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Single particle inductively coupled plasma mass spectrometry for the analysis of inorganic engineered nanoparticles in environmental samples
Trends in Environmental Analytical Chemistry 9 (2016) 15–23
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Trends in Environmental Analytical Chemistry journal homepage: www.elsevier.com/locate/teac
Single particle inductively coupled plasma mass spectrometry for the analysis of inorganic engineered nanoparticles in environmental samples Francisco Labordaa,* , Eduardo Boleaa , Javier Jiménez-Lamanab a Group of Analytical Spectroscopy and Sensors (GEAS), Institute of Environmental Sciences (IUCA), University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain b Laboratory of Bioinorganic Analytical and Environmental Chemistry (LCABIE), UMR5254, CNRS-UPPA, Hélioparc, 2 Av. President Angot, 64053 Pau Cedex 09, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2015 Received in revised form 10 February 2016 Accepted 10 February 2016
The analysis of engineered nanoparticles in environmental samples involves their detection followed by their quantification and characterization. The development of robust and reliable methods for achieving these objectives is one of the main challenges of analytical chemistry, and single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) is considered one of such promising methods. The success of SP-ICP-MS lies on the fact that different types of analytical information can be obtained by using any commercial instrument. A priori, the following information related to one or more specific elements can be obtained: (i) qualitative information about the presence of particulate and/or dissolved forms, (ii) quantitative information as particle number as well as mass concentrations and (iii) characterization information about the mass of element/s per particle and particle size. The transformations that engineered nanoparticles can undergo under environmental conditions and the occurrence of natural particles of similar composition bring additional challenges. The aim of this review is to present the current situation of SP-ICP-MS for the analysis of inorganic engineered nanoparticles in environmental samples and the approaches needed to cope with complex environmental problems. ã 2016 Elsevier B.V. All rights reserved.
Keywords: Single particle Inductively coupled plasma mass spectrometry Engineered nanomaterials Engineered nanoparticles Environment
Contents 1. 2. 3. 4.
5.
6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumentation and software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flux and number concentration of particles . . . . . . . . . . . . . . . . . 4.1. Acquisition frequency and reading time . . . . . . . . . . . . . . . . . . . . 4.2. Composition and size of particles . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Analytical information and performance . . . . . . . . . . . . . . . . . . . . . . . . . Determination of element mass/particle size . . . . . . . . . . . . . . . . 5.1. Determination of particle number concentration . . . . . . . . . . . . . 5.2. Determination of dissolved species . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Environmental applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SP-ICP-MS based approaches for the analysis of environmental samples
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16 16 17 18 18 18 18 18 18 19 19 19 20
Abbreviations: AF4, asymmetrical flow field flow fractionation; EDS, energy-dispersive X-ray spectroscopy; EELS, electron energy loss spectroscopy; ENM, engineered nanomaterial; ENP, engineered nanoparticle; HDC, hydrodynamic chromatography; ICP-MS, inductively coupled plasma mass spectrometry; MDG, monodisperse droplet generator; NP, nanoparticle; SP-ICP-MS, single particle inductively coupled plasma mass spectrometry; TEM, transmission electron microscopy; TMAH, tetramethyl ammonium hydroxide; XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction. * Corresponding author. E-mail address: fl[email protected] (F. Laborda). http://dx.doi.org/10.1016/j.teac.2016.02.001 2214-1588/ ã 2016 Elsevier B.V. All rights reserved.
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8.
F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
7.1. Screening of particles and dissolved species . . . . . . . Identification and sizing of particles . . . . . . . . . . . . . 7.2. Coupling of SP-ICP-MS to size separation techniques 7.3. Differentiation of engineered and natural particles . 7.4. Current limitations and future prospects . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction The widespread use of engineered nanomaterials in a range of applications and products is leading to their release into the environment, raising serious concerns about their potential impact [1]. For this reason, realistic risk assessments are needed to ensure the sustainable development and use of ENMs [2]. Environmental risk assessments of ENMs involve the use of exposure models, whose predictions should be validated through the detection, characterization and quantification of these ENMs, and/or their derivatives, in the compartments of interest. However, all these analyses are hampered by the complexity of the environmental sample matrices, the background levels of naturally occurring particulate materials and the expected concentrations of ENMs, down to ng L1 and ng kg1 [3]. This means that it is not currently possible to fully validate modeled data on ENMs in the environment, and both modeling and analytics must be used together to advance the risk assessment of ENMs for the time being [4]. Although techniques used for studying environmental colloids and particles were used as starting point [5] for the analysis of ENPs, and despite the significant progress made in recent years [6], the development of innovative techniques, methods and approaches to overcome the difficulties listed above remains a challenge for analytical scientists. In this context, single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) is one of the innovative and emerging analytical methods that is contributing to overcome this challenge [7]. SP-ICP-MS combines the benefits of an element specific atomic spectrometry technique, like ICP-MS, with those of a particle counting technique, because measurements are performed in a particle-by-particle basis. The strength of SP-ICP-MS lies in the different types of analytical information that can be provided at relevant environmental concentrations from natural and engineered NPs, excepting carbon based nanomaterials due to the intrinsic low sensitivity for this element in ICP-MS. The objective of this review is to present the state-of-the-art of SP-ICP-MS, its current limitations and future potential, as well as the approaches based on this methodology to cope and solve the challenges and problems related to the analysis of ENMs in environmental samples.
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21 21 21 21 22 22 22
present just in the discrete particles. Then, if a sufficiently dilute suspension of particles is nebulized into the plasma, the pack of ions generated from each single particle can be detected. Fig. 1 shows the time-resolved signals, with a duration of about 300– 500 ms, recorded at different acquisition frequencies. By using fast data acquisition (104 Hz, i.e. using reading times 100 ms), the transient signal produced by each particle can be recorded (Fig. 1a). Working at lower frequencies (i.e. at reading times in the milliseconds range), the packs of ions are measured as pulses, as it is shown in Fig. 1b and c. ICP-MS signals are recorded during several minutes, obtaining time scans as shown in Fig. 2a, which consist of a number of events, recorded as transient signals or pulses, depending on the selected acquisition frequency, above a steady baseline. Whereas the intensity of each event is due to the ions detected from each particle, the baseline is due to the background at the mass recorded or to the presence of dissolved forms of the element measured. Raw time scans can be processed by plotting the event intensity vs. the event intensity frequency, obtaining histograms as shown in Fig. 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 particles themselves. Theoretical basis of single particle detection applied to ICP-MS were outlined by Degueldre et al. and Favarger [8] and summarized by Laborda et al. [7]. The basic assumption behind SP-ICP-MS is that each recorded event represents a single particle. If this assumption is true then the number of events counted (np) during an acquisition time (ti) is directly related to the number
2. Fundamentals Samples are most commonly introduced into ICP-MS instruments as liquids by using a nebulization system which produces an aerosol of polydisperse droplets. Once the droplets are inside the plasma, solvent evaporates and the resulting particles are volatilized, atomized and subsequently ionized. Ions are extracted through the interface into the mass spectrometer, where they are separated according to their mass/charge ratio and detected. Soluble forms of an element are distributed homogenously within solution, and hence in the aerosol droplets. Thus the flux of element entering the plasma and travelling to the detector as ions can be considered constant, producing a steady signal during the reading period. By contrast, if a suspension of particles is nebulized, the element is not distributed homogenously, being
Fig. 1. Simulated time resolved ICP-MS signals from a particle suspension at different data acquisition frequencies (reading times). (a) 10,000 Hz (100 ms), (b) 1000 Hz (1 ms), (c) 100 Hz (10 ms). Not to scale.
