ISSN 0267-9477

Journal of Analytical Atomic Spectrometry

0267-9477(2010)25:9;1-9

HOT PAPERNiemax et al.Measurement of element mass distributions in particle ensembles applying ICP-OES

EDITORIALHaraguchi and FurutaAnalytical atomic spectrometry in Japan over the last 25 years

www.rsc.org/jaas Volume 25 | Number 9 | September 2010 | Pages 1361–1492

Celebrating

25 years

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PAPER www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry

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Measurement of element mass distributions in particle ensembles applyingICP-OES†

Ayrat Murtazin,a Sebastian Grohb and Kay Niemax*a

Received 7th April 2010, Accepted 25th May 2010

DOI: 10.1039/c004946h

End-on inductively coupled plasma optical emission spectrometry (ICP-OES) of the relative mass

distributions in ensembles of spherical SiO2 particles with small and large size variations is presented.

The particles are introduced into the ICP by injection of monodisperse microdroplets generated from

diluted particle suspensions. The spectroscopic signals, taking into account the peak or the integrated

intensities of the Si 288.16 nm line, are compared with mass distribution measurements of

corresponding SiO2 particle ensembles performed by Scanning Electron Microscopy (SEM). It is found

that the relative mass distributions measured by ICP-OES are in good agreement with SEM

measurements if the spectroscopic signal to noise ratio is sufficiently large indicating that intensity

variations due to different particle trajectories in the ICP are less important than expected.

Introduction

Recently, it was demonstrated in our laboratory1 that the

element masses of spherical SiO2 and Au particles in the micro-

and nanometre size range can be measured with good accuracy

by ICP-OES. The particles were introduced by monodisperse

micro-droplets into the plasma and the line emission of the

atomized particles was compared with the emission obtained

from droplets of the same size as the droplets with particles but

made from respective element standard solutions of known

concentrations. While the mean intensity ratios measured with

particles and standard solutions were in very good agreement

with the beforehand known element masses in the particles and

solution droplets, it was unclear whether this new chemical

particle characterization technique can be used for rapid analyses

of the mass distributions of particles or size distributions if the

particles have spherical shape.

In the recent paper,1 the peak as well as the integrated

emission intensities from the elements of the atomized particles

were evaluated. Both quantities which are proportional to the

particle mass showed intensity distributions which might not

only be due to the size distributions of the spherical particles but

also to possible differences of the particle trajectories in the ICP

and to the intensity noise if the signals are close to the detection

limit. To separate the different contributions, the mass distri-

butions of the particle ensembles of interest have to be known

with good statistical accuracy. In the present paper,

aDepartment of Analytical Chemistry and Reference Materials, FederalInstitute for Materials Research and Testing (BAM),Richard-Willst€atter-Strasse 11, 12489 Berlin, Germany. E-mail: kay.niemax@bam.debLeibniz-Institut f€ur Analytische Wissenschaften—ISAS at the TechnischeUniversit€at Dortmund, Bunsen-Kirchhoff-Strasse 11, 44139 Dortmund,Germany

† Research has been performed within the network Plasma-AnalyteInteraction Working Group (PAIWG). PAIWG is a collaborativeeffort of the University of Florida (Gainesville, USA) and the FederalInstitute for Materials Research and Testing (BAM, Berlin, Germany),jointly funded by NSF and DFG.

This journal is ª The Royal Society of Chemistry 2010

representative ensembles of spherical SiO2 particles with good

but also suboptimal size distributions are measured by Scanning

Electron Microscopy (SEM). The mass distributions obtained

by SEM are then compared with the intensity distributions

measured with respective representative particle ensembles by

ICP-OES.

It has to be noted that the introduction of monodisperse

droplets of standard solution into flames and plasmas has

already a long history. It has been studied by different research

groups in the past. We like to refer here to the pioneering

investigations of Hieftje et al.,2,3 Olesik and Hobbs,4,5 and Lazar

and Farnsworth.6,7 More details on earlier publications are pre-

sented in our recent paper.1

Experiment

The spectroscopic arrangement (see Fig. 1) and experimental

details are described in detail in a recent paper of this journal.1

Therefore, we refer to that paper and present here only the most

important features of the experiment.

