ex–o – oxide bond energy,• Eion+Eexc – sum of the first ionization energy and line excitationenergy,

• sum of Evap+Eatom+Eion+Ex−o=ΣE4,• sum of Eion+Ex−o

• sum of Evap+Eatom+Eion+Ex−o+Eexc=ΣE4+Eexc.

The data matrix of the variables analyzed was subjected to PCA inorder to obtain certain differentiation between the samples and not asthe data reduction method. Hence, the inclusion of the variables thatcould be considered redundant in the input set for the PCA analysisdid not turn out cumbersome for analysis, while providing usefulinsights. Correlation matrices were calculated, and correlationcoefficients of the line intensity enhancements with all other variablesare presented in Fig. 1. For both data sets, the correlations range fromweak to medium. Nevertheless, when estimating the significance ofthe correlation coefficients found between the variables, one also hasto take care of the number of degrees of freedom in the data set.Having in mind the number of cases (50, for each data set) included inthe study, one finds this correlation significant at the commonly usedalpha level of 0.05. These correlation coefficients are high enough tosatisfy the conditions for PCA analysis.

Consequently, it can be observed that the intensity enhancementsof atomic lines show better correlation with different variables thanthose of ionic lines. The enhancements of atomic lines have the bestcorrelation, which is negative, with the first ionization energy of anelement Eion (−0.415) and somewhat weaker with the sum Eion+Eexc (−0.368). Conversely, the enhancements of ionic lines have thebest correlation, which is positive, with the sum Eion+Ex−o (0.402)and somewhat weaker with Ex−o (0.373). In the case of atomic lines,this means that enhancement of the emission intensity decreaseswith increasing first ionization energy (Eion) of an element. In thecase of ionic lines, on the other hand, enhancement of emissionintensity increases with the increasing sum Eion+Ex−o. Also, it isclear that the energies of vaporization and atomization have a small

Table 2Intrinsic physical properties of the elements in the marked groups in PC1–PC2 scoreplot, ionic lines, Fig. 2a. In each group the elements are listed according their atomicnumber.

Z Element Eion, eV Ex−o, eV Eion+Ex−o, eV Atom ground stateconfiguration

I 21 Sc 6.56 7.06 13.62 [Ar]3d14s2

22 Ti 6.83 6.97 13.80 [Ar]3d24s2

23 V 6.75 6.52 13.27 [Ar]3d34s2

39 Y 6.22 7.46 13.68 [Kr]4d15s2

42 Mo 7.09 5.81 12.96 [Kr]4d55s1

57 La 5.58 8.28 13.86 [Xe]5d16s2

58 Ce 5.54 8.24 13.78 [Xe]4f15d16s23 0 2

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contribution to the emission intensity enhancement of atomic andionic lines. This suggests that the later processes are moreenergetically efficient (energetically less demanding) compared tothe processes of atom excitation, ionization, and oxide dissociation.The excitation energy, Eexc, was also included as an individualvariable into the preliminary analysis, but weak correlations wereobtained for both atomic and ionic lines (− 0.242 and − 0.144,respectively). Much better correlations were evaluated takingvariable Eion+Eexc, Fig. 1. Therefore, the excitation energy isexcluded from the discussion as an individual variable.

Three principal components (PC) for both atomic and ionic linesshould be considered in describing the calculated variances.

59 Pr 5.47 7.80 13.27 [Xe]4f 5d 6s64 Gd 6.15 7.45 13.60 [Xe]4f75d16s2

92 U 6.19 7.87 14.06 [Rn]5f36d17s2

Mean±SD 6.2±0.6 7.3±0.8 13.6±0.3II 66 Dy 5.94 6.29 12.23 [Xe]4f105d06s2

67 Ho 6.02 6.33 12.35 [Xe]4f115d06s2

68 Er 6.11 6.37 12.48 [Xe]4f125d06s2

Mean±SD 6.02±0.08 6.33±0.04 12.4±0.1III 4 Be 9.32 4.51 13.83 [He]2s2

12 Mg 7.65 3.76 11.41 [Ne]3s2

25 Mn 7.43 4.18 11.61 [Ar]3d54s2

26 Fe 7.90 4.05 11.95 [Ar]3d64s2

48 Cd 8.99 2.44 11.43 [Kr]4d105s2

3.1. Ionic lines

The cumulative variance of the first three principal componentsis 96.14%. The first component accounts for 59.35% of the totalvariance and the second one for 26.37%. Two score plots, PC1–PC2and PC1–PC3, show significant grouping among the elements. Fourgroups are clearly recognizable in both plots, Fig. 2. However, inter-group constituents are somewhat different, Tables 2 and 3. By

Fig. 2. a) PC1–PC2 score plot, b) PC1–PC3 score plot for ionic lines. In both plots, thegroups are formally indicated by appropriate Roman numbers. The group constituentsin PC1–PC2 and PC1–PC3 are listed in Tables 2 and 3, respectively.