F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
17
element per particle (mP):
(a)
IP ¼ K ICPMS K M mP
ð2Þ
event intensity
where KICPMS is the detection efficiency, which represents the ratio of the number of ions detected versus the number of atoms introduced into the ICP. KM (=ANAv/MM) is a factor related to the element measured, where A is the atomic abundance of the isotope considered, NAv the Avogadro number, and MM the atomic mass of the element. Eq. (2) can be related to the size of the particle if the composition, shape and density of the particle are known. For instance, for a solid, homogeneous and spherical particle, Eq. (2) can be written as: 1 3 IP ¼ prX P K ICPMS K M d 6
ð3Þ
where d is the diameter, r the density and XP the mass fraction of the element in the particle. Eqs. (1) and (2) summarize the fundamentals behind single particle ICP-MS. Quantitative determinations of particle number concentrations are based on the linear relationship between the number of events and the number concentration (Eq. (1)); whereas, the intensity of the particle events is proportional to the mass of analyte per particle (Eq. (2)), or to the third power of the diameter for solid, spherical, and pure particles (Eq. (3)), allowing the determination of element mass per particle and size distributions, respectively.
time
(b) number of events
background/ dissolved element
3. Instrumentation and software
NPs event intensity
number of events
(c)
diameter, nm Fig. 2. (a) Time scan of a nanoparticle suspension containing dissolved forms of the element contained in the nanoparticle. (b) Event intensity frequency histogram of data from (a). (c) Size distribution of spherical nanoparticles calculated from the second intensity distribution in (b).
concentration of particles (CP): nP ¼ hneb Q sam ti C P
ð1Þ
where hneb is the nebulization efficiency and Qsam the sample introduction flow rate. On the other hand, the intensity of each event (IP) is proportional to the number of atoms of the element monitored in each detected particle, and hence to the mass of
Originally, SP-ICP-MS was developed with quadrupole instruments. Working with these instruments, the frequency of data acquisition was dependent on applicable reading times (selected through the quadrupole dwell time per isotope) and the transmission and storage of data. In practice, dwell times were limited to the millisecond range, being 3–10 ms commonly used [7], which implies recording the particle events as pulses. Most recently, fast scanning quadrupole instruments have become commercially available, being able of working with dwell times in the microsecond range (100–10 ms) and hence recording particle events as more or less resolved transient signals [9]. As an additional feature, fast scanning quadrupole instruments can monitor up to two isotopes/elements in the same particle by combining the use of microseconds dwell times and removing the settling time [9]. Double focusing instruments can also be run in fast scanning mode by scanning the acceleration voltage instead of the magnetic field, allowing the use of 100 ms reading times [10]. An ICP time-of-flight mass spectrometer with a temporal resolution of 30 ms and full multielement capability was developed recently [11], and it is commercially available at present. Liquid samples are usually introduced in ICP-MS instruments by using different types of pneumatic nebulization systems, although most recently monodisperse droplet generators have been used for introduction of nanoparticle suspensions [12,13]. MDGs provide nebulization efficiencies close to 100%, at sample flow rates of hundreds of nL min1, whereas conventional nebulization systems achieve nebulization efficiencies of 2–5% at samples flow rates in the range of ca. 0.1–1 mL min1. Once data have been acquired, they have to be processed to get the different types of information that SP-ICP-MS can provide. Different protocols have been developed for processing raw data [14–17]. These protocols can be implemented by the users, exporting raw data to in-house programs and spreadsheets. A dedicated spreadsheet is freely available from RIKILT (http://www. wageningenur.nl/en/Expertise-Services/Research-Institutes/rikilt/ show/Single-Particle-Calculation-tool.htm) for calculation of particle sizes and concentrations from data acquired at milliseconds
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dwell times. On the other hand, manufacturers are incorporating to the software of their instruments, specific modules for acquisition and processing of single particle data, making easier the analysis of nanoparticles to unskilled users. In any case, users must be aware about a number of practical considerations to obtain reliable results, that will be discussed in the next section. The use of available reference materials (e.g. RM 8013 AuNPs or RM 8017 AgNPs from NIST) is highly recommended for the calculation of the nebulization efficiency and number concentration calibrations, as well as for internal validation of the measurement procedure. 4. Practical considerations The rapid development of SP-ICP-MS makes this technique available to a broad range of users, who should consider several key issues to get reliable information from a particle suspension analyzed by SP-ICP-MS. 4.1. Flux and number concentration of particles When a sufficiently diluted suspension is introduced into an ICP-MS, particles arrive into the plasma randomly and their flux must be low enough to record separate particle events, as it is shown in Fig. 1 for three different acquisition frequencies. The probability of more than one particle being measured within a reading time can be estimated by Poisson statistics [15,18], and it decreases with decreasing the reading time and/or the flux of particles. When using acquisition frequencies of 10,000 Hz or higher (reading times 100 ms) the occurrence of multiple particle events is related to the overlapping of the transient signals generated by individual particles, as it can be seen in Fig. 1a (event 4), which can be reduced by decreasing the flux of particles. If acquisition frequencies of 1,000 Hz or lower are used (reading times 1 ms) the flux must be even lower to avoid the measurement of two or more particles within a reading time, as it is depicted in the events 3 and 4 of Fig. 1c. When reading times slightly longer than the duration of the transient signal are used (e.g., 1 ms), the risk of measuring a fraction of the whole signal is increased, as it is shown in Fig. 1b (events 2 and 4). The flux of particles through the plasma depends ultimately on the particle number concentration of the suspension analyzed, but also on the sample flow rate and the nebulization efficiency of the introduction system. In practice, particles number concentrations below ca. 108 L1 are commonly used to reduce the occurrence of multiple-particle events. However, the concentrations to avoid the occurrence of two or more particle events must be established for each instrumental setup, through its sample flow rate and the nebulization efficiency, and the reading time selected. Although this concentration can be calculated to minimize the attainable uncertainty [15], as a first approximation, a visual inspection of the histograms helps to detect the occurrence of these events, which produce an additional NP distribution at intensities double than the original one. 4.2. Acquisition frequency and reading time We have seen in section 3 that SP-ICP-MS can be performed with conventional scanning quadrupole instruments (acquisition frequency <1,000 Hz, reading time >1 ms) or fast scanning quadrupole instruments (acquisition frequency <105 Hz, reading time >10 ms and no settling time). The use of reading times in the microsecond range allows to monitor more than one isotope per particle both with fast scanning quadrupole and TOF instruments. Under these acquisition conditions, the flux of particles and hence their number concentration can be higher, which involves wider
linear ranges, better precision due to counting statistics and smaller bias due to the occurrence of 2 particle events [19]. On the other hand, the presence of high concentrations of dissolved species or the occurrence of high background levels has a lower adverse effect on the capability to detect small particles [9,19], although the capability to quantify correctly the dissolved species is impaired. 4.3. Composition and size of particles Eqs. (2) and (3) involve a linear relationship between the signal and the mass of element per particle or the third power of the diameter, respectively. However, this linear relationship can be missed at high masses/diameters if particles are partially vaporized [18,20], as it has been reported for Au or SiO2 particles larger than 150–200 nm [21] and 1–2 mm [18,22], respectively. For a set of ICP operating conditions (RF power, central channel gas flow rate and sampling depth), the vaporization rate for each type of particle depends on its molecular mass and density, whereas the boiling point determines the point in the plasma where the particle starts to vaporize [21]. From a practical point of view, the nature of the particles must be considered when determining particle size or mass, especially in relation to the standards used for calibration. 5. Analytical information and performance Whereas ICP-MS just provides information about elemental composition and element mass concentration, SP-ICP-MS allows to get in addition: (i) Qualitative information about the presence of particulate and/ or dissolved forms of specific elements. (ii) Characterization information about the mass of element/s per particle, which can be converted into particle size as long as information about the composition, shape and density of the particles is known or assumed. (iii) Quantitative information as particle number concentrations, as well as mass concentrations of the dissolved and/or particulate forms.
5.1. Determination of element mass/particle size When NP standards with known mass per NP or size are available, Eqs. (2) and (3) can be used to construct the corresponding calibrations to obtain information about the mass per particle and the size of NP in the samples, respectively [23]. In the case NP standards are not available, the procedure developed by Pace et al. [24] must be followed. It is based on the use of dissolved standards of the element measured, whereas calculations involve to know the nebulization efficiency and the sample flow rate to determine the mass of analyte per NP and hence the NP diameter, assuming that the element from the dissolved standard solution and from a NP behave comparably in the ICP. With monodisperse droplet generators, known masses of element can be delivered into the plasma by generating droplets of fixed diameter from solutions of dissolved standards [25]. By using any of these procedures, NP intensity distributions can be transformed into size distributions, as shown in Fig. 2c. The detection of particles is associated to the capability of identifying the events from the NPs over the baseline produced by the background (Fig. 2a). The basic approach for identification of NP events consists in applying 3s or 5s threshold criteria, where s is the standard deviation of the baseline. In a similar way, size detection limits are usually calculated by using a 3s criterion. By using this criterion and the instrument sensitivity, Lee et al.