Monodisperse microdroplets are generated from particle

suspensions applying a commercial piezo-electric droplet

dispenser (Type MD-E-201 H with dispenser head MD-K-150,

microdrop Technologies, Norderstedt, Germany). The dispenser

head is equipped with a gas nozzle (dispenser gas flow) around

the 30 mm diameter orifice producing droplets with diameters of

about 50 mm for the chosen piezo-electric pulse parameters

(voltage and length). The variation of the diameter for successive

droplets is smaller than 1% as claimed by the manufacturer. For

visual control of droplet production, diameter and velocity, the

droplets are stroboscopically illuminated and measured with

a commercial microscope equipped with a CCD also delivered by

microdrop Technologies. The accuracy of the droplet diameter

measurements is about �3 mm, limited by the pixel size of the

CCD. The droplet generation rate was mostly 20 Hz. In very few

cases, it was 25 Hz and in one 50 Hz. Within the experimental

error there was no effect of the different droplet rates on the

measured line intensities as long as the dispenser gas flow was not

changed.

J. Anal. At. Spectrom., 2010, 25, 1395–1401 | 1395

Fig. 1 Experimental arrangement used for SiO2 particle characteriza-

tion and the time respective position dependent intensity of the Si 288.16

nm line.

Table 1 SiO2 particles provided by microparticles GmbH (Berlin) withRSD of the diameters measured with the Coulter Multisizer II

Product name Nominal diameter/mm RSD (%)

SiO2-F-L1516 1.06 7.6SiO2-F-L1513 1.11 4.4SiO2-F-B1060 1.16 4.2SiO2-F-L1280 1.55 2.3SiO2-F-L1561 2.06 2.2SiO2-82103 2.08 6.1

‡ Although the uncertainties of the particle diameters exceed the digitsafter the decimal points (see Table 1) they are not rounded for betterdiscrimination of the samples. Moreover, the diameters given arenominal diameters used by the production company.

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Once the stability of droplet generation is established, the

droplets made from the particle suspensions are transported by

a dispenser gas flow of 0.21 L min�1 (Ar) directly through the

injector tube (diameter at the end: 1 mm) of the vertically

mounted ICP torch into the plasma. A silicone O-ring between

the dispenser head and the injector tube is applied to tighten the

units. The injector flow rate is lower than usually applied in ICP

spectrometry because the droplet transport is unstable for flow

rates larger than �0.3 L min�1. On the other hand, the residence

time of the droplets with particles in the ICP has to be as long as

possible so that atomization and plasma equilibration is

accomplished.8

The ICP with power supply unit was taken from a prototype

ICP-instrument (ELEMENT 1, Finnigan MAT, Bremen,

Germany). The operation conditions during the ICP-OES

measurements in Ar are as follows: 1000 W rf forward power, 16

L min�1 plasma gas flow rate, and 1 L min�1 auxiliary gas flow

rate. The spectral emission was collected end-on and focused

onto a quartz fiber guiding the radiation to a 1000 M SPEX

monochromator (Jobin-Yvon). The intensity at the mono-

chromator exit slit is detected by a fast photomultiplier (EMI

9789 QA). The signals were amplified by a Stanford Research

SR570 amplifier (time constant: 53 ms), digitized (National

Instruments DAQPad-6015), and processed.

The SiO2 particles were analyzed by tuning the Spex mono-

chromator to the Si(I) 288.16 nm emission line. Although it was

shown before that the line intensities measured with mono-

disperse microdroplet injection are very reproducible in time,1,8,9

1396 | J. Anal. At. Spectrom., 2010, 25, 1395–1401

the Ca(II) emission line at 393.36 nm was simultaneously recor-

ded in some of the experiments applying a second mono-

chromator,1 not shown in Fig. 1. A small amount of Ca solution

was added to the suspensions so that the Ca concentration was

�100 ng mL�1. The Ca(II) signals served as a marker for the

presence of droplets in the ICP and to monitor the transport

stability of the droplets.