Mean±SD 8.2±0.8 3.8±0.8 12.0±1.0IV 20 Ca 6.11 4.17 10.28 [Ar]3s2

56 Ba 5.21 5.82 11.03 [Xe]6s2

62 Sm 5.64 5.86 11.50 [Xe]4f 65d06s2

63 Eu 5.67 4.96 10.63 [Xe]4f 75d06s2

70 Yb 6.25 4.12 10.37 [Xe]4f 145d06s2

Mean±SD 5.8±0.4 5.0±0.8 10.8±0.5

careful analysis of the group constituents, it is possible to noticesimilarities among their intrinsic properties.

The results, Tables 2 and 3, indicate that the constituents of group Iin PC1–PC2 are a composite of the constituents of group I and group IIin PC1–PC3. The constituents of group I in PC1–PC3 (Sc, Y, and Gd) are

Table 3Intrinsic physical properties of the elements in the marked groups in PC1–PC3 scoreplot, ionic lines, Fig. 2b. In each group the elements are listed according their atomicnumber.

Z Element Eion, eV Ex−o, eV Eion+Ex−o, eV Atom ground stateconfiguration

I 21 Sc 6.56 7.06 13.62 [Ar]3d4s2

39 Y 6.22 7.46 13.68 [Kr]4d5s2

64 Gd 6.15 7.45 13.60 [Xe]4f75d6s2

Mean±SD 6.3±0.2 7.3±0.2 13.63±0.04II 22 Ti 6.83 6.97 13.80 [Ar]3d24s2

23 V 6.75 6.52 13.27 [Ar]3d34s2

42 Mo 7.09 5.81 12.96 [Kr]4d55s57 La 5.58 8.28 13.86 [Xe]5d6s2

58 Ce 5.54 8.24 13.78 [Xe]4f5d 6s2

59 Pr 5.47 7.80 13.27 [Xe]4f35d06s2

92 U 6.19 7.87 14.03 [Rn]5f36d7s2

Mean±SD 6.2±0.6 7.4±0.9 13.6±0.4III 4 Be 9.32 4.51 13.83 [He]2s2

26 Fe 7.90 4.05 11.95 [Ar]3d64s2

66 Dy 5.94 6.29 12.23 [Xe]4f105d06s2

67 Ho 6.02 6.33 12.35 [Xe]4f115d06s2

68 Er 6.11 6.37 12.48 [Xe]4f125d06s2

Mean±SD 7.1±1.5 5.5±1.1 12.6±0.7IV 12 Mg 7.65 3.76 11.41 [Ne]3s2

20 Ca 6.11 4.17 10.28 [Ar]3s2

25 Mn 7.43 4.18 11.61 [Ar]3d54s2

48 Cd 8.99 2.44 11.43 [Kr]4d105s2

56 Ba 5.21 5.82 11.03 [Xe]6s2

62 Sm 5.64 5.86 11.50 [Xe]4f65d06s2

63 Eu 5.67 4.96 10.63 [Xe]4f75d06s2

70 Yb 6.25 4.12 10.37 [Xe]4f145d06s2

Mean±SD 6.6±1.3 4.4±1.1 11.0±0.5

Fig. 3. Side-on scans of steady-state plasma column obtained by its successiveshifting relative to the entrance slit. For clarity, only half of the lateral profiles arepresented: (a) YII 371.029 nm, Eexc=3.52 eV, (b) YI 410.236 nm, Eexc=3.09 eV(c) YO 597.20 nm, Eexc=2.07 eV. Intensity values are normalized to surfaces underthe corresponding profiles. The Y concentration is 200 μg ml−1.

