Analysis of environmental lead contamination: comparison of LIBS field and laboratory instruments

Ž .Spectrochimica Acta Part B 56 2001 777�793

Analysis of environmental lead contamination:comparison of LIBS field and laboratory instruments �

R.T. Wainner a,1, R.S. Harmonb, A.W. Mizioleka,�, K.L. McNesby a,P.D. Frenchc

aUS Army Research Laboratory, AMSRL-WM-BD, Aberdeen Pro�ing Ground, MD 21005-5069, USAbUS Army Research Office, Research Triangle Park, NC 27709-2211, USA

cADA Technologies, Inc., Littleton, CO 80127-4107, USA

Received 10 October 2000; accepted 13 April 2001

Abstract

The Army Research Office of the Army Research Laboratory recently sponsored the development of a commercialŽ .laser-induced breakdown spectroscopy LIBS chemical sensor that is sufficiently compact and robust for use in the

Ž .field. This portable unit was developed primarily for the rapid, non-destructive detection of lead Pb in soils and inpaint. In order to better characterize the portable system, a comparative study was undertaken in which theperformance of the portable system was compared with a laboratory LIBS system at the Army Research Laboratorythat employs a much more sophisticated laser and detector. The particular focus of this study was to determine theeffects on the performance of the field sensor’s lower spectral resolution, lack of detector gating, and the multiplelaser pulsing that occurs when using a passively Q-switched laser. Surprisingly, both the laboratory and portable LIBSsystems exhibited similar performance with regards to detection of Pb in both soils and in paint over the 0.05�1%concentration levels. This implies that for samples similar to those studied here, high-temporal resolution time gatingof the detector is not necessary for quantitative analysis by LIBS. It was also observed that the multiple pulsing of thelaser did not have a significant positive or negative effect on the measurement of Pb concentrations. The alternativeof using other Pb lines besides the strong 406-nm line was also investigated. No other Pb line was superior instrength to the 406-nm line for the latex paint and the type of soils used in the study, although the emission line at220 nm in the UV portion of the spectrum holds potential for avoiding elemental interferences. These results are

� This paper was presented at the 1st International Congress on Laser Induced Plasma Spectroscopy and Applications, Pisa,Italy, October 2000, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference.

1Present address: Physical Sciences Inc., 20 New England Business Ctr, Andover, MA 01810-1077, USA.� Corresponding author. Tel.: �1-410-306-0861; fax: �1-410-306-1909.

0584-8547�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 5 8 4 - 8 5 4 7 0 1 0 0 2 2 9 - 4

( )R.T. Wainner et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 777�793778

very encouraging for the development of lightweight, portable LIBS sensors that use less expensive and lesssophisticated laser and detector components. The portable LIBS system was also field tested successfully at sites ofdocumented Pb contamination on military installations in California and Colorado. � 2001 Elsevier Science B.V. Allrights reserved.

Keywords: Laser-induced breakdown spectroscopy; Lead; Laser diagnostics; Atomic spectroscopy; Atomic emission;Plasma

1. Introduction

Ž .Lead Pb poisoning may be one of the mostprevalent diseases of environmental and occupa-tional origin. Its toxic effects, which include

Žanemia, neurological problems which may be.largely irreversible , colic, renal and reproductive

problems, are various and can be severe. Childrenand developing fetuses are especially at risk forPb poisoning. Just a few �g�dl can cause devel-opmental problems in children, while �100�g�dl can cause death in children and seriousproblems and reduced longevity in adults. Thesephysiological effects are compounded by the factthat lead accumulates in bones and other areas ofthe body and has a concentration half-life of

� �approximately 15 days 1,2 . Lead is used in agreat variety of industrial applications; such as instorage batteries, paints, the production of chemi-

� Ž . �cals, gasoline additives e.g. Pb C H , ammuni-2 5 4Žtion, and various other metal products e.g. solder,

.weights, pipes, etc. . Lead can find its way intoŽreceptor organisms through the air via respira-

.tion and through the ingestion of water and food.Since the US Environmental Protrection AgencyŽ .EPA mandated the reduction of the amount ofPb that may be added to gasoline during the1980s, much reduced Pb concentrations presentlyoccur in the air within the US than in previousdecades. However, the burning of gasoline is stilla significant global source for atmosphereic Pb,together with the combustion of oil and coal, and

� �the smelting of Pb ores, steel, and iron 3 . Theleaching of soluble Pb into soils and groundwaters also is of much public concern, especiallyin areas of high environmental Pb concentrationlike those near mines, smelters, Pb-using indus-tries, battery and waste repositories, and areas ofhigh automobile traffic and fossil fuel combus-

tion. A significant addition to these sources canbe Pb that leaches from older buildings that con-tain a high Pb content in their paint or plumbing.Lead has been listed as a pollutant of concern toEPAs Great Waters Program due to its persis-tence in the environment, the potential to bioac-cumulate, and its toxicity to humans and the

� �environment 4 . EPA regulations limit Pb indrinking water to 0.015 mg�l. The US NationalInstitute for Occupational Safety and HealthŽ .NIOSH recommends that workers not be ex-posed to Pb levels of more than 100 �g�m3 forup to 10 h. As Pb is also released to the air withautomobile exhaust, the EPA limits the amountof Pb that can be in leaded gasoline to 0.1 g of

Ž .Pb�gallon of gasoline 0.1 g�gal , and in un-leaded gasoline to 0.05 g�gal. The US Consumer

Ž .Product Safety Commission CPSC does not al-low the amount of Pb in most paints to be more

� �than 0.06% 3 .The legacy of hazardous substances and waste

by-products developed, tested, and manufacturedby the US military during the preparations fortwo world wars and throughout most of the subse-quent half-century, the impact of military trainingactivities, and the improper disposal of liquid andsolid waste materials from normal day-to-day mil-itary operations has had a significant adverse im-pact on the environment. Presently, there aremore than 10 000 sites on some 1200 present and