F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
19
Table 1 Reported limits of detection expressed as mass per particle and size (diameter) for selected elements and different instrumental setups. Nanoparticle
Isotope
Ag
107
Al2O3 Au
Ag
27
Al Au
197
Co Cr Er2O3 FeOOH SiO2
59
ThO2 TiO2
Th Ti Ti 48 Ti 238 U – 90 Zr
Co Cr Er 57 Fe 28 Si 53
164
232
47
48
U ZnO ZrO2
LOD Mass (fg)
Size (nm)
0.012–0.044 0.005 0.023–0.148 0.030 0.034–0.081 0.002–0.022 0.022–0.050 0.13 3.8 8.7 10 5.2 5.2 2.4 0.36 0.026–0.17 0.011 0.005 0.077 0.755
13–20 10 16–30 30 15–20 6–13 13–17 30 100 130 200 200 200 80 65 27–50 20 10 32 70
estimated attainable size detection limits for NPs composed of 40 different elements [26]. Table 1 summarizes reported mass and size detection limits for metal and oxide based NPs. Metal NP in the range of 10–20 nm can be detected by SP-ICPMS, whereas the sizes increase for oxides up to several hundred nanometers, depending on their stoichiometry and composition. With respect to mass, limits of detection down to attograms per particle can be achieved. Double focusing instruments, with better ion transmission and hence higher detection efficiency, as well as lower background signals, provide the best detection limits. With quadrupole instruments, the use of dwell times in the microsecond range and collision/reaction cells for isotopes subjected to polyatomic background interferences also could contribute to lower attainable detection limits. When analyzing NPs of sizes close to their detection limit, intensity distributions from NPs and background (fig. 2b) can be overlapped, which prevents from obtaining the full NPs size distribution, like in Fig. 2c. In the case of NPs below the size detection limit, intensity histograms show a single distribution which appears as a tailed background distribution [23], making impossible to obtain any reliable information. 5.2. Determination of particle number concentration Because NP number concentration standards are not available, spherical, solid and pure NP suspensions of known average diameter and mass concentration (e.g., NIST RM 8013 Au NPs 60 nm) are used for calibration according to Eq. (1). These calibrations are suitable for the quantification of any type of suspension because number concentrations are independent of the NP nature. Number concentration detection limits are directly related to the number of particles that can be delivered to the plasma, and hence to the nebulization efficiency and the sample flow rate, but also to the time spent counting particles, i.e. the acquisition time [7]. In practice, number concentration detection limits in the range of 106 L1 are reported with current ICP-MS instruments and acquisition times around one minute. 5.3. Determination of dissolved species Mass concentration of dissolved species can be determined by summing up the intensity of the baseline events recorded or,
Instrument
Ref.
Quadrupole Double focusing TOF Quadrupole Quadrupole Double focusing TOF Quadrupole Quadrupole Double focusing Quadrupole Quadrupole Double focusing Quadrupole Quadrupole Quadrupole Double focusing TOF Quadrupole Quadrupole
[22,23,32,34,48–51] [13,22] [11,52] [8] [12,22,39] [13,16,22,53] [11,52] [54] [54] [55] [8] [22] [22] [56] [32] [22,59] [22] [11] [37] [57]
alternatively, by considering the average intensity of the baseline and interpolation in the corresponding calibration with dissolved standards. Detection limits are equivalent to those obtained in conventional ICP-MS. 6. Environmental applications Because SP-ICP-MS is still an emerging methodology, more than one half of the works published up to now are related to fundamental issues, although the number of applied works is steadily increasing, moving from proofs-of-concept to complex studies and the monitoring of environmental compartments. Table 2 summarizes selected applications of SP-ICP-MS in environmental samples. Most of the applications involve the analysis of water samples, which are analyzed directly or after a suitable dilution. Moreover, many of these applications are related to fate studies of ENMs under controlled laboratory or micro/ mesocosms set-ups, using pristine ENPs of selected sizes and coatings. In this regard, SP-ICP-MS was used in combination with ultrafiltration, cloud point extraction and AF4-ICP-MS to understand the persistence and transformations of Ag NPs in a littoral lake mesocosms [27]. Size distributions and particle concentrations obtained showed that Ag NPs underwent dissolution although clear evidences of agglomeration were not observed. The agglomeration process of Ag NPs in artificial seawater was studied by combining the information provided by AF4-UV–vis and SP-ICP-MS [28]. The dissolution of Ag NPs in different types of waters has been studied by SP-ICP-MS, tracking the changes of NP diameters over time [29,30]. The behavior of Ag and Au NPs in simulated washing procedures has also been investigated by combining size distributions determined by SP-ICP-MS and visual inspection by TEM [31]. SP-ICP-MS has been used to study the effectiveness of drinking water treatments for removal of Ag, Au and TiO2 ENPs under simulated conditions [32]. Apart from analysis involving spiked ENPs, SP-ICP-MS has been applied to the detection and determination of native particles in waters. Ag, Ti and Ce-bearing particles were detected in urban runoff waters [33] and wastewater effluents [14]. In the same way, Mitrano et al. [34] detected and quantified Ag-bearing particles in influent and effluent samples from a waste water treatment plant at the ng L1 level in the presence of dissolved Ag. Farkas et al. [35] detected the presence of Ag NPs in the effluent of a nanosilver producing washing machine, although the majority of NPs resulted
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Table 2 Selected application of SP-ICP-MS to environmental samples analyzed directly or after a suitable treatment. Samples Waters Surface water
Nanoparticle Size (nm)
Instrument
Dwell time (ms)
Analytical information
Sample treatment
Ref.
–
Quadrupole
10
NP screening
–
[33]
–
Double focusing
0.1
NP screening NP number concentration
–
[14]
Waste water Waste water
Ag-bearing Ti-bearing Ce-bearing Ag-bearing Ti-bearing Ce-bearing Ag-bearing Ag-bearing
– –
Quadrupole Quadrupole
20 3
– –
[34] [22]
Lake water
Ti-bearing
–
Quadrupole
10
–
[36]
River water Well water Drinking water River water Waste water
Ti-bearing
–
Quadrupole
0.1
–
[32]
Zn-bearing
–
Quadrupole
0.5
Ion exchange resin (on-line coupling)
[37]
Effluent from washing machine
Ag
–
Double focusing
1/4
NP/dissolved screening NP/dissolved mass concentration Size distribution NP screening NP number concentration Dissolved concentration NP number concentration Size distribution Dissolved concentration NP number concentration Size distribution NP screening NP number concentration
–
[35]
Ag Ag
70 50
Quadrupole Quadrupole
3 10
– –
[28] [27]
River water Well water Drinking water Drinking water
Ag Au TiO2 Au
100
Quadrupole
0.1
–
[32]
Quadrupole
10
HDC (on-line coupling)
[42]
River water Waste water Drinking water River water Waste water Simulated washing procedure River water Waste water
ZnO
30 60 50
Quadrupole
0.5
60 100
Quadrupole
10
Ion exchange resin (on-line coupling) –
[37]
Ag
Size distribution NP number concentration NP mass concentration Size distribution Dissolved concentration NP number concentration Size distribution Mass per particle vs. NP size distribution NP number concentration Size distribution Size distribution
Au Ag Ag
60 100 80
Quadrupole
3/5
–
[31]
Quadrupole
5
–
[30]
Ag
30 70 60 100
Quadrupole
–
Size distribution
Water leaching
[58]
Quadrupole
10
NP number concentration NP mass concentration Size distribution NP number concentration NP mass concentration Size distribution NP number concentration Size distribution
TMAH digestion
[38]
TMAH digestion
[38]
Enzymatic digestion
[39]
Waste water
Spiked waters Artificial seawater Lake water
Exposed organisms Lumbriculus variegatus Lumbriculus variegatus
Ag Au
Daphnia magna
Ag Au
100
Quadrupole
10
Tomato plants
Au
40
Quadrupole
0.1
to be below 20 nm. Ti-bearing particles were also quantified in lake water during a 12-month period [36], associating the seasonal variability to the release of TiO2 NPs from sunscreens. The determination of NPs is hindered by the presence of dissolved forms of the element present in the particles. Detection, characterization and quantification of Zn-bearing nanoparticles in surface and waste waters, containing up to 100-fold concentration of dissolved zinc, has been achieved by on-line removal of the dissolved zinc with an ion-exchange resin prior to the SP-ICP-MS measurements [37]. Dealing with solid samples always involves some sort of sample treatment to render the particles into suspension. Alkaline and enzymatic digestions are commonly used to digest biological materials, preserving the core of inorganic NPs and allowing the direct detection, quantification and size characterization of the NPs themselves. Size distributions, nanoparticle number
NP number concentration Size distribution Mean diameter NP mass concentration
[29]
concentrations and mass concentrations were determined after TMAH digestion in Daphnia magna and Lumbriculus variegatus spiked with Ag and Au NPs [38]. The method was applied to D. magna exposed to 100 nm Ag and Au NPs, showing similar size distributions for the NPs accumulated into the organisms. An alternative to TMAH is the use of enzymatic digestions. Dan et al. [39] studied the uptake of Au NPs in tomato plants by digesting the shoots with pectinases, and showing that tomato plants can uptake and transport Au NPs as intact nanoparticles. 7. SP-ICP-MS based approaches for the analysis of environmental samples Pristine inorganic ENPs show a defined chemical composition of their core, with specific coatings and monodisperse size distributions in most cases, allowing their direct analysis by SP-ICP-MS.
