PAPER www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry
Evaluation of sample pretreatment methods for multiwalled and single-walledcarbon nanotubes for the determination of metal impurities by ICPMS,ICPOES, and instrument neutron activation analysis
Karl X. Yang,*a Michael E. Kitto,ab John P. Orsini,a Kamal Swamiab and Shaun E. Beacha
Received 29th April 2010, Accepted 22nd June 2010
DOI: 10.1039/c0ja00012d
The manufacture of carbon nanotubes (CNTs) relies on the use of transition metal catalysts. The
presence of metals in CNTs has been shown to critically affect the physical, chemical and surface
properties of the material for applications in areas such as gas sensors and microcolumns. Once CNTs
are released into the environment, the bioavailability of the metals is of concern, in the context of
potential human toxicity. In the present study, methods were developed to determine the metals’
concentrations in single-walled and multiwalled CNTs (SWCNT and MWCNT, respectively). The
metals’ concentrations in the SWCNT and MWCNT were determined by inductively coupled plasma
optical emission spectrometry (ICPOES) and mass spectrometry (ICPMS), after the CNTs had been
pretreated with one of the three extraction/digestion methods: water extraction, dilute acid (1% HNO3)
extraction, and microwave acid digestion. The total metal concentrations were determined by
instrumental neutron activation analysis (INAA). The metals in CNTs were found to have poor
solubility in water and dilute acid, suggesting that the role of CNT metals in cytotoxicity may be limited
due to their limited bioavailability, and that metals encapsulated in the CNTs could have potential use
as tracers for CNTs, in biological or toxicological studies. Microwave acid digestion can achieve a near-
complete extraction of metals from the CNTs, and thus is a suitable cleaning method, when high-purity
CNTs are desired. Microwave acid digestion followed by ICPOES analysis produced results closer to
those obtained by INAA than to those obtained by ICPMS; the latter method was subject to
nonspectral interference induced by carbon residues in the sample solution.
Introduction
Carbon nanotubes are relatively new carbon allotropes with
structures similar to fullerenes and graphite. The manufacture of
CNTs relies on the use of transition-metal catalysts. Even though
the ‘‘purified’’ CNTs available commercially have been processed
so as to reduce the amounts of metals, as impurities, significant
amounts of metals remain in the final products.1 The presence of
metals in CNTs has been shown to critically affect the physical,
chemical, and surface properties of the material in applications
such as gas sensors2 and microcolumns.3,4 Once CNTs are
released into the environment, the bioavailability of these metals
was also a great concern with respect to human toxicity.5 Some
nanomaterials that enter animal tissues can pass through cell
membranes or cross the blood–brain barrier;6 those character-
istics can be beneficial, in the delivery of therapeutically active
molecules, but they can also allow harmful materials to gain
access to the most protected organs in the human body. The
toxicity of CNTs originates from a suite of characteristics: their
large surface-to-mass ratio, surface activity, fiber length, and
metal content, as well as the presence of organic compounds on
the surface.7 The metals typically found on the surface of CNTs
aLaboratory of Inorganic and Nuclear Chemistry, Division ofEnvironmental Health Sciences, Wadsworth Center, New York StateDepartment of Health, P. O. Box 509, Albany, NY, 12201-0509. E-mail:[email protected] of Environmental Health Sciences, School of Public Health,The University at Albany, P. O. Box 509, Albany, NY, 12201-0509
1290 | J. Anal. At. Spectrom., 2010, 25, 1290–1297
include Co, Fe, Ni, and Mo, all of which have documented
a toxic effect on human health, especially in relation to the
pulmonary system.