Ž .past Department of Defense DoD installationsin the territorial US that are contaminated and inneed of cleanup, the cost of which is estimated tobe in excess of $25�30 billion. A significant num-ber of these sites are characterized by toxic metal

Žcontamination e.g. Sb, As, Ba, Be, Cd, Cr, Pb, Ni,.Se, Ag, U, V and Zn that the World Health

Ž .Organization WHO has linked to adverse hu-

( )R.T. Wainner et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 777�793 779

man health effects. These metals are found in avariety of materials and forms on military installa-

Ž .tions, such as the paint on building walls Pb ,Ž .firing range residues Pb, U , and waste products

from a variety of industrial manufacturing platingŽ .and coating operations Cd, Cr, Ni for military-

unique equipment such as weapon systems.As concern for the environment has grown

within the US public sector over the past threedecades, so has the level of environmental legisla-tion and regulation. Under the terms of the Fed-eral Facilities Compliance Act of 1993, every gov-ernment department and agency is governed byfederal environmental regulations to the sameextent as the private sector. Military activitiesthat have an adverse impact on the environmentrepresent very real costs to the armed services.These costs include fines paid to state regulatoryagencies, time and resources used to clean upspills of hazardous substances, lost use of trainingor testing areas, and delays in operations andmilitary readiness. The Army, specifically, isunique amongst the military services in that it hasboth military and civil responsibilities. At present,the Army is the custodian of more than 12 millionacres of testing and training lands, a significantportion of which is within fragile desert or wet-land ecosystems and the Army is responsible forthe maintenance of navigable waterways and floodcontrol structures through its civil works mission.The American public expects the Army to respon-sibly manage the resources entrusted to it and theArmy leadership sees this stewardship and pro-tection of the environment as an integral part ofits mission.

Toxic metal monitoring techniques that arerecognized by the EPA, OSHA, NIOSH, and uti-lized routinely by researchers and commerciallaboratories have mostly included X-ray fluores-

Ž . Ž .cence XRF , atomic emission spectrometry AESŽ .via inductively-coupled plasma ICP or mi-

Ž .crowave plasma MP , and atomic absorptionŽ .spectrometry AAS via flame, electrothermal, or

graphite furnace dissociation. Mostly, these arelaboratory-based methodologies. Only a compara-tively minor effort has been made to adaptlaboratory technologies for direct, in-field mea-surement. Laboratory-based analytical techniques

usually offer a rigorous approach to quantitativemeasurement of metal concentrations, but re-quire extensive time, effort, and cost associatedwith sample collection, packaging, transportation,and preparation for analysis, as well as a lengthyturnaround time for the analysis. A field-basedapproach that can provide a rapid capability forin situ, real-time metal detection and analysis willovercome the excessive cost and time delay issuesassociated with laboratory analysis. Current

�field-portable instruments including X-ray fluo-Ž .�rescence XRF partially achieve this end, but

have limitations, due to issues such as the use of aŽradioactive source prohibited at some facilities

.and imposing shipping restrictions and an inabil-Ž .ity to detect some light elements e.g. Be .

As reported by the US DoD Tri-Service Envi-Ž .ronmental Quality Strategy Plan DoD, 1993 ,

there is an urgent DoD need for rapid, cost-effective methodologies to identify specific siteswhere toxic metal contamination exists and rapidlyand cost-effectively assess the level of contamina-tion. Furthermore, there exists a requirement toverify specific remediation actions and monitorsites where containment or natural remediation isthe chosen course of action. Cited in the plan asimportant needs related to site characterization

Ž .are: i improved sensor technologies and fieldanalytical methods for real-time, in situ analysisand status reporting for all types of media at

Ž .contaminated sites; ii standard test methods forŽ .validating monitoring devices; iii Pb detection in

Ž .soil along building perimeters; iv small armsŽ .range pollution assessment technologies; and v

open burning�open detonation emissions andfallout measurement during disposal of unwantedor unusable munitions.

Ž .Laser-induced breakdown spectroscopy LIBS ,an atomic emission spectroscopic technique, is anemerging analytical capability that offers theprospect of non-destructive, in situ, rapid andhighly selective and sensitive detection and analy-sis of both natural and man-made materials. Thefoundation for LIBS is a solid-state, short-pulsedlaser that is focused on a sample to generate ahigh-temperature plasma. Upon cooling, the ex-cited atomic, ionic, and molecular fragments pro-duced within the plasma emit radiation that is


characteristic of the elemental composition of thematerial in the volatilized sample. Fiber optictechnology offers the potential for designingportable LIBS analyzers. The LIBS technique hasproven capable of detecting many metals of envi-ronmental concern in both natural and anthro-pogenic materials. Because off-site analysis is un-necessary, the measurement complexity is greatlyreduced and there is no chance for sample loss orcross-contamination during transport or compli-cated preparations for laboratory analysis. Addi-tional LIBS advantages include the ability to con-

Žduct standoff distance measurements perhaps up. Žto 100 m which allows access to difficult i.e.

