Laser-induced breakdown spectroscopy of alkali metalsin high-temperature gas

Alejandro Molina, Peter M. Walsh, Christopher R. Shaddix, Shane M. Sickafoose,and Linda G. Blevins

Laser-induced breakdown spectroscopy (LIBS) measurements of alkali in the high-temperature exhaustof a glass furnace show an attenuation of the Na and K LIBS signals that correlates with the stoichi-ometry of the bath gas surrounding the spark. The results are explained as being due to (1) a strongincrease in the concentration of atomic Na and K, resulting in neutral line signal absorption by theseatoms, and to (2) a change of phase of the major Na- and K-containing species from an aerosol to a gaseousphase when the gas mixture becomes fuel rich, resulting in a reduced LIBS emission intensity. LIBSsampling at lower temperatures, or in a consistently oxidizing environment, or both are suggestedstrategies for circumventing these difficulties. © 2006 Optical Society of America

OCIS codes: 140.3440, 160.2750, 280.1100, 280.1740, 300.6360.

1. Introduction

In laser-induced breakdown spectroscopy (LIBS) anintense laser pulse generates a microplasma at tem-peratures sufficient to dissociate the material in thespark into atoms and ions. Electron–ion collisionsproduce a spectrally broad continuum that dominatesoptical emission within the first few hundred nano-seconds after spark formation. As the plasma tem-perature and electron density decay 0.5–2 �s afterspark formation, spectral line emission from excitedelemental ions dominates the emission spectra. Inthe last stages of the spark �2–50 �s�, line emissionfrom excited neutral atoms and small molecules be-comes dominant.1,2 Time-gated spectroscopic analy-sis of the atomic or ionic spark emission coupled withsuitable calibration permits quantification of elemen-tal concentration.3

The ability of LIBS to determine the in situ ele-mental concentrations of metals has led to the appli-cation of this technique to conditions that differconsiderably from its initial application in the labo-ratory. For example, LIBS has been used in the eval-

uation of the composition of coal particles at elevatedtemperatures and pressures4–6 and to determineaerosol concentrations in the exhaust of thermal-waste treatment units,6–8 glass furnaces,9 pulp millrecovery boilers,9 and electric power generation boil-ers.9 Recently LIBS was applied in the determinationof equivalence ratio in spark-ignited engines10 and isbeing developed as a potential analysis tool for use onfuture spacecraft missions to Mars11,12 and Venus.13

The application of LIBS to severe environmentshas prompted the study of the effects that changesin bath-gas composition5,11–17 temperature,5,11–14,17,18

laser energy,4,19 particulate and humidity levels,14

and phase state of species20 have on the sparkproperties and on the spectral-specific emission ofcertain elements. In high-temperature, high-alkali-concentration environments, such as those present insome industrial furnaces, additional considerationscan become of primary importance. Under these con-ditions, minor variations in the bath-gas stoichiome-try can generate (1) significant concentrations ofatomic alkali atoms in the bath gas, attenuating theemission from neutral alkali atoms in the laser spark,and (2) changes in the chemical nature and phase ofthe alkali-containing species in the system. In thispaper we report the observation of such phenomenain a series of measurements in the exhaust gas froma large glass furnace. Laboratory experiments, chem-ical equilibrium calculations, and spectral simula-tions of neutral-line absorption are also reported insupport of the interpretation of the glass-furnacedata.

A. Molina (amolina@sandia.gov), C. R. Shaddix, S. M. Sickafoose,and L. G. Blevins are with Sandia National Laboratories, Livermore,California 94550. P. M. Walsh is with the University of Alabama atBirmingham, Birmingham, Alabama 35294.

Received 28 June 2005; revised 16 December 2005; accepted 10January 2006; posted 17 January 2006 (Doc. ID 63041).

0003-6935/06/184411-13$15.00/0© 2006 Optical Society of America

20 June 2006 � Vol. 45, No. 18 � APPLIED OPTICS 4411

Our goal in the paper is to emphasize the potentialsignificance of signal absorption and phase changeon LIBS measurements of alkali metals in high-temperature environments. Signal absorption, al-though it is common in atomic absorption spectroscopyin flames,21 was not previously considered in LIBSanalyses. The importance of phase change was re-cently demonstrated by Hohreiter and Hahn20 in lab-oratory experiments but has not been reported in theapplication of LIBS to industrial environments. In thepaper we also propose strategies to mitigate the ob-served sensitivity of alkali LIBS signals to bath-gasstoichiometry.

2. Experiment

A. LIBS Technique

The laser used in the experiments was a ContinuumQ-switched, Nd:YAG laser operated at 1064 nm andwith a repetition rate of 5 Hz. Typical single-pulseoutput was 420 mJ, with a 10 ns pulse duration. Thelaser beam was focused with a 25 mm diameter,100 mm focal-length lens that was also used to collectthe emitted plasma radiation into a bifurcated fiberoptic, as described by Hahn et al.22

A water-cooled steel shroud provided protection ofthe focusing lens from fouling and thermal stress.Nitrogen flowed through the probe at a rate of 9.6standard liters per minute to prevent aerosol deposi-tion on the optics. The standoff distance from thespark to the probe entrance was 5.6 cm. Blevins etal.9 presented a more detailed description of thepurge system.

