Single Particle Inductively Coupled Plasma Mass Spectrometry: A
Powerful Tool for Nanoanalysis
Single particle inductively coupled plasma mass spectrometry is an emergent ICPMS method for
detecting, characterizing, and quantifying nanoparticles. Although the number of applications
reported to date is limited, the relatively simple instrumental requirements, the low number
concentration detection levels attainable, and the possibility to detect both the presence of dissolved
and particulate forms of an element make this methodology very promising in the nanoscience related
areas.
Francisco Laborda,* Eduardo Bolea, and Javier Jiménez-Lamana
Group of Analytical Spectroscopy and Sensors, Institute of Environmental Sciences, University of Zaragoza, 50009 Zaragoza, Spain
F. Laborda
Single particle inductively coupled plasma mass spectrometry
(SP-ICPMS) takes advantage of the well established
elemental technique of ICPMS but performing measurements
on a “particle by particle” basis. Single particle analysis using
ICP optical emission spectrometry was first reported in 1986.1
The methodology was initially adopted for analysis of aerosol
and airborne particles1−5
and suspensions of microparticles6
and cells7
being implemented in ICPMS instruments from 1993
to improve the attainable sensitivity of optical emission.8
Afterward, the feasibility of SP-ICPMS for analysis of colloidal
and microparticle suspensions was demonstrated by Degueldre
et al. in a series of papers.9−13
More recently, the rapid increase
in the development, production, and use of engineered
nanomaterials has led to renew the interest on single particle
ICPMS as an alternative to other available techniques for
detection, determination, and characterization of nanomateri-
als.14−16
Whereas nanomaterials are playing an increasing role in
many fields, the knowledge about their potential impact on
human health and the environment as well as the development
of regulations and legislation for their control is being outpaced.
It is recognized that innovative analytical approaches are
necessary for monitoring the presence of nanomaterials in
environmental and biological media, assessing their potential
impact and supporting regulations.17−19
In this context,
analytical chemistry is facing new challenges by regarding
nanomaterials as analytes and not just as samples. In
comparison with conventional analytes, the metrology of
nanomaterials involves not only their detection and quantification
but also their physicochemical characterization. One of
the first challenges arises from the solid-state properties of
nanomaterials. In addition to their chemical composition, a
number of physical properties can be used for their characterization,
which includes size, shape, surface charge, or surface
area. Concentration levels are another critical challenge, which
are especially significant when analyzing samples at environmentally
relevant concentrations, where mass concentration
detection limits below parts per billion should be achieved.
Finally, theses challenges have to be multiplied by the number
of nanomaterials and the potential samples to be analyzed,
which include from raw nanomaterials and industrial and
consumer products to environmental and biological samples
containing these nanomaterials.
Single particle ICPMS can be considered one of the
innovative and emerging analytical approaches demanded by
nanostakeholders. Although it is mainly used with suspensions
of nanoparticles (NP), which are nanomaterials with all three
external dimensions in the size range from 1 to 100 nm, it can
be also applied to colloids and microparticles up to several
micrometers as well as other nanomaterials with external
dimensions in these ranges, like nanoplates and nanofibers.
Single particle ICPMS is able to provide information about
the elemental chemical composition of noncarbon nanomaterials
(carbon based nanomaterials are excluded due to the
intrinsic low sensitivity of this element in ICPMS) as well as
their number concentration, size, and the number size
distribution. Because its dynamic range can be extended up
to the micrometers region, polydispersed systems as well as
aggregation or agglomeration processes may be studied. In
addition, dissolved forms of the constituent elements of the
nanoparticles can be detected and determined.15
Finally, the
low detection limits of ICPMS make single particle-ICPMS a
Published: December 5, 2013
Feature
pubs.acs.org/ac
© 2013 American Chemical Society 2270 dx.doi.org/10.1021/ac402980q | Anal. Chem. 2014, 86, 2270−2278
suitable method for the analysis of environmental samples at
environmentally relevant concentrations.
■ ICPMS AND NANOPARTICLES
ICPMS is considered one of the most versatile elemental
techniques, providing rapid multielement analysis and low
detection limits for a large range of samples on a routine basis.
Although samples can be analyzed in any aggregation state,
liquid is the most common. Liquid samples are introduced into
the ICPMS instrument by using a nebulization system,
consisting of a nebulizer and a spray chamber, which produces
an aerosol of polydisperse droplets. Once the droplets are into
the plasma, solvent evaporates, forming solid particles, which in
turn are vaporized and their elements atomized and ionized.
Ions are extracted through the interface into the mass
spectrometer, where they are separated according to their
mass/charge ratio and detected. Alternatively, suspensions can
be introduced by using monodisperse droplet generators and
in-house built introduction systems.20−23
Soluble forms of an element are distributed homogenously
within a solution, even at very low concentrations. Thus the
mass of element entering the plasma per unit of time and
traveling to the detector as ions can be considered constant,
producing a “steady” signal during the reading period. By
contrast, if the sample contains NPs, the element is no longer
distributed homogenously, being present as discrete groups of
atoms. Then, if a sufficiently dilute suspension of NPs is
nebulized into the plasma, a single pack of ions will be
generated when each NP is vaporized, atomized, and ionized,
which results in a transient signal of less than about 0.5 ms
length.24
Figure 1 shows the time-resolved signals of an element
being introduced into the ICPMS in dissolved form (part a) or
as NPs (part b). From a quantitative point of view, particles
with diameters below ∼1 μm are completely vaporized in the
time they spent in a plasma under standard conditions,24
allowing the same sensitivity for suspensions or solutions of the
same mass concentration.