. Žcontaminated locations, a small sample size �1.�g which allows for discreet elemental analysis

of individual particles, and the analysis of certainelements outside the capability of other current

Ž .field portable techniques such as XRF .LIBS is not a new technique. The pioneering

� �work by Cremers and Radziemski 5,6 began inthe early 1980s, while studies of laser-inducedbreakdown go back to the early 1960s. A thoroughreview of LIBS development and applicationsthrough the mid-1990s was performed by Song et

� �al. 7 . Recently, LIBS has experienced a resur-gence of attention. Because of its simple anddirect nature, the LIBS technique is an optimalcandidate for use as a sensor, employed in the

Ž .field e.g. environmental monitoring , in an indus-Žtrial process e.g. as quality control in an assem-

.bly line , or in settings that are adverse to humanŽ .health e.g. nuclear reactors . Research has al-

ready been performed on numerous environmen-tal samples, including water, soil and paint� �5,6,8�15 . Other materials receiving attention viaLIBS have included mined ore, coal, steel andother metals, glass, concrete, pharmaceuticals,polyesters, artwork, and atmospheric particulates.

Ž .The US Army Research Laboratory ARL hasbeen active in LIBS research for over a decade.The early work was focused on the use of pulsed

Ž .UV excimer lasers for the production of sparks� �in gas flows 16 , while more recently the labora-

tory has developed LIBS for the detection ofhalon replacement compounds and refrigerants� �17 . In the past few years a significant growth hasbeen identified in the number of LIBS applica-

tions of potential military interest. Applicationareas include the detection of toxic metals insoils, waters, and airborne particulate matter aswell as the detection of hazardous chemicals suchas explosives and toxic chemical�biologicalagents. Part of the ARL effort in developingLIBS sensor technology involves collaborationwith industry towards the commercialization ofthis technology. This is being done through the

Ž .Small Business Innovation Research SBIR pro-gram.

During the initial stage of a SBIR programaward sponsored by the ARL Army ResearchOffice, ADA Technologies, Inc. of Littleton, Col-orado developed and manufactured a field-por-table LIBS instrument aimed specifically at thedetection of Pb in paint and soil. The prototypeinstrument was validated under laboratory condi-tions through the analysis of samples with differ-ing Pb contents. During the second stage of theproject, the portable LIBS instrument was up-graded and field tested at Fort Carson Military

Ž .Reservation FCMR in Colorado, and SierraŽ .Army Depot SIAD in California for the detec-

tion of Pb in contaminated soils and Pb in paintflakes and on painted surfaces. This LIBS instru-ment was also able to detect Pb collected onPM-10 air filters from local air monitoring sta-tions in Panama City, Panama. A major issue thatneeded to be resolved concerning this portableLIBS instrument is how does its performancecompare to that of a sophisticated laboratoryLIBS system which uses an actively Q-switchedlaser and a gated detector. This paper describesthe results of a detailed comparison of perfor-mance of the two systems for determining Pb inboth soils and in paint. The quantitative compar-ison with other analytical techniques will be thesubject of a follow-on study.

2. Experimental

2.1. Portable system

A rigorous program of LIBS measurements ofPb in paint and soil was undertaken at ARL withboth the ADA portable instrument that was uti-


lized in the field tests and with the laboratoryŽ .LIBS system described below Section 2.2 . The

ADA LIBS instrument is based on a prototypedesign developed by Cremers’ group at Los Ala-

� �mos National Laboratory 18 . As this LIBS sys-tem is a commercial product, only componentspecifications, but not make�model, are provided

Ž .here. As configured Fig. 1 , the prototype ADAŽ .instrument consists of two parts: i a laser-bearing

sample probe which also contains an optical fiberŽ .for signal collection; and ii a central

detector�analyzer unit that houses the spec-trometer�detector, timing, power, and data ac-quisition and analysis equipment. The laser powercables and fiber optic connect the two units. Thecomplete LIBS instrument is contained within a23�51�38-cm aluminum case. This system hasbeen operated from both a standard 12-V snow-mobile battery and 115-V AC current. The fiberoptic cable collects and transmits the light to a

Žsmall spectrograph 1�6 m f.l., 2400 g�mm, 250-.nm blaze with a thermoelectrically-cooled, 250�

Ž .12 element CCD 24 �m pixel . This yields a20-nm spectral range. The portable LIBS detectorsystem also has low readout noise and dark cur-rent, providing a 75 000 count dynamic range,though the A�D conversion is performed at 12bits. Data are stored in a palmtop-type personalcomputer.

For field use the laser�fiber probe unit termi-nates in a flat plate with a small hole in it at thelaser focal point. This plate is also connected to asafety pressure switch to inhibit laser firing whenno surface is present. For this investigation,laboratory measurements using the portable sys-

Ž .tem employed a sample carousel rotational stageŽ .that also holds the probe without the head at a

Ž .variable distance from the sample see Fig. 1 .This distance was nominally fixed at the same

Ž .distance lens to sample as occurs with the probehead present.

The laser is passively Q-switched and providesa nominal 15-mJ energy per pulse at 1064 nm.The nominal single pulse lamp energy thresholdfor lasing is 7 J and the pulse width is 4 ns. Thebeam diameter is 3 mm, with a beam divergenceof 1 mrad. Upon focusing by a 45-mm focallength lens, the laser spot size is 60 �m. The laseris fired once every 4 s. The instrument uses a0.5-mm round bundle to linear array fiber optic totransport the light from the plasma to the spec-trograph. No focusing lens is used in front of thefiber optic to collect the light and the width of the

Ž .individual fibers 100 �m, NA 0.22 defines theaperture for the light input to the spectrograph.The LIBS analyzer is run off an independentclock circuit, which triggers the laser, the detec-tor, and the data collection system. This was

Fig. 1. Schematic diagram of the ADA portable LIBS analyzer. The wand containing the laser head is normally free. The rotationstage is employed for these measurements to present, ‘hands-free’, a new surface sample to each laser shot.


necessary to keep the power systems for the de-tector and the laser completely separated to avoidfalse triggers of the data acquisition system dueto the high voltage power supply. The detectorsare read immediately prior to the plasma event toclear any readings from ambient light since theprevious plasma event. These readings are notrecorded. The detectors are then read immedi-

Ž .ately after the plasma event i.e. within a few msand these readings form the basis for the spectralanalysis.