The accurate determination of metal concentrations in CNTs
is essential to the understanding of not only the physical and
chemical characteristics of the CNTs, but also the extent to which
the CNTs interact with endogenous biological molecules, upon
host exposure. The uptake of CNTs by cells or the distribution in
vivo will be affected by the CNTs’ complexing, possibly involving
the metals in the nanomaterial, with the environmental and/or
recipient factors. Both the leachable and CNT-bound metals
merit careful investigation; the two types affect the properties of
CNTs differently, especially in relation to human toxicity. Few
publications have entailed the accurate determination of metal
contaminants in CNTs. Ishikuro et al.8 investigated the use of
several sample dissolution procedures for the determination of
metallic elements in carbon nanotubes by inductively coupled
plasma optical emission spectrometry. The microwave-assisted
dissolution procedure was found to dissolve the CNTs
completely with a mixture of nitric and perchloric acids. Metallic
elements (Fe, Cr, Mo, Al, and Ni) in CNTs were determined and
good reproducibility was achieved. Ge et al.9 used neutron acti-
vation analysis (NAA) to determine total metal concentrations in
single-walled and multiwalled CNTs. The results from NAA
were compared with those from ICPMS analysis. Different
sample pretreatment methods, involving dry ashing coupled with
acid extraction, wet digestion, and a combination of dry ashing
and acid digestion, were used prior to ICPMS analysis. In the
This journal is ª The Royal Society of Chemistry 2010
absence of a true reference material for CNTs, the NAA results
were used as the best estimate of the ‘‘true value’’ of the metallic
impurities in CNTs. The ICPMS instrument was deemed to be
ideal for a routine analytical laboratory, if the CNT samples were
pretreated with a prolonged microwave digestion with high
concentration of acids, or with a combination of dry ashing and
acid digestion. Bizzi et al.10 used a microwave-induced combus-
tion technique11 to digest CNTs prior to determination of the
halogen (Br, Cl, F, and I) contents by ICPMS.
In the present study, commercially available SWCNT and
MWCNT samples were pretreated and metal concentrations
were compared among three extraction methods: water extrac-
tion, extraction with dilute acid and microwave acid digestion.
Extraction with water simulates leaching of metals in human
body once the CNTs are inhaled; crucial to the understanding of
the properties of different types of metal containing carbon
nanotubes in relation to pulmonary toxicology. Extraction with
1% (v/v) HNO3 simulates leaching of metals in an acidic envi-
ronment; the study will shed light on the bioavailability and
mobility of CNTs metals once the nanomaterial is released into
the environment. Extraction with 8% (v/v) HNO3 + H2O2 +
microwave digestion is designed for total leaching of metals.
Microwave acid digestion is a sample pretreatment method that
is routinely used prior to determination of metals in various
matrices by the use of analytical atomic spectroscopic methods,
such as ICPOES and ICPMS. The digestion technique also has
potential use as a cleaning method to leach out most or all of the
metal impurities for applications of the CNTs in toxicology
studies,12 and analytical microcolumn or preconcentration
absorbent material,3 where high purity CNTs are desirable,
although many sample pretreatment processes were reported to
have altered the structures of the CNTs.13 In the present study,
the CNTs were also analyzed by the use of instrumental neutron
activation analysis (INAA) for determination of the total metal
contents from which the extraction efficiencies were calculated
and compared.
Experimental
Extraction/digestion methods
Three aliquots, approximately 5 mg each, of each of the two
CNTs (Sigma-Aldrich, St. Louis, MO) were weighed out for each
of the three extraction/digestion methods. A total of eighteen
CNT samples were prepared. Sample blanks, spikes and a Stan-
dard Reference Material (SRM 1648, Urban Particulate Matter)
were also included in all preparation procedures. For the water
extraction experiment or the dilute acid extraction experiment,
each aliquot of about 5 mg of CNT was mixed with 25 ml of
distilled-deionized water (18.2 MU-cm; Barnstead International,
Dubuque, IA) or 1% HNO3 (v/v), made up from the
concentrated nitric acid (Ultrex; J.T. Baker, Phillipsburg, NJ), in
a 50-ml polypropylene centrifuge tube, which was then sonicated
in a water bath (Bransonic 2200; Branson Cleaning Equipment,
Shelton, CT) for one hour.
A MARS 5 microwave oven (CEM Corporation, Matthews,
NC) was used for the microwave digestion of the CNT samples.
Details of the microwave system have been described else-
where.14,15 About 5 mg of CNT was mixed with 2 ml of the
This journal is ª The Royal Society of Chemistry 2010
concentrated HNO3 + 4 ml of H2O2 (30–32%, semiconductor
grade; Aldrich Chemical Company, Inc., Milwaukee, WI). The
samples were digested in two steps. In step 1, the temperature was
ramped to 170 �C in 20 min with the application of 1200 W of
power, followed by a dwell time of 5 min at 170 �C. In step 2, the
temperature was ramped to 185 �C in 5 min with the application
of 1200 W of power, followed by a dwell time of 15 min at 185 �C.
