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Table 2. Limits of Detection for Halon Alternatives Examined for

This Study
Halon-14* Halon-1301* FM-200° HFC-134a
ysme+b
m (slope) 2,080 1.721 3.313 Lis
b Gy intercept) 42,114 41,156 37,228 35,217
R? 0.9990 0.9730 —0.9842—(0.9988
LOD (ppmy’ 490 750 170 530
on = 5.

°Twenty background measurements.

gate width for a plasma formed in 4.23-ppTh FM-200
in nitrogen. Laser energy was 60 md per pulse, and
the gate delay was 0.6 ys. The gate width varied
from 0.2 to 20 ps. Figure 5 shows that no further
improvement in signal-to-noise ratio for fluorine
emission was achieved for gate widths greater than
2.5 ps. Further studies indicated that the lowest
(i.e., most sensitive) limits of detection were at a gate
width of 0.5 ps.

D. Limit of Detection
LOD’s were determined from the following equation:

LOD = 38,/m, (1)

where S, is the standard deviation of 20 background
measurements and m is the slope of the signal-
intensity versus concentration-calibration curve.
The LOD is the level at which it is possible to con-
clude that the analyte is present. From Table 2 we
find that FM-200 exhibits the lowest limit of detec-
tion (170 parts in 10°), whereas the highest is that of
Halon-1301 (LOD, 750 parts in 10°).

Figure 6 shows LIBS atomic-fluorine emission in-
tensity at 685.6 nm versus concentration for all four
compounds studied. Correlation coefficients for
these calibration curves are listed in Table 2.

Figure 7 shows the single-shot LIBS spectrum of
Halon-1301 (9.16 ppTh) measured over the spectral
region from 670 to 720 nm. As we stated above, the
spectra for all compounds were identical in this spec-
tral region, indicating that identification of the par-

Halon

Haton-1901

WC 1340

‘Signal Intensity (arbitrary units)

ee ee en ee
‘Concentration { poem)
Fig. 6. Concentration versus LIBS atomic-fluorine emission in-
tensity for the four compounds analyzed for this study; ppm, parts
in 10°.

60000,
F

2
a
2
g
}

0000

* 684.6 nm
seone r . .-
ero 80 680 700 Ho 720

Wavelength (nm)

Fig. 7. Single-shot Halon-1301 spectrum: 9.17 ppTh, 0.6-us de-
tection delay, 0.5-s integration time.

ent molecule from LIBS spectra measured over this
spectral region is not feasible, although such identi-
fication is not necessary for our applications. This
spectrum indicates that the LIBS detector should be
useful for real-time concentration measurements in
full-scale fire-suppression tests for which the re-
quired sensitivities are in the 1-100-ppTh range.

5. Conclusion

In this paper we have demonstrated the wide appli-
cability of the LIBS technique to measure concentra-
tions of a new generation of fire-extinguishing agents
and refrigerants. To the best of our knowledge, we
have presented the first quantitative LIBS spectra of
CF,Br (Halon-1301), C3F;H (FM-200) and C,F,H,
(HFC-134a), all commercially and environmentally
important compounds. Limits of detection for these
halocarbons are somewhat greater (less sensitive)
than those previously reported for fluorocarbons.
We believe that this decrease in sensitivity is caused
by the use of fiber optics to collect the LIBS radiation
as well as by the use of a linear diode array to mea-
sure the LIBS emissions. This loss in sensitivity
relative to lens coupled systems that use photomul-
tiplier tubes for detection is compensated for by the
ability of the present system to make measurements
in hostile and remote environments.

This study was supported by the U.S. Army Tank
Automotive Command (Steven McCormick, project
manager) in conjunction with the U.S. Department
of Defense Next Generation Fire Suppression Tech-
nology Development Program. We thank R. Reed
Skaggs for technical assistance. E. D. Lancaster
was a National Academy of Sciences—National Re-
search Council Postdoctoral Research Associate.

References

1. D. J. Wuebbles, P. S, Connell, and K. 0. Patten, “Evaluating
the potential effects of halon replacements on the global envi-
ronment,” in Halon Replacements: Technology and Science,
A. W. Miziolek and W. Tsang, eds. ACS Symp. Series 611,
59-71 (1995).