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the only elements that show extremely large intensity enhancementsby a factor of about 200, typically [1]. These elements exhibitimportant inherent similarities. First of all, they have very similarvalues of Eion and Ex−o (6% maximum difference), and particularly thesum Eion+Ex−o (less than 0. 5% difference). Moreover, their oxidebond energies are greater than the first ionization energies, Ex−oNEion,Table 3. More importantly, they all have analogous ground statevalence configurations, nd1 (n+1)s2 (Sc, Y), plus half-filled (n−1)f7

in the case of Gd, Table 3.The constituents of group II in PC1–PC3, Table 3, show very similar

mean values of Eion, Ex−o and Eion+Ex−o as constituents of group I, andthe relationship of themean values is the same as in group I, Ex−oNEion .However, the individual values of Eion and Ex−o deviatemuchmore fromthe mean values and for most elements the differences Ex−o–Eion areconsiderably greater (about 2.5 eV) than those for elements of group I(about 1 eV). The exceptions are V and Ti, whose oxide bond energiesand ionization energies are very close, and Mo, where EionNEx−o,Table 3. In terms of valence electronic configuration, all constituents,except La, differ from configuration nd1 (n+1)s2. Some constituentshavemore thanoneelectron in thedorbital (Ti, V,Mo),while somehaveone or no electrons in that orbital, but also under half-filled f orbital (Ce,Pr, and U). All of these elements, including La, exhibit smallerenhancement in comparison to Sc, Y, or Gd.

The constituents of group II in PC1–PC2 (Dy, Ho, and Er) are asubset of group III in PC1–PC3, Tables 2 and 3, respectively. Thesethree elements also have close values of Eion, Ex−o, and of Eion+Ex−o.Their oxide bond energies are also greater than the ionizationenergies, but energy differences are less than 0.5 eV. The specificityof their valence electronic configuration is an empty d orbital and overhalf-filled 4f orbital, plus 6s2, Table 2. Besides Dy, Ho, and Er, the groupIII in PC1–PC3 includes Be and Fe, Table 3, but those elements alsobelong to group III in PC1–PC2, where, according to their properties,they more rightfully belong, Table 2.

Constituents of groups III and IV in the PC1–PC2 plot are mergedinto group IV in PC1–PC3 (with the exception of Be and Fe). Thoseconstituents show a trend of increasing differences between Eion andEx−o, while at the same time, EionNEx−o, except for Ba and Sm. Also,the sum of Eion+Ex−o decreases. The diversity concerning the valenceelectronic configuration in relation to nd1 (n+1)s2 increases insidethe group, Table 3.

Findings based on PCA for ionic lines make possible differentiationof at least four assemblies of elements with specific intrinsiccharacteristics. Despite the similarity and periodicity in Eion and Ex−o,and consequently, Eion+Ex−o, the slight differences in their character-istics seem sufficient to distinguish the various behaviors in terms ofchange in emission intensities generated by fast lateral shifting of arcplasma. Although ground state valence electron configurations werenot included as variables, the results finally lead to significantcorrelation between the three characteristics: ionic line intensityenhancements, sum of the atom ionization energy and the oxidebond energy, and the atom valence configuration. This mainly pertainsto valence electronic configurations of the d-block and f-blockelements. Their common tendency is to form oxides, i.e. oxophilicity.Results of numerous studies indicate that neutral lanthanoid metal-oxide molecules (LnO) are formed with two valence 5d electrons,rather than one 5d and one 6s electron [17–21]. An analogous behavioris found for actinoid neutral oxidemolecules (AnO) but with a 6d and a7s electron.

In the case of lanthanoid, since the spin-paired electrons in the 6s2

valence orbital are unavailable for oxygen bonding, promotion of oneof the two 6s electrons into the outer valence orbital should occur. Insuch a way, the realized divalent configuration 5d2 6s1 is suitable forlanthanoid gas-phase metal-monoxide covalent bonding. Promotionenergies for such a configuration significantly vary within thelanthanoid series, indicating additional diversity in their affinitytowards gas-phase oxygen bonding [18].

Of all the measured elements, Sc, Y, Gd, and La have an analogousatom ground state valence configuration (d1s2), Table 2, and they allexhibit the largest intensity enhancement effect (La somewhat less). Tothis groupmay also be added lutetium (Lu), 4f14 5d1 6s2, actinium (Ac),6d1 7s2, and curium (Cm), 5f7 6d1 7s2, which we could not measure.Therefore, it is possible to expect comparable intensity enhancements ofthese three elements, or, based on the correlations between lanthanoidand actinoid oxide bond energies [18–21], mutually comparableintensity enhancements of the entire actinoid series. The latter isindicated by the classification of uranium (U) into group I, Table 2.