The samples remained in the microwave oven while they cooled
back down to room temperature before the vessels were vented
and opened. The samples were then diluted to 25 ml with the
distilled-deionized water. Before analysis, all samples were
filtered through a metals-free, pure PTFE (2mm) fiber filter
(Environmental Express, Mt. Pleasant, SC).
ICPMS analysis
The ICPMS system was an Agilent 4500 (Agilent Technologies,
Palo Alto, CA), equipped with ChemStation Software and
a Cetac Technologies ASX 500 autosampler (CetacTechnologies,
Omaha, NE). The instrument was tuned daily using both auto-
tuning and manual tuning; the tuning solution consisted of 10 mg
l�1 each of Li, Y, Ce, and Tl in 1% HNO3. Ion counts greater than
20 K s�1, 60 K s�1, and 40 K s�1 for Li, Y, and Tl, respectively,
with relative standard deviations better than 3%, were deemed
satisfactory. The ratio of doubly charged to singly charged ions
(70Ce2+/140Ce+) was maintained below 1%; the oxide ratio
(156CeO+/140Ce+) was maintained below 3%. Elements included
in the analysis were: Be, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, As, Se, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Tl, and Pb;
the respective m/z values used were 9, 23, 24, 27, 39, 43, 47, 51, 53,
55, 57, 59, 60, 65, 66, 75, 82, 88, 95, 107, 111, 118, 123, 137, 205,
and 208. Three repeated readings were taken per mass for all
elements, with an integration time of 0.1 s per reading for all
elements except As and Se; for these two, an integration time of
1.0 s was used. An internal standard solution, consisting of 1 mg
l�1 of Sc, Y, In, and Tb, assigned to analyte masses of <61, 63–88,
95–137, and 205–209, respectively, was used throughout the
experiment. Each analytical run began with a series of calibration
solutions (diluted from standards manufactured by Spex Certi-
Prep, Metuchen, NJ), followed by an independent quality
control (QC) (diluted from standards manufactured by Inorganic
Ventures, Christiansburg, VA), low and high QCs, 10 samples,
a set of QCs to continue validating the calibration curve, and 10
more samples followed by a set of QCs, and so on. We followed
EPA Method 200.8, which requires that the analytical errors not
to exceed 10% for most QCs.
ICPOES analysis
The ICPOES system was an Optima 3300 manufactured by
Perkin Elmer (Norwalk, CT), equipped with a dual detector
system that covered a wavelength range from 165 to 782 nm; it
was operated with the ICP WinLab32 Software. The detection
wavelengths were chosen following the manufacturer’s recom-
mendation for Al, Sb, As, Ba, Be, Cd, Cr, Co, Cu, Fe, Pb, Mn,
Mo, Ni, Se, Ag, Sr, Tl, Sn, Ti, V, Zn, Ca, Mg, K, and Na. The
flow rates for the plasma, auxiliary, and nebulizer gases were 15,
0.5, and 0.55 l min�1, respectively. The generator power was 1500
W. The peristaltic pump sample flow rate was 1.8 ml min�1. The
J. Anal. At. Spectrom., 2010, 25, 1290–1297 | 1291
spectrometer’s minimum and maximum integration times were 5
and 50 s, respectively. The instrument was tuned daily before
quantitative analysis, with a 1 mg l�1 of Mn solution in 1%
HNO3. A reading exceeding 1.8 M was considered satisfactory.
The sequence of each analytical run was similar to what was
described above for ICPMS. We followed EPA Method 200.7,
which requires that the analytical errors not exceed 5–10% for
most QCs.
INAA analysis
The SWCNT sample was analyzed for metals in duplicate
(12.2 mg and 23.9 mg). The MWCNT sample was analyzed in
triplicate (15.1 mg, 16.6 mg, and 17.7 mg). Each of the CNT
samples was placed into a polyethylene bag. The bagged samples
were irradiated for 10 h at a thermal neutron fluence rate of 1.2�1014 cm�2 s�1 in the nuclear research reactor at the National
Institute of Standards and Technology (Gaithersburg, MD).