Although NP suspensions behave and can be managed in
ICPMS like conventional solutions, the measurement of
individual NPs implies to record signals with respect to time
at high acquisition frequencies. By using fast data acquisition
systems (greater than 104
−105
Hz), detailed information about
the transient signal produced by each NP can be obtained
(Figure 1b). However, commercial ICPMS instruments with
quadrupole or single collector sector field mass spectrometers
typically do not attain these high frequencies, and signals are
measured as individual pulses with acquisition periods of one or
more milliseconds, as it is shown in Figure 1c,d. These time
scans are recorded during several seconds or minutes, and they
consist of a number of spikes above a steady baseline, as it is
shown in Figure 2a. Whereas each spike is due to the pack of
ions from a NP, the baseline is due to the background or the
presence of dissolved forms of the element measured. Raw time
scans can be processed by plotting the signal intensity vs the
signal intensity frequency, obtaining histograms as shown in
Figure 2b, where the first distribution is due to the background
and/or the presence of dissolved forms of the element
measured and the second to the NPs.
■ BASIC PRINCIPLES
The basic assumption behind SP-ICPMS is that each recorded
pulse represents a single NP. If this assumption is true then the
frequency of the pulses is directly related to the number
concentration of NPs and the intensity of each pulse is
proportional to the mass of element, in fact to the number of
atoms, in each detected NP. Theoretical basis of single particle
detection applied to ICPMS were outlined by Degueldre et
al.9−13
for NP suspensions continuously introduced through
conventional nebulization systems.
The relationship between the signal R (ions counted per time
unit) and the mass concentration of a solution of an element M
(CM
), which is nebulized into an ICPMS, can be expressed as25
=R K K K Cintro ICPMS M
M
(1)
Kintr (= ηnebQsam) represents the contribution from the sample
introduction system, through the nebulization efficiency (ηneb)
and the sample uptake rate (Qsam). KICPMS is the detection
efficiency, which represents the ratio of the number of ions
detected versus the number of atoms introduced into the ICP
and involves the processes of ionization, sampling through the
ICPMS interface, as well as the transmission through the mass
spectrometer. KM (= ANAv/MM) includes the contribution from
the element measured, where A is the atomic abundance of the
isotope considered, NAv the Avogadro number, and MM the
atomic mass of M.
For suspensions of spherical, solid, and pure NPs, the
elemental mass concentration CM
NP is expressed as
πρ= ⎜ ⎟
⎛
⎝
⎞
⎠
C
d
X N
4
3 2
NP
M
3
NP NP
(2)
where d is the NP diameter, ρ the density of the NPs, XNP the
mass fraction of the element in the NP (equal to 1 for a metallic
NP), and NNP the NP number concentration. Equation 1 can
be adapted for the nebulization of this NP suspension as
=R K K K K Nintro ICPMS M NP NP (3)
where KNP (= 4/3πρ(d/2)3
XNP) includes the properties of the
NPs.
Single particle ICPMS involves the use of suspensions
sufficiently diluted in order to detect just one NP per reading.
Under such conditions, the flux of NPs reaching the plasma
Figure 1. Time resolved ICPMS signals from a solution (a) and a
nanoparticle suspension (b, c, and d) of the same element. Frequency
of data acquisition: (a and b) × 100, (c) × 3, and (d) × 1 (not in
scale).
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(QNP) and hence the frequency of NPs detected (f NP) is given
by the first contribution of eq 3, where
η= =f Q Q NNP NP neb sam NP (4)
When measurements are acquired in time-resolved mode,
each reading lasts for a period equal to the dwell time (tdwell).
Under such conditions, and if just one NP is detected during a
single reading (ηnebQsamNNPtdwell = 1), rNP (= KICPMSKMKNP)
represents the total counts per reading and NP, which can be
expressed with respect to the diameter of the NP as
πρ=r X K K d
1
6
NP NP ICPMS M
3
(5)
or to the mass of M per NP (mNP):
=r K K mNP ICPMS M NP (6)
Equations 4−6 summarize the fundamentals behind single
particle ICPMS. Quantitative determinations of NP number
concentration are based on the linear relationship between the
frequency of NP events and the number concentration (eq 4);
whereas, the signal intensity of the NP events is proportional to
the mass of analyte per NP (eq 6) or to the third power of the
NP diameter for solid, spherical, and pure NPs (eq 5), allowing
the determination of analyte mass per NP and size
distributions, respectively.
■ PRACTICAL ASPECTS
When a NP suspension is introduced by conventional
nebulization, NPs arrive in the plasma randomly and their
flux must be low enough in order to obtain separate NP signals,
as it is shown in Figure 1b. If acquisition frequencies of 1000
Hz or lower are used (dwell times of 1 ms or longer) the flux
must be even lower to avoid the measurement of two or more
NPs within a dwell time, as it is depicted in the second event of
Figure 1d. Although the use of low dwell times (e.g., 1 ms) can
be considered an useful option to prevent this problem, the risk
of measuring a fraction of the whole signal is increased. These
NPs partially detected are indistinguishable from the entire
signal produced by a smaller NP, as it is shown in Figure 1c
(NPs 4 and 5 starting from the left).
