During the recycling of end-of-life waste electric andelectronic equipment �EOL-WEEE�, downgrading ofvaluable technical polymers has to be avoided by sep-aration of the collected material into fractions of highpurity. Most EOL-WEEE pieces are doped with sev-eral additives to improve their mechanical, electrical,and chemical properties. Common additives areflame retardants, antioxidants, light stabilizers, fill-ers, dyes, and pigments. The concentration of theseadditives varies typically from traces to several per-cent.1 Waste pieces that contain brominated flameretardants �BFRs� and additives with heavy metalshave to be automatically identified and removed fromthe waste stream and recycled, because their pres-ence causes significant environmental problems forwaste management. To establish an economicallyfeasible recycling process that meets these require-ments high-speed automatic sorting systems both toidentify polymers and to detect critical additives at a
The authors are with the Fraunhofer-Institut fur Lasertechnik,Steinbachstrasse 15, D-52074 Aachen, Germany. R. Noll’s e-mailaddress is [email protected].
Received 20 January 2003; revised manuscript received 18 July2003.
rate of several parts per second are needed. As partof the European project Sure-Plast,2 a prototype au-tomatic identification and sorting conveyor line formaterial-specific sorting of EOL-WEEE pieces hasbeen set up and evaluated. The detection unit ofthis automatic sorter line is a multisensor system forrapid identification of a polymer matrix and detectionof the additives that it contains. The detection unitcomprises three spectroscopic modules, based onlaser-induced breakdown spectrometry �LIBS�, near-infrared spectrometry, and mid-infrared spectrome-try. In this paper we focus our attention on theinvestigation of LIBS for analysis of heavy metalsand BFRs in EOL-WEEE pieces before recycling, thesetup of a LIBS analyzer for automatic sorting, andthe evaluation of the LIBS analyzer for on-line detec-tion of heavy metals and brominated flame retar-dants in moving EOL-WEEE pieces.
Frequently applied methods for the determinationof metals and flame retardants are atomic absorptionspectrometry, inductively coupled plasma opticalemission spectrometry �ICP-OES�, and electrochem-ical techniques.3 However, these methods requirethe time-consuming step of sample dissolution beforethe analysis and are therefore not applicable for high-speed applications. Therefore direct sampling ana-lytical methods such as laser ablation massspectrometry,4 glow discharge spectrometry,5,6 slid-ing spark discharge optical emission spectrometry,7
and x-ray fluorescence spectrometry8 are of greaterinterest. However, these methods are not feasiblefor on-line application with desired detection rates ofseveral parts per second. LIBS, however, has thepotential for on-line high-speed applications as arapid analysis method without sample contact. Theidentification of polymer matrices by use of LIBS hasalready been demonstrated for certain polymers.9,10
Bromium has been detected with LIBS in pure solidorganic compounds, and a helium atmosphere in thelaser–target interaction zone has been found to im-prove detection significantly.11 LIBS detection ofheavy metals in polymers has been reported forsingle-pulse plasma excitation under laboratory con-ditions and recently was applied to experiments thatincluded a double-chain extruder.12,13 However, tosort EOL-WEEE waste according to its content ofhazardous elements before recycling requires thatthe waste pieces be analyzed while they are movingon conveyor belts at speeds of as much as 1 m�s.This requires a LIBS setup that is capable of com-pensating for variations in distance between the fo-cusing optics and the surface of the waste piece.
The detection of heavy metals by LIBS as describedin this paper is focused on lead, chromium, cadmium,and mercury, which are found in pigments, nucleat-ing agents, and heat stabilizers. Brominated flameretardants are detected by use of bromium and addi-tionally by the use of antimony as a component of thewidely used synergist Sb2O3. Because of the re-quirement for LIBS analysis to be applied for on-linemeasurements under industrial conditions, all mea-surements are constrained to be conducted in air un-der normal atmospheric conditions. Signal-to-noiseratio–enhancing detection in low-pressure or noblegas atmospheres is not applicable in this case.
2. Polymer Reference Samples
A set of reference samples prepared by Gaiker, Viz-caya, Spain, was used for the LIBS investigationsdescribed here and for calibration.2 The polymermatrix was acrylonitrile butadiene styrene �ABS�,and the reference samples contained the elementsCd, Cr, Hg, and Pb in concentrations of 50–10,000�g�g, Sb in concentrations of 50–100,000 �g�g, andBr in concentrations of 50–300,000 �g�g. Further,virgin �i.e., unadulterated� samples with concentra-tions c � 1 �g�g for all these elements were providedfor calibration.
