Effect of temperature and CO 2 concentration on laser-induced breakdown spectroscopy measurements of...

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Spectrochimica Acta Part B

Effect of temperature and CO2 concentration on laser-induced breakdown

spectroscopy measurements of alkali fume

Alejandro Molina a,*, Christopher R. Shaddix a, Shane M. Sickafoose b,

Peter M. Walsh c, Linda G. Blevins a,1

aHydrogen and Combustion Technologies, Sandia National Laboratories, P.O. Box 969 MS 9052, Livermore CA 94550, USAbAnalytical Material Science, Sandia National Laboratories, P.O. Box 969 MS 9403, Livermore CA 94550, USA

cUniversity of Alabama at Birmingham, Department of Mechanical Engineering, BEC 257; 1530 Third Avenue South, Birmingham, AL 35294, USA

Received 1 December 2004; accepted 15 June 2005

Available online 21 July 2005

Abstract

Laser-induced breakdown spectroscopy (LIBS) was used in the evaluation of aerosol concentration in the exhaust of an oxygen/natural-

gas glass furnace. Experiments showed that for a delay time of 10 As and a gate width of 50 As, the presence of CO2 and changes in gas

temperature affect the intensity of both continuum emission and the Na D lines. The intensity increased for the neutral Ca and Mg lines in the

presence of 21% CO2 when compared to 100% N2, whereas the intensity of the Mg and Ca ionic lines decreased. An increase in temperature

from 300 to 730 K produced an increase in both continuum emission and Na signal. These laboratory measurements were consistent with

measurements in the glass furnace exhaust. Time-resolved analysis of the spark radiation suggested that differences in continuum radiation

resulting from changes in bath composition are only apparent at long delay times. The changes in the intensity of ionic and neutral lines in the

presence of CO2 are believed to result from higher free electron number density caused by lower ionization energies of species formed during

the spark decay process in the presence of CO2. For the high Na concentration observed in the glass furnace exhaust, self-absorption of the

spark radiation occurred. Power law regression was used to fit laboratory Na LIBS calibration data for sodium loadings, gas temperatures,

and a CO2 content representative of the furnace exhaust. Improvement of the LIBS measurement in this environment may be possible by

evaluation of Na lines with weaker emission and through the use of shorter gate delay times.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Laser-induced plasma; Spectroscopy; LIBS; Spectrochemical analysis; Bath gas interference

1. Introduction

A number of industrial combustion systems are adopting

oxygen-enhanced combustion to improve heat transfer

characteristics and reduce emissions of NOx [1]. In

0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.sab.2005.06.005

* Corresponding author. Tel.: +1 925 294 3088; fax: +1 925 294 2276.

E-mail addresses: [email protected] (A. Molina),

[email protected] (C.R. Shaddix), [email protected]

(S.M. Sickafoose), [email protected] (P.M. Walsh),[email protected] (L.G. Blevins).1 Present address: National Science Foundation, 4201 Wilson Boulevard,

Room 525, Arlington, Virginia 22230.

particular, retrofit of air/natural-gas to oxygen/natural-gas

in glass-container furnaces has been a common practice since

the beginning of the 90’s. In these furnaces, sand, lime, soda

ash and other raw materials that form the glass are melted and

flow underneath long flames that burn below a radiant,

refractory crown. In addition to yielding immediate benefits,

such as production increase, reduction of pollutant emissions

and improved glass quality, oxygen/natural-gas combustion

in glass furnaces has been found to cause an increase in

refractory corrosion. It is believed that the increased

corrosion is a consequence of higher concentrations of alkali

metals, mainly sodium and potassium, that volatilize from the

glass melt, coupled with higher water concentration, as

occurs due to reduced gas volume in oxy-glass furnaces [2,3].

60 (2005) 1103 – 1114

A. Molina et al. / Spectrochimica Acta Part B 60 (2005) 1103–11141104

The importance of alkali volatilization in refractory corrosion

motivates the evaluation of the effect that furnace parameters

have on the concentration of volatilized Na and K in glass

furnaces. Ca, Al andMg are also apparent in the glass furnace

exhaust, though at concentrations lower than Na and K.

The effectiveness of laser-induced breakdown spectro-

scopy (LIBS) as a technique for the in situ evaluation of

elemental concentration in environments as different as

Mars [4,5], Venus [6] or industrial boilers and furnaces [7,8]

makes it an ideal technique for the evaluation of Na and K

concentrations in glass furnace exhaust. No matter where

LIBS is applied, the evaluation of elemental concentration

by LIBS requires the preparation of calibration plots under

carefully controlled conditions. For the case of alkali metals,

Radziemski et al. [9] showed that the evaluation of Na

concentration in air by LIBS was possible by the generation

of a calibration plot that was non-linear, even for a log–log

plot of spectrometer signal versus Na (Ag/m3) concentration.