The bifurcated fiber bundle transmitted the emittedlight to two separate spectrometer systems: a Mul-tichannel Instruments echelle spectrometer (resolu-tion, ���� � 4000) that was coupled to a 12 bit, 1280� 1024 intensified charge-coupled device (ICCD)camera and an Acton 0.25 m Czerny–Turner spec-trometer (resolution, 0.12 nm) coupled to a PrincetonInstruments ICCD. Both ICCD intensifier gates wereset for a 10 �s delay relative to the laser pulse tooptimize detection of neutral alkali emission lines.The gate duration was 50 �s for the echelle spectrom-eter and 150 �s for the Czerny–Turner spectrometer.Whereas the echelle spectrometer permitted detec-tion over a continuous range from 200 to 850 nm, thedetection range for the Czerny–Turner spectrometerwas 24 nm. The higher sensitivity of the Czerny–Turner spectrometer yielded acceptable signal-to-noise ratios for single-shot spectra, whereas eachcollected spectrum for the echelle spectrometer wasthe average of 500 consecutive sparks. The resultspresented below are those for the echelle spectrome-ter, unless otherwise stated.

We calculated spectrally integrated LIBS signalsby integrating the spectrometer’s signal over a de-fined pixel range and correcting for background, us-ing the average baseline of featureless intervalsadjacent to the main peaks. Baseline signals weretypically �0.1% of peak signals. Table 1 shows thespectral regions analyzed in this study. Each set of

sodium and potassium D lines and calcium 393.4 and396.8 nm lines was combined as one area in the anal-ysis. In addition to the Na D lines, the excited-stateNa doublet at 818.3 and 819.5 nm was analyzed.However, the 818.3 nm line was not included in thearea evaluation because N emission at 818.5 and818.3 nm interferes with its detection.

B. Glass-Furnace Measurements

The field measurements were performed in the ex-haust of an oxygen–natural-gas container-glass fur-nace at the Gallo Glass Company plant in Modesto,California. One sampling port was located 15 m fromthe furnace outlet, in the vertical flue duct of theexhaust system. This location is referred to as “up-stream” in this paper. The cross-sectional area of thevertical flue duct was 12.3 m2. Typical values of gastemperature and velocity at the upstream locationwere 1380 K and 0.8 m s�1, respectively. The secondsampling port (referred to as “downstream”) was lo-cated in the horizontal flue duct, after cooling anddilution of the exhaust stream with an air-assistedwater spray. At the downstream location the cross-sectional area was 0.33 m2, the typical gas tempera-ture was 715 K, and the gas velocity was 31 m s�1.

In conjunction with the LIBS measurements, gassamples were collected by use of a water-cooled sam-pling probe and a vacuum pump. Water was removedfrom the sampled combustion products by a cold trap.A conventional portable gas analyzer system wasused to analyze the dry, extracted gases for O2 (zir-conia cell), CO (nondispersive infrared spectroscopy),and SO2 (nondispersive infrared spectroscopy). Dur-ing the glass-plant measurement campaign, the fur-nace was generally operated just on the lean side ofthe natural gas–oxygen stoichiometry, but for someperiods the furnace was operated slightly fuel rich.Air infiltration through the sides and the end of thefurnace as well as into the flue-gas duct made theactual stoichiometry at the sampling point dependenton both the furnace firing conditions and the ambienttemperature.24 Lower limits on the average K, Na,and Ca concentrations in the furnace during the testswere 18 � 1, 114 � 6, and 5 � 1 parts in 106 (ppm)on a molar basis, respectively, as measured by induc-tively coupled plasma mass spectroscopy for analysis

Table 1. Spectral Lines, Energy Levels,a and Analysis Ranges Usedin This Work

LineWavelength

(nm) Ei–Ej (cm�1)AnalysisRange

Na I 588.9950 0–16 973.638 588.5–590.0Na I 589.5924 0–16 956.172Na I 818.3255 16 956.172–29 172.889 Not analyzedNa I 819.4824 16 973.368–29 172.839 819.2–819.8K I 766.4911 0–13 042.876 764.0–772.0K I 769.8974 0–12 985.170Ca II 393.3663 0–25 414.40 393.0–393.7Ca II 396.8469 0–25 191.51 396.5–397.2

aRef. 23.

4412 APPLIED OPTICS � Vol. 45, No. 18 � 20 June 2006

of metals concentrations in the gas-analyzer conden-sate. These results represent the average and thestandard deviation of condensate samples collectedduring three 10-h time intervals. The measuredlower-limit Na concentration is of the same order asthat measured for industrial glass furnaces by Buck-ley et al.25 by a combination of absorption of Na Dlines in the furnace and equilibrium calculations andby Rice et al.26 by extractive sampling and excimerlaser photofragmentation spectroscopy.

C. Laboratory Furnace Experiments

The laboratory experiments were conducted in an up-flow laminar flow reactor driven by the exhaust of adiffusion-flamelet-based flat flame burner (Henckenburner) with a square cross section �5.08 cm on eachside). Figure 1 shows a schematic of the experimentalsetup. Mixtures of O2�N2 and H2�C2H2�N2 enteredthe burner through oxidizer and fuel streams, respec-tively. A 10 standard liters per minute N2 flow wasused to entrain a solution of 56.2 g NaOH�L in waterthrough a TSI Model 9302A atomizer that generatedan aerosol stream. The aerosol was injected into thecombustion products through holes located on twoopposite sides of a quartz chimney resting on top ofthe burner. Two different flow conditions were usedto produce oxidizing and reducing conditions in theburner effluent, while a constant total volumetricflow was maintained. The Na concentration in theburner product flow was 100 ppm for both cases.Table 2 lists the gas concentrations for the oxidizingand reducing environments, as calculated by chemi-cal equilibrium. The gas temperature was 1430 and1530 K for oxidizing and reducing cases, respectively,based on radiation-corrected fine-wire thermocouplemeasurements.

Because Na deposits formed on the quartz chimneyduring the course of the experiments, the LIBS laserbeam was directed through an open �2.54 cm� sectionin the chimney to the reactor centerline. Ambient airinfiltration through the open section caused a reduc-tion in gas temperature and dilution of the burnerflue-gas stream in the flow adjacent to the opening.Although there is no evidence that the air infiltrationinfluenced the LIBS spark at the chimney centerline,it reduced the effective path length over which signalabsorption from Na atoms in the hot flow could occur.The 100 K difference in centerline gas temperatureunder oxidizing and reducing conditions obscures thequantitative analysis of stoichiometry effects but al-lows a meaningful qualitative analysis to be made.