F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
However, once these ENPs have been exposed to the environment they are subjected to different transformations [40], which difficult their identification and subsequent characterization and quantification, requiring more complex approaches. Fig. 3 summarizes the potential transformations that ENPs can undergo in aqueous environmental media. 7.1. Screening of particles and dissolved species The basic approach for screening the presence of particulate and/or dissolved forms of an element/s consists of analyzing the sample directly, or after a suitable dilution, by applying any standard SP-ICP-MS method. A time scan similar to that shown in Fig. 2a is obtained. By comparing this time scan with another from a blank solution, the presence of dissolved species of the element monitored can be confirmed by the increase in the continuous baseline, whereas the occurrence of pulses is an indication of particles being present. 7.2. Identification and sizing of particles Whereas SP-ICP-MS provides direct evidence of the presence of particulate forms of an element, determination of the size of the particles involves knowing their nature, namely their chemical composition, shape and density, to convert the mass of element per particle into the corresponding size. If this information is not available, an estimation of the particle size can be done based on the assumption of a specific shape and composition [37]. In any case, when primary particles have undergone a process of aggregation or agglomeration, SP-ICP-MS is just capable of providing information about the aggregated/agglomerated particles, in this latter case if the primary particles have not unbound when diluted. To obtain the additional information about the nature of the particles, supplementary techniques have to be used. TEM can provide information about the shape of the particles, their aggregation or agglomeration state, and about the elemental composition and crystal structure when combined with EDS and XRD, whereas additional information about oxidation states can be obtained in combination with EELS. XAS also allows to obtain element-specific information to determine the chemical
21
composition, oxidation state and structure of the chemical species containing the element/s under study. XAS has been successfully used to study the fate and transformation of different ENPs in a variety of environmental samples (Ref. [6] and references therein). Recently, it has been used in combination with SP-ICP-MS to identify and size natural arsenic-bearing particles, consisting of scorodite (FeAsO47H2O), which were involved in the transport of arsenic [41]. As in the case of TEM, XAS is limited to concentrations in the range of mg kg1 or higher. 7.3. Coupling of SP-ICP-MS to size separation techniques In spite of the chemical composition and the shape of the ENPs under study are known, SP-ICP-MS just provides information about the inorganic core of the nanoparticle and hence about the core dimensions. This means that if the ENPs consist of an inorganic core and an organic coating, or they have been associated to natural particulate matter through heteroaggregation/heteroagglomeration, their hydrodynamic diameters will differ from the sizes calculated from the measured mass per particle. Whereas field flow fractionation and hydrodynamic chromatography can be coupled to ICP-MS to separate and detect inorganic NPs according to their hydrodynamic diameter, the coupling of these separations techniques to SP-ICP-MS can provide a broader set of information, correlating core size information with hydrodynamic diameters and quantifying NPs not just in massbasis but also as number concentrations, as well as to differentiate homo- from heteroaggregates/agglomerates [4]. The potential of this approach has been demonstrated by the on-line coupling of HDC to SP-ICP-MS [42] for the analysis of Au NPs mixtures spiked in drinking water, which produces three dimensional chromatograms containing information about number concentration, metal content per NP and diameter. Up to now, AF4 has just been off-line coupled to SP-ICP-MS for the analysis of selected fractions collected from the separation of Ag NPs extracted from spiked chicken meat [43]. 7.4. Differentiation of engineered and natural particles Detection, characterization and quantification of engineered nanoparticles in environmental samples is complicated by the
Fig. 3. Transformations of pristine inorganic ENPs in contact with an environmental aqueous medium. NOM: natural organic matter; NPM: natural particulate matter.
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F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
ubiquitous presence of different types of natural particles, covering a broad range of sizes. Although SP-ICP-MS provides specific detection of metal(loid) containing particles, the differentiation of engineered and natural particles containing the same element remains a challenge. To distinguish ENPs from natural nanoparticles of similar composition, Von der Kammer et al. [44] proposed the use of elemental ratios. The underlying principle is that natural NPs contain significant amounts of other elements, showing different ratios which can even be related to the geographical location or to the parent materials, whereas synthetic NPs are pure elemental materials (e.g. Ag, Au, CeO2) or have fixed multielement composition (e.g. S/Cd in quantum dots, Ag/Au in core/shell bimetallic NPs). In this way, the Ti/Al ratio has been used for identification of engineered TiO2 nanoparticles analyzing bulk samples by conventional ICP-MS [36], and the use of the Ce/La ratio has also been proposed with a similar purpose [45]. More recently, the availability of fast scanning quadrupole instruments has allowed to implement this approach in combination with SP-ICP-MS, monitoring elemental ratios in individual particles, allowing the identification of engineered and natural NPs in a particle-by-particle basis [9,45]. This approach is suitable of being applied to TOF instruments, as well as to the measurement of isotopic ratios, when isotopically labeled ENPs are involved or isotopic shifts from natural isotopic ratios occur. 8. Current limitations and future prospects In spite of the features of SP-ICP-MS, users must be aware that solving analytical problems related to ENMs in environmental samples by using SP-ICP-MS involves its combination with other techniques in most cases. This is due to intrinsic limitations of this methodology, basically the fact of determining the content of element/s in particles regardless of their nature, but also to instrumental limitations. In this way, multielement SP-ICP-MS has become available with commercial fast scanning quadrupole and TOF instruments, although attainable mass per particle/size detection limits remains the most significant limitation for the widespread use of SP-ICP-MS. For quadrupole and TOF instruments, detection of NPs in the low nanometer range would involve to improve current detection efficiencies 2–3 orders of magnitude, at the level of the best reported efficiencies for double focusing instruments [46]. However, it is expected that the ongoing development of ICP-MS instrumentation will allow to overcome this gap in the near future. Before SP-ICP-MS could be considered a mature methodology, additional work is needed to standardize it, including the assessment of the reliability and variability of size and concentration determinations, as well as of the transferability of the methods among laboratories [47]. In any case, the number of users and environmental applications is steadily increasing, mainly because the nano-environmental community is aware that the only currently known methods that are both selective and sensitive enough to analyze inorganic ENMs in complex environmental samples are ICP-MS based [4]. Acknowledgements This work was supported by the Spanish Ministry of Economy and Competitiveness, project CTQ2012-38091-C02-01. JJL acknowledges the European Commission for receiving funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 660590.