The results derived from the analysis indirectly confirm that theremust be a large amount of gas-phase oxides of these metals in steady-state arc plasma, which significantly alter the spatial distribution andreduce the intensity of their ionic and atomic spectra. This isdemonstrated by lateral intensity profiles of steady-state plasma atwavelength of atomic (YI) and ionic (YII) lines and molecule (YO)band head, Fig. 3. Yttrium oxide (YO) emission extends over thecolder plasma periphery, whereas yttrium ion (YII) and atom (YI)intensity emission profiles are situated much closer to the hot plasmacore. In general, at this point we may propose that the first ionizationenergy affects the inner slope of the atomic line intensity profile ofyttrium, whereas yttrium oxide bond energy affects the outer slope.The resulting YI intensity profile could certainly be disturbed. Insteady-state plasma YI emission profile will be quenched, narrowed,with its maximum shifted closer to the hot plasma core than it wouldregularly be without yttrium–oxide formation [21]. With this in mind,the resulting YII intensity profile could certainly be determined by thesecond ionization energy (the inner slope) and the ion–atomrecombination equilibrium, and with atom yttrium–yttrium oxidedissociation equilibrium (the outer slope). When the hot, oscillatorydriven arc core penetrates into the plasma periphery (the yttriumatom, and especially the yttrium-oxide loaded region), the YOmolecule dissociation and atom ionization occurs, inducing aconsiderable ionic line emission enhancement.

Accordingly, we propose two main mechanisms that govern theemission intensity enhancement of ionic lines. The first includes adominant contribution of oxide bonding, and less pronouncedcontribution of the first ionization energy. The second mechanism isthe opposite.

In the first case, in steady-state plasma, the metal-oxide occur-rence extensively depresses the atomic line intensities throughweakening the influence of the ionization energy. In oscillating

Fig. 4. Intensity enhancement of atomic lines (solid) and ionic lines (open) correlatedwith Eion and Eion+Ex−o, respectively. Various symbols correspond to the specifiedgroups, Tables 4 and 2, respectively. The mutually opposing linear trends are indicatedby dashed lines. For both atomic and ionic lines the outstanding elements are marked.Sc, Y, and Gd are particularly prominent.

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plasma, the resulting line enhancement is mostly due to metal-oxidemolecule dissociation, and much less due to extra atom ionization(note, Ex−oNEion, for Y, Sc and Gd, Tables 2 and 3).

In the second case, in steady-state plasma, the first ionizationenergy dominantly affects the ionic line intensities, as the influence ofoxide bonding is much less pronounced (oxides do not occur, or areless stable). In oscillating plasma, the resulting line enhancement ismostly due to extra atom ionization, and less or not at all due to oxidedissociation (note, Ex−obEion for most elements in groups III and IV,Table 2).

Obviously, those two extreme examples are considerably depen-dent on parameters Eion and Ex−o, as well as on their ratio. Betweenthose two extremes, there are many elements exhibiting a wide rangeof different mutual contributions of those two parameters in theintensity enhancement caused by fast oscillating arc plasma. Evident-ly, the oxide bond strength and the ionization energy are involved inthe effects observed, and the contribution of each process will dependon many parameters, including various thermodynamic properties inmechanisms of the reaction of a gas-phase metal atom with oxygen.This explains the difficulty of obtaining better correlation and ageneralized conclusion that would describe the behavior of each ofthe measured elements, i.e. their spectral lines. Of course, bothquantities Eion and Ex−o, are primarily related to the atom ground stateelectron configuration.

Clearly, the behavior of scandium (Sc), yttrium (Y), and gadolinium(Gd) are found to be consistent with the first proposed mechanismwith respect to the valence electron configuration and their para-meters, Eion and Ex−o. Only lanthanum (La) slightly deviates from theexpected trend and shows inferior enhancement, which may be theconsequence of a significant difference between of Eion and Ex−o ifcompared to Sc, Y, and Gd, Table 2.