In the evaluation of aerosol concentration in industrial

furnaces, one common approach [7,10,11] has been to

extrapolate the results (in mass units per actual volume) of

calibration plots performed in the laboratory to the flue gas,

where the temperature may be several times that in the

laboratory and where the gas composition differs somewhat

from that of air or pure nitrogen used in the laboratory. This

methodology has been justified, in part, by a study by Yalcin

et al. [12]. These authors showed that the bath gas

composition, laser energy, particle levels and humidity had

little effect on the spark temperature and electron number

density at short delay times and in the center of the spark,

where local thermodynamic equilibrium (LTE) conditions

could be assumed to apply. However, recent studies have

shown that temperature, pressure and bath gas composition

can affect LIBS signals for gases [13,14], aerosols [11,15],

fine particulate matter [8] and solid samples [16–18].

We used LIBS to evaluate the effects of furnace

parameters on alkali volatilization in an oxygen/natural-gas

glass furnace [19]. At the available measurement locations in

the furnace exhaust, the gas is composed of 33–45% H2O,

15–37% CO2, 1–6% O2, 14–39% N2 (from air infiltration)

and trace compounds such as SO2, CO and NO. The gas

temperature at the sampling points varies from 720 to 1380K.

These conditions are markedly different from the traditional

atmospheres used for LIBS calibrations of aerosol systems

[10,20]. The literature on the effects of gas composition on

the LIBS evaluation ofmetal concentration in solid samples is

extensive [21–29], though most studies have focused on the

effects of using Ar, He, N2 or air at different pressures (at

room temperature). These studies have shown that for LIBS

interrogation of solid samples there is a complex interaction

between the extent of ablated material and the location of the

plasma with respect to the solid sample. Ar produces higher

line and background intensities than He at low pressures (¨5

Torr). The reason for this is that Ar favors the cascade-like

growth that occurs during plasma generation due to the higher

molecular mass (M =40) and lower ionization energy

(E =14.5 eV) than He (M=4, E =23.4 eV) [25]. However,

as pressure increases, the higher energy absorbed by Ar

shields the solid surface from incoming laser radiation, and

therefore there is a decrease in the amount of sample ablated

[26] as well as a displacement of the electronic breakdown

away from the sample [25]. This combined effect produces

lower line intensities in Ar than He as the pressure approaches

one atmosphere.

For LIBS analysis of submicron fume particles, as

considered here, the relevant physics does not involve the

complexity of plasma–surface interactions, so different

trends may be expected. Also, in the LIBS analysis of

alkali metals, long time delays (on the order of 10 As afterthe laser pulse) are typically used in the collection of the

LIBS spectra, to optimize the peak-to-background ratio. The

effects of gas composition on late-time evolution of the laser

spark may well differ from trends apparent with short time

delays. Therefore, we investigated the influence of CO2

concentration and gas temperature on the LIBS analysis of

alkali fume, approximating conditions in the exhaust of an

oxygen/natural-gas glass furnace.

2. Experimental

2.1. LIBS technique

A Q-switched, Nd-YAG laser with a 10 ns pulse width, a

typical output energy of 360 mJ, and a repetition rate of 5 Hz

was operated at the fundamental frequency (1064 nm). A

100-mm-focal-length-quartz lens was used both to form the

spark and to collect the emitted plasma radiation into a UV-

quality fiber through the use of a broadband pierced mirror

and a focusing mirror [30]. The fiber was coupled to a

Multichannel Instruments echelle spectrometer with a 12-bit,

1280�1024 intensified charge-coupled device (ICCD)

camera. The combination of the echelle spectrometer and

the ICCD provides sensitivity over a continuous range from

200 to 850 nm with a constant spectral resolution of k /Dk =4000. The spectrometer was calibrated every one hour

by use of a fiber-coupled Hg lamp. An ICCD intensifier gate

delay (td) of 10 As and a gate duration (tg) of 50 As were usedfor the measurement of alkali metal concentrations. These

gate settings are optimized for detection of the Na D lines, as

previously noted by Cremers and Radziemski [31] and

confirmed in the current study. To obtain statistically

independent results, each spectrum corresponds to 500

consecutive sparks. Ten sparks were accumulated on the

ICCD array, and 50 accumulated signals were summed to

give the final spectrum.

In the analysis below, the peak area was calculated

according to Eq. (1), where hi is the measured intensity for

pixel i, ki the wavelength for pixel i, Base the average

intensity for two featureless intervals adjacent to the main

peak and Dk the length of the peak window interval. For

the Na D lines the reference baseline intervals were

0

10

20

30

40

50

60

575 585 595 605 615 625 635 645

wavelength (nm)

tran

smis

sion

(%

) 12

34

0

2

4

6

inte

nsity

(a.

u.)

Fig. 2. Comparison of laboratory sodium aerosol LIBS spectrum (thick line)

and 590 and 635-nm bandpass filters.

A. Molina et al. / Spectrochimica Acta Part B 60 (2005) 1103–1114 1105

(587.00 to 587.75 nm) and (591.00 to 591.75 nm) and the

peak intervals were (588.25 to 589.25 nm) and (589.25 to

590.5 nm) for the Na lines at 588.99 and 589.59 nm

respectively.

Area ¼X

0:5� hi þ hiþ1ð Þ � kiþ1 � kið Þð Þ � Base� Dk

ð1Þ

For measurements in the glass furnace exhaust, a 7.9-cm

outer diameter, 91-cm-long water-cooled probe was used to

perform LIBS measurements away from the wall surfaces.