3. Signal Absorption Calculations

Simulation of the effect of the bath gas on the trans-mitted spark spectrum was based on the Beer–Lambert law [Eq. (1) below], as applied to theradiation emitted from the spark �I0���� and absorbedby a uniform-density medium over a constant pathlength (L, in centimeters):

I��� � I0���exp���0f���NL�, (1)

where �0is the absorption cross section at line fre-

quency �0, f(�) is the Voigt line shape function [Eq. (2)below], and N is the media concentration (atoms percubic centimeter). The calculation of the line shapefunction and of the Doppler ��D, collisional ��C, andresonance ��R half-intensity widths (all in hertz) fol-lowed the theoretical description by Winefordner etal.27 that was recently applied to LIBS results28,29:

f��� �a�

��

�exp��y2�

a2 � �x � y�2dy, (2)

x �2�� � �0�

��D�ln 2, (3)

a ���C � ��R

��D�ln 2, (4)

Fig. 1. Schematic of laboratory furnace experiments.

Table 2. Estimated Concentrations in the Laboratory-FurnaceExperiments

Molecule Condition

Condition

Oxidizing Reducing

CO2 Molecular fraction 0.08 0.05O2 Molecular fraction 0.04 0.00N2 Molecular fraction 0.73 0.78H2O Molecular fraction 0.15 0.11H2 Molecular fraction 0.00 0.03CO Molecular fraction 0.00 0.03Na ppm 101.4 100.8

20 June 2006 � Vol. 45, No. 18 � APPLIED OPTICS 4413

��C �2C

2NF

�2RT 1M �

1MF

�1�2

, (5)

��R �2R

2N RT

M 1�2

, (6)

��D �2�2R ln 2

c �oTM1�2

. (7)

The integral in Eq. (2) was calculated according tothe method described by Humlicek.30 In Eq. (5), NF

refers to the number density (inverse centimeters) ofspecies different from the emitting atom. M and MF

are the molecular weights of the emitting and foreignspecies, respectively. C

2 and R2 (square centimeters)

represent the cross sections for collision and reso-nance broadening, respectively, c is the speed of lightin vacuum (centimeters per second), T is the temper-ature (kelvin), and R is the universal gas constant(erg per kelvin per mole). Table 3 shows the colli-sional cross sections for the Na D lines used in oursimulations, as selected from the review of Nefedov etal.31 Cross sections at 2000 K were used for all mol-ecules except NO, for which only data at 583 K havebeen reported. The resonance broadening cross sec-tion for the Na D1 line is 164 10�14 cm2, as mea-sured by Popov and Ruzov.32 However, given the lowconcentration of Na atoms, the effect of resonancebroadening was negligible compared with that of col-lisional broadening by the major gas species.

�0was calculated according to27

�o�

2�ln 2 �02gm At

83�2��D gn, (8)

where At (inverse seconds) is the Einstein coefficientfor spontaneous emission, gm and gn are the statisticalweights of the excited and ground states, respec-tively, and �0 is the peak wavelength (centimeters).The values for At and g were taken from the NationalInstitute of Standards and Technology atomic spectradatabase.23 The calculated values of �0

at 1373 K are285 10�14 cm2 and 573 10�14 cm2 for the D1 andD2 lines, respectively. Note that the value of �0

forthe D1 line is of the same order as that measured byPopov and Ruzov32 for collisional broadening at580 K.

4. Results

A. Measurements in the Glass-FurnaceUpstream Location

Figure 2 shows the variation of the spectrally inte-grated LIBS signals (peak areas) for Na, K, and Ca,together with O2 and CO concentrations (dry), asmeasured at the upstream location in the glass-furnace exhaust. This 7 h interval is characteristic ofseveral others that occurred during a 200 h samplingcampaign. In Fig. 2(a) the Na and K peak areas showa steep decrease during the 4–5 and 6–7 h intervals.In contrast, the Ca peak area remains constant in thesame intervals. As is evident from Fig. 2(b), the steepdecrease in Na and K areas coincides with periods oflow O2 and high CO concentration caused by changesin furnace stoichiometry.

Figure 3 presents correlation plots of the Na D, Na819.5 nm, K, and Ca LIBS peak areas with O2 con-centration collected over a 100 h time interval. TheLIBS peak areas were normalized by the averagevalue when the O2 concentration was more than2.5%. Figure 3 shows that the LIBS peak areas for Naand K decrease with decreasing O2 concentration forO2 concentrations less than 2.5%. Interestingly, COwas present in the exhaust gas whenever the O2 con-centration was lower than 2.5%. In contrast to the Naand K signals, the variation in the Ca LIBS peak areawith O2 concentration for O2 less than 2.5% is negli-gible. Extractive sampling measurements of alkaliconcentration, by analysis of the gas-analyzer con-densate, showed that the average Na, K, and Ca con-

Table 3. Collisional Cross Sections (Å2) for Major Speciesfor Na D Linesa

Molecule D1 D2 T (K)

H2O 244 248 2000CO2 194 199 2000N2 174 175 2000CO 194 199 2000NO 282 231 583

aFrom Ref. 31.

Fig. 2. Variation in Na, K, and Ca LIBS peak areas and in O2 andCO concentrations (dry) with time during measurements in glass-furnace exhaust at the upstream location.