2.2. Lab system

The bench-top system in the ARL laboratorywas configured to be similar to the portable sys-tem in many ways, but has some significant dif-ferences. In this setup, the output pulse of an

Žactively Q-switched Nd:YAG laser Big SkyCFR200, 12-ns pulse, 3.0-mm diameter, 2.8-mrad

.divergence, 10�300 mJ is turned downward by aprism and focused by a 100-mm lens at the sam-ple, which sits on a rotational stage. The LIBSemission is collected and delivered to a 0.3-m

Žspectrograph Acton, 300i, 1200 g�mm, 500-nm.blaze by a round bundle to linear array fiberŽ .optic 190�1100 nm . The collection end of the

Ž .fiber 1.0-mm diameter, 200-�m fibers, NA 0.22is located 7 mm from the laser focal point on thesample at an angle 50� from normal. No focusinglens is employed. At the other end, the line of

Ž .fibers is focused onto the entrance slit 50 �m bya coupling apparatus employing a 120 mm f.l.concave mirror. The dectector employed at theoutput is a thermoelectrically-cooled, intensified

ŽCCD camera Princeton Instruments, ICCD-Max,.256�1024 element, 26-�m pixel that is gateable

down to 13 ns. The low dark current and readoutnoise provides a dynamic range of 62 500 andA�D conversion is performed at 16-bit resolu-

Ž .tion. The camera is triggered with a delay by aflashlamp sync pulse from the laser. The differ-ences in hardware for the two LIBS systems aresummarized in Table 1.

2.3. Lab measurement methodology

The laser was run at one of two energies, 42 or15 mJ, nominally the higher of the two, unlessotherwise noted. The lower energy was used attimes for comparison to results from the portablesystem, which has comparable pulse energy, butthe variation in laser energy at this lower settingis greater. The data presented are generally sin-

Table 1Major differences in hardware specifications for the ADA portable system and laboratory system

Lab system Component�category Portable system

Active Q-switch Laser Passive Q-switchaHigher max. energy 15 mJ�pulse

200 �m fiber Optical fiber 100 �m fiber

0.3 m f.l. 1200 g�mm Spectrograph 0.15 m f.l. 2400 g�mmgrating. 50 �m slit grating. 100 �m slit

ICCD. 16 bit dyn. Detector CCD. 12 bit dyn. range.range. 26 �m pixel 24 �m pixel

0.18 nm Resolution 0.35 nm

Full control Detector grating �100 �s to ��10 msŽ .8 ns resolution from laser pulse

Higher Cost�weight Lower

a Ž .45 mJ when operated in the triple pulse mode see text .


gle-shot spectra or software averages of an accu-mulated number of laser shots. Time gating ofthese signals varied and is noted. In some cases,time-resolved LIBS signals were acquired. These

Žemployed 100 ns integrations from t�0 laser.pulse begins to 3 �s, stepping by 100 ns, with

100-shot averaging.The soil samples were hand pressed into

Ž .aluminum cups �1 cm deep . These includedsamples from several locations at the SIAD site,as well as a red clay soil from Aberdeen, Mary-

Žland. Samples of latex paint Wal MartŽ ..ColorPlace Interior Latex Flat Wall Paint white

were created with two thick coats on a 5�5-cmthin aluminum surface. This painted plate waselevated to the same height as the top surface ofthe soil samples for analysis. The sample stagewas rotated at a speed that would present a newsample to the laser pulse for each new shot. Anumber of samples of the local soil and paintwere doped with varying concentrations of PbŽ .expressed in %, w �w in the form of finePb sample

Ž .metal powder �20 �m or with a solution con-Ž .taining dissolved Pb NO . All samples were al-3 2

lowed to dry thoroughly prior to analysis.

3. Results and discussion

3.1. Lab measurements

The most obvious difference in the hardware ofthe portable and laboratory systems involves thetiming of the laser pulse and signal detection. Thelaboratory system has optimal control over thesetimings, as it employs a fast-gating detector andan active Q-switch on the laser. As the opposite istrue of the portable system, the only sure way toget a consistent LIBS signal with this instrumentis to use a long exposure on the CCD, whichcaptures the entire duration of the flashlamp andsome time beyond. Early in the laser breakdownprocess, the signal is dominated by broadband

Žcontinuum bremsstrahlung emission due to free. Želectron transitions . This emission is brief �0.5

.�s , but very intense. The ungated detector in-cludes this contribution, motivating an analysis ofwhether the sensitivity of detecting a particularatomic emission is negatively affected by such adetection scheme. At the least, the signal to back-

Ž .ground ratio S�B will be worse with the contin-uum emission included. Fig. 2 shows the temporal

Ž .history of the LIBS signal 340�460 nm spectrum

Fig. 2. Temporal history of LIBS spectra of solid lead surface. A 25-shot averaging and 100-ns signal integration is employed foreach spectra. The first spectrum employs a signal integration that begins with the onset of the laser and subsequent spectra are

Ž .recorded at 100-ns increment delays from that time up to 2.2 �s .


generated on a solid lead surface. The laboratorysystem is employed with 25-shot averaging and100-ns gating at 100-ns increments in the detec-tion delay. The time steps shown are from 0 to 2.2�s delay from the onset of the laser pulse. Thecontinuum emission in the first 100 ns is verylarge in comparison to signals at later times.Although difficult to see on this graph, a Starkeffect is clearly evident at early times in thebreadth of the lines and �0.4 nm shift to longerwavelengths. This is in good agreement with the

� �observations of other research 20 . Also of inter-est in the first spectrum is the evidence of photon

Žabsorption by Pb at the wavelengths i.e. transi-.tion energies that later transition into Pb emis-

Žsion lines 357.3, 364.0, 368.4, 374.0 and 405.8.nm . The lower energy level for all these transi-

tions is in the 10�20 000-cm�1 region and themagnitude of the absorption is on the order of15%. Thus, the potential exists for self-absorptionfrom the atomic emissions at later times, but atlow Pb concentrations this should not be signifi-cant.