N2 flowed through the probe at a rate of 9.6 slpm to prevent

fouling of the focusing lens by alkali fume. Experiments

verified that at this flow rate the N2 did not affect the spark

in either the laboratory or glass furnace sampling environ-

ments. Blevins et al. [7] and Hahn et al. [30] present a more

detailed description of the LIBS system.

2.2. Tube furnace

A tube furnace (0.85-m heated section) with an alumina

flow tube (internal diameter=0.10 m, length=1.38 m) was

used in the high-temperature experiments. The position of

the alumina tube in the furnace was such that one end of the

tube coincided with the end of the furnace heated section

(see Fig. 1). Certified standard solutions of alkali metals

were entrained in a standard pneumatic-type medical

nebulizer (similar to the one described by Hahn et al.

[20]). The nebulizer was connected through a 1.27 cm OD

stainless steel tube to a 2.54 cm OD expansion that had a

coflow (70 slpm) of N2 or N2/CO2 that prevented aerosol

deposition on the walls. 7.4 slpm of N2 flowed through the

nebulizer.

The laser spark was focused at the center of an annular

piece of refractory with a 2.54 cm inner diameter and a

thickness of 1.45 cm located at the end of the furnace (to

provide better jetting action of the furnace effluent). The gas

temperature at the spark location was measured with a 1 /32

in. type-K thermocouple with the coflow gas and nebulized

stream flowing through the furnace. The average of the gas

temperature at entrance and exit of the annular piece was

considered to be the gas temperature at the spark location.

aerosol and coflow entrance

spark location

2.5 10.2

85.153.4

alumina flow tube

exit flow aperture

Fig. 1. Schematics and dimensions (in cm) for laboratory experiments using

a tube furnace.

2.3. Time-resolved spark intensity

For determination of the time-dependent spark behavior,

the emitted light was collected with a 2.5 cm focal length/

2.5 cm diameter lens, a set of bandpass filters and a Thor

Labs PCA-155 photodiode detector, coupled to a 500 MHz

digital oscilloscope. Two bandpass filters with nominally

identical FWHM and similar peak transmittances were

selected to pass the 589 nm Na doublet signal and a

featureless region close to the Na lines, representing the

continuum emission from the LIBS spark. Fig. 2 shows the

transmission regions for the 590-nm (Na region) and 635-

nm (baseline) bandpass filters and compares them to a

typical laboratory LIBS spectrum for sodium aerosol. The

Na region filter captures the complete Na line, while the

baseline filter does not present any interference. During the

experiments, the average signal of 200 sparks was recorded

for each filter.

3. Results

3.1. CO2 and temperature effects on Na signals

Fig. 3a shows that the presence of 21% CO2 (N2 balance)

increases the sodium peak area, (ANa), and produces a

strong increase in the intensity of the baseline (Fig. 3b) close

to the Na peaks, (BNa), when compared to a spectrum

collected under similar Na concentration, but under 100%

N2. Integration over the Na peak area yields a ratio of

sodium areas (AC02Na /AN2

Na) of 1.27T0.03 and the ratio of

average baseline values (BC02Na /BN2

Na) of 1.92T0.06. Fig. 4shows the emission decay for the 635-nm bandpass filters

for 100% N2 and the mixture of 21% CO2 in N2 at 298 K.

Because the spectrometer measurements were collected

from 10 to 60 As after the laser pulse, the measurement of

temporal decay was focused on this region. In Fig. 4a, the

presence of 21% CO2 increases the signal through the 635-

nm filter, particularly between 15 and 50 As. Fig. 4b shows

results with the 590-nm bandpass filter. The variation with

bath composition observed in Fig. 4b is similar to the one

observed in Fig. 4a. The presence of CO2 increases the

0

5

10

15

20

25

30

35

I 635

(m

V)

1234

N2

21 % CO2

a.

background

0

5

10

15

20

25

30

35

I 590

(m

V)

N2

21 % CO2

b.

signal + background

0

5

10

15

20

25

30

35

10 30 50 70 90 110 130 150

t (µs)

I 590

- I

635

(mV

) 12

34

N2

21% CO2

c.

signal

Fig. 4. Time-resolved analysis of spark emission during Na aerosol flow at

different gas bath compositions: 21% CO2/N2 balance (triangles) and

100% N2 (circles). The gas temperature was 298 K and the Na

concentration was 33,300 Ag m�3. a. 635-nm filter; b. 590-nm filter, c.

I590– I635 Symbols are used to distinguish lines labels and do not represent

actual data points.

0.01

0.1

1

10

587 588 589 590 591

wavelength (nm)

inte

nsi

ty (

a.u.)

1234

300 K

730 K

Fig. 5. Effect of bath gas temperature on peak and baseline signal.

Experiments in tube furnace with N2 as bath gas. Na concentration was

12,800 Ag m�3 and 12,400 Ag m�3 at 300 K (squares) and 730 K

(diamonds), respectively.

0

1

2

3

4

587.5 588 588.5 589 589.5 590 590.5 591

wavelength (nm)

inte

nsity

(a.

u.)

1234

N2

CO2

a.