The probability of more than one NP being measured within
a dwell time can be estimated by Poisson statistics.24
Because
this probability decreases with decreasing the dwell time and/or
the flux of NPs, compromise conditions can be selected which
guarantee the counting of an adequate number of one-NP
events. In practice, dwell times between 3 and 10 ms are
commonly used. Under such acquisition conditions, the NP
number concentration below 108
L−1
must be used to reduce
the occurrence of two-NP events,15
although this concentration
depends in last instance on the nebulization system used,
through its nebulization efficiency and the sample uptake rate
(eq 4).
■ DETERMINATION OF NANOPARTICLE SIZE
In single particle analysis, the detection of NPs depends on two
factors: (i) their number concentration, which should be high
enough to allow counting a minimum number of events, and
(ii) the size or the element mass per NP, which should be large
enough to generate a pulse of ions detectable by the
spectrometer.
If there is no limitation in the number of NPs, their detection
is associated to the capability of identifying the pulses from the
NPs over the baseline produced by the continuous background
in a time scan like shown in Figure 2a. The smallest pulse
height that can be distinguished from the background
determines the smallest detectable NP mass that can be related
to the smallest detectable NP size. Alternatively, NP pulses can
be identified in a frequency histogram (Figure 2b) as events
located at higher intensities than the background distribution.
In any case, size detection limits (LODsize) can be established
by25
σ
πρ
=
⎛
⎝
⎜
⎞
⎠
⎟
X K K
LOD
18
size
B
NP ICPMS M
1/3
(7)
when a 3σ criterion is used, where σB is the standard deviation
of the continuous background.
Equation 7 shows that LODsize depends basically on the
detection efficiency. Thus, the improvement in the ionization
conditions for elements with low ionization potentials as well as
the increase on sampling or transmission of ions through more
efficient instrumental designs can reduce the current size
detection limits. Table 1 summarizes reported size and mass
detection limits for metal and oxide based NPs. Metal NPs over
∼20 nm can be detected by SP-ICPMS, whereas the sizes
increase for oxides, depending on their stoichiometry. With
respect to mass, limits of detection are about tens of attograms
per NP.
On the other hand, the upper size limit for particle
characterization in SP-ICPMS is limited by the selective
removal of large particles in the spray chamber26
or their
incomplete vaporization in the plasma.24
In any case, 1−5 μm
are the maximum sizes recommended, which are sufficient for
Figure 2. (a) Time scan of a nanoparticle suspension containing dissolved forms of the element contained in the nanoparticle. (b) Pulse intensity
frequency histogram of data from part a.
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NPs and small aggregates characterization but may omit larger
aggregates.
For elements which show significant contributions from
plasma polyatomic ions (e.g., Si, Ti, Fe...) or from dissolved
forms, both as part of the sample or as contamination, the
magnitude of the continuous baseline affects negatively the size
detection limits, through the background standard deviation
(σB
2
= RBtdwell). Thus, in such cases, the selection of longer
dwell times involves an additional increase of size detection
limits.27,28
For pure, solid, and spherical NPs, size information can be
obtained by calibration with NP size standards of the same
chemical composition by using eq 5. The logarithmic plot of
the median pulse intensity for each NP standard versus its
diameter provides a straight line with a slope close to 3,15
as it is
shown in Figure 3. Alternatively, these standards can be used to
construct a mass calibration (eq 6) to obtain information about
the mass per NP for heterogeneous NPs.
When size standards are not available, Pace et al.16
have
developed a procedure based on the use of dissolved standards
of the element measured to determine the mass of analyte per
NP and hence the NP diameter. The procedure, which is
summarized in Figure 4, assumes that once in the plasma,
atoms from a dissolved standard solution and from a NP
behave comparably for the same element. It involves knowing
the nebulization efficiency, which is estimated by using a NP
suspension of known number concentration, as the ratio of the
detected NPs with respect to the calculated number of NPs
nebulized.
■ QUANTIFICATION OF NANOPARTICLES: NUMBER
CONCENTRATION
Working with NPs of detectable sizes, the frequency of NP
events counted is directly related to the NP number
concentration (eq 4). For a fixed NP number concentration
(NNP), the number of events (N) can be increased proportionally
by increasing the total acquisition time (ti):
η=N Q tNineb sam NP (8)
The counting of NP events can be assimilated to an ideal
Poisson counting process with zero blank, whose signal
detection limit can be rounded to 3 counts. Thus the number
concentration detection limit is associated to the capability of
counting three NP events, which can be directly related to the
number concentration limit of detection (LODNP) through eq
8:
η
= ×
Q t
LOD 3
1
i
NP
neb sam (9)
Equation 9 shows that number concentration detection limits
can be enhanced by improving nebulization efficiency,
increasing the sample flow rate, and/or using longer acquisition
times. In practice, NP detection limits in the range of 106
L−1
are reported with current ICPMS instruments.25,29
Number concentrations are independent of the NP nature,
and calibrations are performed by using available number
concentration standards. In practice, spherical solid and pure
NP suspensions of known average diameter and mass
Table 1. Size and Element Mass Detection Limits Reported
for Selected Nanoparticles Determined by SP-ICPMS
nanoparticle LODsize LODmass ref
Ag 18 nm 32 ag 15
<20 nm <44 ag 20
20 nm 44 ag 34,28
33 nm 200 ag 21
30 nm 535 ag 22
Au 21 nm 94 ag 21
25 nm 158 ag 13
17 nm 331 ag 22
U 10 nm 106 ag 22
Al2O3 30 nm 30 ag 9
ZrO2 70 nm 755 ag 10
ThO2 80 nm 2.4 fg 11
TiO2 100 nm 1.3 fg 9
FeOOH (goethite) 200 nm 10 fg 9
Figure 3. NP diameter/mass calibration vs 107
Ag pulse intensity. Gray
dashed lines: limits of detection (3σ criterion). Reprinted with
permission from ref 15. Copyright 2011 The Royal Society of
Chemistry.