Following several days of decay, spectra were collected at least
seven times for each sample, over a period of a month, using two
p-type, coaxial, high-purity germanium detectors in low-back-
ground shields. The absolute detector efficiencies were 40% and
80%, relative to a 300 � 30 0 NaI(Tl) detector, and the resolutions
were 1.9 keV at 1333 keV. Spectra were collected and analyzed by
the Genie-VMS Spectroscopy System (Canberra Industries Inc.,
Meriden, CT) and a multiplexed system coupled to an Ethernet
network, as described elsewhere.16 Compton background and
spectral interferences due to some elemental enrichments in the
nanotubes (e.g., high Co concentration in the SWCNT sample)
prohibited measurement of some commonly observed decay
products. Four standard reference materials, SRM 1570a
(Spinach), SRM 1571 (Orchard Leaves), SRM 1572 (Citrus
Leaves), and SRM 1632a (Eastern Coal), were included in the
irradiation and counting quality assurance. The polyethylene bag
blank was subtracted from each sample result.
Results
ICPOES results
Table 1 shows the metal concentrations in MWCNT and
SWCNT, from each of the three extraction methods, followed by
ICPOES analysis. For the MWCNT, H2O-sonication extracted
a small amount of measurable Ni into the solution, while other
metals concentrations were below the quantification level
Table 1 Mean concentrations of metals in MWCNT and SWCNT samplesanalysisa,b
Metal
Multi-walled Carbon Nanotubes (MWCNT)
H2Osonication/mg g�1
1%HNO3
sonication/mg g�1
8% acidmicrowave/mg g�
Cr nd nd 12 � 3Co nd nd ndCu nd nd 40 � 12Fe nd 68 � 5 960 � 190Mo nd nd ndNi 56 � 8 130 � 33 3100 � 830Zn nd nd 100 � 100
a nd: not detectable. b Each mean concentration and standard deviation was
1292 | J. Anal. At. Spectrom., 2010, 25, 1290–1297
(3 times the limit of detection). Sonication with 1% HNO3
increased the solubility of metals; the concentration of Ni more
than doubled; the concentration of Fe was high enough to be
quantified. Microwave acid digestion increased the metals’
solubility dramatically. As a result, Cr, Cu and Zn levels became
measurable in the solution. The solubility of Ni and Fe increased
by 24 and 14 fold, respectively, when compared to the level
achieved in the 1% HNO3-sonication extraction method.
Compared to MWCNT, the SWCNT contained greater total
amounts of metal impurities, but elements differed between the
two. Co and Mo had the highest concentrations among the
metals. Water-sonication leached out 500 and 680 mg g�1 of Co
and Mo, respectively; the levels of other metals were too low to
be quantified. Sonication with 1% HNO3 increased the leachable
Co, Fe, and Mo to 760, 400, and 930 mg g�1, respectively.
Microwave acid digestion further increased the leaching of
metals: the soluble concentrations of Cr, Co, Fe, Mo, Ni, and Zn
increased to 50, 25 500, 2000, 9000, 40, and 110 mg g�1, respec-
tively. Microwave acid digestion enhanced the solubility of Co,
Mo and Fe to the greatest extent, by 33, 10, and 5 times,
respectively, when compared to the 1% HNO3/sonication
extraction method. The greatly increased leaching power by
microwave acid digestion supports findings from previous
reports1 that the metals are encapsulated by the carbon shells;
these shells isolate the metals from the acid solvents, thereby,
limiting the metals’ solubility by the H2O-sonication and 1%
HNO3-sonication methods. Microwave acid digestion appar-
ently dissolved greater amounts of CNT-bound metals, possibly
by breaking up the carbon shells, to allow the acid solution to
come to full contact with the encapsulated metal particles, and
the solubility of the metals was thereby drastically increased.
Despite their limited solubility under mild leaching conditions,
the CNT-bound metals, especially those present in high
concentrations, such as Mo and Co in SWCNT and Ni in
MWCNT, have the potential to be released into the surround-
ings; although the proportionate amounts leached are low, they
nevertheless might be high enough to be biologically significant.17
The variable solubility of CNT metals could have contributed to
the conflicting results that have been reported with respect to the
cytotoxicity of CNTs,18 given that the toxicity of the CNTs is
dependent on the bioavailability of the CNT-bound metals.