If the atom ground state valence configuration deviates from theabove, nd1 (n+1)s2 where n=3, 4, 5 and 6, respectively, the atomaffinity towards gas-phase metal oxide covalent bonding is ratherreduced. In these cases, the intensity emission profiles in the steady-state plasma are not much or, negligible affected by oxide formation,but primarily by the first ionization energy, and the effect of emissionenhancement is weaker. Therefore, the role of the gas-phase metal-oxide bond energy in the entire effect can be considered as rangingfrom substantial (Sc, Y, Gd, and somewhat La) to unimportant (Cd andsomewhat Fe). Again, this confirms the oxygen-related chemicalbehavior of the lanthanoid series. Although f electrons are chemicallyinactive in oxide bonding [18], and all the lanthanoids, except La, Ce,Gd, and Lu, have an empty 5d orbital, they demonstrate diversedegrees of emission enhancement. Observe that the lanthanoid seriesitself is split into three groups, Table 2. By taking into account differentvalues of promotion energy needed to change the electronicconfiguration from 5d1 6s2 to 5d2 6s1, the number and compositionof the groups we observed becomes clearer. Brenner et al. [10]obtained the same number of groups of lanthanoid series, but thegroup constituents slightly disagree with our.

On the other hand, the similarities between Y and the lanthanoidsare very strong, and, chemically, yttrium resembles these elementsmore closely than its neighbor in the periodic table. The chemistry ofscandium (Sc) is also very closely related to that of yttrium, andexplains why scandium was classified as a lanthanoid-like element.Keeping these facts in mind, analogous behavior in terms of emissionintensity of Sc and Y to some lanthanoids, Gd primarily, in steady-state and oscillating DC plasma is explainable.

In summary, an overview of all the measured intensity enhance-ments versus the first ionization energies for atomic lines, and the sumof the first ionization and oxide bond energies for ionic lines, isrepresented in Fig. 4. In both cases the quality of fits presented inFig. 4. has to be understood in the context used in Fig. 1.

Group constituents of ionic lines, labeled with reference to Table 2,fit along the upward linear trend, except Be and Yb, which deviate

significantly. The elements with the largest ionic line emissionenhancement, Sc, Y, and Gd are highlighted. The deviation of Bemay be attributed to specific oxygen-related chemical properties ofberyllium, even in comparison to other alkaline-earth elements. Also,Yb shows a quite different behavior from any other lanthanoid and thethermodynamics of its reaction with oxygen are more similar to thatof the alkaline earth elements [20,21].

In the case of ionic lines, a good classification of elementsaccording to the sum of the first ionization and oxide bond energiesleads to a proper correlation with the atomic electronic ground statevalence configurations, i.e. affinity towards gas-phase oxide forma-tion. In the case of atomic lines, a less constructive classification of theelements related to Eion and the atomic ground state configurationswas achieved.

3.2. Atomic lines

In the case of atomic lines, the cumulative variance of the firstthree components is 90.34%. The first component accounts for 49.29%of the total variance, the second one 28.73%. Only the single score plot,PC1–PC3, shows significant grouping among the elements included.Data on other PC–PC plots are more scattered and too complex for anyconstructive conclusion. Five groups of elements can be recognized inthe PC1–PC3 score plot, Fig. 5. The contents of the groups are listed inTable 4.

With the atomic lines, no exact relationship between the intrinsicproperties of the group constituents is evident as in the case of ioniclines. This pertains to the intra-group similarity of the intensityenhancements, ionization energies, and atom ground state electronicconfigurations. For example, all constituents of group I are theelements of high ionization energies set, ranging from 6.8 eV to15.8 eV, Table 4, but they express quite different intensity enhance-ments. The emission enhancements vary from about ten-fold (Ti, V),and even emission depressions occur (P, Ar).

The most numerous group IV shows similar lack of uniformity.Although ionization energies of those elements are less scattered, theintensity enhancements range within one order of magnitude, andmostly overlap with the constituents of group I, Fig. 4. Thecharacteristics of group III are similar, even though it is clearlyseparated from the others, Fig. 5. Apparently, there are no recogniz-able trends in Eion and ground state electronic configurations within

Fig. 5. PC1–PC3 score plot for atomic lines. The groups are formally indicated by theappropriate Roman numbers. Some groupmembers are labeled. The complete list of thegroup constituents is given in Table 4.

933M.M. Stoiljković et al. / Spectrochimica Acta Part B 65 (2010) 927–934

the constituents of groups I, III, and IV, so they can be collectivelyconsidered as one group.