The mass distributions of the SiO2 particle ensembles were

determined using a Quanta 200 F scanning electron microscope

(FEI). The evaluations of the SEM pictures regarding the particle

sizes were made applying the free scientific image processing

software ImageJ v1.42q.10 The SEM image was imported into the

program, the internal scale calibrated with the respective scales

of the SEM-pictures, and the threshold function of the program

applied to produce a binary black and white image. After

applying of the watershed function, the ‘‘Analyze Particles’’

option of the program was used for particle numbering and

displaying the outer contours of the particles. The particle shape

pictures were inspected at the computer screen and particles

excluded which obviously were fragments, attached to each other

(e.g., twin-particles), had artefacts due to the evaluation proce-

dure, or were not totally displayed because they were on the

brink of the SEM-picture. Fragments and twin-particles were in

particular observed in the particle suspensions with broad size

distributions. The volumes of the remaining particles were then

calculated assuming spherical symmetry. Finally, the Si mass

determined taking into account the SiO2 density of 1.85 g cm�3.

Samples

Diluted suspension of monodisperse SiO2 particles was prepared

from commercially available suspensions and from suspensions

which are not for sale because of relatively broad particle

distributions. All suspensions were provided by microparticles

GmbH, Berlin, Germany. The product names, the nominal

diameters, as well as the relative standard deviations (RSD) of

the diameters as quoted by the company are given in Table 1. The

particle size distributions were determined by the production

company applying a Coulter Multisizer II instrument.11 The SiO2

suspensions with 1.16, 1.55, and 2.06 mm particle diameters had

already been used in the preceding paper1 while the additional

1.06 mm, 1.11 mm, and 2.08 mm suspensions were new.‡ The 1.11

mm particles had a small size distribution (RSD: 4.4%) while the

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distributions of the 1.06 mm and 2.08 mm particles were relatively

broad (RSDs: 7.6% and 6.1%, respectively).

The undiluted suspensions with known particle numbers were

put in an ultrasonic bath for about 30 min to minimize

agglomeration and sedimentation of particles. In the next step,

the suspensions were diluted with deionized water in order to

have on average less than 1 particle in each of the 50 mm droplet

produced by the droplet generator. After each dilution step and

before analysis the samples were again treated in the ultrasonic

bath for �20 min. Once the suspensions were properly diluted

and the generated droplets visually controlled with the micro-

scope, the droplet dispenser was directly attached to the injector

of the ICP torch so that the droplets could be transported by the

dispenser gas flow into the plasma.

Results and discussion

The probability of particle inclusion in the monodisperse droplets

ICP-OES measurements of size or particle mass distributions in

suspensions are only possible if the probability of finding more

than one particle in the droplet is practically zero. In the recent

Fig. 2 The probability of finding SiO2 particles in the monodisperse 50 mm d

number in the suspension to the number of droplets representing the total volu

the open are theoretical data expected from the experimentally prepared partic

the particle sizes, and the number of droplets injected to the ICP are given in

This journal is ª The Royal Society of Chemistry 2010

paper on micro- and nanoparticle characterization1 was already

mentioned that the probability of finding particles in mono-

disperse droplets is approximately given by Poisson statistics.

Here, the probability of finding none (x¼ 0), one (x¼ 1) or more

particles (x ¼ 2, 3,.) in the droplets is given by

PlðxÞ ¼lx

x!e�l;

where l is the ratio of the number of particles in a suspension to

the number of droplets representing the total volume of the

suspension. The precondition for application of Poisson statistics

is that the particles are uniformly distributed in the suspension,

i.e., that there is no particle agglomeration and precipitation.

In order to investigate the validity of Poisson statistics for SiO2

particles in the size range up to about 2 mm, suspensions of

different particle sizes and different l were prepared applying the

dilution and ultrasonic preparation procedure described in the

Experiment section. Fig. 2 shows the comparison between

measured (full symbols) and calculated probabilities (open

symbols) taking into account Poisson statistics and the prepared

particle concentrations. As can be seen, the agreement between

the experimental and theoretical data points is very good,

roplets for diluted suspensions characterized by l, the ratio of the particle

me of the suspension. The full symbols represent experimental data, while

le concentrations in the suspensions. The formula of the Poisson statistics,

the insets.

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supporting the assumption that Poisson statistics is valid for

suspensions with SiO2 particles, at least for particles in the size

range 0.83–2.06 mm. The numbers of the injected droplets are

given in the insets of Fig. 2. They vary between 713 and 2556

droplets.