4414 APPLIED OPTICS � Vol. 45, No. 18 � 20 June 2006

centrations in the furnace were fairly independent ofthe O2 concentration in the furnace. Figures 3(a) and3(b) show that, whereas the areas of both the Na Dand Na 819.5 nm lines decrease as the O2 concentra-tion decreases below 2.5%, the effect is more pro-nounced for the Na D lines. In fact, at 0% O2concentration the regression line in Fig. 3(a) yields aNa D peak area of 0.3, while Fig. 3(b) gives a value of0.7 for the Na 819.5 nm line.

Figure 4 shows details of spectra that correspondto points A, B, and C highlighted in Fig. 3. The A, B,

and C spectra represent periods with 0%, 0.8%, and4.3% O2 concentrations, respectively. Spectra A andB in Figs. 4(a) and 4(b) were multiplied by a factor tofacilitate comparison. From Fig. 4(a) it is evidentthat, as O2 concentration decreases, the intensity ofthe Na D lines �3s2 ← 3p2P� decreases. However,spectrum A �0.0% O2� not only shows a lower inten-sity than spectrum C �4.3%� but reveals a significantchange in the spectrum shape, as is evident from thehigher reduction of LIBS signal at the peak center-lines (589.0 and 589.6 nm) compared with the signalat an intermediate wavelength �589.3 nm�. SpectrumB, at an O2 concentration between those of spectra Aand C, shows an intermediate behavior, with reduc-tion in intensity and a less evident change in spectralshape. Figure 4(b) presents two Na emission lines,which correspond to transitions that originate from amore excited electronic state that do not decay to theground state �3p2P ← 3d2D�. The fractional decreasein the emission peak area for the low-O2 spectrum (A)in Fig. 4(b), is less than that observed for the Na Dlines. Furthermore, the variation in line shape under

Fig. 3. Variation of Na D lines, Na 819.5 nm, K, and Ca LIBSpeak areas (normalized by the average area for O2 � 2.5%) with O2

concentration (dry) during measurements in glass-furnace exhaustat the upstream location. Curves are linear fits to the data for theranges 0–2.5% O2 and 2.5–6% O2. Squares represent locations forpoints A, B, and C for which spectra are shown in Fig. 4.

Fig. 4. Emission spectra for points A, B, and C in Fig. 3. (a) Na Dlines, (b) Na 818.3 and 819.5 nm lines, (c) Ca 393.4 and 396.8 nmlines. The spectra in (a) and (b) were multiplied by the indicatedfactors to facilitate comparison.

20 June 2006 � Vol. 45, No. 18 � APPLIED OPTICS 4415

reducing conditions is negligible in Fig. 4(b) com-pared with Fig. 4(a). Figure 4(c) shows that the mag-nitude and shape of the Ca lines remain essentiallyunchanged (within a noise level) for the spectra ob-tained at different O2 concentrations. For all the spec-tra in Fig. 4, the continuum emission, as observedfrom featureless regions in the spectra, did not showany significant changes for the different O2 concen-trations.

B. Measurements in the Laboratory Furnace

Figure 5 shows the variation in Na D and Na 819.5peak areas as measured in the laboratory-furnaceexperiments for the oxidizing and reducing condi-tions described in section 2. The Na D peak areastrongly decreases for reducing conditions, while theNa 819.5 nm line remains constant. Figure 6 showssample spectra of the Na D and Na 819.5 nm linesthat correspond to the reducing and oxidizing condi-tions, respectively, in Fig. 5. For reducing conditions,not only does the integrated peak area for the Na Dlines decrease but the center of the emission linesshows evidence of line inversion (i.e., a local dip inemission intensity). In contrast, the intensity andshape of the Na 819.5 nm lines do not show any sig-nificant difference for reducing and oxidizing condi-tions. These experiments demonstrate that, at theconditions in the laboratory-furnace experiments, a

shift in the bath-gas stoichiometry from lean to richresults in a sharp reduction in the Na D-line LIBSsignals, while the Na 819.5 signal remains essen-tially invariant.

5. Discussion

The results shown in Figs. 2–6 suggest the existenceof one or more processes that cause a decrease in theLIBS Na and K D signals under reducing conditions,while they do not affect the Ca signal. At the sametime, the Na 819.5 nm lines show a decrease for theglass-furnace measurements and no change for theexperiments in the laboratory furnace. The reductionin Na signal for the glass-furnace measurements isstronger for the Na D lines than for the Na lines at819.5 nm and involves a significant modification ofthe spectral peak shape. The fact that the Ca signaldoes not show a significant change with O2 concen-tration suggests that the laser spark intensity, size,and duration did not change appreciably over thisrange of conditions and therefore are not the cause ofthe observed alkali signal decrease. This conclusion isalso supported by the invariance of the continuumemission intensity over this range of conditions. Fur-thermore, the observation of alkali D-line reductionboth in the exhaust of a glass furnace and in thelaboratory furnace suggests that this phenomenondoes not result from changes specific to the glass-making process but rather is related only to the bath-gas composition.

A. Equilibrium Calculations

To understand the variations in the Na speciationthat occur in the system, we performed equilibriumcalculations with the NASA CEA equilibrium codeand thermodynamic data base.33 For our purposes,an equilibrium approximation is reasonable for theexhaust of the glass furnace given the high temper-atures, relatively long residence times, and high de-gree of mixing that occur in the furnace.

The simulation used the ratio of O2 to methane�O2�CH4� as the characteristic parameter control-ling the furnace stoichiometry. A value of O2�CH4 of2.00 represents stoichiometric conditions. In theequilibrium calculations, O2�CH4 was varied from1.95 to 2.05. This range is typical of that observed inglass-furnace exhaust. For the gas mixture, SO2 wasalso considered, given the importance of sulfur onNa chemistry. The SO2 concentration was set to320 ppm, the average value measured during the in-terval shown in Fig. 2, corrected for H2O dilution. TheNa concentration was assumed to be 100 ppm andthe concentration of Ca 1 ppm, consistent with ex-tractive samples taken from the glass-furnace fluegas. Previous researchers25 found Na concentrationsin the glass furnace varying from 15 to 173 ppmand reported an approximate Ca concentration of0.1 ppm.