Fig. 3 illustrates the temporal history of the Pbsignal for the 405.8-nm peak minus backgroundŽ .monitored at 414.0 nm and temporal history ofthe ratio of that signal to the background level.Again, discrete measurements represent

100 ns � averaged signals. The measurement atzero time delay is negative due to the signalabsorption. The S�B ratio increases due to thequicker decrease of the continuum emission com-pared to the atomic emission, then decreaseswhen the background stabilizes at the level due todetector noise. This ratio remains large for manymore microseconds. Provided there are no othermajor contributors to the background signal, such

Žas room or sun light small for this fiber-coupled.setup , inclusion of the signal at very long delays

does not seriously affect the S�B ratio and helpsmaximize the overall magnitude of the emissionof interest. A large effect on the acquired spec-trum is seen when the first 100 ns of signal iseither included or excluded. If the signal detec-tion begins promptly with no time delay, then100% of the atomic emission at 405.8 nm iscollected. Using paint samples doped with 1% Pbpowder as an example, the integrated S�B ratio is�0.5. If detection is delayed 100 ns, then �90%of the Pb signal is collected and the S�B ratioincreases to �1.6. This is a dramatic improve-ment in contrast. However, for a given detectorresponsivity, the magnitude of the Pb signal isessentially the same in either case. If detectorsaturation can be avoided, integration of the en-

Žtire temporal signal as is required with the un-

Fig. 3. Temporal history of the 405.8-nm Pb emission signal strength and signal-to-background ratio. Signal strength is negative atŽ .t�0 due to atomic absorption of the broadband emission bremsstrahlung . Circles represent Pb signal, triangles represent signal to

background ratio.


.gated detector is entirely sufficient. This assumesthat one cannot control the amount of lightreaching the detector, the responsitivity, or gain.This is true for the portable system as configured.If the emission signal could be magnified to takeadvantage of the full dynamic range of the detec-tor, then a 100-ns to a few �s detection timedelay would be optimal.

Fig. 4 shows the results of employing both thelaboratory and portable setup on paint samplesdoped with the fine Pb powder. A 100-ns delayand 20-�s gate were employed with the labora-tory system. The spectra are averages of 25 sin-gle-shot signals. The difference in spectral resolu-tion is apparent. This is a concern because thereare interfering lines very near the Pb line ofinterest and even a noticeable contribution rightat 405.8 nm which is likely due to a small amountof Pb that is already in the paint. Accurate Pbdoping level measurements must subtract thisbackground reading. Absolute Pb concentrations,however, should be referenced to a backgroundthat is the average of the levels of the two valleysto either side of the Pb line. The problem createdby the lesser resolution of the portable system isevident for the 1% Pb case. The background

Žcontribution from neighboring emissions Ti, in.this case becomes hard to determine without the

other lesser-doped cases as a reference. However,with such background information, and despitethe lesser resolution, the sensitivity of the portablesystem to the Pb contaminant is approximatelyequal to that of the costlier, laboratory system. Atworst, the effective difference may be a factor oftwo. Some uncertainty certainly is present and thevariation in the background signals suggests theshot-to-shot spectra have significant variation andthe averaging was not quite sufficient or that thematrix was inhomogeneous.

Shot-to-shot repeatability of a LIBS signal de-pends on a large number of factors, some ofwhich are systematic. The stability of the laserpulse intensity is important. In terms of thelaser�surface interaction, the steadiness of thesurface location and angle to the beam path, aswell as the resulting position in the detectionoptic’s field of view, play an important role. Thesample itself can have a strong effect on shot-to-

Fig. 4. LIBS emission spectra from paint samples doped withŽ . Ž .Pb powder for the a laboratory and b portable systems. A

baseline contribution to the 405.8-nm emission line is evidentin the undoped sample. Spectra are 25-shot averages. Theentire LIBS event is captured by the portable system, while

Žthe laboratory system employs a 100-ns delay from the laser.onset and a 20-�s gate.

shot signal variation in the form of what aregenerally referred to as ‘matrix’ effects. Inessence, what is important is the consistency ofthe sample presented to the focused laser beam.The sample smoothness, homogeneity, density,

Ž .and grain�particle size if applicable can varyŽgreatly for different samples or as in this re-

.search for different locations on a given sample.The laser fluence stability per pulse was actuallyrather good for the lasers of both systems. How-ever, a problem exists for the passively Q-switchedlaser at certain energies. As the flashlamp energyis increased for this laser, the output laser energyremains essentially constant until enough lampenergy exists in the laser cavity to support the


Fig. 5. Characteristics of the laser employed in the portableŽ .LIBS unit. a Time history of the laser emission with varying

flashlamp energy. The flashlamp energy varies linearly withthe potentiometer on the portable unit. The 4-ns laser pulses

Žare resolvable on this time scale and appear longer than they.actually are due to the use of a slow decay photodiode for the

detection of scattered laser light. Temporal repeatability isgood only to �10 �s, as shown by the repeated data at 8.0

Ž .setting. b LIBS spectra of soil sample OPF13SS from theŽ .Old Popping Furnace site at Sierra Army Depot SIAD , CA

as the flashlamp energy, and thus number of pulses, is varied.Spectra are 10-shot averages.