3. LIBS Setup
The experimental setup for the study of detectionparameters and excitation conditions is shown in Fig.1. The pulses of a Q-switched Nd:YAG laser �Pow-erlight PL8030, Continuum, Santa Clara, Califor-nia�, which can be operated in single-pulse and indouble-pulse mode, are focused onto the surface of thespecimen by a single lens with focal length f � 220mm. The laser has the following parameters:wavelength, � � 1064 nm; typical pulse energy, EL �350 mJ; pulse duration �FWHM�, �L � 7 ns; repetitionrate, � � 30 Hz. For single-pulse plasma excitation
the beam waist of the focus is positioned inside thesample volume at s � 12 mm underneath the sur-face of the specimen. The emission of the laser-induced plasma is received by an optical fiber with acore diameter of 600 �m and a length of 2 m. Thereceiving aperture of the fiber is protected by a quartzwindow and positioned at a distance d � 60 mm fromthe laser-induced plasma. The plasma radiation isguided to an echelle spectrometer �ESA 3000EV�i,LLA Instruments GmbH, Berlin-Adlershof, Germa-ny� equipped with a microchannel plate electro-opticgateable image intensifier and a megapixel full-frameCCD camera �Kodak KAF 1001, Eastman Kodak,Rochester, New York�. The plasma emission is de-tected during a defined gating interval at wave-lengths from � � 200 to � � 780 nm with a spectralresolution of � � 5 to � � 19 pm. The integratedintensities are digitized and transferred to a personalcomputer for data processing.
Detection of Br in air at atmospheric pressure isdifficult because the most sensitive Br emission linesare in the vacuum-ultraviolet spectral region atwavelengths � � 170 nm, e.g., at 148.85, 153.17, or163.34 nm.14 Because of the strong absorption ofthese spectral emissions by the ambient atmospherethe application of these vacuum ultraviolet lines isnot feasible for an on-line LIBS analyzer. Hence Bris detected by use of the � � 827.24-nm emission lineas the most sensitive Br emission line in the spectralrange 200 nm � � � 900 nm. With the availableechelle spectrometer it is not possible to detect Bremission at 827.24 nm.
Therefore, for high sensitivity and also for high-speed measurements, the echelle spectrometer is re-placed by a Paschen–Runge polychromator�Spectrotest, Spectro GmbH, Kleve, Germany�. Inthis polychromator, the spectral emission lines Cd228.80 nm, Cr 425.43 nm, Hg 253.65 nm, Pb 405.78nm, Sb 259.80 nm, and Br 827.24 nm and the 0th-order signal are detected by photomultiplier tubes.For each laser-induced plasma, the photomultipliersignals are processed by multichannel integrationelectronics �Model MCI, Fraunhofer-Institut fur La-sertechnik, Aachen, Germany�, which integrate the
intensities of all spectral lines simultaneously. Theposition and width of the integration time gates canbe programmed individually for the different chan-nels. The use of a Paschen–Runge polychromator incombination with the multichannel integration elec-tronics allows for measuring frequencies as high as1000 Hz.15 The time-integrated line intensities aredigitized and transmitted to the control PC for fur-ther data processing.
4. LIBS Experiments
The objective of the investigations described in whatfollows is to minimize the limit of detection �LOD� forthe elements Cd, Cr, Hg, Pb, Sb, and Br within theconstraints of on-line application. This can beachieved by maximizing the signal-to-noise ratio�SNR� of the respective background-corrected inte-grated line intensities. If spectra are taken with theechelle spectrometer, both the line intensity and theSNR of an emission line can be determined from onerecorded spectrum. The spectral background andits noise within the wavelength range of the emissionline are determined by the mean signal level and itsstandard deviation in wavelength regions adjacent tothe emission line. The background-corrected lineintensity is then calculated by standard background-correction procedures. The expectation value andits variation are determined by repeated measure-ments.
The features of the Br line at 827.24 nm require theuse of the Paschen–Runge polychromator, which wasalso used for all Sb 259.80-nm measurements be-cause Sb is contained in the same set of referencesamples as Br. Using the Paschen–Runge polychro-mator does not permit the spectral background andthe background-corrected line intensity to be deter-mined with one measurement. Here, repeated mea-surements with one specimen of high and with onespecimen of low analyte concentration are performed.Then the analytical resolving power A of each analyteline is calculated, defined here as
A ��Ihigh � �Ilow
s2�Ihigh� � s2�Ilow��1�2 ��Ianalyte
s��Ianalyte�, (1)
where �Ihigh and �Ilow are the mean intensities of theanalyte line measured with the specimens containingthe analyte at high �chigh� and low �clow� concentra-
tions and s�Ihigh� and s�Ilow� are the respective empir-ical standard deviations. For a low analyteconcentration clow close to or less than the detectionlimit, analytical resolving power A is similar to theexpected mean SNR.