Figure 7 shows the variation of the predictedequilibrium concentration of Na-containing speciesas a function of O2�CH4 at 1380 K, the average tem-perature of the data measured in Fig. 2. Similar cal-

Fig. 5. Variation in LIBS peak area for Na D and Na 819.5 nmlines during experiments in the laboratory furnace. For times of�20 min (open symbols) the conditions were oxidizing, and fortimes of �20 min (filled symbols) the conditions were reducing.

Fig. 6. Detail of Na D and Na 819.5 nm lines for oxidizing (B) andreducing (A) conditions in the laboratory furnace. Note the differ-ent scale for the Na 819.5 nm line.

4416 APPLIED OPTICS � Vol. 45, No. 18 � 20 June 2006

culations performed at the minimum �1360 K� andmaximum �1400 K� temperatures of the same inter-val showed similar trends to those found at 1380 K.For oxidizing conditions �O2�CH4 � 2.01� most ofthe Na is present as Na2SO4, in both liquid andgaseous forms. As the stoichiometry drops belowO2�CH4 � 2.01, the Na2SO4 is converted to NaOHand Na, which are both gaseous species at thistemperature. The equilibrium between Na2SO4 andNaOH is controlled by the following global reaction,as is well documented in the literature34:

Na2SO4�g, l� � H2O ⇔ 2NaOH�g� � SO2 � ½O2.(9)

As the oxygen concentration decreases (lowerO2�CH4), the equilibrium of reaction (9) is displacedtoward the right. Under reducing conditions, S occurspredominantly as H2S and therefore does not formNa2SO4. For lower values of O2�CH4, NaOH partiallydecomposes to form Na and OH radicals. This reac-tion is favored by the low OH concentrations underfuel-rich conditions. In fact, the Na atom changesfrom 0.04 ppm for O2�CH4 � 2.05 to 17 ppm forO2�CH4 � 1.95.

Figure 8 presents a plot similar to Fig. 7 but forCa-containing species. In contrast to the resultsfor Na, the overwhelming majority of Ca is present

in condensed phases [cubic CaO(Cr) and rhombicCaSO4�II�] throughout this range of stoichiometry.Although some Ca exists in the gas phase �Ca�OH�2�,its concentration is 1 order of magnitude lower thanthat in the condensed phase and decreases as O2�CH4increases, particularly for O2�CH4 � 2. Also, al-though there are changes in the dominant chemicalspecies, Ca remains in molecular forms at all tem-peratures. The predicted concentration of Ca atoms isalways less than 0.01 part in 109 for all the casesstudied.

The analysis of K-containing species (not pre-sented) shows a behavior similar to that observed forNa, with an increase in K atom and KOH concentra-tion and a change from condensed to gas species asO2�CH4 decreases. In summary, the equilibrium cal-culations for glass-furnace conditions show that areduction in O2�CH4 displaces the equilibrium of al-kali species from condensed to gas phases and favorsthe formation of atomic forms, while Ca remains pre-dominantly in molecular condensed forms.

Equilibrium calculations were also conducted forthe laboratory-furnace experiments for the gas com-positions in Table 2 and the measured gas tempera-tures. The results show that for oxidizing conditionsNaOH is the dominant species �99 ppm�, with lessthan 1 ppm of Na. For reducing conditions the NaOHconcentration is predicted to be 30 ppm; and the Naconcentration, 71 ppm. Because of the absence of SO2in the laboratory experiments, Na2SO4 is not formedand, therefore, only gas-phase Na-containing speciesare present in the system.

B. Radiation Absorption by Alkali Atoms

The simultaneous occurrence under reducing condi-tions of (1) a decrease in Na and K D-line peak areas,(2) changes in spectrum shape, and (3) the calculatedappearance of alkali atoms suggests that radiationabsorption by Na and K atoms is largely responsiblefor the observed changes in the signals for the alkaliD lines with stoichiometry variations. At this point itis important to distinguish three optical absorptionprocesses that can occur during LIBS spark forma-tion and signal detection. The first process corre-sponds to the absorption of continuum radiationwithin an optically thick plasma during the first fewhundred nanoseconds after spark formation. Thisself-absorption of spark radiation is responsible forthe initial characteristics of the spark, but it does notproduce spectral-specific interference such as that ob-served in our experiments.

The second absorption process occurs within thespark as well, after the initial continuum emissionhas begun to subside, and it is caused by self-absorption of those emitting species at sufficientlyhigh concentrations. This absorption is responsiblefor the change in slope in LIBS calibration plots thatoccurs at high analyte concentrations (i.e., the curveof growth).28,29,35

Although this process of self-absorption has spec-tral specificity and occurs at the high alkali concen-

Fig. 7. Predicted equilibrium concentration of major Na-containing species at 1380 K. Na2SO4(l) denotes a liquid form ofNa2SO4; all other species are in a gas phase.

Fig. 8. Predicted equilibrium concentration of major Ca-containing species at 1380 K. CaO(Cr) and CaSO4(II) representcubic and rhombic solid forms, respectively; Ca(OH)2 is in a gasphase.

20 June 2006 � Vol. 45, No. 18 � APPLIED OPTICS 4417

trations that are present in the exhaust of glassfurnaces,17 it is independent of the form of the speciesthat exists before spark formation, provided that thelaser pulse has enough energy to transform all ele-ments into ions and neutral atoms.

In the third absorption process, atomic species sur-rounding the spark absorb the radiation generated inthe spark as it passes through the bath gas to thedetection optics. This third process is a non-plasma-related process; its occurrence depends not on theplasma characteristics but on the concentration ofatomic species present in the gas layer between thespark and the light-collection optics.