2, W. L. Grosshandler, R. G. Gann, and W. M. Pitts, eds., “Eval-
uation of Alternative In-Flight Fire Suppressants for Full-Scale

20 March 1999 / Vol. 38, No. 9 / APPLIED OPTICS 1479


Spectroscopic analysis of fire suppressants and
refrigerants by laser-induced breakdown spectroscopy

Edwin D. Lancaster, Kevin L. McNesby, Robert G. Daniel, and Andrzej W. Miziolek

Laser-induced breakdown spectroscopy is evaluated as a means of detecting the fire suppressants CF,Br,

C,F,H, and CF, and the refrigerant C,F,H,.

The feasibility of employing laser-induced breakdown

spectroscopy for time- and space-resolved measurement of these agents during use, storage, and recharge
is discussed. Data are presented that demonstrate the conditions necessary for optimal detection of

these chemicals.
OCIS codes:

1. Introduction

Certain halogenated hydrocarbons (halons), such as
CF ;Br (Halon-1301) and CF,ClBr are among the
most effective fire suppressants. Unfortunately, the
production of these bromine-containing fire suppres-
sants has been banned by international agreement?
because these substances have been implicated as
contributing to stratospheric ozone depletion. Part
of the ongoing effort of choosing a replacement sup-
pressant is the determination of how a given suppres-
sant is dispersed within a clutter-filled environment
(such as the crew compartment aboard a military
vehicle) and how the concentration of suppressant
changes with time after the chemical has been re-
leased. Also, there is great interest in developing
field sensors for monitoring unwanted release (leaks)
of these chemicals.

Many candidate replacement suppressants are flu-
orinated hydrocarbons.? In an initial study we have
used laser-induced breakdown spectroscopy (LIBS) to
measure atomic-fiuorine emission from candidate re-
placement suppressants (CF,, CF;H, CF,H», and
C,F5H).3 These measurements were used to deduce
limits of detection (LOD) for these compounds. The
reasons for choosing LIBS for these measurements
are that the technique allows laser delivery and col-

When this research was performed, the authors were with the
U.S. Army Research Laboratory, AMSRL-WM-BD, Aberdeen
Proving Ground, Maryland 21005-5066. E. D. Lancaster is now
with the National Ground Intelligence Center, 220'Seventh Street,
N.E., Charlottesville, Virginia 22902. The e-mail address for
A.W. Miziolek is miziolek@arl.mil.

Received 30 July 1998; revised manuscript received 21 Decem-
ber 1998.

1476 APPLIED OPTICS / Vol. 38, No. 9 / 20 March 1999

140.3440, 120.1880, 300.6390, 300.0300.

lection optics to be fiber coupled to provide access to
remote or hostile environments as well as that the
data collection rate is determined by the repetition
rate of the laser. This means that LIBS-based mea-
surements of suppressant concentrations can be
made rapidly and in situ during full-scale fire sup-
pressant testing or for detection of leaks from storage
containers. Although there are commercially avail-
able halocarbon (refrigerant) detectors that use de-
tection techniques other than UV-visible plasma
emission, there is still a need for a halocarbon sensor
with certain desirable characteristics such as near
real-time response, remote operation, and freedom
from other chemical interference.

The research presented here extends our initial
study in two important directions. First, it includes
three important compounds not previously studied:
Halon-1301; C3F,H (FM-200), a leading replacement
candidate for fire suppression; and C,F,H, (HFC-
134a), a common replacement refrigerant. Second,
unlike in the initial study, in which we captured
the LIBS spectra by using a monochromator—
photomultiplier tube system, in this study we con-
centrate on using and systematically optimizing the
photodiode array-spectrograph system for the pur-
pose of utilizing it in real-world applications.