delivery of a second pulse and then a third.Therefore, the output laser energy increases indiscrete jumps from 15 to 30 to 45 mJ. At border-line flashlamp energy settings, the number ofoutput pulses can become unpredictable. Theseborderline energies can be established, and thusare somewhat predictable, but can also depend onlamp life and laser temperature. Fig. 5a illustratesthe occurrence and timing of laser pulses forvarious potentiometer settings to the flashlamp.The signals are recorded with a photodiode with a

slow signal decay, so the 4-ns pulses can be visual-Ž .ized on this time scale. Graph b shows how the

signals add linearly with the number of laserpulses. Such variation can be avoided if the num-ber of laser pulses can be kept constant. How-ever, if firing the laser at the same surface loca-tion is acceptable, multiple pulses may be usefulfor signal enhancement. Increasing the energy ofa single laser pulse has been shown to increase

� �the LIBS signal non-linearly 9,10 .The shot-to-shot signal variation is, in fact,

rather large for the data shown in Fig. 4, withmuch of this due to the sample inhomogeneity.Some sample-to-sample variation also likely hadsome influence from the different degrees offlatness of the painted aluminum samples. Over-

Fig. 6. Shot-to-shot signal variation of the Pb LIBS signal atŽ .405.8 nm in 1% Pb-doped paint samples for a the laboratory

Ž .system with Pb powder dopant; b the portable system withŽ .Pb powder dopant; and c the laboratory system with Pb

Ž .NO dopant. Signals are background-subtracted. Relative3 2Ž .standard deviation R.S.D. for the three cases are 35%, 45%,

and 17%, respectively.


all, the relative standard deviation for therecorded signals of both systems was similar,�40%. Shot-to-shot background-subtracted Pbsignals are shown for each setup with the 1% Pbpowder sample in Fig. 6a,b. As noted above, both

Ž .Pb powder and Pb NO were utilized as sample3 2dopants. This was done in order to determinewhat sort of matrix effect might be engenderedon the signal due to the character and species of

Ž .the Pb source itself. As can be seen in Graph cof Fig. 6, the dispersion of the Pb in the paint isenhanced with the use of a dissolved Pb speciesover a solid particulate. Considering the relativesize of the laser focal spot to the Pb particle size,it should be possible to achieve a relatively uni-form sample with the Pb powder also. It is clearthat much of the sample inhomogeneity for thisdopant must come from particle agglomeration orinsufficient mixing. In actuality, Pb species maybe found in anthropogenic sources in a number ofdifferent forms and varying solubility, and may betransported to human contact in either particu-late or aqueous form.

In fact, the SIAD soil samples analyzed in thefield during this study most likely have Pb con-tamination in the form of PbO from furnace ash.By contrast, the soils around the buildings onFCMR may contain Pb in the form of morewater-soluble compounds. It is Pb contaminationof this latter form that should be of more regula-tory concern from a cleanup standpoint since it isthe very soluble lead compounds that are mostdangerous to living organisms. Fig. 7a,b displaysthe results of the LIBS analysis of the local Ab-erdeen soil samples with varying concentrations

Ž .of added Pb NO . Again, the spectra are 25-shot3 2averages. Not surprisingly, the soil producessomewhat greater signal variation, most likely dueto its physical heterogeneity. More importantly,however, the spectral region of interest at 405.8nm is clear of any interfering lines, with the

Ž .closest being an Fe I emission at 406.4 nm. Thisfactor improves the detection limit to the order of0.01%. Also included in the graph of the datafrom the portable system is a measurement madeof the soil with the highest Pb concentration fromthe SIAD location. The comparison illustratesthat this Pb level is �0.5%, though this is sig-

Fig. 7. LIBS emission spectra from local red clay soil samplesŽ . Ž . Ž .doped with Pb NO for the a laboratory and b portable3 2

systems. Also included with the spectra for the portable sys-tem is a spectrum obtained from the SIAD soil sampleOPF13SS. Note the difference in Sr concentration for the twosoils. Spectra are 25-shot averages.

nificantly different from the reported concentra-� �tion 19 . As would be expected from clay soils

derived from granitic sources, these two soils aresimilar except for a higher Sr concentration in theSIAD soil. This suggests that the spectral regionaround 405.8 nm may remain free of other atomicinterferences for most clay soils.

It is clear that improved Pb sensitivity for paintŽsamples which usually contain Ti and any num-

.ber of rare metals in the pigments may requirebetter resolution or perhaps an analysis basedupon the use of alternate Pb emission lines withfewer interferences. Table 2 details a list of otherpotential strong lead lines, the relative intensities


Table 2� � Ž .Major atomic emission lines for Pb as reported in Meggers et al. 21 DC arc data and the relative strength of those emissions as

Ž .observed in this research for a solid lead surface and a 1% Pb NO -doped latex paint sample3 2

Wavelength Energy levels Line strength�1Ž . Ž .nm cm Ž . Ž .DC arc LIBS pure lead LIBS in paint

217.00 0�46 069 160 10 1a220.35 14 081�59 448 40 60 5

b239.38 10 650�52 412 50 20 0b,c247.64 7819�48 189 40 30 0b257.73 10 650�49 440 30 20 0

261.42 7819�46 069 210 170 110c280.20 10 650�46 329 290 660 200c283.31 0�35 287 280 270 70b357.27 21 458�49 440 30 460 0b363.96 7819�35 287 160 610 0b,c368.35 7819�34 960 410 800 0b374.00 21 458�48 189 80 435 0

405.78 10 650�35 287 1000 1000 1000

a Singly-ionized state.b No detectable emission.cStrong interfering emission present at wavelength of interest.