0.01

0.1

1

10

587.5 588 588.5 589 589.5 590 590.5 591

wavelength (nm)

inte

nsity

(a.

u.)

1234

N2

CO2

b.

Fig. 3. Effect of the presence of 21% CO2/N2 balance (triangles) on peak

and baseline signals during tube furnace experiments when compared to

100% N2 (circles). Gas temperature was 735T10 K and balance gas was

nitrogen. Na concentration was 12,400 Ag m�3 for 100% N2 and 12,000 Agm�3 for 21% CO2. Fig. 3b uses a logarithmic scale in the ordinate to

highlight changes in baseline.


signal, particularly in the region between 15 and 60 As. Fig.4c shows the difference between the 590- and 635-nm

filters, demonstrating that the presence of CO2 increases the

Na D line signal. The ratio of the areas under the curve

between 10 and 60 As for the lines in Fig. 4c is 1.27, in goodagreement with the spectrometer results.

Fig. 5 illustrates the effect of bath gas temperature on Na

emission. The Na concentration in Ag m�3 (i.e., mass

loading per actual gas volume, at temperature) is similar at

both temperatures. The comparison is based on actual gas

volume since, for an invariant spark volume, equal masses

of Na per actual gas volume should yield the same LIBS Na

signal. A comparison based on actual gas volume, therefore,

accounts for the changes in concentration due to the

decrease in gas density with temperature.

In Fig. 5, as temperature increases, the Na peak signals

increase (A730Na /A

298Na =2.59T0.07). At the same time, the

baseline shows an increase in intensity (B730Na /B298

Na =

1.54T0.08). A time-resolved spark intensity experiment

also showed an increase in signal from the 635- and 590-nm

bandpass filters due to the increase in gas temperature from

298 to 730 K.

Additional evidence that the presence of CO2 and

temperature affects the baseline can be obtained from the

spectra collected in the glass furnace exhaust, because LIBS

measurements were conducted before (upstream) and after

(downstream) dilution and cooling of the furnace exhaust

with an air-assisted water spray (see Table 1). The results of

the laboratory experiments suggest that the upstream

measurement (higher CO2 and temperature) should show a

higher background and larger peak area than the down-

stream measurement. Fig. 6 shows a comparison of spectra

collected at both locations, demonstrating that the baseline

signal upstream is higher than downstream. Although there

Table 1

Typical conditions at the two sampling locations

Location T Vel. Typical gas composition

K m/s H2O (%) O2 (%) N2 (%) CO2 (%) SO2 (ppm) dry NO (ppm) dry

Upstream 1378 0.8 33–45 1–6 14–33 27–37 40–1000 230–640

Downstream 721 30.8 ¨35 ¨6 ¨39 ¨15 N.A. ¨80


are changes in the Na line peak intensity, a comparison of

Na line sensitivities is not straightforward, because of

dilution that occurs between the two locations.

3.2. CO2 effects on Mg and Ca signals

Fig. 7 shows that the intensity of the Mg II (279.5 nm and

280.3 nm) lines decreases when CO2 is present in the gas

mixture. This seems to contradict the previous experiments

that showed that the presence of CO2 increases the Na D line

signals. However, Fig. 7 also shows that the Mg I neutral line

(285.2 nm) behaves similarly to the Na D lines (i.e., its

intensity increases when CO2 is present). The singly-ionized

Ca II lines (317.9 nm and 318.1 nm) also show a decrease in

intensity when CO2 is present, whereas the intensity of the

neutral Ca I line (422.6 nm) increases in the presence of CO2

(see Fig. 8). Given that the Na D lines are neutral lines, the

results consistently show that the effect ofCO2 on line intensity

at long delay times depends on the ionic state of the element.

0.001

0.01

0.1

1

10200 250 300 350 400 450 500 550 600 650 700 750 800 850

wavelength (nm)

inte

nsity

(a.

u.)

123

4

upstream

downstream

a.

0.01

0.1

1

10

586 588 590 592

wavelength (nm)

inte

nsity

(a.

u.)

1234

upstream

downstream

b.

Fig. 6. Comparison of downstream and upstream LIBS spectra from the

glass furnace exhaust. a. complete spectrum; b. Na region. The complete

spectrum was filtered to reduce number of data points and facilitate

comparison. Periodic variations are apparent in the baseline of the complete

spectrum because of differences in the orders of the echelle spectrometer.

3.3. CO2 and temperature effects at short delay times

Fig. 9 shows the time-resolved spark intensity at the early

stages of spark formation for two bandpass filters, with 21%

CO2 or N2 as bath gases, and for 298 and 730 K. A neutral

density filter was used in front of the photodiode detector to

prevent saturation. It is apparent that the gas composition

does not cause any difference in spark intensity at short

delay times. However, the spark intensity after the initial

peak is higher at 298 K than at 730 K. Note that at the early

stages after the spark formation (t <0.5 As) the radiation

intensity is dominated by an intense background continuum

due to bremsstrahlung radiation from electron–ion colli-

sions [31,32]. Therefore, the intensity detected by the 590-

nm bandpass filter at short delay times is not proportional to

the Na concentration, but represents the continuum intensity.

The trending in continuum intensity at short delays is

opposite to the trend found at long delays, where emission

was strongest for sparks formed in the higher temperature

bath gas.