Figure 4. Data processing scheme for counting (path A) and sizing
(paths A and B) nanoparticles using SP-ICPMS. (A-1) Raw data of
unknown sample, (A-2) sorted and binned raw data to separate pulses
from the background, (B-1) calibration curve of dissolved standards
created for particle size calculation, (B-2) transformed calibration
curve from concentration to mass per event. Reprinted from ref 16.
Copyright 2011 American Chemical Society.
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concentration (e.g., NIST RM 8013) are used for number
concentration calibration, being suitable for the quantification
of any type of suspension. Because the occurrence of two NP
events increases at high number concentrations, the use of eq 8
should be limited to a maximum number concentration which
guarantees the linear relationship between the number of
events and the number concentration.25
■ DISSOLVED vs NANOPARTICLES
When a time scan is processed as a frequency distribution
histogram (Figure 2b), specific information about the
occurrence of dissolved and NP forms of the element measured
can be obtained. If both distributions are fully resolved, the first
one shows a Poisson profile, whereas the second one resembles
the size distribution of the NPs, which tends to be log-
normal.15
When the distributions are not resolved, due to the
high concentration of dissolved forms or the small size of the
NPs, the Poisson distribution is lost and a tailed profile is
obtained.15,25
Although the element mass concentration in a NP
suspension can be determined by ICPMS working in standard
mode, mass concentration can be determined in single particle
mode by summing up the pulse intensity of the events recorded
or by integrating the corresponding histograms (sum of the
pulse intensities multiplied by the number of pulses, in Figure
2b). If such integration is performed for the different
distributions obtained in the histograms, the content of the
element as NPs and as dissolved forms can be obtained in
terms of mass concentration if both distributions are fully
resolved.
■ APPLICATIONS
Although SP-ICPMS is considered an emergent methodology
yet, its current applications cover different fields: (i) detection
and quantitation of NPs at environmental relevant concentrations,
(ii) sizing and size distribution of nanomaterials, (iii)
studies on stability of NPs, and (iv) migration and dissolution
of NPs from solid matrixes.
Detection and Quantitation of Nanoparticles in
Environmental Samples. The capability of SP-ICPMS for
detection of NPs in environmental samples has been tested by
the analysis of wastewaters from treatment plants.28,30
Mitrano
et al.30
detected and quantified both dissolved and nanoparticulated
silver at the ng L−1
level in inffluent and effluent
samples from a wastewater treatment plant. Figure 5 shows SPICPMS
time scans that confirm the presence of dissolved as
well as particulate silver in the wastewater samples. Tuoriniemi
et al.28
detected NPs containing silver, cerium, and titanium in
wastewater effluent samples at concentrations of 2000−30 000
mL−1
, which were in agreement with predicted concentrations
in Europe, although the specific nature of the particles was not
determined.
As a consequence of the use of consumer products
containing NPs, they can be released into the environment.
Thus silver NPs were detected in the effluent of a commercially
available nanosilver washing machine by SP-ICPMS.31
The
concentration of silver containing particles was determined to
be ∼108
particles mL−1
. The majority of the silver NPs were
below 20 nm; however, a small amount of larger particles was
also present in the effluent. Because of the low number
concentration detection limits of SP-ICPMS, Coleman et al.32
were able to detect the retention of NPs in invertebrates
Lumbriculus variegatus exposed to 70 nm silver NPs.
Although carbon-based NPs cannot be detected directly by
SP-ICPMS, Reed et al.33
have used the residual catalyst metals
(cobalt, ytrium, molybdenum, nickel) contained in carbon
nanotubes for their detection at ng L−1
levels. In spite of
quantitative results that could not be obtained, the method
allowed one to screen the release of carbon nanotubes from a
polymeric matrix.
Sizing and Size Distribution of Nanomaterials. Pace et
al.34
have shown that SP-ICPMS is able to size silver NPs,
across different sizes and number concentrations, with accuracy
similar to other commercially available techniques, namely,
dynamic light scattering (DLS), differential centrifugal
sedimentation (DCS), nanoparticle tracking analysis (NTA),
and transmission electron microscopy (TEM).
Loeschner et al.35
have used ICPMS in single particle mode
to obtain the number size distribution of silver NPs in meat
digestates. When compared to TEM, some differences were
observed for the size distributions although similar results were
obtained for the maxima, as it can be shown in Figure 6. These
differences arose from the size detection limit attained by SPICPMS
that hindered the detection of NPs smaller than 15 nm.
Also the fact that just the diameters of the primary particles
were measured by TEM, whereas aggregates/agglomerates
were also measured by SP-ICPMS, can justify them.