Some members of group II (Ca, Al, Y and Ba) roughly expressseveral 10-fold enhancements, i.e. the biggest among all theelements considered. Elements Ca, Al, and Y show similarity in thevalues of their Eion, but have various atom ground state configura-

Table 4Intrinsic physical properties of the elements in the groups of the PC1-PC3 score plot, atomic

Z Element Eion, eV

I 4 Be 9.325 B 8.3014 Si 8.1515 P 10.4918 Ar 15.7622 Ti 6.8323 V 6.7533 As 9.79mean±SD 9.4±2.9

II 12 Mg 7.6513 Al 5.9920 Ca 6.1139 Y 6.2256 Ba 5.2182 Pb 7.42mean±SD 6.5±1.0

III 42 Mo 7.0975 Re 7.8378 Pt 8.96mean±SD 8.0±1.0

IV 24 Cr 6.7725 Mn 7.4326 Fe 7.9027 Co 7.8828 Ni 7.6429 Cu 7.7331 Ga 6.0046 Pd 8.3447 Ag 7.5848 Cd 8.9949 In 5.7979 Au 9.2281 Tl 6.1183 Bi 7.28mean±SD 7.5±1.0

V 3 Li 5.3919 K 4.34mean±SD 4.9±0.7

tions. Conversely, alkaline-earth metals Mg, Ca, and Ba haveanalogous atom ground state configurations, but the trend of theirEion is in negative linear correlationwith the emission enhancements(Ba expresses the largest enhancement). Note that in the case ofionic lines, Mg, Ca, and Ba are also classified within the same group,Table 3.

The constituents of group V are alkali metals Li and K, Fig. 5. Bothare easily ionizable elements with the same ns1 ground stateelectronic configuration, Table 4. They deviate significantly from therecognizable linear trend, showing low emission enhancements,Fig. 4. Due to low Eion, the depleted volume inside the arc column islarge when compared to the applied amplitude of the arc motion, sothat the hot arc core moves in the partially depleted region [1]. As aresult, the intensity enhancement is small. Furthermore, Ga, In, and Tl,similarly diverge, showing lower emission enhancements, Fig. 4. Theyall have the same (n−1)d10 ns2 np1 ground state electronicconfiguration and similar values of ionization energies (~6 eV).However, they are classified as constituents of group IV.

For atomic lines, the observed deviation from the decreasing lineartrend, Fig. 4, is the consequence of atom depletion in the centralplasma column by atom species in the steady-state arc, which isgenerally assumed for all elements with low first ionization energies.According to the supposed mechanism [1], the atom line enhance-ment is expected to be smaller as the depleted volume is largercompared to the amplitude of the arc column shifts in magnetic field.

Once more, the exceptional behavior of Be should be noted.Emission enhancements of both atomic and ionic Be lines significantlydeviate from the expected trends, Fig. 4. Although Eion of the berylliumatom is very high, and Ex−o is very low, the enhancements are similar

lines case, Fig. 5. In each group the elements are listed according their atomic number.

Ex-o, eV Eion+ Ex-o, eV Atom ground state configuration

4.51 13.83 [He]2s2

8.38 16.68 [He]2s22p8.29 16.44 [Ne]3s23p2

6.21 16.70 [Ne]3s23p3

— — [Ne]3s23p6

6.97 13.80 [Ar]3d24s2

6.52 13.27 [Ar]3d34s2

4.99 14.78 [Ar]3d104s24p3

15.1±1.53.76 11.41 [Ne]3s2

5.30 11.29 [Ne]3s23p4.17 10.28 [Ar]4s2

7.46 13.68 [Kr]4d5s2

5.82 11.03 [Xe]6s2

3.96 11.38 [Xe]4f145d106s26p2

11.1±0.55.81 12.90 [Kr]4d55s6.5 14.33 [Xe]4f145d56s2

4.06 13.02 [Xe]4f145d96s13.4±0.8

4.45 11.22 [Ar]3d54s4.18 11.61 [Ar]3d54s2

4.05 11.95 [Ar]3d64s2

3.98 11.86 [Ar]3d74s2

3.96 11.60 [Ar]3d84s2

2.79 10.52 [Ar]3d104s3.66 9.66 [Ar]3d104s24p3.95 12.29 [Kr]4d10

2.28 9.86 [Kr]4d105s2.44 11.43 [Kr]4d105s2

3.32 9.11 [Kr]4d105s25p2.3 11.52 [Xe]4f145d106s— – [Xe]4f145d106s26p3.49 10.77 [Xe]4f145d106s26p3

11.0±1.03.46 8.85 [He]2s2.88 7.22 [Ar]4s

8.0±1.0

934 M.M. Stoiljković et al. / Spectrochimica Acta Part B 65 (2010) 927–934

and amount to about 10-fold. As declared previously, that stems fromthe characteristics of its electronic configuration and small atom size.