Absolute and relative mass distributions of SiO2 particle

ensembles with narrow and broad size dispersions

Fig. 3 shows SEM exposures of two different ensembles of SiO2

particles with narrow and broad size distributions. This

Fig. 3 SEM pictures of 2.06 (top) and 2.08 mm (bottom) SiO2 particle

ensembles showing the different dispersions of the particle samples.

Fig. 4 Size evaluation of SEM exposures using the ImageJ v1.42q

software. The section was taken from the SEM picture of the 2.08 mm

SiO2 particles in Fig. 3.

1398 | J. Anal. At. Spectrom., 2010, 25, 1395–1401

difference can already be seen by visual inspection. While the

2.06 mm particles at the top of the figure are obviously quite

uniform in size, the 2.08 mm particles at the bottom have slightly

different diameters. Moreover, there are particle fractions and

several particles in the 2.08 mm ensemble which consist of two

particles which are glued together (twin-particles).

The size and finally the mass distributions of both classes of

particles were evaluated applying the image processing software

ImageJ v1.42q as described in the Experiment section. For

example, Fig. 4 displays the contours of 2.08 mm particles in four

sub-section of Fig. 3 (bottom). The contours which are used for

the calculation of the particle volume are result of the image

processing program as described above. The particles are

numbered because this allows discriminating individual particles

from evaluation. As can be seen in Fig. 3, the particles are

predominantly rounded. However, one can immediately observe

fragments (see, e.g., particle 790 and 814 in Fig. 4b and c,

respectively), twin particles (703/708 in Fig. 4a), or artefacts due

to the evaluation which appear as small protuberances (e.g., 698,

768, and 859 in Fig. 4a, b, and c, respectively) and incomplete

contours (95 in Fig. 4d). Fragments, twin-particles and particles

with incomplete contours were excluded from evaluation while

particles having contours with only small protuberances were

taken into account when the estimated errors in particle mass

determination were only small.

The Si mass distributions of 2.06 and 2.08 mm SiO2 particle

ensembles evaluated from SEM pictures are plotted in form of

histograms in the upper parts of Fig. 5 and 6, respectively. Three

sub-ensembles were individually evaluated for each particle sort.

Their individual data with the respective RSDs are given in

Table 2 together with the values for the total ensembles measured.

Histograms of the peak as well as of the integrated intensities

measured with 2.06 and 2.08 mm particle ensembles by ICP-OES

are presented in the lower parts of Fig. 5 and 6, respectively. It

can be seen that the shapes of the relative mass distributions

measured by OES are very similar to the absolute mass distri-

butions determined by SEM if we neglect the intensity distribu-

tions at low and high values. The peaks at �7 mA and �9 mA ms

in Fig. 5 and �9 mA and �8 mA ms in Fig. 6 are due to

contamination by smaller SiO2 particles which had been depos-

ited in the dispenser head from earlier measurements. In all later

measurements the dispenser head was also very carefully cleaned

before analysis to avoid particle contaminations from earlier

measurements. Therefore, peaks at lower intensities due to

contamination by other particle ensembles could not be observed

in later measurements (see, e.g., Fig. 7). In case of the 2.08 mm

particles the intensities at high values are due to twin particles of

different mass, while small fragment particles contribute to the

low intensities. The disregard of fragment signals as well as of

twin-particles is allowed in the comparison of ICP and SEM

measurements because both sorts of particles were not taken into

account in the evaluation of the SEM pictures. In addition to the

histograms in Fig. 5 and 6, the results of the ICP-OES

measurements are given in Table 3a and b, respectively. The

tables do not only present the results of the total ensembles

measured but also of the five sub-ensembles measured with 2.06

and 2.08 mm particles.

Comparing the data given in Tables 2 and 3, the relative

standard deviations of the mass distributions measured by SEM

This journal is ª The Royal Society of Chemistry 2010

Fig. 5 Comparison of the absolute Si mass distribution for the 2.06 mm SiO2 particles measured by SEM (top) and the relative mass distributions

derived from the peak as well as from the integrated intensities of the Si 288.16 nm line (bottom).