Absorption by atoms in the bath gas is not signif-icant in traditional LIBS applications, because neu-tral atom and atomic ion concentrations are low inmost conventional environments. However, at hightemperatures neutral alkali atoms can be present insignificant concentrations, as shown in Subsection5.A. This third process of signal absorption was pre-viously analyzed in the fields of laser photofragmen-tation spectroscopy36 and flame spectroscopy.21

Figure 9 shows a comparison of the LIBS spectracollected from the glass-furnace exhaust under oxi-dizing and reducing conditions, together with spectracalculated from Eq. (1). The simulated spectra usedthe measured spectrum under oxidizing conditions(with negligible presumed attenuation between thespark and the collection optics) and determined theattenuation of this signal source under reducing con-ditions. The path length in the simulation was thedistance from the spark to the entrance of the water-cooled probe �5.6 cm�. Five-hundred-shot mean spec-tra collected with the Czerny–Turner spectrometerwere selected for this analysis because this spectrom-eter had a higher spectral resolution. (Both spectrom-eter systems detected similar variations in D-linespectral shape under oxidizing and reducing condi-tions.) In Fig. 9 the simulated spectra are presentedas a function of the assumed concentration of Naatoms in the sampled flow stream. These simulations

suggest that a Na atom concentration of �10 ppmproduces a decrease in the spectrally integrated Nasignal of the same magnitude as that observed in theglass-furnace exhaust. Because the equilibrium Naatom concentration varies from a few parts in 106 forO2�CH4 � 2.00 to 17 ppm when O2�CH4 is 1.95, a Naatom concentration of 10 ppm in the furnace underreducing conditions is reasonable.

It is evident from Fig. 9 that the shape of the at-tenuated line emission is imperfectly predicted. Someof this difference may be attributed to uncertaintiesin the line broadening cross sections. However, itappears that the most important reason for the dif-ference in measured and modeled spectral shapes is aconsequence of using representative, time-averagedLIBS signals. A detailed examination of single-shotspectra suggests that poor mixing in the sampled flowand shot-to-shot jitter in the spectrometer dispersionon the ICCD array (i.e., jitter in the pixel locationsassociated with a given emission line, probablycaused by mechanical vibration inducing displace-ment in the spectrometer grating) lead to significantbroadening of the detected peak and filling-in of thecalculated dip at the line center. Figure 10 showsexamples of 250 consecutive shots taken during oxi-dizing [Fig. 10(a)] and reducing [Fig. 10(b)] condi-tions. While the spectra for oxidizing conditions showsome dispersion, the scatter of the line intensities isconsiderably higher for reducing conditions. Further-more, in Fig. 10(b) there are a significant number ofspectra with intensities well above the average value

Fig. 10. Examples of single-shot Czerny–Turner spectrometerspectra for (a) oxidizing and (b) reducing conditions. The averagespectra appear as white curves.

Fig. 9. Comparison of spectra measured under oxidizing (contin-uous darker curve) and reducing (open symbols) conditions in theglass furnace (Czerny–Turner spectrometer) with spectra calcu-lated from Eq. (1) for the indicated values of Na atom concentration(ppm). Dashed curve, simulation after application of the Gaussianfunction from instrument broadening with a Na atom concentra-tion of 10 ppm.

4418 APPLIED OPTICS � Vol. 45, No. 18 � 20 June 2006

and comparable to the values obtained in oxidizingconditions.

Figure 11 shows the probability-density function(pdf) of the Na peak maximum intensity for 2 h in-tervals of reducing and oxidizing conditions. Whilethe distribution for oxidizing conditions behaves sim-ilarly to a Gaussian function, the plot for reducingconditions shows a tail at high intensities that rep-resents spectra for reducing conditions with shapesand intensities similar to those of oxidizing condi-tions, as the single shots in Fig. 10 show. These high-intensity spectra correspond to periods when the localNa atom concentration in the bath gas is low, pre-sumably because of imperfect mixing and the tran-sient existence of pockets of fuel-lean gases.

Figure 12 gives additional evidence of the effect ofvariations in the shot-to-shot centerline, showing thepdf of the location of the maximum intensity in theregion of the 589.0 nm Na peak under oxidizing andreducing conditions for 2 h intervals (53,500 shots).Under oxidizing conditions, the frequency distribu-tion can be represented by a Gaussian centered at589.0 nm, whereas under reducing conditions thenormal peak maximum at 589.0 nm becomes a well(a local minimum), and two maxima, at 588.9 and589.1 nm, appear. The location of the well corre-sponds to the location of the predicted maximum lineinversion in Fig. 9. The maxima at 588.9 and589.1 nm are consistent with the presence of localmaxima at both sides of the location of line inversion,as was predicted in Fig. 9 for a Na concentration of10 ppm.

The pdf of the Na peak location obtained under theoxidizing conditions shown in Fig. 12 can be used torepresent broadening by spectrometer jitter. If thisfunction is applied to the calculated spectrum for aconcentration of 10 ppm, the spectrum obtained(dashed curve in Fig. 9) is closer to the experimentalvalue than the spectrum calculated without instru-ment broadening. However, to accurately predict themeasured spectrum under reducing conditions it is

necessary to account for the variations in bath-gasabsorption that are evident in Figs. 10 and 11.