4. Discussion

4.1. Variation of continuum and line intensity due to the

presence of CO2

The results show that temperature and the presence of

CO2 affect the signal intensity during the application of

LIBS for the detection of aerosol streams containing Na, Mg

and Ca. To understand these effects, it is important to

consider the evolution of the spark radiation with time.

The variations in signal with bath gas composition found

in the current study do not contradict previous authors (e.g.

Yalcin et al. [12]) that concluded that the effect of bath gas

composition on the spark electron number density and

temperature is minor. In fact, Fig. 9 shows that the time-

resolved decay of spark radiation in N2 and 21% CO2 is the

same for a delay time of less than 0.5 As. In the study by

Yalcin et al. [12], delay times between 0.35 As and 2 As wereused, with exposure times equal to 30% of the delay time.

For these short values of delay and exposure, one might

expect the effect of bath gas composition on the spark

emission to be minor. Gleason and Hahn [15] and Buckley

[11] also observed less LIBS signal quenching by O2 for

shorter delay times.

At long delay times (10 As or longer), our results showsignificant effects of the bath gas composition on both the

0

1

2

3

4

5

6

279.25 279.5 279.75 280 280.25 280.5

wavelength (nm)

inte

nsity

(a.

u.)

1234

0

0.1

0.2

0.30.4

0.5

0.6

0.7

0.8

285 285.25 285.5

inte

nsity

(a.

u.)

279.

6 M

g II

280.

3 M

g II

285.

2 M

g I

Fig. 7. Variation in Mg lines for different bath gas composition: 21% CO2 (triangles), 100% N2 (circles). The gas temperature was 710 K and Mg concentration

was 11 mg m�3. Note different vertical axes.


continuum emission and the alkali line intensities. The

similar shape of the profiles of the time-resolved decay of

Na signal intensity for N2 and CO2 in Fig. 4c suggests that

the difference in Na emission strength is not related to the

temporal evolution in the recombination of Na ions and

electrons (the recombination yields excited Na neutral atoms

that then radiate). Our understanding of the increase in Na

peak signal with CO2 in the bath gas (or with gas

temperature) would be clearer with the evaluation of spark

temperature by the Boltzmann or Saha equations [12].

Given the evidence of self-absorption of the detected alkali

lines, as discussed below, we used Mg lines to determine the

spark temperature. Although Boltzmann plots have been

employed in the evaluation of spark temperature under self-

absorption conditions [33], the energy spread in a single

ionization state is normally too low in LIBS applications to

guarantee accurate temperature evaluation [12]. However, a

Saha–Boltzmann approach increases the energy spread by

allowing the combination of different ionization stages. Fig.

10 shows the result of a Saha–Boltzmann plot for the

spectra in Fig. 7, following the procedure described by Bye

and Scheeline [34]. Table 2 shows spectroscopic data, from

the NIST Atomic Spectral data base [35], of the ionic and

neutral lines used in the calculations. Only low-intensity

lines were selected to avoid any interference of self-

absorption in the data. In particular, the strong Mg II lines

0

0.5

1

1.5

317.50 317.75 318.00 318.25 318.50

wavelength

inte

nsi

ty (

a.u.)

1234

31

7.9

Ca

II

31

8.1

Ca

II

422.

Fig. 8. Variation in Ca lines for different bath gas composition: 21% CO2 (triangle

11 mg m�3. Note different vertical axes.

at 279.6 and 280.3 nm that appear in Fig. 7 were discarded

in the calculation. In Fig. 10, n and i refer to neutral and

ionic lines respectively, A is the transition probability for the

excited state, k is the transition wavelength, g the statistical

weight of the upper level of the transition, I is the peak area

of the ionic and neutral lines, me the rest mass of the

electron, h Planck’s constant, k Boltzmann constant, E the

energies of the upper states, EIP the ionization potential of

the neutral state and DE is a correction to the ionization

potential for plasma interactions. The slope in Fig. 10 is

� (kT)�1, where T is the electron temperature. The electron

number density (ne) is found from the y-axis intercept,

which is equal to ln(ne�1T3 / 2).

Table 3 shows the calculated electron temperature and

number density and the standard error of the best linear fit.

For the calculated uncertainty estimate, the electron temper-

ature is the same for N2 and 21% CO2. This observation,

coupled with the observed insensitivity of the early spark

behavior to CO2 (Fig. 9), suggests that the presence of CO2

does not significantly affect the spark temperature, even at

fairly long delay times when thermal conductivity effects

may be expected to become important. This does not come

as a surprise given the relative similarity in the main

physical properties of N2 and a 21% CO2/N2 mixture (Table

4). Although there is considerable difference in gas density

and heat capacity between CO2 and N2, the difference

(nm)

0

0.5

1

1.5

2

2.5

3

25 422.5 422.75 423 423.25

inte

nsi

ty (

a.u.)

42

2.6

Ca

I

s), 100% N2 (circles), gas temperature was 710 K and Ca concentration was

0

0.15

0.3

0.45

0.6

0.75

sign

al (

V)

1234

a.