2. Background

LIBS is a well-established technique* and has been
widely discussed in the literature. Briefly, a pulsed
laser beam is focused to a fluence of approximately
108-10° W/cm? on or in the sample. In this focal
region, dielectric breakdown of the sample occurs.
Continuum absorption of the laser radiation by free
electrons produced during the dielectric breakdown
further ionizes and heats the sample, generating the


Collimator /Fiber Plasma

—_ id co

sos ||

1084-0 +}
| Pulsed Laser

| Rastation

Fig. 1. Schematic of the instrumentation.

plasma by cascade ionization. The emission of the
atomic and molecular constituents, which are excited
within the plasma, is collected with lenses or fiber
optics, spectrally resolved, and measured with a pho-
tomultiplier or an optical multichannel analyzer.
ical LIBS plasma temperatures are in the

20,000-25,000 K range for gases at atmospheric
pressure.>

Although LIBS is most commonly employed for sol-
ids analysis,®” such as in measurement of metal con-
tamination of soils, paint, metals, and steels, early
studies* demonstrated the feasibility of the technique
for detection of fluorine-containing species. Addi-
tionally, LIBS analysis of gaseous samples has been
reported for the detection of chlorine, fluorine, sulfur,
phosphorus, arsenic, and mercury in air.?-!! Re-
cently the authors’ laboratory employed LIBS to de-
termine limits of detection for a series of fluorinated
methanes [CF, (Halon 14) CF;H, CF2H,].3

In the research reported in this paper we used
optical fibers to collect radiation emitted by the laser
and then transferred it to the monochromator. Op-
tical fibers have been utilized by other researchers for
the remote analysis of metals in the environment.!21%
These fibers have also been used in a probe arrange-
ment for both transfer of the plasma generating pulse
and the collection of emitted radiation.1+

3. Experiment

A schematic of the experimental apparatus is shown.
in Fig. 1. Laser radiation at a wavelength of 1.06
wm is supplied by a Nd:YAG laser (Quanta-Ray
DCR2A). The laser repetition rate is 10 Hz. The
laser radiation is passed through a 25-mm-diameter
plano—convex lens with a focal length of 50 mm and
is brought to a focus 2 mm above the outlet port of the
flow apparatus used to deliver the analyte. Plasma
emission is collected with an f/4 quartz lens, colli-
mated, and focused directly into a 1-mm-aperture
bifurcated optical fiber bundle consisting of 10 indi-
vidual fibers in a cylindrical shape at the light-
collection point and a rectangular configuration at
the other end. The output from the optical fiber
bundle is spectrally resolved by a spectrograph
(Jarell-Ash Monospec 27) and measured with an in-
tensified photodiode array detector (Princeton Instru-
ments IRY1024G). The grating is used in first
order, and the effective resolution is ~0.7 nm.
Single-shot spectra were obtained for each of these

Relative Intensity

Wavelength (nm)

Fig. 2, Time-delay spectra: neat Halon-1301, integration time
0.2 ps, gate delay 0.0-3.0 ys in 0.2-ps steps.

measurements. All spectra were obtained in a
single-shot manner. :

The sample delivery device was described previous-
ly.15 The device consists of a 0.318-cm (O.D.)
stainless-steel tube placed inside a 0.953-cm (O.D.)
stainless-steel tube. Analyte gas is delivered by the
center tube at a flow rate of 50 standard cubic centi-
meters per minute (sccm). The outer tube delivers
argon gas flowing at 500 sccm. The argon gas is
used as a sheath to confine the analyte to the focal
volume of the laser. We mounted the sample deliv-
ery device upon a three-dimensional stage to adjust
the location of the analyte stream relative to the laser
focal volume. For the limit of the detection studies
described here, the analyte sample was mixed with
ultrapure nitrogen. Gas flows were controlled by
mass-flow meters (Tylan). All gases were supplied
by Matheson, Inc., and were used without further
purification. The mass-flow controllers were cali-
brated with a wet test meter.

4. Results and Discussion

In general, initial laser-induced plasma emission con-
sists of continuum radiation for several hundred
nanoseconds. This continuum emission is typically
much more intense than atomic and molecular emis-
sions, which occur sequentially from one to several
microseconds following plasma formation. Figure 2
shows typical time-resolved laser-induced break-
down spectra for neat Halon-1301 with an argon
sheath gas. Visible in this LIBS spectrum are emis-
sion lines from fluorine and argon.