A. Single-Pulse Plasma Excitation
The influence of laser pulse energy on SNR in single-pulse plasma excitation was investigated. For theelements Cd, Cr, Hg, and Pb the echelle spectrometerand for Sb and Br the Paschen–Runge polychromatorwere used for signal detection. The plasma emis-sion was detected in a fixed integration time gatetint � 10 �s starting with a time delay tdelay � 1 �safter the laser-induced plasma’s ignition. The re-sults for all investigated elements are summarized inTable 1. For Cd 228.80 nm, increasing the laserpulse energy from EL � 50 to EL � 300 mJ by a factorof 6 improves the SNR by 94%. At the same time,the background-corrected line intensity increases bya factor of 20. The significantly smaller improve-ment of the SNR compared with that of the line in-tensity is due to the increasing electron density in theplasma, yielding a higher spectral background level.Still, the expected detection limit is less by almost afactor of 2. For Hg 253.65 nm, the SNR could also beapproximately doubled, increasing the pulse energyfrom EL � 50 to EL � 300 mJ. The most distinctimprovement could be achieved for the Br 827.24-nmemission line, where detection was not possible witha pulse energy of EL � 50 mJ. Increasing the pulseenergy from EL � 100 to EL � 350 mJ improved theSNR by more than a factor of 7.
In contrast, for Sb 259.80 nm, no significant vari-ation of A was found with increasing laser pulse en-ergy. For the emission lines of Cr 425.43 nm and Pb405.78 nm, even a decrease in SNR of approximately13–40% was observed for higher pulse energies.Hence the temporal evolution of the SNR has to beconsidered for these emission lines, and suitable in-tegration time gates for signal detection have to bedetermined.
Figure 2 shows the Pb 405.78-nm emission line fora width of the integration time gate of tint � 0.5 �s attime delays tdelay � 1, 3, 20 �s. Figure 3 shows thecorresponding SNR as a function of the time delayranging from 1 to 20 �s. Whereas the overall signalintensity decreases as a function of time, the SNR
Table 1. Relative Change of SNR or Analytical Resolving Power A with Increasing Laser Pulse Energya
Element � �nm�a clow ��g�g� chigh ��g�g� EL1 �mJ� EL2 �mJ�Relative Change of
a�, emission wavelength of the analyte; clow and chigh, analyte concentrations in the ABS reference samples; EL1 and EL2, laser pulseenergies; A, analytical resolving power Eq. �1��.
rises at early time delays of tdelay � 1 �s to tdelay � 3�s and decreases slowly for time delays up to 20 �s.
Regarding signal integration, the temporal evolu-tion of both the SNR and the line intensity have to beconsidered. High line intensities with low SNR atearly delay times contribute much more backgroundto the integrated line intensity than the low line in-tensities at larger time delays, which show a higherSNR. The noise during the decay of the analyte sig-nal with low SNR at large delay times can be ne-glected because of the low overall intensity of the lineemission.
The intention is to maximize the SNRs of the inte-grated background-corrected line intensitiesIanalyte i�tdelay, tint� for each analyte line i by settingthe optimum time delay tdelay and integration timegate tint. Using a Paschen–Runge polychromator,we can determine the optimum parameters by sam-
pling the photomultiplier signals directly with an os-cilloscope and calculate analytical resolving power Afor any possible time delay and integration time gate�tdelay, tint� within the measured time range. Thisapproach has already been demonstrated for highanalyte concentrations.16 However, for element con-centrations less than 1000 �g�g, the photomultipliersignals are too weak to be recorded. Hence we usedthe multichannel integration electronics to samplethe temporal evolution of the background-correctedanalyte signal Ianalyte�tdelay, tint�; we used a samplinginterval tint for a series of time delays tdelay. Foreach spectral emission line, repeated measurementswere performed at different time delays tdelay thatwere varied in steps of tdelay � 1 �s with a fixedsampling interval tint � 1 �s. We used analytesignals Ianalyte�tdelay, tint� and their empirical stan-dard deviations sanalyte�tdelay, tint� to calculate the ex-pected analytical resolving power A Eq. �1�� for those�tdelay, tint� combinations, which can be formed byconsecutive sampling intervals. For these �tdelay,tint� combinations we calculated analyte intensityIanalyte by summing the sampled intensitiesIanalyte�tdelay, tint�:
Ianalyteacc�tdelay, tint� � �
td�tdelay
tdelay�tint� tint
Ianalyte�td, tint�, (2)
where Ianalyteacc�tdelay, tint� is the analyte signal cal-
culated for time delay tdelay and integration time gatetint we call this intensity accumulated intensity inwhat follows to clarify the difference from the mea-sured intensity in the sampling interval Ianalyte�tdelay, tint�� and Ianalyte�tdelay, tint� is the measured meanintensity in the fixed sampling interval tint at delaytd � tdelay. The empirical standard deviation of theaccumulated analyte signal is determined by
sanalyteacc�tdelay, tint� �
� �td�tdelay
tdelay�tint� tint
s2Ianalyte�td, tint���1�2
. (3)
In the calculation of the standard deviation, the co-variance of measurements in consecutive samplingintervals is neglected. This assumption is based onthe experimental observation that a temporally re-solved photomultiplier signal of a single plasma emis-sion shows a mean curve that is distorted by strongwhite noise. This noise is due to statistical pro-cesses in the plasma emission, variations of the la-ser’s peak power, variations in the absorption of thelaser pulses in the specimen, and photomultipliernoise. For single-shot evaluation, the variationscaused by this noise dominate the variations in themean signal curve, which we determine by averaginga large number of plasma emissions. For single-shot evaluation, the covariance of the measured in-tensities in adjacent detection windows is thereforeassumed to be small. With Ianalyte
acc and sanalyteacc,
Fig. 2. Temporal evolution of the Pb 405.78-nm emission line forsingle-pulse plasma excitation with laser pulse energy EL � 250mJ. Detection with an echelle spectrometer with integration timegate tint � 0.5 �s at time delays of tdelay � 1, 3, 20 �s. ABSreference sample, cPb � 10,000 �g�g.