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Contents lists available at ScienceDirect
Trends in Environmental Analytical Chemistry journal homepage: www.elsevier.com/locate/teac
Single particle inductively coupled plasma mass spectrometry for the analysis of inorganic engineered nanoparticles in environmental samples Francisco Labordaa,* , Eduardo Boleaa , Javier Jiménez-Lamanab a Group of Analytical Spectroscopy and Sensors (GEAS), Institute of Environmental Sciences (IUCA), University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain b Laboratory of Bioinorganic Analytical and Environmental Chemistry (LCABIE), UMR5254, CNRS-UPPA, Hélioparc, 2 Av. President Angot, 64053 Pau Cedex 09, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2015 Received in revised form 10 February 2016 Accepted 10 February 2016
The analysis of engineered nanoparticles in environmental samples involves their detection followed by their quantification and characterization. The development of robust and reliable methods for achieving these objectives is one of the main challenges of analytical chemistry, and single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) is considered one of such promising methods. The success of SP-ICP-MS lies on the fact that different types of analytical information can be obtained by using any commercial instrument. A priori, the following information related to one or more specific elements can be obtained: (i) qualitative information about the presence of particulate and/or dissolved forms, (ii) quantitative information as particle number as well as mass concentrations and (iii) characterization information about the mass of element/s per particle and particle size. The transformations that engineered nanoparticles can undergo under environmental conditions and the occurrence of natural particles of similar composition bring additional challenges. The aim of this review is to present the current situation of SP-ICP-MS for the analysis of inorganic engineered nanoparticles in environmental samples and the approaches needed to cope with complex environmental problems. ã 2016 Elsevier B.V. All rights reserved.
Keywords: Single particle Inductively coupled plasma mass spectrometry Engineered nanomaterials Engineered nanoparticles Environment
Contents 1. 2. 3. 4.
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumentation and software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flux and number concentration of particles . . . . . . . . . . . . . . . . . 4.1. Acquisition frequency and reading time . . . . . . . . . . . . . . . . . . . . 4.2. Composition and size of particles . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Analytical information and performance . . . . . . . . . . . . . . . . . . . . . . . . . Determination of element mass/particle size . . . . . . . . . . . . . . . . 5.1. Determination of particle number concentration . . . . . . . . . . . . . 5.2. Determination of dissolved species . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Environmental applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SP-ICP-MS based approaches for the analysis of environmental samples
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Abbreviations: AF4, asymmetrical flow field flow fractionation; EDS, energy-dispersive X-ray spectroscopy; EELS, electron energy loss spectroscopy; ENM, engineered nanomaterial; ENP, engineered nanoparticle; HDC, hydrodynamic chromatography; ICP-MS, inductively coupled plasma mass spectrometry; MDG, monodisperse droplet generator; NP, nanoparticle; SP-ICP-MS, single particle inductively coupled plasma mass spectrometry; TEM, transmission electron microscopy; TMAH, tetramethyl ammonium hydroxide; XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction. * Corresponding author. E-mail address: fl[email protected] (F. Laborda). http://dx.doi.org/10.1016/j.teac.2016.02.001 2214-1588/ ã 2016 Elsevier B.V. All rights reserved.
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7.1. Screening of particles and dissolved species . . . . . . . Identification and sizing of particles . . . . . . . . . . . . . 7.2. Coupling of SP-ICP-MS to size separation techniques 7.3. Differentiation of engineered and natural particles . 7.4. Current limitations and future prospects . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction The widespread use of engineered nanomaterials in a range of applications and products is leading to their release into the environment, raising serious concerns about their potential impact [1]. For this reason, realistic risk assessments are needed to ensure the sustainable development and use of ENMs [2]. Environmental risk assessments of ENMs involve the use of exposure models, whose predictions should be validated through the detection, characterization and quantification of these ENMs, and/or their derivatives, in the compartments of interest. However, all these analyses are hampered by the complexity of the environmental sample matrices, the background levels of naturally occurring particulate materials and the expected concentrations of ENMs, down to ng L1 and ng kg1 [3]. This means that it is not currently possible to fully validate modeled data on ENMs in the environment, and both modeling and analytics must be used together to advance the risk assessment of ENMs for the time being [4]. Although techniques used for studying environmental colloids and particles were used as starting point [5] for the analysis of ENPs, and despite the significant progress made in recent years [6], the development of innovative techniques, methods and approaches to overcome the difficulties listed above remains a challenge for analytical scientists. In this context, single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) is one of the innovative and emerging analytical methods that is contributing to overcome this challenge [7]. SP-ICP-MS combines the benefits of an element specific atomic spectrometry technique, like ICP-MS, with those of a particle counting technique, because measurements are performed in a particle-by-particle basis. The strength of SP-ICP-MS lies in the different types of analytical information that can be provided at relevant environmental concentrations from natural and engineered NPs, excepting carbon based nanomaterials due to the intrinsic low sensitivity for this element in ICP-MS. The objective of this review is to present the state-of-the-art of SP-ICP-MS, its current limitations and future potential, as well as the approaches based on this methodology to cope and solve the challenges and problems related to the analysis of ENMs in environmental samples.
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21 21 21 21 22 22 22
present just in the discrete particles. Then, if a sufficiently dilute suspension of particles is nebulized into the plasma, the pack of ions generated from each single particle can be detected. Fig. 1 shows the time-resolved signals, with a duration of about 300– 500 ms, recorded at different acquisition frequencies. By using fast data acquisition (104 Hz, i.e. using reading times 100 ms), the transient signal produced by each particle can be recorded (Fig. 1a). Working at lower frequencies (i.e. at reading times in the milliseconds range), the packs of ions are measured as pulses, as it is shown in Fig. 1b and c. ICP-MS signals are recorded during several minutes, obtaining time scans as shown in Fig. 2a, which consist of a number of events, recorded as transient signals or pulses, depending on the selected acquisition frequency, above a steady baseline. Whereas the intensity of each event is due to the ions detected from each particle, the baseline is due to the background at the mass recorded or to the presence of dissolved forms of the element measured. Raw time scans can be processed by plotting the event intensity vs. the event intensity frequency, obtaining histograms as shown in Fig. 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 particles themselves. Theoretical basis of single particle detection applied to ICP-MS were outlined by Degueldre et al. and Favarger [8] and summarized by Laborda et al. [7]. The basic assumption behind SP-ICP-MS is that each recorded event represents a single particle. If this assumption is true then the number of events counted (np) during an acquisition time (ti) is directly related to the number
2. Fundamentals Samples are most commonly introduced into ICP-MS instruments as liquids by using a nebulization system which produces an aerosol of polydisperse droplets. Once the droplets are inside the plasma, solvent evaporates and the resulting particles are volatilized, atomized and subsequently ionized. Ions are extracted through the interface into the mass spectrometer, where they are separated according to their mass/charge ratio and detected. Soluble forms of an element are distributed homogenously within solution, and hence in the aerosol droplets. Thus the flux of element entering the plasma and travelling to the detector as ions can be considered constant, producing a steady signal during the reading period. By contrast, if a suspension of particles is nebulized, the element is not distributed homogenously, being
Fig. 1. Simulated time resolved ICP-MS signals from a particle suspension at different data acquisition frequencies (reading times). (a) 10,000 Hz (100 ms), (b) 1000 Hz (1 ms), (c) 100 Hz (10 ms). Not to scale.