� �of those emissions from DC arc data 21 , and therelative intensities observed from the solid leadtarget and in 1% Pb-doped paint with the labora-tory system. Very few Pb emissions are detectablein the paint matrix. The best alternative to 405.8nm appears to be at 220.4 nm, though the respon-sivity for most camera�fiber combinations tapersoff strongly in this region. As illustrated in Fig. 8,the nearest interference, for this particular paint,at least, occurs at 220.9 nm. Alternatively, the lineat 261.4 nm is fairly strong, but there is an inter-fering emission at 261.2 nm.

A final note should be made in regard to avoid-ing detector saturation and maximizing the usefuldynamic range. The portable system, as config-ured, has a fixed geometry for the detection fiberand laser delivery, no means for varying theamount of light delivered to the spectrograph,and no means of varying the sensitivity of thedetector. However, the portable system is opti-mally configured with the detection fiber aimedslightly above the point where the laser illumi-nates the surface and the laser focal point issomewhat above the surface. The broadbandbackground is significantly reduced in this man-ner, yet occupies half of the dynamic range of thedetector. The detector can be easily saturated if

Žthe laser fluence at the surface is increased e.g.by an extra laser pulse or raised portion of the

.sample or the surface comes more into view ofŽ .the fiber e.g. raised portion of the sample . With

the sampling wand head with fixed plate attachedfor field sampling, the latter concern should notbe a problem. Significant signal decreases, though,

Ž .can still occur for low spots valleys on a roughsample. This can be avoided by making sure sam-ples are pressed flat. Imaging, with a collection

Fig. 8. LIBS emission spectra in the UV from paint samplesŽ .doped with Pb NO . Pb emission is apparent at 220.4 nm3 2

and, to a lesser extent, at 217.0 nm.


optic, of a region at least as large as the fullplasma might also reduce the signal variation.Also, the signals reported by the portable systemare integrations over the vertical dimension of atwo-dimensional detector. As the light is likely tobe non-uniformly distributed in this direction, theability to interrogate the full array, or some mea-sure of warning, should be available to avoidsaturation of individual pixels. However, as con-figured, the portable system performs quite well.

3.2. Qualitati�e field measurements

The location selected for the LIBS field de-monstration and test at SIAD is an area known asthe ‘Old Popping Furnace’ site. The soil in thisarea of the installation consists of a thin surficial

cover of highly permeable, windblown sand thatoverlies low permeability unconsolidated lacus-trine and fluvial lake bed sediments and alluvialfan deposits of sand, silt, and clay. From 1942 intothe mid-1950s, a furnace at this location was usedfor the demilitarization of small arms ammunitionby burning. Metal casings and Pb were recoveredfrom the furnace operation and the remainingash and residue was buried locally at shallowdepths. The furnace was dismantled and removedsome time after operations ceased, but the con-crete pad on which the furnace was situated re-mains at the site. There is also physical evidencethat small amounts of small arms ammunitionwere burned on the surface in the surroundingarea.

An environmental survey of the site was con-

� �Fig. 9. Site geography and Pb concentration data for surface soils at the Old Popping Furnace site at Sierra Army Depot, CA 19 .


ducted by Harding Lawson Associates under con-� �tract to SIAD 19 . As a part of the survey, 27

surface soil samples were analyzed for their con-centration of 23 trace metals including Pb. Inaddition, 11 subsurface samples from five bore-holes were also analyzed for Pb content. Thissurvey recognized elevated and highly variableconcentrations of Pb across the Old Popping Fur-nace site. Compared to a background soil Pb levelat SIAD of 3.8 ppm, the Pb contents for the 27surface soil samples, which were collected up to400 m away from the furnace site, were found to

Žrange from 20 to 180 000 ppm by Data Chem.Analytical Laboratories . The subsurface soil

samples exhibited much lower Pb values of �130ppm, with Pb concentrations declining to localambient soil values below a 2-m depth. In gen-eral, Pb concentrations were highest to the eastof concrete pad in the prevailing downwind direc-tion and decreased with distance from the fur-

Ž .nace site Fig. 9 , an observation which is consis-tent with the deposition of Pb on the soil surfacefrom furnace stack emissions. However, the high-est soil lead levels were observed in an area some200�300 m south-east of concrete pad, suggestingthat this location may have been a site of furnaceash disposal.

The portable LIBS system was tested on sam-ples of surface soil collected down to a depth of�1 cm that were sieved to remove large parti-cles. A split of each sample was formed into afirm pellet in an aluminum dish using a smallhydraulic press. The head of the LIBS analyzerwas placed on the soil pellet for analysis, the laserfired, and the laser head then moved incremen-tally by hand to a new spot on the pellet for the

Ž .next shot. Two sets of tests were made: i onewhich involved the collection of a LIBS spectrumfor 10 localities across the entire Old Popping

Ž .Furnace area; and ii a second Pb emission-onlysurvey along the south-east traverse of Fig. 9.

The focus of the LIBS instrument test at FCMRwas the old hospital complex area. The exteriorsand interiors of these cinder block buildings werepainted with a Pb-base paint until the 1970s.Woodwork on the interior of the building initiallywas painted multiple times with Pb-base paintsand then subsequently painted over several more

times with lead-free paint. Peeling of paint fromthe exteriors of the buildings is widespread and itwas assumed that Pb may have been leached fromthe exterior painted surfaces into the top layers ofthe soil immediately adjacent to the buildings.

Ž .The portable LIBS unit was tested on paint on aan exterior wooden porch that had been added to

Ž .the building in the 1980s; b inside the buildingŽ .on an interior wall; c a window sill and frame;

Ž .and d on soils around one building. In thewindow sill and frame tests, the laser repeatedlyfired on the same spot in order to progressivelyremove thin amounts of material and ultimately‘drill’ through the cumulative paint layers to theunderlying wood.