Fig. 3. SNR of the background-corrected Pb 405.78-nm emissionline as a function of the time delay of the integration window fromtdelay � 1 �s to tdelay � 20 �s for single-pulse plasma excitation witha laser pulse energy of EL � 250 mJ. Detection with an echellespectrometer with integration time gate tint � 0.5 �s. ABS ref-erence sample, cPb � 10,000 �g�g.
analytical resolving power A for the parameters�tdelay, tint� is then calculated by
A�tdelay, tint� �Ianalyte
acc�tdelay, tint�
sanalyteacc�tdelay, tint�
�1
RSDanalyteacc . (4)
It is equivalent to the inverse relative standard de-viation RSDanalyte
acc of the accumulated analyte sig-nal. The lowest LOD is expected at maximumanalytical resolving power A�tdelay, tint� or the lowestRSDanalyte
acc.The method described above allows one to deter-
mine the optimum integration interval for raw LIBSsignals. However, using internal standardization,i.e., evaluating the ratios of integrated line intensi-ties of analyte and reference lines, yields higher sta-bility for quantitative LIBS measurements.17
Hence it is of interest to know whether the optimumdetection intervals for raw line intensities can also beapplied to internal standardization. In this case theparameters �tdelay, tint� can be optimized individuallyfor the analyte and the reference emission line. In-ternal standardization is described by
Ianalyteacc �
Ianalyteacc�tdelay
a, tinta�
Ireferenceacc�tdelay
r, tintr�
, (5)
where the superscript a in �tdelaya, tint
a� denotes thatthese parameters belong to the analyte line and thesuperscript r refers to the reference line. For therelative variance,
RSDanalyteacc 2 � � sanalyte
acc
Ianalyteacc�2
��sanalyte
acc�2
�Ianalyteacc�2 �
�sreferenceacc�2
�Ireferenceacc�2
� 2s2�Ianalyte
acc, Ireferenceacc�
IanalyteaccIreference
acc . (6)
The first two terms on the right-hand side of Eq. �6�describe the relative variance of the integrated valuesfor the analyte and the reference line, as they areobtained by individual optimization of the integrationtime gate. The last term in Eq. �6� is the normalizedcovariance of analyte and reference line, both inte-grated in individual integration time gates. In casesof parameter fluctuations such as laser pulse energy,absorption of laser radiation, or plasma emission,RSDanalyte
acc of the standardized signal will be signif-icantly lower than RSDanalyte
acc of the raw analytesignal.17 This is so because of the covariance of ana-lyte and reference signal when both reflect the influ-ence of the parameter fluctuations in a similar way.The dependence of RSDanalyte
acc on the applied detec-tion intervals �tdelay
a, tinta� and �tdelay
r, tintr� is not so
obvious. The first two terms on the right-hand sideof Eq. �6� vary significantly with the setting of thetime delay and the integration time gate. This isevident from Figs. 2 and 3 and has been described in
other publications.18 The temporal evolution of theanalyte and of the reference line intensity are com-parable and differ mostly in parameters of the sameprincipal, nonnegative intensity curve. Hence thepositive normalized covariance between analyte andreference signal is expected to vary much less withthe choice of the individual integration time gates�tdelay
a, tinta� and �tdelay
r, tintr� than the sum of the
individual RSDs of analyte and reference signal.Consequently the optimized signal integration for in-ternal standardization is expected to be achieved bythe individual optimized detection intervals for theanalyte and the reference line.
In the experiments, the temporal evolution of theline emission was sampled for time delays tdelay � 0.5�s as long as 80 �s with a fixed sampling interval tint � 1 �s. Five measurements with fifty repeti-tions each were conducted at different sample spotsby use of laser pulses with a pulse energy of EL � 350mJ on high- and low-concentration samples. For theinvestigation of the emission lines of Cd 228.80 nm,Cr 425.43 nm, Hg 253.65 nm, and Pb 405.78 nm, thehigh-concentration specimen contained chigh � 1000�g�g and the low-concentration specimen containedclow � 50 �g�g of analyte. For the emission lines ofSb 259.80 nm and Br 827.24 nm the analyte concen-tration in the high-concentration specimen was c �100,000 �g�g and in the low-concentration samplewas c � 300 �g�g. Figure 4 shows the calculatedbackground-corrected intensity Ianalyte
acc�tdelay, tint�for Pb 405.78 nm Eq. �2�� as a function of the end ofthe integration time gate �tdelay � tint� for severaltime delays tdelay. For simplification, this intensityis denoted IPb 405.78 nm in this and further figures.As expected, we achieved higher accumulated,background-corrected intensities by setting earliertime delays tdelay. Increasing integration time gatetint for a fixed time delay tdelay yielded a monotonicincrease of the signal with saturation behavior forlarge �tdelay � tint�.