730 K

298 K

0

0.15

0.3

0.45

0.6

0.75

0 0.1 0.2 0.3 0.4 0.5

t (µs)

sign

al (

V)

1234

b.

730 K

298 K

Fig. 9. Comparison of time-resolved spark radiation for N2 (triangles) and

21% CO2 (squares) as bath gases for two gas temperatures: 298 K (filled

symbols) and 730 K (open symbols). a. 590-nm filter; b. 635-nm filter and

symbols are used to distinguish lines labels and do not represent actual data

points. CO2 and N2 lines for a similar temperature overlap.


between pure N2 and 21% CO2/N2 is less than 15%. Other

properties important in the spark decay process such as

thermal conductivity and the ratio of specific heats [25] are

very similar for N2 and 21% CO2.

One effect that the presence of CO2 might be expected to

have on the temporally evolving spark plasma is on the

population of free electrons and ions in the system. Given

that the Saha–Boltzmann calculation (Table 3) showed that

the presence of 21% CO2 increased the electron number

density by an order of magnitude, we examined the

possibility that the presence of CO2 increases the free

-53

-52

-51

-50

-49

-48

-47

-46

9 9.5 10 10.5

(Ei - En -EI

Y

Fig. 10. Saha–Boltzmann plot for the Mg lines in Table 2. Experiments in tube

710T10 K and Mg concentration was 11 mg m�3. Y=ln (((gn ,jAn ,jk i ,kIi ,k)/(gi ,kAi

electron concentration in the LIBS spark, causing changes in

the distribution of ions after spark formation.

A semi-quantitative estimate of the effect that the

presence of 21% CO2 has on the LIBS spark properties

can be obtained from equilibrium calculations. For this

purpose we used the NASA CEA equilibrium code and

thermodynamic database [36], assuming a constant mass

load of Mg or Na (2.88�10�5 mol fraction) in the two bath

gases used in the experiments: 100% N2 and 21% CO2/79%

N2. The temperature range for the calculations was based on

the electron temperature calculations described above and

measurements by Radziemski et al. [9] for sparks formed in

air at laser energies varying from 60 to 300 mJ. These

authors found that at a delay time of 10 As the electron

temperature varied from 8000 to 9000 K and the free

electron number density for this condition was approx-

imately 4�1016 cm�3. Because the gate time varied from

10 to 60 As in our experiments, we focused our analysis on

the temperature range of 5000 to 12,000 K assuming that the

electron temperature determined by the Saha–Boltzmann

approach using the high-energy states described in Table 2

is biased towards the high-temperature end of the measured

gate time.

Fig. 11 shows the predicted equilibrium number density

of electrons, neutral and ionic species of Mg and Na for

100% N2 and 21% CO2. For a temperature of 9000 K, a

free electron number density of 1�1016 cm�3 is calcu-

lated, close to the value determined by Radziemski et al.

[9]. Fig. 11 shows that the free electron number density is

greater in the presence of CO2 for plasmas temperatures

below 12,000 K. Also, in the presence of CO2 the number

density of neutral species is higher. In contrast, the number

density of ionic species is higher in the presence of 100%

N2. These trends are in agreement with our experimental

results, which show an increase in neutral line intensity

and a decrease in ionic line intensity in the presence of

21% CO2 (Figs. 7 and 8).

11 11.5 12 12.5

P - ∆E) (eV)

furnace with N2 (circles) and 21% CO2 (triangles). Gas temperature was

,kkn ,jIn ,j)) ((2(2pmek)2/3)/(h3))). See text for explanation of different terms.

Table 2

Lines and properties used in the measurement of spark temperature

Wavelength Ai�10�7 gi E Wavelength Ai�10�7 gi E

nm s�1 eV nm s�1 eV

Mg II 279.08 4 4 8.86 Mg I 277.67 1.31 5 7.18

Mg II 292.86 1.2 2 8.65 Mg I 277.83 1.76 3 7.17

Mg II 293.65 2.3 2 8.65 Mg I 278.14 5.3 1 7.17

Mg I 278.30 2.16 3 7.17

Mg I 285.21 4.95 3 4.35

Mg I 309.69 0.56 7 6.72

Data from Ref. [35].

Table 4


An increase in free electron number density in the

presence of CO2 occurs in the decaying spark due to the

lower ionization energy of the species formed when CO2 is

present (see Table 5). Fig. 12 shows the predicted

equilibrium number density of ionic species for both bath

gas atmospheres. Clearly the number of species that produce

a significant number of ions in this temperature range is

higher in the presence of CO2 (Fig. 12b). Species with low

ionization energy (such as NO, O, and C) are formed when

CO2 is present, increasing the electron number density by

remaining as ions as the plasma decays. For trace species,

such as Na and Mg, that do not significantly contribute to

the total ion population, a higher free electron number

density results in lower concentrations of Mg+ and Na+,

since the atom–ion equilibrium of these species is displaced

to the atomic forms of Mg and Na.

The trends evident in Figs. 11 and 12 can explain the

observed difference in the background emission of the LIBS

signal when CO2 is present as well as the change in the ionic

and atomic line intensities. A higher electron number

density in the latter stages of spark decay (when CO2 is

present in the bath gas) produces greater bremsstrahlung

emissions and, therefore, higher continuum background

signal. At the same time, a higher free electron concen-

tration tends to displace the equilibrium of Na+ and Mg+

species to the neutral forms, causing higher signal of Na and

Mg neutral lines while the intensity of the Na+ and Mg+

lines decreases.