Although the formation of metal oxides in plasma would reducethe population of the free atoms and ions, the effects of the metaloxide molecular species on the excitation conditions in the steady-state plasma is not well characterized [21]. We suppose that the newconditions produced by fast plasma oscillatory shifts are much morefavorable for extra analyte ionization rather than for atom excitation,resulting in generally larger enhancements of ionic lines than theatomic lines.

4. Conclusions

Multivariate statistical analysis has been demonstrated on manyatomic and ionic lines of various elements that the emissionenhancements induced by oscillating DC arc plasma can be satisfac-torily correlated with two intrinsic properties of an element. Inaddition, these properties, the first ionization energy of the atom(Eion) and its metal oxide bond energy (Ex−o), synergistically affectline intensity emission and its spatial distribution in steady-state arcplasma, and intensity enhancement in the oscillating DC plasma.Because of close values and periodicity of these properties, as well asspecific chemical behavior of some elements, the emission enhance-ment effect appears to be complex and it is hard to propose a singleexplanation. In the case of ionic lines, the larger part of the effect canbe accounted for by the affinity towards metal oxide formation,determined by the atom ground state electronic configuration. In thecase of atomic lines, however, no analogous correlation can beconfirmed. This indicates the possibility of different mechanisms ofemission enhancement for atomic and ionic lines. The complexity isalso demonstrated through obviously linear, but medium, correlationsbetween the atomic and ionic line emission enhancements and thekey properties, Eion and Eion+Ex−o, respectively. However, the resultsof the analysis clearly indicate the role of stable gas-phase metal-oxide formation in DC plasma and the impact it may have on theintensity emission distribution of atomic and ionic lines in steady-state plasma.

In the case of atomic lines, the interpretation of the mechanism ofemission enhancement is no different from already proposed. Theplasma zone close to the arc axis is partially depleted of the analyteatom species. This is induced by the radial electric field in steady-stateplasma (radial demixing) and maximum emission and the width ofthis zone is primarily determined by the first ionization energy of theatom.When the arc core brought into lateral oscillation by an externalmagnetic field in order to penetrate the enclosed colder gas contain-ing higher concentrations of the analyte vapor and aerosol particles,extra excitation and atom emission enhancement occurs. In steady-state plasma, if the ionization energy of an analyte is higher, thedepleted zone is narrower and the intensity of atomic emission is lessaffected by demixing, i.e. the enhancement effect is smaller in theoscillating plasma. In fact, just the opposite of this behavior can beexpected. Consequently, a descending linear correlation between theatomic emission enhancements and first ionization energies shouldexist. However, there are several exceptions to this trend, which canbe contributed to the lack of strength of the applied magnetic field, i.e.insufficient lateral amplitude of the arc core oscillations. If thedepleted central zone is wide enough due to small ionization energyof an element, the arc column mostly oscillates in the depleted zone,and the atom line enhancements are smaller. In addition, theexceptions could originate from the specific chemical properties ofan element, not only due to its ionization energy.

In the case of ionic lines, however, in addition to the first ionizationenergy, metal-oxide formation can significantly affect lateral dis-placement and intensity emission reduction of an analyte in steady-

state DC plasma. The origin of oxygen may be through waterdissociation or through the inlet of air. In cases of some elements,this is reflected through the formation of significant amounts of stablegas-phase oxides at plasma periphery. Sc, Y, Gd, and partially La havesuch a unique electronic configuration and show a strong affinitytoward oxygen bonding. These elements provide the largest emissionenhancements. An upward linear correlation between the ionic lineemission enhancements and the sums of the first ionization energieswith oxide bond energies imply that emission enhancement is mostlythe consequence of metal oxide dissociation, and less a consequenceof extra atom ionization.

Acknowledgement

Financial support by the Ministry of Science of the Republic ofSerbia, Contract No. 142065 and 142047 (I.P.) is acknowledged.

References

[1] M.M. Stoiljković, M.S. Pavlović, J. Savović, M. Kuzmanović, M. Marinković, Study ofaerosol sample interaction with DC plasma in the presence of oscillating magneticfield, Spectrochim. Acta Part B 60 (2005) 1450–1457.