Figure 5 shows the calculated RSD of the accumu-lated signal for several delays tdelay as a function of
Fig. 4. Calculated background-corrected line intensity of Pb405.78 nm as a function of the end time of integration time gate�tdelay � tint� for several time delays tdelay.
the end of integration time gate �tdelay � tint�. In-creasing �tdelay � tint� reduces RSDPb 405.78 nm for ev-ery measured time delay tdelay; hence the detectionlimit is improved. The lowest RSDPb 405.78 nm for thePb 405.78-nm emission line can be achieved for tdelay �2 �s. For earlier signal integration with tdelay � 0.5,1 �s, the lowest achievable RSDPb 405.78 nm is signifi-cantly larger. This reflects the high amplitude of
noise introduced by early signal integration, whichcannot be compensated for by later integral contribu-tions with lower amplitude. Figure 6 illustrates theminimum of RSDPb 405.78 nm as a function of tdelay fora fixed end of the integration time gate of �tdelay � tint�� 80 �s. A minimum of RSDPb 405.78 nm occurs attdelay � 2 �s.
We used the method described to determine theoptimum parameters �tdelay, tint� for all investigatedemission lines, and the results are listed in Table 2.
B. Double-Pulse Plasma Excitation
For a comparative assessment of the achievable de-tection limits we studied double-pulse excitation forLIBS analysis of the elements Cd, Cr, Hg, Pb, Sb, andBr in technical polymers. To improve the analyticalperformance of LIBS, several authors have studiedthe influence of double-pulse excitation on the dy-namic of the laser-induced plasma and its spectralemission for various materials.19–25
Some authors have demonstrated that for metalmatrices the line intensity and the detection sensi-tivity can be significantly improved for interpulseseparations in the range 0.5–80 �s, depending on thematrix and on the ambient gas conditions.21,19,25
The signal enhancement of the line emission ob-served for double-pulse excitation of steel samples isonly a weak function of the interpulse separation inthe interval from 2.5 to 80 �s.19 Whether this is alsovalid for polymer samples is not known until now.Investigations to clarify this issue are beyond thescope of this paper. In our experiments, two sepa-rate driver units for the Pockels cell in the laser headof the Nd:YAG laser generate the two laser pulseswith an adjustable interpulse separation of t �18–30 �s. The typical pulse widths �FWHM� of thelaser pulses are �1 � 12 ns and �2 � 15 ns for the firstand the second pulses, respectively. The pulse en-ergy ratio of the first to the second laser pulse isdenoted E1�E2. The burst energy, i.e., the total en-ergy of a double pulse, is given by EBurst � E1 � E2.Time delays tdelay given for double-pulse excitationrefer to the second pulse. Maximum plasma emis-sion was found when the laser was focused s � 5mm below the sample surface.
The temporal evolution of the emission lines was
Fig. 5. Calculated RSDPb 405.78 nm of the Pb 405.78-nm integratedline intensity as a function of the end of integration time gate �tdelay
� tint� for several time delays tdelay.
Fig. 6. Calculated RSDPb 405.78 nm of the integrated line intensityof Pb 405.78 nm as a function of delay tdelay for a fixed end ofintegration time gate �tdelay � tint� � 80 �s.
Table 2. Detection Limits According to the 3s Criterion for Single- and Double-Pulse Plasma Excitationand the Respective Parameters �tdelay, tint� under Laboratory Conditions
found to differ significantly from that in single-pulselaser-induced plasmas. The spectral background ofthe Pb 405.78-nm emission line at time delays tdelay �3 �s is reduced significantly by use of double-pulseexcitation; compare Fig. 7 with Fig. 2. No signifi-cant emission could be detected for time delaystdelay � 25 �s, and it is evident that the background-corrected line intensity decreases faster for double-pulse plasma excitation than for single-pulse plasmaexcitation. The SNR of the Pb 405.78-nm line isshown in Fig. 8 as a function of the time delay. Asan important difference from single-pulse plasma ex-citation �Fig. 3�, the SNR does not increase for in-creasing delay times of 1–4 �s. Both the intensityand the SNR of the Pb 405.43-nm emission decreasewith the time delay, as was also observed for theemission lines Cd 228.80 nm, Cr 425.43 nm, and Hg253.65 nm. According to the discussion above it isclear that, inasmuch as both the line intensity and
the SNR decrease with time delay tdelay, the lowestRSDPb 405.78 nm is achieved for the shortest time delaywithin the sampled time interval. Hence the lowestdetection limit is expected for the shortest time delay.As integration time gate tint was found to be of lessimportance for the expected detection limit, we ad-justed it to detect the significant parts of the lineemission. The resultant settings for time delay tdelayand integration time gate tint are listed in Table 2.