Figure 3 illustrates the spectral emission from
neat Halon-1301. This figure shows the bromine
emission lines in the 460-480-nm wavelength re-
gion. All spéctra obtained for the compounds of
interest were identical except in this region. For
these studies the fluorine emission line at 685.6 nm
(3S"P5,.-3P*D$,2) is used for signal optimization
and signal-to-noise ratio calculations. This line
was chosen because it is the strongest fluorine emis-
sion line in this spectral region? and because it is
free from interference from oxygen, nitrogen, and
argon. For optimum sensitivity it is necessary to
minimize contributions to the signal from the con-

20 March 1999 / Vol. 38, No. 9 / APPLIED OPTICS 1477


Signal intensity (arbitrary units}

Signal Intensity (arbitrary units)

Cr a
Wavelength (nm)
Fig. 3. LIBS spectra of Halon-1301.

tinuum radiation while maximizing the analyte
signal-to-noise ratio. The factors that must be con-
sidered in achieving optimum balance are discussed
below.

A. Optimization of Gate Delay

Previous studies of fluorine emission from halocar-
bons? delayed signal collection by 2 ws after the laser
pulse. Because the time during which the diode ar-
ray detector intensifier is on is termed the gate width,
the delay between laser firing and collection of signal
is called the gate delay. For these studies the
signal-to-noise ratio for emission from the atomic-
fluorine line at 685.6 nm was measured as a function
of gate delay for an incident laser energy of 60 mJ per
pulse. Integration time (i.e., the gate width) was 0.5
us. Figure 4 shows that, for the compounds studied
here, the optimum gate delay was 0.6 4s. The un-
certainty of the data shown in Fig. 4 is approximately

«so pe “Pra ha
=
o
§ x)
3 aw
B x0
ol
=

Detection Delay (ys)

Fig. 4. Signal-to-noise ratio for atomic-fluorine emission at 685.6
nm as a function of gate delay for the gated, intensified diode-array
detector used in these studies. For each of the compounds stud-
ied, optimum delay was determined to be 0.6 ps.

1478 APPLIED OPTICS / Vol. 38, No. 9 / 20 March 1999

Table 1. Dependence of Signal-to-Noise Ratio on Laser Energy for
Atomic-Fluorine Emission at 685.6 nm

Energy (mJ/Pulse) Signal/Noise
40 229
50 247
60 251
72 157
80 119
90 231
100 245
110 209
120 184

15%. We have found that the uncertainty level for
all the data that we report in this paper is in the
15-20% range. The signal-to-noise ratios for all ex-
periments were calculated from the average of five
peak measurements and the rms noise from 20 blank
measurements.

B. Dependence of Signal-to-Noise Ratio on Laser Power

We measured the emission intensity from a plasma
occurring in a nitrogen—FM-200 gas mixture contain-
ing 4.23 parts in 10° (ppTh) of FM-200 at various
laser pulse energy levels to determine the laser en-
ergy that maximized signal-to-noise ratio for emis-
sion from atomic fluorine at 685.6 nm. Table 1
shows that for FM-200 the laser pulse energies that
yield the best signal-to-noise ratio for fluorine emis-
sion were observed at 60 and 100 mJ/pulse (Table 1).
A pulse energy of 60 mJ/pulse was chosen for further
optimization studies because we observed that at
greater than approximately 80 mJ/pulse the plasma
was breaking apart or segmenting. This phenome-
non is well known in the laser breakdown field, where
at a sufficiently high laser pulse energy the single
microplasma converts into multiple spatially distinct
microplasmas.

C. Optimization of Gate Width

To determine the optimum signal integration time
(gate width) we measured the signal-to-noise ratio for
atomic-fluorine emission at 685.6 nm as a function of

M200

160000 See

2 s40000
= 120000
100000
&

% 80000
} 0000
2 40000
é

Oe
0.00 5.00 1000 1500 ©2000-2500
Gate Width (us)

[Sanat Signal = SigiNoise

Fig. 5. Dependence of signal-to-noise ratio on integration time.

SignalNolse