The European Commission (EC) published in 2011 a
definition of the term “nanomaterial” for regulatory purposes
based on the size of the NPs and their number size
distribution.36
In this respect, a report by the Institute of
Reference Materials and Measurements of the EC,37
related to
the requirements on measurements for the implementation of
this definition, included SP-ICPMS in addition to other current
available methods due to its intrinsic potential. Recently,
Laborda et al. have applied SP-ICPMS to estimate the number
size distribution of different commercial nanomaterials in the
context of the EC definition with successful results.25
Stability of Nanoparticles. NPs can undergo a number of
transformations under biological or environmental conditions.
Primary NPs can form agglomerates or aggregates of bigger
sizes, being reversible just in the first case. On the other hand,
some metal-containing NPs (silver, zinc oxide) can dissolve,
releasing soluble species. As we have stated throughout the
paper, SP-ICPMS is capable of handling NPs as well as their
Figure 5. Evidence of dissolved silver (elevated continuous background)
and nanoparticulate silver (pulses) in wastewater measured by
SP-ICPMS. Reprinted with permission from ref 30. Copyright 2012
Wiley.
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transformation products, both as aggregates and dissolved
forms. Following this trend, SP-ICPMS has been used for
quantitation of dissolved silver from silver NPs under
laboratory conditions.38
The stability of silver NPs in algal growth medium has been
studied by Pace et al.34
In Figure 7 it can be observed that the
appearance of a secondary distribution at sizes lower than the
original one after incubation, what was hypothesized as being
due to formation of silver chloride after oxidation of silver NPs.
By using SP-ICPMS in combination with DLS and SEM,
Walczak et al.39
have studied the fate of silver NPs during
gastrointestinal digestion. The study concluded that after gastric
digestion, the number of NPs dropped significantly due to the
formation of clusters composed of primary silver NPs and
chlorine (not observed by SP-ICPMS), but it rose back to
original values after the intestinal digestion and disagglomeration
of the clusters.
Migration and Dissolution of Nanoparticles from
Solid Matrixes. SP-ICPMS has been applied in studies on
the releasing and dissolution of silver NPs from plastic food
storage containers.40,41
Although the methodology has not
been fully exploited, it has allowed to distinguish the presence
of NPs from dissolved silver in different food simulants (water,
ethanol, acetic acid) used in the migration studies as well as the
relative mass distribution of both silver forms.
■ COUPLING OF SP-ICPMS TO SIZE SEPARATION
TECHNIQUES
In last instance, SP-ICPMS is just able of measuring the
elements present in a particle. This means that two different
NPs with the same element content behave in SP-ICPMS in
the same way. This can be the case for NPs of different shapes
or NPs with the same shape and core diameter but showing
different coatings. Transmission electron microscopy can
provide information about the shape of inorganic NPs, in
order to convert the mass per NP information obtained by SPICPMS
into size/diameter information. However, for inorganic
NPs coated with an organic moiety, other suitable techniques
for providing the size of the actual NP are needed.
Field flow fractionation (FFF) and hydrodynamic chromatography
(HDC) are two separation techniques which
separates particles according to their hydrodynamic diameter.
These techniques, when coupled to SP-ICPMS can provide an
additional selectivity that SP-ICPMS lacks.42
Up until now, just
HDC has been coupled online to SP-ICPMS, confirming the
potential of this approach.43
Figure 8 shows the threedimensional
chromatogram obtained by HDC-SP-ICPMS,
which provides information about the number concentration
and metal content of NPs as well as about their size. Despite
that the coupled technique has just been applied to pure gold
NPs, it is suitable of being used with heterogeneous NPs of
different nature.
Although FFF has not been coupled online to SP-ICPMS,
Loeschner et al.35
have measured fractions collected from
separations performed by asymmetric flow field flow fractionaFigure
6. Normalized number size distributions based on (a) SPICPMS
and (b) TEM for the pristine silver nanoparticles and for silver
nanoparticles in meat after enzymatic digestion. Reprinted with
permission from ref 35. Copyright 2013 Springer.
Figure 7. Nanoparticle size distribution of 100 nm silver nanoparticles
in algal growth medium before and after incubation at different silver
concentrations: (a) at 0 h, (b) after 144 h at 1000 μg L−1
, (c) after 144
h at 50 μg L−1
, and (d) after 144 h at 6 μg L−1
. Reprinted from ref 34.
Copyright 2012 American Chemical Society.
Figure 8. Three-dimensional hydrodynamic chromatogram obtained
from 30 and 60 nm gold nanoparticles detected online by ICPMS in
single detection mode. The chromatogram provides information about
number concentration, metal content per nanoparticle, and nanoparticle
diameters. Reprinted from ref 43. Copyright 2012 American
Chemical Society.
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tion by SP-ICPMS (offline coupling AsFlFFF−SP-ICPMS). In
this work, selected fractions from enzymatically digested meat
samples spiked with silver NPs were measured by ICPMS in
single particle mode to obtain the corresponding number size
distributions and to detect the occurrence of dissolved silver, as
it is shown in Figure 9.
Ion-mobility spectrometry (IMS) is a separation technique
based on a combination of mass, charge, size, and shape of
previously ionized species. The technique has been used with
success for the analysis of nanosize species such as macromolecules,
viruses, and NPs by using condensation particle
counting detectors. Recently, nanoelectrospray IMS has been
offline coupled to an ICPMS operating in single particle
mode,44
demonstrating the feasibility of the hyphenated
technique for element-specific size determination of gold NPs.
■ LIMITATIONS AND FURTHER INSTRUMENTAL
DEVELOPMENTS
As previously stated, the signal produced by a single NP in SPICPMS
is proportional to the mass of the element in the NP.