F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
17
element per particle (mP):
(a)
IP ¼ K ICPMS K M mP
ð2Þ
event intensity
where KICPMS is the detection efficiency, which represents the ratio of the number of ions detected versus the number of atoms introduced into the ICP. KM (=ANAv/MM) is a factor related to the element measured, where A is the atomic abundance of the isotope considered, NAv the Avogadro number, and MM the atomic mass of the element. Eq. (2) can be related to the size of the particle if the composition, shape and density of the particle are known. For instance, for a solid, homogeneous and spherical particle, Eq. (2) can be written as: 1 3 IP ¼ prX P K ICPMS K M d 6
ð3Þ
where d is the diameter, r the density and XP the mass fraction of the element in the particle. Eqs. (1) and (2) summarize the fundamentals behind single particle ICP-MS. Quantitative determinations of particle number concentrations are based on the linear relationship between the number of events and the number concentration (Eq. (1)); whereas, the intensity of the particle events is proportional to the mass of analyte per particle (Eq. (2)), or to the third power of the diameter for solid, spherical, and pure particles (Eq. (3)), allowing the determination of element mass per particle and size distributions, respectively.
time
(b) number of events
background/ dissolved element
3. Instrumentation and software
NPs event intensity
number of events
(c)
diameter, nm Fig. 2. (a) Time scan of a nanoparticle suspension containing dissolved forms of the element contained in the nanoparticle. (b) Event intensity frequency histogram of data from (a). (c) Size distribution of spherical nanoparticles calculated from the second intensity distribution in (b).
concentration of particles (CP): nP ¼ hneb Q sam ti C P
ð1Þ
where hneb is the nebulization efficiency and Qsam the sample introduction flow rate. On the other hand, the intensity of each event (IP) is proportional to the number of atoms of the element monitored in each detected particle, and hence to the mass of
Originally, SP-ICP-MS was developed with quadrupole instruments. Working with these instruments, the frequency of data acquisition was dependent on applicable reading times (selected through the quadrupole dwell time per isotope) and the transmission and storage of data. In practice, dwell times were limited to the millisecond range, being 3–10 ms commonly used [7], which implies recording the particle events as pulses. Most recently, fast scanning quadrupole instruments have become commercially available, being able of working with dwell times in the microsecond range (100–10 ms) and hence recording particle events as more or less resolved transient signals [9]. As an additional feature, fast scanning quadrupole instruments can monitor up to two isotopes/elements in the same particle by combining the use of microseconds dwell times and removing the settling time [9]. Double focusing instruments can also be run in fast scanning mode by scanning the acceleration voltage instead of the magnetic field, allowing the use of 100 ms reading times [10]. An ICP time-of-flight mass spectrometer with a temporal resolution of 30 ms and full multielement capability was developed recently [11], and it is commercially available at present. Liquid samples are usually introduced in ICP-MS instruments by using different types of pneumatic nebulization systems, although most recently monodisperse droplet generators have been used for introduction of nanoparticle suspensions [12,13]. MDGs provide nebulization efficiencies close to 100%, at sample flow rates of hundreds of nL min1, whereas conventional nebulization systems achieve nebulization efficiencies of 2–5% at samples flow rates in the range of ca. 0.1–1 mL min1. Once data have been acquired, they have to be processed to get the different types of information that SP-ICP-MS can provide. Different protocols have been developed for processing raw data [14–17]. These protocols can be implemented by the users, exporting raw data to in-house programs and spreadsheets. A dedicated spreadsheet is freely available from RIKILT (http://www. wageningenur.nl/en/Expertise-Services/Research-Institutes/rikilt/ show/Single-Particle-Calculation-tool.htm) for calculation of particle sizes and concentrations from data acquired at milliseconds
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F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
dwell times. On the other hand, manufacturers are incorporating to the software of their instruments, specific modules for acquisition and processing of single particle data, making easier the analysis of nanoparticles to unskilled users. In any case, users must be aware about a number of practical considerations to obtain reliable results, that will be discussed in the next section. The use of available reference materials (e.g. RM 8013 AuNPs or RM 8017 AgNPs from NIST) is highly recommended for the calculation of the nebulization efficiency and number concentration calibrations, as well as for internal validation of the measurement procedure. 4. Practical considerations The rapid development of SP-ICP-MS makes this technique available to a broad range of users, who should consider several key issues to get reliable information from a particle suspension analyzed by SP-ICP-MS. 4.1. Flux and number concentration of particles When a sufficiently diluted suspension is introduced into an ICP-MS, particles arrive into the plasma randomly and their flux must be low enough to record separate particle events, as it is shown in Fig. 1 for three different acquisition frequencies. The probability of more than one particle being measured within a reading time can be estimated by Poisson statistics [15,18], and it decreases with decreasing the reading time and/or the flux of particles. When using acquisition frequencies of 10,000 Hz or higher (reading times 100 ms) the occurrence of multiple particle events is related to the overlapping of the transient signals generated by individual particles, as it can be seen in Fig. 1a (event 4), which can be reduced by decreasing the flux of particles. If acquisition frequencies of 1,000 Hz or lower are used (reading times 1 ms) the flux must be even lower to avoid the measurement of two or more particles within a reading time, as it is depicted in the events 3 and 4 of Fig. 1c. When reading times slightly longer than the duration of the transient signal are used (e.g., 1 ms), the risk of measuring a fraction of the whole signal is increased, as it is shown in Fig. 1b (events 2 and 4). The flux of particles through the plasma depends ultimately on the particle number concentration of the suspension analyzed, but also on the sample flow rate and the nebulization efficiency of the introduction system. In practice, particles number concentrations below ca. 108 L1 are commonly used to reduce the occurrence of multiple-particle events. However, the concentrations to avoid the occurrence of two or more particle events must be established for each instrumental setup, through its sample flow rate and the nebulization efficiency, and the reading time selected. Although this concentration can be calculated to minimize the attainable uncertainty [15], as a first approximation, a visual inspection of the histograms helps to detect the occurrence of these events, which produce an additional NP distribution at intensities double than the original one. 4.2. Acquisition frequency and reading time We have seen in section 3 that SP-ICP-MS can be performed with conventional scanning quadrupole instruments (acquisition frequency <1,000 Hz, reading time >1 ms) or fast scanning quadrupole instruments (acquisition frequency <105 Hz, reading time >10 ms and no settling time). The use of reading times in the microsecond range allows to monitor more than one isotope per particle both with fast scanning quadrupole and TOF instruments. Under these acquisition conditions, the flux of particles and hence their number concentration can be higher, which involves wider
linear ranges, better precision due to counting statistics and smaller bias due to the occurrence of 2 particle events [19]. On the other hand, the presence of high concentrations of dissolved species or the occurrence of high background levels has a lower adverse effect on the capability to detect small particles [9,19], although the capability to quantify correctly the dissolved species is impaired. 4.3. Composition and size of particles Eqs. (2) and (3) involve a linear relationship between the signal and the mass of element per particle or the third power of the diameter, respectively. However, this linear relationship can be missed at high masses/diameters if particles are partially vaporized [18,20], as it has been reported for Au or SiO2 particles larger than 150–200 nm [21] and 1–2 mm [18,22], respectively. For a set of ICP operating conditions (RF power, central channel gas flow rate and sampling depth), the vaporization rate for each type of particle depends on its molecular mass and density, whereas the boiling point determines the point in the plasma where the particle starts to vaporize [21]. From a practical point of view, the nature of the particles must be considered when determining particle size or mass, especially in relation to the standards used for calibration. 5. Analytical information and performance Whereas ICP-MS just provides information about elemental composition and element mass concentration, SP-ICP-MS allows to get in addition: (i) Qualitative information about the presence of particulate and/ or dissolved forms of specific elements. (ii) Characterization information about the mass of element/s per particle, which can be converted into particle size as long as information about the composition, shape and density of the particles is known or assumed. (iii) Quantitative information as particle number concentrations, as well as mass concentrations of the dissolved and/or particulate forms.
5.1. Determination of element mass/particle size When NP standards with known mass per NP or size are available, Eqs. (2) and (3) can be used to construct the corresponding calibrations to obtain information about the mass per particle and the size of NP in the samples, respectively [23]. In the case NP standards are not available, the procedure developed by Pace et al. [24] must be followed. It is based on the use of dissolved standards of the element measured, whereas calculations involve to know the nebulization efficiency and the sample flow rate to determine the mass of analyte per NP and hence the NP diameter, assuming that the element from the dissolved standard solution and from a NP behave comparably in the ICP. With monodisperse droplet generators, known masses of element can be delivered into the plasma by generating droplets of fixed diameter from solutions of dissolved standards [25]. By using any of these procedures, NP intensity distributions can be transformed into size distributions, as shown in Fig. 2c. The detection of particles is associated to the capability of identifying the events from the NPs over the baseline produced by the background (Fig. 2a). The basic approach for identification of NP events consists in applying 3s or 5s threshold criteria, where s is the standard deviation of the baseline. In a similar way, size detection limits are usually calculated by using a 3s criterion. By using this criterion and the instrument sensitivity, Lee et al.