Fig. 6 Comparison of the absolute Si mass distribution for the 2.08 mm SiO2 particles measured by SEM (top) and the relative mass distributions

derived from the peak as well as from the integrated intensities of the Si 288.16 nm line (bottom).

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Table 2 Determination of the Si mass of (a) 2.06 mm, (b) 2.08 mm, and(c) 1.16 mm SiO2 particles evaluating three sub-ensembles each from SEMpictures

Nominaldiameter/mm No. Particles m(Si)/pg RSD (%)

(a) 2.06 1 489 4.17 8.22 551 4.18 6.63 807 4.07 7.5Total 1847 4.13 7.5

(b) 2.08 1 497 4.03 18.92 761 4.06 17.53 716 4.01 16.5Total 1974 4.07 17.6

(c) 1.16 1 945 0.83 10.92 895 0.81 10.63 1179 0.80 11.0Total 3019 0.81 10.9

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and by ICP-OES are close together for both sorts of particles.

While the RSDs of the peak and integrated intensities were 7.6%

and 6.2% for the 2.06 mm SiO2 particles and 18.5% and 19.1% for

2.08 mm SiO2 particles, the RSDs of the mass measurements by

SEM were 7.5% and 17.6%, respectively.

Fig. 7 is presenting the results for the 1.16 mm SiO2 particle

suspension (see Table 1). Here, the agreement between the SEM

and ICP-OES distributions is not as good as for the larger 2.06

and 2.08 mm SiO2 particles shown in Fig. 5 and 6. The relative

mass distributions measured by ICP-OES are slightly broader

than the distribution evaluated from SEM pictures. Further-

more, the secondary peak structure at 9.5 pg in the SEM mass

plot, which was observed in all SEM evaluations of sub-ensem-

bles, is obviously smeared in both intensity plots. The relative

Fig. 7 Comparison of the absolute Si mass distribution for 1.16 mm SiO2 part

derived from the peak as well as from the integrated intensities of the Si 288.

1400 | J. Anal. At. Spectrom., 2010, 25, 1395–1401

standard deviations of the spectroscopic measurements were 17%

and 16% whereas a RSD of �11% was found for the SEM

distribution. Table 2c and Table 3c are presenting the results

obtained for sub- and total ensembles by SEM and ICP-OES,

respectively. The reason for the difference and for the absence of

the secondary mass peak in the spectroscopic measurements is

the relatively small signal to noise ratio (S/N ¼ 14) which

significantly broaden the distribution of the 1.16 mm particles. It

means that such small S/N cannot be neglected in the evaluation

of the relative mass distribution. In cases of the 2.06 and 2.08 mm

particles the S/N was about 130.

It is interesting to note that the RSD of the relative mass

distributions of the 2.06 and 2.08 mm particles obtained from the

spectroscopic peak intensity measurements (about 7.6% and

18.5%, respectively) are in reasonable agreement with the relative

standard deviations of the mass distribution which can be

calculated taking into account the RSDs of the diameters

measured by microparticles GmbH using the Coulter Multisizer

II instrument (6.6% and 18% for 2.06 and 2.08 mm, respectively).

However, the RSD of the mass distribution of 1.16 mm particles

calculated from the RSD of the diameter in Table 1 is �12.5%.

This value is significantly smaller than the RSD derived from the

intensity measurements (�17%, see Table 3c) but in rough

agreement with the RSD of the SEM measurement (�11%). As

pointed out above, the larger RSD of the spectroscopic

measurements is due to the contribution by the intensity noise.

The peak intensity measurements were taken for comparison

because they are generally more robust than measurements of the

integrated intensity at least for the evaluation of solid particles.

The reason is that the length of emission signals from particles is

sensitively dependent on the transport gas flow as shown

icle ensembles measured by SEM (top) and the relative mass distributions

16 nm line (bottom).