This means that the composition, density, and shape of the NP
must be known to get information about its size. Thus
combination of SP-ICPMS with imaging techniques, like
electron or atomic force microscopy, is recommended.
At present, SP-ICPMS is particularly suitable for NPs
consisting of one element only and sizes higher than ∼20
nm. In order to enhance size detection limits and implement
multielement capabilities for heterogeneous NPs, improved
commercially available ICPMS instruments are needed.
With respect to improving size detection limits, eq 7 shows
that the detection of smaller sized NPs involves the use of
ICPMS instruments with higher detection efficiency (ratio of
ions detected per NP vs atoms per NP). Detection efficiencies
of current quadrupole instruments are around 10−6
counts per
atom,21
although detection efficiencies of 10−4
counts per atom
have been reported by introducing desolvated aerosols.20
In any
case, reducing size detection limits down to the 1-nm range
requires ICPMS instruments with detection efficiencies around
10−3
counts per atom.
Because the duration of the transient signal generated by an
individual NP is in the range of hundreds of microseconds, the
multielement detection of transient signal generated by
individual NPs involves the use of simultaneous or fast
scanning instruments, able of recording full spectra of a mass
range of interest at frequencies around 105
Hz.45
Apart from
specialized multicollector ICPMS instruments, other commercially
available options for simultaneous multielement detection
would be time-of-flight spectrometers (TOF-ICPMS) and
double focusing sector field instruments equipped with array
detectors. However, the currently available instruments do not
permit the continuous monitoring at acquisition times below
several tens of milliseconds,22
being useless for multielement
SP-ICPMS. Recently, Borovinskaya et al. have developed a
prototype TOF-ICPMS capable of quasi-simultaneous multielement
detection at acquisition times of 33 μs and with
detection efficiencies similar to those of quadrupole-based
instruments.22
Figure 9. (a) AsFlFFF-ICPMS fractogram of an enzymatically digested meat sample containing silver nanoparticles and fractions collected for offline
SP-ICPMS analysis. (b) Number size distributions for all fractions and for the enzymatic digested (bulk) sample. Reprinted with permission from ref
35. Copyright 2013 Springer.
Table 2. Analytical Information Available from Samples Containing Dissolved and Nanoparticle Forms of an Element Analyzed
by SP-ICPMS
analytical information signal units standards
detection dissolved nanoparticle histogram distribution (no. events vs pulse
intensity)
none
dissolved + nanoparticle
characterization elemental composition pulse intensity none
mass distribution mean mass per
nanoparticle
histogram distribution (no. events vs mass) fg nanoparticles (mass) or dissolved standard
size distribution mean diameter histogram distribution (no. events vs
diameter)
nm nanoparticles (diameter) or dissolved
standard
quantitation nanoparticle number concentration number of events per distribution L−1
nanoparticles (diameter and mass
concentration)
element mass concentration number of events × pulse intensity per event ng L−1
dissolved standard
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Quadrupole and single collector sector field instruments,
working in single particle mode, are usually limited to monitor
just one isotope due to the sequential scanning nature of these
mass analyzers. However, fast scanning modes have recently
been proposed both for sector field46
and quadrupole
instruments.47
An acquisition frequency of 104
Hz (acquisition
time of 100 μs) has been reported using a sector field ICPMS,
whereas 106
Hz has been attained with a commercial
quadrupole, which involves acquisition times down to 1 μs.
Nevertheless, attainable settling times between isotopes can be
a limitation for multielement detection in a single nanoparticle.
Liquid sample introduction in commercial ICPMS instrument
is based on the use of pneumatic nebulization systems,
which involve the random arrival of polydisperse aerosol
droplets into the plasma. Alternatively, monodisperse droplet
generators have been used for introduction of NP suspensions
in single particle mode.20−23
These generators are able to
produce pulsed microdroplets at adjustable frequency (up to
100 Hz) and droplet size (below 100 μm) and are coupled to
custom-built delivery systems for transport of the droplets to
the plasma. These systems provide nebulization efficiencies
higher than 95%, although at extremely low sample flow rates.
Their main feature is the possibility of performing size/mass
calibrations just by using dissolved standards and knowing the
diameter of the droplets.21,23
■ CONCLUSIONS
Table 2 summarizes the different types of information that can
be obtained from SP-ICPMS. By analyzing a sample in an
ICPMS running in single particle mode it is possible to detect
both the presence of dissolved and particulate forms of an
element, to determine their respective mass concentrations, as
well as the number concentration of NPs. With respect to
characterization, size information about the average diameter
and the size distribution can be obtained if additional
information about shape and composition is known, otherwise
information about the mass of element per NP will be attained.
Lastly, ICPMS also provides chemical information about the
elemental composition of the nanomaterial analyzed, in
contrast to other techniques like DLS, NTA, or DCS.
Undoubtedly, the main feature of SP-ICPMS is that all the
information summarized in Table 2 is available by using
commercial ICPMS instruments, which makes the methodology
easily accessible to any ICPMS user. In this regard, SPICPMS
is on its way to expand out from the research
laboratories. By way of example, ICPMS manufacturers are
becoming aware of the capability of SP-ICPMS48
and some of
them have already launched application notes for spreading this
methodology.49,50
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: flaborda@unizar.es.
Notes
The authors declare no competing financial interest.