4.2. Effect of temperature on spark intensity

Previous studies [4,25] of the effect of pressure on LIBS

measurements on solid surfaces have concluded that a

higher density gas leads to better plasma confinement and a

reduction in the temperature decay of the plasma (because of

radiative trapping within the plasma). This observation is

Table 3

Calculated (Saha–Boltzmann equation) electron temperature and number

density for different gas mixtures

N2 21% CO2

Te (K) 11,300T730 12,030T600

ne (cm�3) 5.0�1016T2.8�1016 5.7�1017T2.5�1017

r2 0.93 0.96

Selected physical properties of gas mixtures

N2 CO2 21% CO2

Molecular weight 28.02 44.01 31.38

Cp (at 700 K) J mol�1 K�1 30.68 49.62 34.66

Thermal conductivity

(at 600 K)

W m�1 K�1 44.0 41.6 43.5

Density (at 298 K) g cm�3 1.145 1.799 1.282

Cp / (Cp�R) 1.37 1.20 1.34

From Ref. [46].

consistent with the initially lower decay rate in the

continuum emission found here for LIBS measurements at

298 K in comparison to those at 730 K (where the gas

density is 59% lower). However, our data also show that for

later time delays (greater than 10 As), both the continuum

emission and especially the line emission are stronger for

LIBS plasmas produced in higher temperature gas. This

suggests that at these long time delays the plasma in the

high-temperature gas is decaying more slowly than the

plasma in the low-temperature gas. Unfortunately, an

attempt to determine this temperature difference via the

Saha–Boltzmann technique described earlier failed to yield

an acceptable fit for the weaker signals measured at 298 K.

One cause of the slower long time decay for plasmas in the

hotter bath gas is a reduction in the conductive cooling rate.

Further work is clearly required to obtain a better under-

standing of the complicated effects of bath gas temperature

on the LIBS spark signal.

4.3. Calibration plot

In general, one would prefer to collect LIBS signals in

such a way that they were insensitive to variations in the gas

composition and temperature. This would greatly simplify

the LIBS calibration procedure and make a single calibra-

tion curve accurate even as the measurement conditions

varied. The results of the current study suggest that for LIBS

analysis of submicron aerosols it is advisable to keep the

delay and exposure times as short as possible. Short delay

times reduce the differences in spark radiation resulting

from changes in the bath gas composition and, to a lesser

extent, temperature. However, for certain elements (such as

Na), most of the emission occurs relatively late after the

1.E+11

1.E+12

1.E+13

1.E+14

1.E+15

1.E+16

1.E+17

1/cm

3

e-

Mg+

Mg

a.

1.E+10

1.E+11

1.E+12

1.E+13

1.E+14

1.E+15

1.E+16

1.E+17

5000 6000 7000 8000 9000 10000 11000 12000

T (K)

1/cm

3

e-

Na+

Na

b.

Fig. 11. Predicted equilibrium number density for free electrons (e�) and

Mg and Na atoms and ions (Mg+, Na+) for two bath gas compositions:

100% N2 (dotted line, filled symbols) and 21% CO2/N2 balance

(continuous line, open symbols).

1.E+13

1.E+14

1.E+15

1.E+16

1.E+17

1/cm

3 N+

N2+

a.

1.E+13

1.E+14

1.E+15

1.E+16

1.E+17

5000 6000 7000 8000 9000 10000 11000 12000

T (K)

1/cm

3

b.

N+

C+

O+

NO+

N2+

Fig. 12. Predicted equilibrium number density for different ionic species for

two bath gas compositions: a. 100% N2 and b. 21% CO2/N2 balance.

R2 = 1.00

R2 = 0.99

0.E+00

1.E+05

2.E+05

0 5 10 15 20 25 30Na

peak

are

a in

tens

ity (

a.u)

123

4

735 K 298 K ∆N2 ∆N2

4.0. 104 . Na0.62

3.5. 104 . Na0.54


spark formation [31]. Therefore, to optimize signal intensity,

the delay times are typically set long.

Correction for effects of changes in spark intensity is

often attempted by normalization of LIBS signals by total

background signal [37], internal standardization [8,14,38],

or peak-to-base ratio [39,40]. This latter technique has been

widely used for analysis of aerosols [10,15,41] and

proposed by some as an absolute calibration method for

particulate materials [40]. Although theoretical analysis and

experiments [42] have shown that the ratio of peak signal to

baseline does not respond in a linear fashion with concen-

tration, the technique is still considered valuable in the

reduction of shot-to-shot signal variation [39]. Nevertheless,

peak-to-base ratio corrections are not appropriate for the

experiments described here because of the strong influence

of both temperature and CO2 concentration on baseline (i.e.

continuum) intensity and peak intensity, to different extents.

An alternative for such cases is to generate the calibration

plot while mimicking the gas composition and temperature

of the field environment. For application to LIBS sampling

in the exhaust from the oxygen/natural-gas glass furnace it

is advisable to generate calibration plots with the precise gas

compositions and temperatures described in Table 1.