F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
19
Table 1 Reported limits of detection expressed as mass per particle and size (diameter) for selected elements and different instrumental setups. Nanoparticle
Isotope
Ag
107
Al2O3 Au
Ag
27
Al Au
197
Co Cr Er2O3 FeOOH SiO2
59
ThO2 TiO2
Th Ti Ti 48 Ti 238 U – 90 Zr
Co Cr Er 57 Fe 28 Si 53
164
232
47
48
U ZnO ZrO2
LOD Mass (fg)
Size (nm)
0.012–0.044 0.005 0.023–0.148 0.030 0.034–0.081 0.002–0.022 0.022–0.050 0.13 3.8 8.7 10 5.2 5.2 2.4 0.36 0.026–0.17 0.011 0.005 0.077 0.755
13–20 10 16–30 30 15–20 6–13 13–17 30 100 130 200 200 200 80 65 27–50 20 10 32 70
estimated attainable size detection limits for NPs composed of 40 different elements [26]. Table 1 summarizes reported mass and size detection limits for metal and oxide based NPs. Metal NP in the range of 10–20 nm can be detected by SP-ICPMS, whereas the sizes increase for oxides up to several hundred nanometers, depending on their stoichiometry and composition. With respect to mass, limits of detection down to attograms per particle can be achieved. Double focusing instruments, with better ion transmission and hence higher detection efficiency, as well as lower background signals, provide the best detection limits. With quadrupole instruments, the use of dwell times in the microsecond range and collision/reaction cells for isotopes subjected to polyatomic background interferences also could contribute to lower attainable detection limits. When analyzing NPs of sizes close to their detection limit, intensity distributions from NPs and background (fig. 2b) can be overlapped, which prevents from obtaining the full NPs size distribution, like in Fig. 2c. In the case of NPs below the size detection limit, intensity histograms show a single distribution which appears as a tailed background distribution [23], making impossible to obtain any reliable information. 5.2. Determination of particle number concentration Because NP number concentration standards are not available, spherical, solid and pure NP suspensions of known average diameter and mass concentration (e.g., NIST RM 8013 Au NPs 60 nm) are used for calibration according to Eq. (1). These calibrations are suitable for the quantification of any type of suspension because number concentrations are independent of the NP nature. Number concentration detection limits are directly related to the number of particles that can be delivered to the plasma, and hence to the nebulization efficiency and the sample flow rate, but also to the time spent counting particles, i.e. the acquisition time [7]. In practice, number concentration detection limits in the range of 106 L1 are reported with current ICP-MS instruments and acquisition times around one minute. 5.3. Determination of dissolved species Mass concentration of dissolved species can be determined by summing up the intensity of the baseline events recorded or,
Instrument
Ref.
Quadrupole Double focusing TOF Quadrupole Quadrupole Double focusing TOF Quadrupole Quadrupole Double focusing Quadrupole Quadrupole Double focusing Quadrupole Quadrupole Quadrupole Double focusing TOF Quadrupole Quadrupole
[22,23,32,34,48–51] [13,22] [11,52] [8] [12,22,39] [13,16,22,53] [11,52] [54] [54] [55] [8] [22] [22] [56] [32] [22,59] [22] [11] [37] [57]
alternatively, by considering the average intensity of the baseline and interpolation in the corresponding calibration with dissolved standards. Detection limits are equivalent to those obtained in conventional ICP-MS. 6. Environmental applications Because SP-ICP-MS is still an emerging methodology, more than one half of the works published up to now are related to fundamental issues, although the number of applied works is steadily increasing, moving from proofs-of-concept to complex studies and the monitoring of environmental compartments. Table 2 summarizes selected applications of SP-ICP-MS in environmental samples. Most of the applications involve the analysis of water samples, which are analyzed directly or after a suitable dilution. Moreover, many of these applications are related to fate studies of ENMs under controlled laboratory or micro/ mesocosms set-ups, using pristine ENPs of selected sizes and coatings. In this regard, SP-ICP-MS was used in combination with ultrafiltration, cloud point extraction and AF4-ICP-MS to understand the persistence and transformations of Ag NPs in a littoral lake mesocosms [27]. Size distributions and particle concentrations obtained showed that Ag NPs underwent dissolution although clear evidences of agglomeration were not observed. The agglomeration process of Ag NPs in artificial seawater was studied by combining the information provided by AF4-UV–vis and SP-ICP-MS [28]. The dissolution of Ag NPs in different types of waters has been studied by SP-ICP-MS, tracking the changes of NP diameters over time [29,30]. The behavior of Ag and Au NPs in simulated washing procedures has also been investigated by combining size distributions determined by SP-ICP-MS and visual inspection by TEM [31]. SP-ICP-MS has been used to study the effectiveness of drinking water treatments for removal of Ag, Au and TiO2 ENPs under simulated conditions [32]. Apart from analysis involving spiked ENPs, SP-ICP-MS has been applied to the detection and determination of native particles in waters. Ag, Ti and Ce-bearing particles were detected in urban runoff waters [33] and wastewater effluents [14]. In the same way, Mitrano et al. [34] detected and quantified Ag-bearing particles in influent and effluent samples from a waste water treatment plant at the ng L1 level in the presence of dissolved Ag. Farkas et al. [35] detected the presence of Ag NPs in the effluent of a nanosilver producing washing machine, although the majority of NPs resulted
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Table 2 Selected application of SP-ICP-MS to environmental samples analyzed directly or after a suitable treatment. Samples Waters Surface water
Nanoparticle Size (nm)
Instrument
Dwell time (ms)
Analytical information
Sample treatment
Ref.
–
Quadrupole
10
NP screening
–
[33]
–
Double focusing
0.1
NP screening NP number concentration
–
[14]
Waste water Waste water
Ag-bearing Ti-bearing Ce-bearing Ag-bearing Ti-bearing Ce-bearing Ag-bearing Ag-bearing
– –
Quadrupole Quadrupole
20 3
– –
[34] [22]
Lake water
Ti-bearing
–
Quadrupole
10
–
[36]
River water Well water Drinking water River water Waste water
Ti-bearing
–
Quadrupole
0.1
–
[32]
Zn-bearing
–
Quadrupole
0.5
Ion exchange resin (on-line coupling)
[37]
Effluent from washing machine
Ag
–
Double focusing
1/4
NP/dissolved screening NP/dissolved mass concentration Size distribution NP screening NP number concentration Dissolved concentration NP number concentration Size distribution Dissolved concentration NP number concentration Size distribution NP screening NP number concentration
–
[35]
Ag Ag
70 50
Quadrupole Quadrupole
3 10
– –
[28] [27]
River water Well water Drinking water Drinking water
Ag Au TiO2 Au
100
Quadrupole
0.1
–
[32]
Quadrupole
10
HDC (on-line coupling)
[42]
River water Waste water Drinking water River water Waste water Simulated washing procedure River water Waste water
ZnO
30 60 50
Quadrupole
0.5
60 100
Quadrupole
10
Ion exchange resin (on-line coupling) –
[37]
Ag
Size distribution NP number concentration NP mass concentration Size distribution Dissolved concentration NP number concentration Size distribution Mass per particle vs. NP size distribution NP number concentration Size distribution Size distribution
Au Ag Ag
60 100 80
Quadrupole
3/5
–
[31]
Quadrupole
5
–
[30]
Ag
30 70 60 100
Quadrupole
–
Size distribution
Water leaching
[58]
Quadrupole
10
NP number concentration NP mass concentration Size distribution NP number concentration NP mass concentration Size distribution NP number concentration Size distribution
TMAH digestion
[38]
TMAH digestion
[38]
Enzymatic digestion
[39]
Waste water
Spiked waters Artificial seawater Lake water
Exposed organisms Lumbriculus variegatus Lumbriculus variegatus
Ag Au
Daphnia magna
Ag Au
100
Quadrupole
10
Tomato plants
Au
40
Quadrupole
0.1
to be below 20 nm. Ti-bearing particles were also quantified in lake water during a 12-month period [36], associating the seasonal variability to the release of TiO2 NPs from sunscreens. The determination of NPs is hindered by the presence of dissolved forms of the element present in the particles. Detection, characterization and quantification of Zn-bearing nanoparticles in surface and waste waters, containing up to 100-fold concentration of dissolved zinc, has been achieved by on-line removal of the dissolved zinc with an ion-exchange resin prior to the SP-ICP-MS measurements [37]. Dealing with solid samples always involves some sort of sample treatment to render the particles into suspension. Alkaline and enzymatic digestions are commonly used to digest biological materials, preserving the core of inorganic NPs and allowing the direct detection, quantification and size characterization of the NPs themselves. Size distributions, nanoparticle number
NP number concentration Size distribution Mean diameter NP mass concentration
[29]
concentrations and mass concentrations were determined after TMAH digestion in Daphnia magna and Lumbriculus variegatus spiked with Ag and Au NPs [38]. The method was applied to D. magna exposed to 100 nm Ag and Au NPs, showing similar size distributions for the NPs accumulated into the organisms. An alternative to TMAH is the use of enzymatic digestions. Dan et al. [39] studied the uptake of Au NPs in tomato plants by digesting the shoots with pectinases, and showing that tomato plants can uptake and transport Au NPs as intact nanoparticles. 7. SP-ICP-MS based approaches for the analysis of environmental samples Pristine inorganic ENPs show a defined chemical composition of their core, with specific coatings and monodisperse size distributions in most cases, allowing their direct analysis by SP-ICP-MS.