5. Results of Laboratory Measurements
We determined the detection limits for Cd, Cr, Hg,Pb, Sb, and Br in an ABS polymer matrix with LIBSby using the Paschen–Runge polychromator. Fiftylaser bursts onto one spot of the specimen with afrequency of f � 6 Hz were considered one measure-ment. For each specimen, measurements weremade at five different spots. For calibration, single-pulse evaluation with internally standardized lineintensities was applied. This means that, for eachsingle laser-induced plasma, the line intensitiesduring the detection intervals discussed above weredigitized individually and internally standardizedon the 0th order, which was integrated with timedelay tdelay � 1 �s and integration time gate tint �10 �s. From the resultant 50 internally standard-ized line intensities of one measurement, outlierswere removed if they met the Grubbs outlier crite-rion.26 Then the average line intensities were cal-culated for this measurement. From the five meanvalues of the measurements taken at one specimen,the average and the standard deviation were usedfor calibration.
Spatial inhomogeneity of heavy-metal concentra-tions in a polymer matrix is a well-known phenome-non.27 To lessen the effect of spatial inhomogeneityon the calibration, a maximum of one mean value wasremoved before calibration if that value met the Nali-mov outlier criterion,28 which is suitable for statisti-cal populations with N � 3.
For each emission line the detection limit was de-termined according to the 3s criterion.29 Table 2lists LODs with detection interval �tdelay, tint� that weobtained for each emission line that we investigatedby using a laser pulse energy of EL � 350 mJ forsingle pulses and a burst energy of EBurst � 350 mJfor double pulses. Example calibration curves forsingle-pulse plasma excitation, the respective detec-tion limits, background-equivalent concentrations�BECs�, and mean coefficients of determination R2
are shown in Figs. 9 and 10. The corresponding cal-ibration curves for double-pulse plasma excitationare shown in Fig. 11 and 12.
The detection limits for Cr, Hg, Cd, and Pb aresignificantly lower than the maximum permitted con-centration in plastic waste �see Table 3, column 4�and therefore meet the requirements for EOL-WEEErecycling. The detection limits of single- and double-pulse plasma excitation are comparable. This couldbe so because interpulse separations of less than 18�s were not achievable with the laser system used.Further investigations with a two-laser setup might
Fig. 7. Temporal evolution of the Pb 405.78-nm emission line fordouble-pulse plasma excitation with the following parameters:EBurst � 250 mJ, E1�E2 � 1, and t � 30 �s. Detection with anechelle spectrometer with integration time gate tint � 0.5 �s andtime delays tdelay � 1, 3, 20 �s.
Fig. 8. SNR of Pb 405.78 nm for double-pulse plasma excitationas a function of time delay tdelay. Laser burst parameters are asfor Fig. 7. Detection with an echelle spectrometer with integra-tion time gate tint � 0.5 �s and time delays tdelay � 1–20 �s.
give more insight into this issue. For Br 827.24 nm,a detection limit of �1.5 wt. % was achieved in am-bient air. Although the detection limit is signifi-cantly higher than for other elements, it is sufficientfor detection of common concentrations of BFRs inplastics, which typically exceed 3–5 wt. %. Further,the detection limit of 17 �g�g for Sb I 259.80 nm withdouble-pulse laser excitation permits us to detect lowconcentrations of the BFR synergist Sb2O3, which iscommonly added in concentrations of several weightpercent to enhance the flame retarding effect.
6. LIBS Analyzer for On-Line Detection of Additives
A. Autofocusing Unit
Based on the laboratory results, an on-line LIBS an-alyzer was set up to provide the required sensitivityfor heavy-metal detection in polymers. The task ofthe LIBS analyzer is the rapid quantification ofheavy-metal and halogen concentrations in EOL-WEEE pieces moving at speeds of 0.5–1.0 m�s on a
conveyor belt. The major problem in the applicationof LIBS to on-line analysis of real waste EOL-WEEEpieces such as monitors, telephones, and keyboards istheir large variation in size and shape. Unlike inlaboratory experiments, no well-defined measuring
Fig. 9. Calibration curve for Cd 228.80 nm referenced to 0th orderby single-pulse plasma excitation with optimized time delay andtime integration gate; see Table 2. Laser pulse energy, EL � 350mJ.
Fig. 10. Calibration curve for Br 827.24 nm referenced to 0thorder by single-pulse plasma excitation with optimized time delayand time integration gate; see Table 2. Laser pulse energy, EL �350 mJ.
Fig. 11. Calibration curve for Cd 228.80 nm referenced to 0thorder by double-pulse plasma excitation with optimized time delayand time integration gate; see Table 2. Laser burst parameters,EBurst � 350 mJ, E1�E2 � 1.2, and t � 18 �s.