In Table 1, the mean concentration and standard deviation
(SD) were calculated from results of three CNT sample aliquots.
Zinc concentrations were highly variable in the CNT samples,
pretreated with one of three methods, before determination by ICPOES
Single-walled Carbon Nanotubes (SWCNT)
1
H2Osonication/mg g�1
1%HNO3
sonication/mg g�1
8% acidmicrowave/mg g�1
nd nd 50 � 12500 � 40 760 � 50 25500 � 1280nd nd ndnd 400 � 53 2000 � 250680 � 70 930 � 50 9000 � 450nd nd 40 � 11nd nd 110 � 130
calculated from measurements of three aliquots of the CNT sample.
This journal is ª The Royal Society of Chemistry 2010
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This journal is ª The Royal Society of Chemistry 2010 J. Anal. At. Spectrom., 2010, 25, 1290–1297 | 1293
Fig. 2 Photographs of the same solutions after sitting undisturbed for
20 min. Large particles have developed in the solutions due to the
aggregation of the CNT. The aggregation is more prominent in the
MWCNT.
with relative standard deviations (RSDs) close to or greater than
100%. Cr, Cu and Ni showed lower variability in concentrations
in the CNTs, nevertheless, the RSDs were greater than 14%. Iron
showed a consistent RSD of 13% in the SWCNT results;
however, the RSDs were variable in the MWCNT results (8%
and 20%), more sample measurements would be needed to
determine the true variability of Fe in this MWCNT sample. Co
and Mo in the SWCNT showed the lowest variability among the
metals; the RSDs were between 5 and 10%, indicating that the
distribution of these two metals in the SWCNT was more
homogeneous than other metals. Duplicates taken from the same
digestion aliquot were also analyzed, and gave results that agreed
very well with the original results. For example, for the micro-
wave acid digestion of a SWCNT sample, the percent difference
between the duplicate and the original results for Co, Cu, Fe,
Mo, Ni, and Zn was 4%, 10%, 2%, 1%, 4%, and 12%, respec-
tively. These values indicate that the variability in the metal
concentrations was mainly due to actual heterogeneous distri-
butions of the metals in the CNT samples rather than by
imprecision of the ICPOES analytical measurement.
Table 2 shows the spike recoveries of metals in SWCNT and
MWCNT matrices from the three extraction methods, as well as
the recoveries of metals in the Urban Particulate Matter (SRM
1648). Satisfactory recoveries were seen for Al, Sb, As, Ba, Be,
Cd, Cr, Co, Cu, Fe, Pb, Mn, Ni, Se, Ag, Tl, V, and Zn, in both
the MWCNT and SWCNT samples, for H2O-sonication, 1%
HNO3-sonication, and microwave acid digestion. The spike
recovery data showed that the carbon nanotubes in the extrac-
tion matrices contributed little interference to the ICPOES
measurements of the metals, although significant amounts of
carbon nanotubes remained in the solution after the extraction/
digestion and filtration procedures. Fig. 1 shows the microwave
acid digested solutions containing MWCNT and SWCNT
immediately after the solutions had been filtered through the
PTFE (2mm) fiber filter. Only small numbers of particles are
visible in either solution. Fig. 2 shows the same solutions, after
they had sat undisturbed for 20 min. Greater numbers of large
particles are visible in both solutions, due to the aggregation of
carbon nanotubes. The aggregation is more prominent for
MWCNT than for SWCNT. Despite the presence of the particles
in the solution, there was no clogging of the sample introduction
Fig. 1 Photographs of microwave acid digested solutions containing
SWCNT (left) and MWCNT (right) immediately after being filtered
through the PTFE (2mm) fiber filter. Very low numbers of particles are
visible.
1294 | J. Anal. At. Spectrom., 2010, 25, 1290–1297
system, since the aggregation can be easily broken apart by the
physical force generated by the peristaltic pump.
SRM 1648 was used as a quality control for the extraction/
digestion process in the absence of a carbon nanotube standard
reference material. As expected, we found a trend of increasing
recovery of most metals in the SRM, in the order of H2O-soni-
cation, 1% HNO3-sonication, microwave acid digestion. The
recovery result from the microwave acid digestion of the refer-
ence material matched that reported in our previous studies;14,15
therefore, the application of the microwave digestion procedure
to the CNT samples was evidently successful. Although the use
of SRM 1648 as a direct QC for the analysis of CNTs may be
questioned due to the vast difference in the matrix, we demon-
strate here that the SRM can serve as a reference whose metal
recovery results are indicative of the consistency of the micro-
wave acid digestion.