Biography
Dr. Francisco Laborda is Professor of Analytical Chemistry at the
University of Zaragoza. His research focuses on the development of
novel approaches for physicochemical elemental speciation, with
special attention to single detection ICPMS and separation techniques
hyphenated to ICPMS and their application to analytes in the
nanometer−micrometer range for environmental and nanomaterial
analysis. Dr. Eduardo Bolea is Associate Professor in the Department
of Analytical Chemistry at the University of Zaragoza. His research
focuses on the development of methods for the detection, characterization,
and quantitation of analytes in the range of nanometers, based
on field flow fractionation techniques and single particle ICPMS. Dr.
Laborda and Dr. Bolea belong to the Group of Analytical Spectroscopy
and Sensors (GEAS) of the Institute of Environmental Sciences
(IUCA) at the University of Zaragoza. Javier Jiménez-Lamana is a
Ph.D. student in the Group of Analytical Spectroscopy and Sensors.
With a B.Sc. in Chemistry from the University of Zaragoza, his
research focuses on developing analytical methods for the analysis of
nanomaterials.
■ ACKNOWLEDGMENTS
The authors gratefully acknowledge support from the Spanish
Ministry of Economy and Competitiveness (Project CTQ2012-
38091-C02-01).
■ REFERENCES
(1) Kawaguchi, H.; Fukusawa, N.; Mizuike, A. Spectrochim. Acta, Part
B 1986, 41, 1277−1286.
(2) Kawaguchi, H.; Kamakura, K.; Maeda, E.; Mizuike, A. Bunseki
Kagaku 1987, 36, 431−435.
(3) Nomizu, T.; Nakashima, H.; Hotta, Y.; Tanaka, T.; Kawaguchi,
H. Anal. Sci. 1992, 8, 527−531.
(4) Bochert, U. K.; Dannecker, W. J. Aerosol Sci. 1989, 20, 1525−
1528.
(5) Bochert, U. K.; Dannecker, W. J. Aerosol Sci. 1992, 23, S417−
S420.
(6) Garcia, C. C.; Murtazin, A.; Groh, S.; Horvatic, V.; Niemax, K. J.
Anal. At. Spectrom. 2010, 25, 645−653.
(7) Nomizu, T.; Kaneco, S.; Tanaka, T.; Ito, D.; Kawaguchi, H.;
Vallee, B. T. Anal. Chem. 1994, 66, 3000−3004.
(8) Kawaguchi, H.; Tanaka, T. Y. Anal. Sci. 1993, 9, 843−846.
(9) Degueldre, C.; Favarger, P.-Y. Colloids Surf., A 2003, 217, 137−
142.
(10) Degueldre, C.; Favarger, P.-Y.; Bitea, C. Anal. Chim. Acta 2004,
518, 137−142.
(11) Degueldre, C.; Favarger, P.-Y. Talanta 2004, 62, 1051−1054.
(12) Degueldre, C.; Favarger, P.-Y.; Rossé, R.; Wold, S. Talanta
2006, 68, 623−628.
(13) Degueldre, C.; Favarger, P.; Wold, S. Anal. Chim. Acta 2006,
555, 263−268.
(14) Heithmar, E. M.; Pergantis, S. A. Characterizing Concentrations
and Size Distributions of Metal-containing Nanoparticles in Waste Water,
EPA/600/R-10/117, U.S. Environmental Protection Agency, 2010.
(15) Laborda, F.; Jiménez-Lamana, J.; Bolea, E.; Castillo, J. R. J. Anal.
At. Spectrom. 2011, 26, 1362−1371.
(16) Pace, H. E.; Rogers, N. J.; Jarolimek, C.; Coleman, V. A;
Higgins, C. P.; Ranville, J. F. Anal. Chem. 2011, 83, 9361−9369.
(17) Klaine, S. J.; Alvarez, P. J. J.; Batley, G. E.; Fernandes, T. F.;
Handy, R. D.; Lyon, D. Y.; Mahendra, S.; McLaughlin, M. J.; Lead, J.
R. Environ. Toxicol. Chem. 2008, 27, 1825−1851.
(18) Baun, A.; Hartmann, N. B.; Grieger, K. D.; Foss Hansen, S. J.
Environ. Monit. 2009, 11, 1774−1781.
(19) Howard, A. G. J. Environ. Monit. 2010, 12, 135−142.
(20) Franze, B.; Strenge, I.; Engelhard, C. J. Anal. At. Spectrom. 2012,
27, 1074−1083.
(21) Gschwind, S.; Flamigni, L.; Koch, J.; Borovinskaya, O.; Groh, S.;
Niemax, K.; Günther, D. J. Anal. At. Spectrom. 2011, 26, 1166−1174.
(22) Borovinskaya, O.; Hattendorf, B.; Tanner, M.; Gschwind, S.;
Günther, D. J. Anal. At. Spectrom. 2013, 28, 226−233.
(23) Gschwind, S.; Hagendorfer, H.; Frick, D. A.; Günther, D. Anal.
Chem. 2013, 85, 5875−5883.
(24) Olesik, J. W.; Gray, P. J. J. Anal. At. Spectrom. 2012, 27, 1143−
1155.
Analytical Chemistry Feature
dx.doi.org/10.1021/ac402980q | Anal. Chem. 2014, 86, 2270−22782277
(25) Laborda, F.; Jimenez-Lamana, J.; Bolea, E.; Castillo, J. R. J. Anal.
At. Spectrom. 2013, 28, 1220−1232.