LIBS emission spectra for 10 laser shots from aSIAD Old Popping Furnace soil sample is plottedin Fig. 10 as a function of wavelength for the205�235-nm portion of the spectrum. Three dis-tinct peaks are present in the spectra � Al, PbŽ .220.4 nm , and Zn. Al is a primary component ofthe aluminosilicate minerals that form the lacus-trine silts and clay soils at SIAD. The other twoelements are anthropogenic contaminants; Pbfrom the bullets and Zn from the brass shellcasings. The results of the LIBS analysis alongthe south-east traverse are presented in Fig. 11.In this figure, emission intensity of the 220.4-nmwavelength Pb peak is shown as a function of shotnumber for the five SIAD soil samples. The LIBSresponse for 100-ppm and 400-ppm soil standards

Fig. 10. LIBS spectra obtained from 10 soil samples from theŽ .SIAD site showing the emission lines due to Zn 213.8 nm ,

Ž . Ž .Pb 220.4 nm , and Al 228.2 nm .


provided by Los Alamos National Laboratory, asample of elemental Pb on paper, and a blank arealso shown in the figure. The strong Pb enrich-ments that characterize samples OPF12SS andOPF13SS are clearly detected by the portableLIBS system. The strong signal variation seen inthe response spectra for these two samples is aresult of the heterogeneous distribution of Pb inthe soil. This heterogeneity is most likely theresult of the presence of Pb as small but discreteparticles at this location, where the disposal offurnace ash is likely to have occurred.

Examples of LIBS spectra acquired at theFCMR for the window sill are illustrated in Fig.12, where signal response for the 220.4-nm wave-length Pb peak is plotted as a function of shotnumber. The segments of the spectral response

Ž .curve labeled a�e illustrate the results of testsat five different locations on the window sill. Ateach location, the left of the sequence of data areprogressive shots as the laser drilled down throughthe multiple paint layers to the underlying woodsubstrate. The surface layer is a Pb-free paint,hence the rising limb of the data set representsthe transition from the Pb-free to Pb-bearingpaint. The smaller peaks and valleys represent theLIBS Pb response for the different paint layers,which must have variable Pb contents. The fallinglimb of each data set represents the transitionfrom the Pb-bearing paint to the underlying wood

Fig. 11. Successive LIBS signals vs. shot number for the Pbemission line at 220.4 nm from the SIAD site for a 120-msouth-east traverse from the concrete pad. Also shown are thesignals obtained for 100- and 400-ppm soil standards from LosAlamos National Laboratory, a sample of elemental Pb onpaper, and a blank.

Fig. 12. Successive LIBS signals vs. shot number for the Pbemission line at 220.4 nm from the paint on the inside windowsill of Building 76264 at Fort Carson Military ReservationŽ . Ž .FCMR , CO. The areas of the spectrum labeled a�e aredifferent locations sampled on the window sill. Also shown inthe figure are the signals obtained for 100- and 400-ppm soilstandards from Los Alamos National Laboratory just prior tothe paint measurements.

substrate. This experiment clearly demonstratesthe possibility of using the 220.4-nm Pb emissionline for analysis and documents that the LIBSanalysis was able to detect older Pb-base paintthat had been covered with newer, non-Pb paint.Lead was identified in the wall at the same re-sponse levels as measured for the window frame,but was not observed in the painted wood of theporch which had been constructed in the 1980safter the use of Pb-based paint by the installationhad ceased.

4. Conclusions

We have demonstrated that the portable,commercial LIBS system performs nearly as wellas the laboratory setup. A factor of two improve-ment in the resolution of the portable systemwould put it on a par with the costlier laboratorysystem. This might be achieved with the simpleaddition of a narrow slit on the spectrograph.Due to a lesser number of interfering lines at405.8 nm, both systems perform better for the soilsamples than for the paint samples. With thelaboratory system, detection of Pb down to 0.01%Ž .by weight is achievable, although this could beimproved with higher resolution.


The major differences and lower capabilities ofthe components of the portable system appear tonot limit the capabilities of the LIBS technique toa significant extent. Integration of the whole tem-poral emission from the LIBS plasma can stillprovide similar detection sensitivity as delayeddetection. Considering the strength of the contin-uum emission and the variability of this signalwith the physical properties of the sample, controlover the magnitude of the input light or detectorsensitivity would be helpful. Nominally, the sys-tem can be setup with low sensitivity and thedetector can be filled via the two or three pulsemode of the laser. The passively Q-switched laserappears to be sufficient. In this research, thenumber of laser pulses employed did not actuallyaffect the LIBS signal in anyway except to effec-

Ž .tively average a number 1�3 of signals on thedetector.

The use of alternative Pb emission lines wasinvestigated. Many lines were unexpectedly notapparent in the LIBS spectra. In general, no linewas nearly as strong as the emission at 405.8 nm.However, where interfering lines are prevalent atthe strong, visible line, the emission at 202.4 nmmay provide a good alternative. Detection opticsand detector will need to be sensitive in thisregion of the spectrum.

The portable system was also tested in the fieldon soil and paint samples at two military sites.Sensitivity to a 100-ppm standard was demon-strated and detection of various lead levels wasshown, with the strongest signals at least 20 timesgreater than the 100-ppm standard.

Acknowledgements

The authors thank Dr Elizabeth Sagan for as-sistance in the lab and Scott Hamlin for discus-sions and assistance with laser issues. We alsothank Tom Yaroch formerly of ADA Technolo-gies, Inc. for his assistance with the field samplingprogram and Susan Holliday of Sierra Army De-pot and Jeff Linn of Fort Carson Military Reser-vation for their support of the field testing of the

ADA LIBS instrument. The developmental workundertaken by ADA Technologies, Inc. was spon-sored by Army Research Office grants DDAH04-96C-0030 and DAAG55-98-C-0031.

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Documents

Analysis of environmental lead contamination: comparison of LIBS field and laboratory instruments