Figure 13 shows a comparison of the spectra col-lected in the laboratory furnace under oxidizing andreducing conditions, together with the result of aspectral simulation performed as described above butfor the species concentrations given in Table 2. Theinterference path length was assumed to be one quar-ter of the chimney width �1.2 cm�. There is uncer-tainty in this path length, given the infiltration of airthrough the chimney’s open section. Note that themeasured spectrum in Fig. 13 for rich conditionsshows stronger reversals of the Na line centers thanthose in the measured spectrum shown in Fig. 9. Thisresult is expected because the more controlled atmo-sphere in the lab experiments should decrease theshot-to-shot variations that result from spectrometerjitter (probably induced by vibrations) and from localchanges in the gas composition. Similarly, the mea-sured spectrum in Fig. 13 for lean conditions showssharper peaks than were measured when the glass-furnace exhaust was sampled. The spectral simula-tions suggest that a Na atom concentration of10–100 ppm should produce a reduction in spectralintensity similar to that measured under reducingconditions. Equilibrium calculations for the mixturesin Table 2 show that the Na atom concentrationchanges from 0.1 to 67 ppm when conditions arechanged from oxidizing to reducing. Given the uncer-tainty in path length in this experiment, the agree-ment between measurements and experiments issatisfactory. The fact that the calculated spectra forthe glass-furnace conditions as well as for the labo-ratory furnace agree with the experimental measure-ments when expected concentrations of Na atoms arepresent in the bath gas offers strong evidence thatradiation absorption by Na atoms is the primary fac-

Fig. 11. pdf of the maximum Na peak intensity of single shots.Squares, oxidizing; circles, reducing. Czerny–Turner spectrometer. Fig. 12. pdf of the location of the maximum intensity of single

shots for wavelengths close to the Na I 589.0 nm peak. Oxidizing(squares) and reducing (circles) conditions. Czerny–Turner spec-trometer. The curve for oxidizing conditions represents the bestGaussian fit.

20 June 2006 � Vol. 45, No. 18 � APPLIED OPTICS 4419

tor responsible for the observed decrease in intensityof the Na D lines under reducing conditions.

C. Phase Change of Alkali-Containing Species

Although radiation absorption by neutral atoms inthe bulk gas explains the observed variation in spec-tral shape and intensity of the alkali D lines at re-ducing conditions, it does not explain the invariancein spectral shape (in the glass furnace) and peak area(in the laboratory furnace) of the Na 818.3 and819.5 nm lines. Na emission lines in which the lowerstate is electronically excited (i.e., excited-state tran-sitions), such as the 818.3 and 819.5 nm lines, shouldbe much less sensitive to radiation absorption by Naatoms because of the low concentration of electroni-cally excited Na atoms in the bulk gas flow. For in-stance, at 1373 K the Boltzmann equation predictsthat the ratio of excited Na atoms in the 2Po state(corresponding to the lower state for the Na 818.3 and819.5 nm lines) to atoms in the ground state will be3.7 10�8. For a Na concentration of 100 ppm thisrepresents 2.0 107 of Na 3p2P. Given that typicalabsorption cross sections are of the order of 10�12 cm2,the effect of the presence of Na atoms on the 818.3and 819.5 nm lines should be negligible. Surpris-ingly, Fig. 4(b) shows that, under reducing condi-tions, there is a significant decrease in the Na 818.3and 819.5 nm lines.

In a recent paper, Hohreiter and Hahn20 describeda significant decrease in the LIBS atomic carbon sig-nal for interrogation of gas-phase carbon sources incomparison to entrained submicrometer solid carbonparticles. The authors conjecture that particle drageffects during the expansion of the plasma kernelmay lead to a partial concentration of particle-boundcarbon in the spark in comparison to gas-phase spe-cies. These results explain the behavior observed forthe excited-state transitions of Na in the present ex-periments. For the laboratory furnace experiments,in which all Na species are in the gas phase for bothoxidizing and reducing conditions, the variation inline intensity can occur only because of the radiationabsorption caused by the higher Na atom concentra-tion under reducing conditions. Therefore the LIBS

lines for excited-state Na, which are not influenced bysignal absorption by atomic Na, do not show anydifference when they are transitioning from reducingto oxidizing conditions, as is the case in Figs. 5 and 6.

In contrast, for the glass-furnace measurements,the equilibrium calculations (Fig. 7) show a changefrom condensed-phase Na species �Na2SO4�I�� to gas-phase species (NaOH and Na) when the stoichiome-try changes from oxidizing to reducing conditions.According to the results of Hohreiter and Hahn,20 achange from condensed to gaseous species as O2�CH4decreases should cause a reduction in the intensity ofboth neutral and excited lines because it originatesfrom plasma–analyte interactions and not because ofradiation absorption. The results in Figs. 3 and 4support this explanation. The Na 819.5 nm line de-creases in intensity as the O2 concentration decreas-es; however, this decrease is not as strong as thatobserved for the Na D lines. Furthermore, the lineshape for the 819.5 nm line does not change as O2concentration decreases, whereas that of the Na Dlines does. These observations imply that, for theglass-furnace measurements, a portion of the de-crease in the peak area of the alkali D lines withdecreasing stoichiometry may be attributed to theeffect of analyte phase change, but the greater effectfor sufficiently fuel-rich stoichiometries is from alkaliatom absorption of LIBS signals.

In summary, there are two different processes thatcontribute to the variation in intensity of alkali spe-cies LIBS signal with stoichiometry: radiation ab-sorption by alkali atoms and a change in LIBSresponse with phase change. The first process issolely responsible for the decrease in intensity of NaD-line signals in the laboratory-furnace experiments,and, for the glass-furnace experiments, it is the lead-ing contributor to the low signals for the Na D linesfor low O2 concentrations. The second process ex-plains the changes in the excited Na lines (818.3 and819.5 nm) in the glass-furnace measurements as wellas their invariance in the laboratory-furnace experi-ments.