(26) Goodall, P.; Foulkes, M. E.; Ebdon, L. Spectrochim. Acta, Part B
1993, 48, 1563−1577.
(27) Reed, R. B.; Higgins, C. P.; Westerhoff, P.; Tadjiki, S.; Ranville,
J. F. J. Anal. At. Spectrom. 2012, 27, 1093−1100.
(28) Tuoriniemi, J.; Cornelis, G.; Hassellöv, M. Anal. Chem. 2012, 84,
3965−3972.
(29) Mitrano, D. M.; Barber, A.; Bednar, A.; Westerhoff, P.; Higgins,
C. P.; Ranville, J. F. J. Anal. At. Spectrom. 2012, 27, 1131−1142.
(30) Mitrano, D. M.; Lesher, E. K.; Bednar, A.; Monserud, J.;
Higgins, C. P.; Ranville, J. F. Environ. Toxicol. Chem. 2012, 31, 115−
121.
(31) Farkas, J.; Peter, H.; Christian, P.; Gallego Urrea, J. A.;
Hassellöv, M.; Tuoriniemi, J.; Gustafsson, S.; Olsson, E.; Hylland, K.;
Thomas, K. V. Environ. Int. 2011, 37, 1057−1062.
(32) Coleman, J. G.; Kennedy, A. J.; Bednar, A. J.; Ranville, J. F.;
Laird, J. G.; Harmon, A. R.; Hayes, C. A.; Gray, E. P.; Higgins, C. P.;
Lotufo, G.; Steevens, J. A. Environ. Toxicol. Chem. 2013, 32, 2069−
2077.
(33) Reed, R. B.; Goodwin, D. G.; Marsh, K. L.; Capracotta, S. S.;
Higgins, C. P.; Fairbrother, D. H.; Ranville, J. F. Environ. Sci.: Processes
Impacts 2013, 15, 204−213.
(34) Pace, H. E.; Rogers, N. J.; Jarolimek, C.; Coleman, V. A; Gray, E.
P.; Higgins, C. P.; Ranville, J. F. Environ. Sci. Technol. 2012, 46,
12272−12280.
(35) Loeschner, K.; Navratilova, J.; Købler, C.; Mølhave, K.; Wagner,
S.; von der Kammer, F.; Larsen, E. H. Anal. Bioanal. Chem. 2013, 405,
8185−8195.
(36) European Commission. 2011/696/EU: Commission Recommendation
of 18 October 2011 on the definition of nanomaterial. Off.
J. Eur. Commission 2011, 275, 38−40.
(37) Lisinger, T.; Roebben, G.; Gilliland, D.; Calzolai, L.; Rossi, F.;
Gibson, N.; Klein, C. Requirements on measurements for the
implementation of the European Commission definition of the term
″nanomaterial″; Joint Research Centre, Institute for Reference
Materials and Measurements. Publication Office of the European
Union: Luxembourg, 2012.
(38) Hadioui, M.; Leclerc, S.; Wilkinson, K. J. Talanta 2013, 105,
15−19.
(39) Walczak, A. P.; Fokkink, R.; Peters, R.; Tromp, P.; Herrera
Rivera, Z. E.; Rietjens, I. M. C. M.; Hendriksen, P. J. M.; Bouwmeester,
H. Nanotoxicology 2013, 7, 1198−1210.
(40) Goetz, N.; von Fabricius, L.; Glaus, R.; Weitbrecht, V.; Günther,
D.; Hungerbühler, K. Food Addit. Contam., Part A 2013, 30, 612−620.
(41) Echegoyen, Y.; Nerin, C. Food Chem. Toxicol. 2013, 62, 16−22.
(42) Kammer, F.; von der Ferguson, P. L.; Holden, P.; Masion, A.;
Rogers, K. R.; Klaine, S. J.; Koelmans, A.; Horne, N.; Unrine, J. M.
Environ. Toxicol. Chem. 2012, 31, 32−49.
(43) Pergantis, S. A; Jones-Lepp, T. L.; Heithmar, E. M. Anal. Chem.
2012, 84, 6454−6462.
(44) Kapellios, E. A.; Pergantis, S. A. J. Anal. At. Spectrom. 2012, 27,
21−24.
(45) Engelhard, C. Anal. Bioanal. Chem. 2011, 399, 213−219.
(46) Shigeta, K.; Koellensperger, G.; Rampler, E.; Traub, H.;
Rottmann, L.; Panne, U.; Okino, A.; Jakubowski, N. J. Anal. At.
Spectrom. 2013, 28, 637−645.
(47) Badiei, H.; Kajen, K. Single particle analysis using ultra-fast
quadrupole ICP-MS. European Winter Conference in Plasma Spectrometry,
Krakow, Poland, 2013.
(48) Salamon, A. W. Environ. Eng. Sci. 2013, 30, 101−108.
(49) Stephan, C.; Hineman, A. Analysis of NIST Gold Nanoparticles
Reference Materials Using the NexION 300 ICP-MS in Single Particle
Mode; Application Note, Perkin Elmer, Inc., 2012.
(50) Sannac, S.; Tadjiki, S.; Moldenhauer, E. Single particle analysis
using the Agilent 7700x ICP-MS; Application Note 5991-2929EN,
Agilent Technologies, Inc., 2013.
Analytical Chemistry Feature
dx.doi.org/10.1021/ac402980q | Anal. Chem. 2014, 86, 2270−22782278