ICPMS and INAA results
Signal enhancement by carbon. Signal enhancement of metals
by carbon-containing compounds has been well studied.19–23 The
signal enhancement was suggested to result from the increase of
ionization efficiency of the analytes when the population of
highly energetic carbon ions (IP ¼ 11.26 eV) in the plasma was
increased through the introduction of carbon-containing organic
compounds into the sample. Fig. 1 and 2 showed residues of
CNTs in the digested solution. Aggregates were easily dissociated
upon slight shaking of the solution. Once the carbon compounds
derived from the CNTs entered the plasma, some of them would
be quickly decomposed into carbon atoms and eventually into
carbon ions, and would then populate the plasma with highly
energetic C+, thus to enhance the ionization of the analytes.
Therefore, we would anticipate signal enhancement evident in
the ICPMS results, when compared to those of ICPOES where,
for most wavelengths, atomic emission lines other than ionic lines
were used.
ICPMS and INAA results. Table 3 shows the concentrations of
metals in MWCNT and SWCNT determined by INAA,
ICPOES, and ICPMS. The samples were pretreated with
microwave acid digestion prior to ICPOES or ICPMS analysis.
For the SWCNT sample, excellent agreement was seen between
the INAA and ICPOES measurements for Co (25 330 and 25 500
This journal is ª The Royal Society of Chemistry 2010
Table 3 Concentrations of metals in MWCNTs and SWCNTs determined by ICPOES and ICPMS after microwave acid digestion, and by INAAa
Metal
SWCNT MWCNT
INAA/mg g�1 ICP/mg g�1 ICPMS/mg g�1 INAA/mg g�1 ICP/mg g�1 ICPMS/mg g�1
Cr 44 � 4 50 � 12 110 � 31 6.1 � 0.6 12 � 3 32 � 6Co 25 330 � 80 25 500 � 1280 32 000 � 3800 nd nd ndCu nd nd 30 � 13 nd 40 � 12 40 � 13Fe 2300 � 160 2000 � 250 2900 � 500 800 � 40 960 � 190 1030 � 210Mo 9880 � 20 9000 � 450 9700 � 680 4.7 � 0.8 nd 7 � 3Ni nd 40 � 11 80 � 15 nd 3100 � 830 3100 � 870Zn nd 110 � 130 93 � 108 52 � 70 100 � 100 76 � 80
a nd: not detectable.
Fig. 3 Scatter plot of metal concentrations in CNTs (SWCNT and
MWCNT) measured by INAA against concentrations measured by
ICPOES after microwave acid digestion. The straight line shows the best-
fit linear regression from seven pairs of concentration values.
mg g�1, respectively), indicating complete extraction of Co from
the SWCNT by the use of acid microwave digestion. The result
from ICPMS was 24% higher (32 000 mg g�1) than that from
ICPOES, the higher value probably arose from the signal
enhancement effect by the carbon compounds in the solution.