This journal is ª The Royal Society of Chemistry 2010

Table 3 The peak and integrated intensities of the Si 288.16 nm linemeasured by end-on ICP-OES of monodisperse microdroplets of verydiluted suspensions of (a) 2.06 mm, (b) 2.08 mm, and (c) 1.16 mm SiO2

particles

Nominaldiameter/mm Droplets Peaks

Peak intensity Integrated intensity

h/mA RSD (%) A/mA ms RSD (%)

(a) 2.06 25 613 911 55.0 6.5 70.6 5.525 575 553 53.7 7.5 71.0 6.428 250 303 51.1 8.4 71.9 7.425 550 116 52.4 7.7 72.1 6.413 088 15 52.4 10.1 70.0 9.6

118 076 1898 53.8 7.6 71.0 6.2(b) 2.08 10 205 524 53.1 20.5 43.6 20.4

11 940 1064 56.2 19.0 46.5 18.710 230 1078 57.5 17.4 50.4 17.610 345 999 57.5 17.8 51.2 18.210 250 604 55.4 17.7 48.9 18.152 970 4269 56.3 18.5 48.6 19.1

(c) 1.16 10 175 308 6.8 18 6.2 1510 070 293 7.3 16 5.5 1310 740 345 7.2 17 5.7 1510 045 305 6.7 16 6.6 1410 075 328 6.9 15 6.7 1451 105 1579 7.0 17 6.1 16

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recently.8 Therefore, small changes affect the integrated intensity

measurements stronger than the peak intensity measurements.

This is also the reason why the RSDs of the integrated intensity

measurements of the 1.16 mm sub-ensembles were smaller than

the RSD of the total ensemble (see Table 3c). There was a much

larger scatter of the measured integrated intensities than of the

peak intensities for the 1.16 mm sub-ensembles. Similar condi-

tions were also found in the measurements of other particle

ensembles (see, e.g., the scatter of the peak and integrated

intensities of the 2.08 mm sub-ensembles in Table 3b).

On the basis of the present experimental investigation it can be

stated that ICP-OES with microdroplet introduction can be used

for rapid evaluation of the mass distribution of micro- and nano-

particles in liquids. The only requirements for such measurements

are (i) high particle dilutions to exclude signals from two particles

in a droplet, (ii) ultrasonic pretreatment for homogenization of

the suspension and to avoid particle agglomeration and sedi-

mentation, and (iii) sufficiently large S/N ratios of the measured

line intensities to neglect intensity noise.

It is interesting that the variation of the emission line intensity

due to different particle trajectories are obviously of minor

importance. This seems to be plausible because of the relative

large inertia of microdroplets which assures a reproducible and

straight gas transport into the ICP. The situation would be

This journal is ª The Royal Society of Chemistry 2010

certainly different if the SiO2 particles would be introduced as

airborne particles. In particular very small particles are sensitive

to turbulences in the gas transport leading to signal variations

because of difference of the particle trajectories in the plasma.

Conclusion

The comparison of absolute particle mass distribution deter-

mined by the evaluation of SEM images and relative mass

distributions measured by ICP-OES of monodisperse micro-

droplets including these particles revealed that the spectroscopic

method can be applied as long as the S/N ratio of the line

intensities is sufficiently high. Absolute mass distributions can be

given if the ICP-OES signals are calibrated by the line intensities

obtained from monodisperse droplets of the same size but made

from element standard solutions of known concentrations as has

been demonstrated recently.1 To what extend the analytical

figures of merit of the new spectroscopic technique for particle

mass measurements can be transferred to ICP-MS has to be

investigated. ICP-MS would allow characterizing element masses

of particles with diameters below 100 nm because of its higher

detection power compared with ICP-OES.

Acknowledgements

Project funding by the Deutsche Forschungsgemeinschaft

(DFG) is gratefully acknowledged. The authors like to thank Dr

Harald Fiedler of the microparticles GmbH in Berlin for

providing suspensions of SiO2 particles with exceptionally broad

particle size distributions and Dr Alex von Bohlen for the SEM

pictures of the SiO2 particles.

References

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35, 531.4 J. W. Olesik and S. E. Hobbs, Anal. Chem., 1994, 66, 3371.5 J. W. Olesik, Appl. Spectrosc., 1997, 51, A158.6 A. C. Lazar and P. B. Farnsworth, Appl. Spectrosc., 1997, 51, 617.7 A. C. Lazar and P. B. Farnsworth, Appl. Spectrosc., 1999, 53, 457.8 S. Groh, C. C. Garcia, A. Murtazin, V. Horvatic and K. Niemax,

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