Fig. 12. Calibration curve for Br 827.24 nm referenced to 0thorder by double-pulse plasma excitation with optimized time delayand time integration gate; see Table 2. Laser burst parameters,EBurst � 350 mJ, E1�E2 � 1.2, and t � 18 �s. At the time of thecalibration, Br reference samples cBr � 170 mg�g and cBr � 240mg�g were not yet available.
Table 3. LODs of the LIBS Analyzer for Elements in Moving ABSSamplesa
distance to the surface of the specimen is guaranteed,and large variations of the measuring distance forsuccessive samples must be compensated for by theLIBS analyzer. For this task, an autofocusing unitfor LIBS was set up to keep the analyzing laser beamfocused on the sample’s surface; see Fig. 13.
In the autofocusing unit, a laser triangulation sen-sor �Model Cyclop 220, NOKRA GmbH, Baesweiler,Germany� measures the distance between the focus-ing optics and the sample surface with an accuracy of�70 �m and a frequency of 50 Hz. The autofocuscontrol processes the triangulation sensor output andgenerates the control signal for the variable-focal-length optical system �VarioScan, Scanlab, Puch-heim, Germany, with an additional external f � 200mm lens� with a mean focal length of �f � 220 mm.
An example of the signal stabilization achievedwith the autofocusing unit is shown in Fig. 14. Forrising distances of the sample surface to the detect-
ing fiber, both the Pb 405.78-nm and the 0th-ordersignals show the expected intensity decrease. How-ever, the Pb 405.78-nm signal internally standard-ized to the 0th-order signal is constant within 5% overa range of 50 mm. This result could not be achievedwith static focusing. In Fig. 15 the internally stan-dardized Pb 405.78-nm line intensity acquired withstatic focusing is compared to dynamic focusing forvarious distances between the focusing optics and thesample. When static focusing is used, the LIBS sig-nal shows a strong dependency on the deviation fromthe optimum distance to the sample surface. Thisdependency cannot be compensated for by internalstandardization, in agreement with earlier publica-tions.30 In contrast, dynamic focusing can providestandardized line intensities that are constant within5% over a range of 50 mm.
For further reduction of the variation in measuringdistance caused by the shape of the waste piece, thewaste pieces pass by the LIBS analyzer on a tiltedconveyor belt sliding on a side panel. The LIBSmeasurement is carried out through a gap in theconveyor’s side panel; see Fig. 16. However, whilethe waste pieces pass by the LIBS analyzer on thebelt conveyor, the focusing optics has to compensatefor the distance variations that are due to the indi-vidual surface topology of the waste pieces.
Fig. 14. Intensity of Pb 405.78 nm, 0th-order, and ratio �Pb I405.78 nm�0th order� as a function of sample distance for a fixedfiber position obtained with the autofocusing unit. Plasma exci-tation with a double-pulse laser in air of an ABS polymer samplecontaining 2000 �g�g lead. The error bars given by the standarddeviation �SD� are shown enlarged by a factor of 3.
Fig. 15. Comparison of Pb 405.78-nm LIBS signal internallystandardized to 0th-order for static and dynamic focusing.
Fig. 16. Schematic setup of the sensor head �right� at the con-veyor belt �cross section shown at the left�: R, roller of the con-veyor system; S, sample; W, window; M’s, mirrors; WD, wedge tocompensate for beam offset; LB, laser beam for LIBS; AF, autofo-cusing unit; TS, triangulation sensor.
B. Limits of Detection of the On-Line LIBS Analyzer
During on-line measurements, double-pulse laserbursts with a total burst energy of EBurst � 300 mJ,a pulse energy ratio of E1�E2 � 1.2, a pulse separa-tion of t � 30 �s, and a repetition rate of 30 Hz areirradiated onto the EOL-WEEE pieces. The emis-sion of the laser-induced plasma is picked up by anoptical fiber at a distance of 130 mm to the far end ofthe measuring range and is guided to a Paschen–Runge polychromator. The photomultiplier signalsare integrated during an integration time gate oftint � 20 �s after a delay of tdelay � 0.8 �s with respectthe second pulse of the laser burst.
Calibration of the on-line LIBS analyzer was per-formed with the reference samples moving on theconveyor belt at three different measuring distances.Three repeated measurements comprising 20 laserbursts were performed at each measuring distance.The LODs for the heavy metals and halogens that weinvestigated, as determined with the on-line LIBSanalyzer operating at the pilot sorting plant, arelisted in Table 3.
At the pilot sorting plant the required detectionlimit of 100 �g�g was achieved for the elements Cd,Cr, Hg, and Sb. The LOD of 140 �g�g for Pb 405.78nm was slightly higher than this targeted limit. Ingeneral it is evident that the on-line LIBS analyzershowed a lower sensitivity for all spectral lines inves-tigated during the on-line tests than in the laboratoryexperiments described above. This difference is at-tributed to additional disturbing influences duringoperation at the pilot sorting plant. Further, onedamaged lens in the autofocusing unit resulted in adistorted beam profile and in a lower burst energydelivered onto the sample. The detection limit forBr 827.24 nm was higher than the available concen-trations of the reference samples used for calibration;therefore the respective LOD could not be deter-mined. Hence the detection of brominated flame re-tardants had to be accomplished by the detection ofSb in the BFR synergist Sb2O3.