F. Laborda et al. / Trends in Environmental Analytical Chemistry 9 (2016) 15–23
However, once these ENPs have been exposed to the environment they are subjected to different transformations [40], which difficult their identification and subsequent characterization and quantification, requiring more complex approaches. Fig. 3 summarizes the potential transformations that ENPs can undergo in aqueous environmental media. 7.1. Screening of particles and dissolved species The basic approach for screening the presence of particulate and/or dissolved forms of an element/s consists of analyzing the sample directly, or after a suitable dilution, by applying any standard SP-ICP-MS method. A time scan similar to that shown in Fig. 2a is obtained. By comparing this time scan with another from a blank solution, the presence of dissolved species of the element monitored can be confirmed by the increase in the continuous baseline, whereas the occurrence of pulses is an indication of particles being present. 7.2. Identification and sizing of particles Whereas SP-ICP-MS provides direct evidence of the presence of particulate forms of an element, determination of the size of the particles involves knowing their nature, namely their chemical composition, shape and density, to convert the mass of element per particle into the corresponding size. If this information is not available, an estimation of the particle size can be done based on the assumption of a specific shape and composition [37]. In any case, when primary particles have undergone a process of aggregation or agglomeration, SP-ICP-MS is just capable of providing information about the aggregated/agglomerated particles, in this latter case if the primary particles have not unbound when diluted. To obtain the additional information about the nature of the particles, supplementary techniques have to be used. TEM can provide information about the shape of the particles, their aggregation or agglomeration state, and about the elemental composition and crystal structure when combined with EDS and XRD, whereas additional information about oxidation states can be obtained in combination with EELS. XAS also allows to obtain element-specific information to determine the chemical
21
composition, oxidation state and structure of the chemical species containing the element/s under study. XAS has been successfully used to study the fate and transformation of different ENPs in a variety of environmental samples (Ref. [6] and references therein). Recently, it has been used in combination with SP-ICP-MS to identify and size natural arsenic-bearing particles, consisting of scorodite (FeAsO47H2O), which were involved in the transport of arsenic [41]. As in the case of TEM, XAS is limited to concentrations in the range of mg kg1 or higher. 7.3. Coupling of SP-ICP-MS to size separation techniques In spite of the chemical composition and the shape of the ENPs under study are known, SP-ICP-MS just provides information about the inorganic core of the nanoparticle and hence about the core dimensions. This means that if the ENPs consist of an inorganic core and an organic coating, or they have been associated to natural particulate matter through heteroaggregation/heteroagglomeration, their hydrodynamic diameters will differ from the sizes calculated from the measured mass per particle. Whereas field flow fractionation and hydrodynamic chromatography can be coupled to ICP-MS to separate and detect inorganic NPs according to their hydrodynamic diameter, the coupling of these separations techniques to SP-ICP-MS can provide a broader set of information, correlating core size information with hydrodynamic diameters and quantifying NPs not just in massbasis but also as number concentrations, as well as to differentiate homo- from heteroaggregates/agglomerates [4]. The potential of this approach has been demonstrated by the on-line coupling of HDC to SP-ICP-MS [42] for the analysis of Au NPs mixtures spiked in drinking water, which produces three dimensional chromatograms containing information about number concentration, metal content per NP and diameter. Up to now, AF4 has just been off-line coupled to SP-ICP-MS for the analysis of selected fractions collected from the separation of Ag NPs extracted from spiked chicken meat [43]. 7.4. Differentiation of engineered and natural particles Detection, characterization and quantification of engineered nanoparticles in environmental samples is complicated by the
Fig. 3. Transformations of pristine inorganic ENPs in contact with an environmental aqueous medium. NOM: natural organic matter; NPM: natural particulate matter.
22
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ubiquitous presence of different types of natural particles, covering a broad range of sizes. Although SP-ICP-MS provides specific detection of metal(loid) containing particles, the differentiation of engineered and natural particles containing the same element remains a challenge. To distinguish ENPs from natural nanoparticles of similar composition, Von der Kammer et al. [44] proposed the use of elemental ratios. The underlying principle is that natural NPs contain significant amounts of other elements, showing different ratios which can even be related to the geographical location or to the parent materials, whereas synthetic NPs are pure elemental materials (e.g. Ag, Au, CeO2) or have fixed multielement composition (e.g. S/Cd in quantum dots, Ag/Au in core/shell bimetallic NPs). In this way, the Ti/Al ratio has been used for identification of engineered TiO2 nanoparticles analyzing bulk samples by conventional ICP-MS [36], and the use of the Ce/La ratio has also been proposed with a similar purpose [45]. More recently, the availability of fast scanning quadrupole instruments has allowed to implement this approach in combination with SP-ICP-MS, monitoring elemental ratios in individual particles, allowing the identification of engineered and natural NPs in a particle-by-particle basis [9,45]. This approach is suitable of being applied to TOF instruments, as well as to the measurement of isotopic ratios, when isotopically labeled ENPs are involved or isotopic shifts from natural isotopic ratios occur. 8. Current limitations and future prospects In spite of the features of SP-ICP-MS, users must be aware that solving analytical problems related to ENMs in environmental samples by using SP-ICP-MS involves its combination with other techniques in most cases. This is due to intrinsic limitations of this methodology, basically the fact of determining the content of element/s in particles regardless of their nature, but also to instrumental limitations. In this way, multielement SP-ICP-MS has become available with commercial fast scanning quadrupole and TOF instruments, although attainable mass per particle/size detection limits remains the most significant limitation for the widespread use of SP-ICP-MS. For quadrupole and TOF instruments, detection of NPs in the low nanometer range would involve to improve current detection efficiencies 2–3 orders of magnitude, at the level of the best reported efficiencies for double focusing instruments [46]. However, it is expected that the ongoing development of ICP-MS instrumentation will allow to overcome this gap in the near future. Before SP-ICP-MS could be considered a mature methodology, additional work is needed to standardize it, including the assessment of the reliability and variability of size and concentration determinations, as well as of the transferability of the methods among laboratories [47]. In any case, the number of users and environmental applications is steadily increasing, mainly because the nano-environmental community is aware that the only currently known methods that are both selective and sensitive enough to analyze inorganic ENMs in complex environmental samples are ICP-MS based [4]. Acknowledgements This work was supported by the Spanish Ministry of Economy and Competitiveness, project CTQ2012-38091-C02-01. JJL acknowledges the European Commission for receiving funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 660590.
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