The Fe concentration in the SWCNT sample was determined to
be 2300, 2000, and 2900 mg g�1, by INAA, ICPOES, and ICPMS,
respectively. If the value measured by INAA is considered to
represent the total concentration of Fe in the nanomaterial, then
the extraction efficiency for Fe would be about 86% by micro-
wave acid digestion followed by ICPOES measurement. The
ICPMS yielded a concentration higher than did INAA, and the
nonspectral interference or signal enhancement by the carbon-
containing solution, for the former, may again have played a role
in the positively biased result. The concentration of Mo in the
SWCNT sample was measured to be 9880, 9000, and 9700 mg g�1,
by INAA, ICPOES, and ICPMS, respectively. The digestion
recovery rate measured by ICPOES was about 91% when
compared with the INAA result. The result from ICPMS is about
8% higher than that from ICPOES, probably due to the signal
enhancement effect by carbon. The lower IP value for Mo,
relative to Fe and Co, may explain why the signal enhancement
effect for Mo is the smallest, among these three major metal
components in the SWCNT. The Cr level in the SWCNT sample
was determined to be 44, 50, and 110 mg g�1, by INAA, ICPOES,
and ICPMS, respectively. The low concentration and uneven
distribution of Cr in the sample, as well as the small sample
amount, could have contributed to the higher RSDs found for
the ICPOES and ICPMS measurements, relative to the INAA
measurements. The uncertainty of the Cr results from ICPOES
and ICPMS is high. Nevertheless, the values obtained from
ICPMS are significantly higher than those from ICPOES,
partially due to the same signal enhancement effect seen for other
elements discussed previously, and partially due to spectral
interference from 40Ar13C. The levels of Cu, Ni, and Zn were too
low to be measured by INAA, at least with the use of current
experimental arrangement. The time required to transport the
samples from the irradiation facility to the counting laboratory,
and/or the high Compton background, prevented the accurate
measurement of isotopes present at low levels. Therefore, the
concentrations of these metals are not available by INAA. The
level of Cu was too low to be measured by ICPOES, but it was
determined to be 30 mg g�1 by ICPMS due to the latter instru-
ment’s higher sensitivity. The nickel concentration in the
SWCNT was determined to be 40 mg g�1 by ICPOES, and
This journal is ª The Royal Society of Chemistry 2010
80 mg g�1 by ICPMS. The higher value found by ICPMS may
again be due to the signal enhancement effect caused by carbon
compounds in the sample, as well as to spectral interference from59CoH given that the concentration of Co is hundreds of times
higher than that of Ni. The Zn level in the SWCNT sample was
extremely uneven, as shown by the standard deviations of the
results from ICPOES and from ICPMS; more sampling and
measurements would be needed to determine the actual level of
Zn in the SWCNT. The metal levels in the MWCNT sample
differed considerably from those in the SWCNT sample. The Co
level was too low to be quantified in the MWCNT by any of the
three analytical methods, even though the concentration of Co
was the highest among all of metals in the SWCNT. Molyb-
denum, although it had the second highest concentration among
all metals in the SWCNT, had concentrations of 4.7 and 7 mg g�1
in the MWCNT, by the INAA and ICPMS, respectively; the
concentration was too low to be quantified by ICPOES. We
found Ni to be most abundant in the MWCNT; its concentration
was determined to be 3100 mg g�1, by both ICPOES and ICPMS.
The high variability of the concentration value is likely due to the
heterogeneous distribution of Ni in the MWCNT. Unfortu-
nately, the Ni value could not be independently verified by INAA
due to the element’s low thermal-neutron cross section. The
concentration of Fe in the MWCNT sample was determined to
be 800, 960, and 1030 mg g�1 by INAA, ICPOES, and ICPMS,
respectively. The difference was not statistically significant
between the latter two values. It is unclear why the value deter-
mined by INAA is the lowest among the three. One possible
J. Anal. At. Spectrom., 2010, 25, 1290–1297 | 1295
Fig. 4 Scatter plot of metal concentrations in CNTs (SWCNT and
MWCNT) measured by INAA against concentrations measured by
ICPMS after microwave acid digestion. The straight line shows the best-
fit linear regression from eight pairs of concentration values.
Fig. 5 Scatter plot of metal concentrations in SWCNT measured by
ICPOES against concentrations measured by ICPMS after microwave
acid digestion. The straight line shows the best-fit linear regression from
six pairs of concentration values.
Fig. 6 Scatter plot of metal concentrations in MWCNT measured by
ICPOES against concentrations measured by ICPMS after microwave
acid digestion. The straight line shows the best-fit linear regression from
five pairs of concentration values.
explanation is that random contamination with Fe occurred
during the acid digestion process. In MWCNT, as was the case
seen in SWCNT, the concentration of Zn showed great vari-
ability by all three methods. Further analysis is needed for
accurate determination of Zn in MWCNT. There was good
agreement between the ICPOES and ICPMS results for the
concentration of Cu (40 mg g�1) in MWCNT. No INAA data for
Cu are available for comparison, due to the element’s short half-
life (12.7 h). The concentration determined for Cr in the
1296 | J. Anal. At. Spectrom., 2010, 25, 1290–1297
MWCNT sample varied according to the analytical instruments
used. By ICPOES, the reported level was close to the quantifi-
cation level, uncertainty is thus high.