D. Implications for LIBS Measurements of Alkali Metalsat High Temperatures

Although the interference of alkali atoms and sensi-tivity to analyte phase in LIBS measurements de-scribed above clearly complicates the quantification ofNa and K in high-temperature flows, there are severalstrategies available to address this problem. Fig-ure 14(a) shows the Na and K D-line LIBS signals formeasurements in the exhaust of the glass furnace atthe cooler downstream location. Figure 14(b) showsthat, from 5 to 6 h, the CO and O2 concentrationsreached values similar to those in Fig. 2(b), indicatingreducing conditions. However, the Na and K LIBSsignals did not show the steep reduction observed forsampling at the upstream location. The gas temper-ature at the downstream location varied from 700 to730 K during the interval presented in Fig. 14. Equi-librium calculations show that at this temperaturethe Na atom concentration is negligible, even under

Fig. 13. Comparison of spectra measured under oxidizing (con-tinuous darker curve) and reducing (open symbols) conditions inthe laboratory furnace with spectra calculated from Eq. (1) for theindicated values of Na atom concentration (ppm).

4420 APPLIED OPTICS � Vol. 45, No. 18 � 20 June 2006

reducing conditions, and Na2SO4 and Na2CO3, both inthe condensed phase, are the major Na species.

An alternative for effective LIBS measurements ofNa concentration at conditions when the Na atomconcentration is high considers the evaluation of NaII lines, because the Na ion concentration in the bathgas is negligible. However, ionic emission is strongesta few microseconds after spark formation, when con-tinuum emission is still strong, and the intensity ofthe Na II lines is low compared with that of the Na Dlines. Although optimization of delay and gate timesto detect Na II lines is possible, such an approach willgenerally reduce the effectiveness of multielementanalysis and is valid only if there are no phasechanges in the system.

An additional option is to conduct extractive gasanalysis with LIBS. In this approach, gas can be ex-tracted from the furnace and its temperature reducedby dilution and radiation loss to conditions with a neg-ligible concentration of Na atoms and where alkalispecies are in a condensed phase. Although this meth-odology, as with any extractive technique, demandsthe design of a sampling system with minimal sampledeposition, it conserves the near-real-time analysischaracteristics of LIBS.

6. Conclusions

We have demonstrated that measurement of alkalispecies in high-temperature gas by LIBS is affected byattenuation of LIBS signals by alkali atoms in the bathgas and by changes in the phase state of the alkali-containing species. The first phenomenon has not beendocumented in the LIBS literature because, undercommon laboratory conditions, the concentration of al-kali atomic species is negligible. The second processwas recently described by Hohreiter and Hahn.20 The

variation in LIBS signal by these two processes be-comes important in high-temperature chemically ac-tive environments for which the concentration of alkaliatomic species and the phase state of the alkali-containing compounds change with gas compositionand temperature. The particular example presentedhere concerns measurements of sodium and potassiumin the exhaust from a glass furnace. Minor variationsin the oxygen�natural-gas firing ratio in this furnacewere observed to result in substantial reduction in theLIBS peak areas and variation in line shapes for theNa and K D lines, a smaller decrease in the peak areafor Na excited-state lines (818.3 and 819.5 nm), and nochange in the Ca lines. The sharp decrease in the NaD lines when conditions become fuel rich was repro-duced by injection of Na aerosol into a laboratoryflame-driven furnace operated under fuel-lean andfuel-rich conditions. However, these laboratory-scaleexperiments did not reproduce the variations in theexcited-state Na lines that were observed in the glassfurnace. Equilibrium calculations show that at hightemperatures a change in the mixture’s stoichiome-try from fuel-lean to fuel-rich causes a rapid rise inthe alkali atom concentration and a change fromcondensed-phase to gaseous-phase species when S ispresent in the system, as was the case in the glassfurnace. In the absence of S, as concerns the labora-tory experiments, the equilibrium calculations pre-dict a rapid increase in Na atom concentration but nophase change in the system as conditions become fuelrich. Spectral simulations of signal absorption dem-onstrate that the observed extent of signal attenua-tion for the Na D lines in the glass-furnace exhaustand in the laboratory experiments is consistent withsignal absorption by Na atoms at concentrations sim-ilar to those predicted by equilibrium for fuel-richconditions. The gradual decrease in the strength ofthe excited-state Na lines as conditions become fuelrich in the glass furnace probably occurs because ofthe transition from condensed-phase to gas-phase Naspecies in the system. The phase change is also re-sponsible for the initial decrease in the intensity ofthe Na D signal as the O2 concentration decreases,before the atomic Na concentration becomes signifi-cant. The application of LIBS after the treatment ofthe gas sample to reduce gas temperature, to ensurean oxidative environment, or both are alternativesthat permit quantitative LIBS evaluation of alkalispecies in high-temperature gas flows.

This research was performed as part of a projectfunded by the U.S. Department of Energy (DOE) Of-fice of Industrial Technologies, under the direction ofElliot Levine (Glass Industry). Support was also pro-vided by Gallo Glass Company of Modesto, Califor-nia. John Neufeld of Gallo Glass provided technicalassistance and coordination of site measurementcampaigns. Insightful discussions with Thomas Set-tersten of Sandia National Laboratories are acknowl-edged. Doug Scott of Sandia National Laboratoriesprovided essential technical assistance. The sugges-tion by a reviewer of this manuscript to consider the

Fig. 14. Variations in Na and K D-line LIBS peak areas and in O2

and CO concentrations (dry), with time during measurements inglass-furnace exhaust. LIBS sampling occurred in the downstreamlocation, with gas analysis performed in the upstream location.

20 June 2006 � Vol. 45, No. 18 � APPLIED OPTICS 4421

recent results reported by Hohreiter and Hahn20 isgratefully acknowledged. Sandia is a multiprogramlaboratory operated by Sandia Corporation, a Lock-heed Martin Company, for the U.S. DOE’s NationalNuclear Security Administration under contractDE-AC04-94AL85000.

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