Evaluation of sample pretreatment methods for multiwalled and single-walled carbon nanotubes for the...

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


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


(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.


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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.


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


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


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


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


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


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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