ICPOES seems to be more suitable for accurate determina-
tion of metals in CNTs after microwave acid digestion than is
ICPMS. Fig. 3 illustrates the correlation of metal concentra-
tions, as measured by INAA and by ICPOES. Seven pairs of
metal-concentration values, from both SWCNT and MWCNT
materials, were included in the calculation of the linear regres-
sion equation; both the slope and the correlation coefficient (r2)
are close to 1, indicating strong agreement between INAA and
ICPOES. Fig. 4 shows the linear regression calculated from
eight pairs of metal-concentration values, as measured by
INAA and by ICPMS. The slope and the r2 are 1.224 and 0.993,
respectively, indicating a positive bias on the ICPMS measure-
ment as a result of the above-mentioned signal enhancement
effect by carbon (nonspectral interference) or/and polyatomic
spectral interference. The degree of signal enhancement by
carbon differs depending on the type of CNT. Fig. 5 and 6 show
the linear regressions between metal-concentration values, as
measured by ICPMS and by ICPOES, for SWCNT (from six
pairs of metal concentrations) and MWCNT (from five pairs of
metal concentrations), respectively. The two graphs show
similar r2 values, close to unity. Nevertheless, the slopes are
quite different, 1.23 for SWCNT, and 1.01 for MWCNT. These
data indicate that the sample matrix from MWCNT induces
much less signal enhancement effect on the ICPMS results than
does the matrix from SWCNT, possibly because of the particle
size difference between the two CNT types. The tube dimen-
sions of the MWCNT are: O.D. � I.D. � L ¼ (20–30 nm) �(5–10 nm) � (0.5–200 mm), while the diameter and length of the
SWCNT tube are 0.7–1.3 nm and 450–2300 nm, respectively.
The larger the particle size, the lower the likelihood that the
particles penetrated the PTFE fiber filter to reach the sample
solution. Once in the solution, the particles of larger size are less
likely to survive the sample introduction system, especially the
spray chamber, such that they can reach the plasma. Inside the
plasma, more energy and longer time were needed, to break
down the larger MWCNT particles and ionize the carbon
atoms. Together, these factors gave rise to a much smaller
population of carbon ions in the plasma, leading to less signal
enhancement effect, for the MWCNT sample matrix than for
the SWCNT matrix. Complete digestion of CNTs so as to drive
out carbon in the sample solution, has been demonstrated by
other researchers.8,9 In those cases, the signal enhancement
effect by carbon, when samples were analysed by ICPMS was
minimal. Nevertheless, in applications where incomplete diges-
tion of CNTs is encountered, ICPOES may be a better choice
for the determination of major metal impurities. The signal
enhancement by carbon, in ICPMS analysis of CNTs, cannot be
corrected by the reaction/collision cell technology, since the
interference is nonspectral in its nature.
The tendency of metals to remain complexed with CNTs
rather than to be leached into the surrounding biological envi-
ronment can potentially be exploited. For instance, given that
direct and sensitive determination of CNTs remains challenging,
it may be possible to use metals as tracers for the CNTs, in
certain biological and toxicological research areas where the
uptake route and congregation sites of the CNTs could be
This journal is ª The Royal Society of Chemistry 2010
mapped via a tracking of the signals from the encapsulated
metals in the CNTs.
Conclusions
Only small fractions of the metals present in carbon nanotubes
are soluble in water or 1% HNO3; such limited bioavailability
suggests that the cytotoxicity of CNT-associated metals is
limited. Microwave acid digestion dissolved most, if not all, of
the metal impurities present in the CNTs, and thus provides an
effective method for cleaning and producing high-purity CNTs.
Microwave acid digestion followed by analysis by ICPOES
produced more accurate results for major metal components in
the CNTs than did ICPMS; the latter was subject to interference
by carbon residues in the sample solution. Metal impurities can
potentially be used as monitoring tracers for the CNTs, whose
uptake, transportation and aggregation paths in a biological
system are critical to the study of these materials’ toxicity.
Acknowledgements
We wish to thank the operations staff at the NIST reactor
(Gaithersburg, MD) and Dr Robert Gregory Downing, for help
in performing the irradiation. We wish to thank Dr Liang T. Chu
at Wadsworth Center for providing the carbon nanotubes.
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