[2] A.B. Murphy, Thermal plasmas in gas mixtures, J. Phys. D Appl. Phys. 34 (2001)R151.

[3] M.S. Pavlović, M. Marinković, Demixing effect induced by the radial electric fieldin the cylindrically shaped DC plasma. Modeling and experimental investigation,Spectrochim. Acta Part B 53 (1998) 81–94.

[4] M.S. Pavlović, M.M. Kuzmanović, V.M. Pavelkić, M. Marinković, The role ofdemixing effect in analyte emission enhancement by easily ionized elements inDC plasma, Spectrochim. Acta Part B 55 (2000) 1373–1384.

[5] H.F. Pop, J.W. Einax, C. Sarbu, Classical and fuzzy principal component analysis ofsome environmental samples concerning the pollution with heavy elements,Chemom. Intell. Lab. Syst. 97 (2009) 25–32.

[6] D. Kara, Evaluation of trace metal concentrations in some herbs and herbal teas byprincipal component analysis, Food Chem. 114 (2009) 347–354.

[7] R.E. Santos Froes, W. Borges Neto, N.O. Couto e Silva, R.L. Pereira Naviera,C.C. Nascentes, J.B. Borba da Solva, Multivariate optimization by exploratoryanalysis applied to the determination of microelements in fruit juice byinductively coupled plasma optical emission spectrometry, Spectrochim. ActaPart B 64 (2009) 619–622.

[8] Lopez-Molinero, A. Villareal Cabalero, J.R. Castillo, Classification of emissionspectral lines in inductively coupled plasma atomic emission spectroscopy usingprincipal component analysis, Spectrochim. Acta Part B 49 (1994) 677–682.

[9] A. Lopez-Molinero, J.R. Castillo, P. Chamorro, J.M. Muniozguren, Multivariatestatistical characterisation of the tolerance of argon inductively coupled plasmasto organic solvents, Spectrochim. Acta Part B 52 (1997) 103–112.

[10] I.B. Brener, I. Segal, M. Mermet, J.M. Mermet, Study of the depressive effects ofnitric acid on the line intensities of rare earth elements in inductively coupledplasma atomic emission spectrometry, Spectrichim. Acta Part B 50 (1995)333–347.

[11] N. Furuta, Spatial emission distribution of YO, YI, YII and YIII radiation in aninductively coupled plasma for elucidation of excitation mechanisms, Spectrochim.Acta Part B 41 (1986) 1115–1129.

[12] G.L. Long, J.S. Bolton, The effect of propane on atomic spectrometric signals in theinductively coupled argon plasma, Spectrochim. Acta Part B (1987) 581–589.

[13] M.M. Stoiljković, M. Pavlović, J. Savović, M. Kuzmanović, Emission intensityenhancement of DC arc plasma induced by external oscillating magnetic field,Contrib. Plasma Phys. 47 (2007) 670–676.

[14] W.F. Meggers, C.H. Corliss, B.F. Scribner, Tables of Spectral-Line Intensities Part I,Arranged by elements, National Bureau of Standards Monograph 32-Part I, U.S.Government Printing Office, Washington, 1961.

[15] http://physics.nist.gov/PhysRefData/Handbook/periodictable.htm.[16] http://www.webelements.com.[17] G.K. Koyanagi, D.K. Bohme, Oxidation reactions of lanthanide cations with N2O

and O2: periodicities in reactivity, J. Phys. Chem. A 105 (2001) 8964–8968.[18] J.K. Gibson, Role of atomic electronics in f-element bond formation: bond energies

of lanthanide and actinide oxide molecules, J. Phys. Chem. A 107 (2003)7891–7899.

[19] H.H. Cornehl, R. Wasendrup, M. Diefenbach, H. Scwartz, A comparative study ofoxo-ligand effects in the gas-phase chemistry of atomic lanthanide and actinidecations, Chem. Eur. J. 3 (1997) 1083–1090.

[20] M.L. Campbell, Temperature-dependent rate constants for the reactions of gas-phase lanthanides with O2, J. Phys. Chem. A 103 (1999) 7274–7279.

[21] G.C.-Y. Chan, G.M. Hieftje, Investigation of plasma-related matrix effects ininductively coupled plasma-atomic emission spectrometry caused by matriceswith low second ionization potentials–identification of the secondary factor,Spectrochim. Acta Part B 61 (2006) 642–659.