C. Detection Performance
The on-line classification performance of the LIBSsensor was evaluated with 160 measurements of 40real waste monitors. The surfaces of the monitorhousings did not show any lacquer and were notcleaned before measurement. The reference anal-ysis for the concentration of heavy metals and halo-gens in these monitor housings was provided byICP-OES.2 In the real waste EOL-WEEE monitorhousings, no Hg concentration of �1 �g�g was de-tected with the ICP-OES reference analysis. Hencethe classification performance of the LIBS analyzercannot be quantified for Hg. For sorting of the mon-itor housings the concentrations of elements thatwere determined were classified into four categories:�a� 0–100, �b� 100–1000, �c� 1000–2000, and �d��2000 �g�g. For Pb the upper concentration limitof category �a� had to be increased to 140 �g�g be-cause the LOD of Pb 405.78 nm was 140 �g�g. The
results of the on-line classification obtained for mon-itor housings without surface preparation as theymoved on the conveyor belt at a velocity of �0.5 m�sare summarized in Table 4. For Sb, Cd, Pb, and Cr,high classification ratios of as much as 95% wereachieved during the on-line measurements at the pi-lot sorting plant.
7. Conclusions
Laser-induced breakdown spectrometry has beensuccessfully applied to the analysis of heavy metalsand brominated flame retardants in ABS polymersand EOL-WEEE pieces moving on a conveyor belt.Both single-pulse and double-pulse plasma excitationwas investigated in air under atmospheric pressure.A method for optimizing analytical resolving power Awith respect to the temporal evolution of the spectralline emissions by use of a Paschen–Runge polychro-mator has been presented that can be applied for thedetection of concentrations lower than 1000 �g�g.Chosen time delay tdelay was found to be much morecritical for single-pulse than for double-pulse plasmaexcitation, whereas the end time of the integrationtime gate was best set to detect all the significant lineemission. With the laboratory setup the detectionlimits for Cd, Cr, Hg, Pb, Sb, and Br were found to becomparable for single- and double-pulse plasma exci-tation when the respective optimized parameterswere applied. The detection of Br in ABS at atmo-spheric air pressure has been demonstrated, and de-tection limits of 11–15 mg�g were determined fordouble- and single-pulse plasma excitation. Al-though the detection limit for Br is significantlyhigher than for the other elements, it is sufficient fordetection of common concentrations of brominatedflame retardants in plastics, which typically exceed3–5 wt. %. For the elements Cd, Cr, Hg, Pb, and Sb,the detection limits were in the range 2–50 �g�g.
Based on the laboratory results, a LIBS analyzerfor on-line measurements of moving samples was setup and evaluated at a pilot sorting plant for EOL-WEEE pieces. An autofocusing unit with a measur-ing range of 50 mm was incorporated to permitmeasurements of samples as they passed by the LIBSanalyzer with their surfaces at various distancesfrom it. The on-line LIBS analyzer showed less sen-sitivity than the laboratory setup, and the detectionlimit of Br could not be determined. For the ele-ments Cd, Hg, Pb, Cr, and Sb the detection limits
Table 4. Results from the On-Line LIBS Analyzer at the Pilot SortingPlant with Real Waste EOL-WEEE Monitor Housings Moving at a
Velocity of 0.5 m�s
Classification Propertiesa Cd Cr Hgb Pb Sb
Number of correct classifications 135 152 �160� 152 152Correct Classifications �%� 84 95 �100� 95 95
aFor all elements listed, the number of total classifications was160.
bNo Hg concentration of �1 �g�g was found in the real wasteEOL-WEEE test samples with the ICP-OES reference analysis.
were found to be in the range 60–140 �g�g for mea-surements of moving samples. The on-line LIBS an-alyzer has proved that it is possible to determine withhigh accuracy the amounts of Cd, Cr, Pb, and Sb inreal EOL-WEEE monitors moving at 0.5 m�s on aconveyor belt. No specific surface treatment of theEOL-WEEE monitors was required. The high accu-racy of Sb detection provides a reliable validation ofthe presence of brominated flame retardants in tech-nical polymers by detection of the synergist Sb2O3.
The successful operation of this on-line LIBS ana-lyzer in conjunction with a conveyor belt opens a newfield of applications for LIBS. The presented LIBSanalyzer, which provides a measuring range of 50mm with demonstrated detection limits lower than100 �g�g, can be applied to a wide variety of sortingapplications for moving parts. Future research anddevelopment efforts at the Fraunhofer-Institut furLasertechnik will concentrate, e.g., on the develop-ment of a LIBS analyzer for aluminum recycling.
The authors are grateful to the European Union,which supported this research under the contractBRPR-CT98-0783, to the consortium partners of theSure-Plast project, and to the Fraunhofer Society.
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