Table 5

Ionization energy of selected species (from Ref. [46])

Species NO C O N N2

IP/eV 9.26 11.26 13.62 14.53 15.58

However, as we did not have the means of introducing a

controlled mixture with high water vapor concentration (up

to 45%) into our tube furnace, we only considered the

effects of variations of CO2 and temperature.

Fig. 13 shows a calibration plot at two different temper-

atures (298 and 735 K) for a 21% CO2 (N2 balance) bath gas

atmosphere. The Na signal area is larger for higher temper-

atures. Fig. 13 also includes two measurements (open

symbols) when the bath gas was pure N2. In this case, the

signal is lower. Power law regression fits to the data in 21%

CO2 are indicated. The power law is an approximation to the

curve of growth (COG) method [33,43–45] used to model

the non-linear behavior observed when self-absorption

Na mg/m3

Fig. 13. Calibration plot for Na in 21% CO2 (N2 balance) at two different

temperatures: 735 K (circles) and 298 K (triangles). Two data points with

N2 as coflow appear as open symbols. Vertical error bars are of the size of

the symbols and represent one standard deviation of three measurements.

Lines represent power law regression fits.

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12

Na (mg/am3)

Are

a589

.0/A

rea5

89.6

Fig. 14. Variation of the ratio of areas of Na lines at 589.0 and 589.5 nm

with Na concentration.


occurs. According to the COG method, the calibration plot

may be represented by a two-slope line in a log–log plot of

signal vs. element concentration, with the deflection point

depending on the absorption characteristic of the plasma. The

occurrence of Na D line self-absorption for our conditions is

corroborated by the measured ratio of the area of Na line at

589.0 nm to that at 589.5 nm. As Fig. 14 shows, for most of

the range of the calibration plot, the measured ratio is about

1.3, well below the theoretical value of 2, based on the two

times greater degeneracy in the excited state for the line at

589.0 nm. Note that in Fig. 14 the extrapolation of the ratio of

Na lines to low Na concentration tends to the theoretical

value. A similar analysis for the magnesium lines in Fig. 8

also shows evidence of self-absorption. The theoretical ratio

of line intensities for theMg (II) line at 279.6 nm to theMg (I)

line at 280.3 nm is 2, in contrast to the ratio of 1.2 obtained

from the measured peak areas. Self-absorption complicates

quantitative LIBS analysis, because of the non-linear

relationship between peak area and concentration and

because the lower slope of the calibration curve reduces

sensitivity to variations in analyte concentration. However,

the high concentration of Na in the exhaust of glass furnaces

causes self-absorption for the most prominent Na lines. One

alternative is to use lines with low emission probabilities [44].

However, in this study, those lines either had insufficient

intensity, or were plagued by interference from other lines.

5. Conclusions

The analysis of LIBS of Na fume generated in a laboratory

setup at conditions that partially mimic those present in the

exhaust of an oxygen/natural-gas container glass furnace

show that the CO2 concentration and gas temperature affect

the continuum baseline intensity as well as the total Na D line

radiation. The presence of CO2 increases the background

intensity and the peak area for neutral lines, while decreasing

those of Mg and Ca ionic lines. A Saha–Boltzmann analysis

suggests that the presence of CO2 does not affect the

temperature of the decaying spark but significantly increases

the number density of free electrons in the system at long

delay times, and therefore, the bremsstrahlung radiation

responsible for the background continuum. A higher free

electron number density also explains an increase in the

population of neutral versus ionic species for trace elements.

Equilibrium calculations show that the increase in the number

density of free electrons results from the occurrence of atomic

and molecular species with low ionization energies when

CO2 is present in the bath gas.

Similar evidence of bath gas effects on LIBS baseline

intensity was also observed in spectra collected in the

exhaust of an oxygen/natural-gas container glass furnace.

The variations in baseline intensity make it difficult to apply

the traditional peak-to-baseline ratio correction traditionally

used to account for variations in spark intensity. Further-

more, the calibration plot and the variation of the ratio of

intensities of the two Na D lines show that the Na lines are

self-absorbing.

The experimental results show that detection at a

shorter delay and reduced exposure time decreases the

variation in spark intensity due to the presence of CO2 and

may aide in reducing the effects of variations in gas

temperature. However, LIBS detection of alkali metals is

optimized at long delay times. Improvement of the Na

detection system should include the evaluation of Na

spectral lines with lower emission probabilities, to reduce

the effects of self-absorption.

Acknowledgement

The project was funded by the U.S. Department of

Energy (DOE) Office of Industrial Technologies, Glass

Industry of the Future Program, under the direction of Elliot

Levine. Support was also provided by Gallo Glass Company

of Modesto, California. John Neufeld of Gallo Glass

provided technical assistance and coordination of site

measurement campaigns. Doug Scott of Sandia National

Laboratories provided essential technical assistance. Sandia

is a multiprogram laboratory operated by Sandia Corpo-

ration, a Lockheed Martin Company, for the United States

Department of Energy’s National Nuclear Security Admin-

istration under Contract DE-AC04-94AL85000.

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