Direct Online Determination of Laser-Induced Particle Size Distribution

Subscriber access provided by UNIVERSITY OF CONNECTICUT

Article

Direct on-line determination of laser induced particle size distribution by ICPMS Ariane Donard, Fanny Claverie, Fabien Pointurier, Céline Blitz Frayret, Barbora Svatosova, and Christophe Pecheyran Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01041 • Publication Date (Web): 08 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

Direct on-line determination of laser induced particle size distribution by ICPMS Ariane Donard1,2, Fanny Claverie1*, Fabien Pointurier2, Céline Blitz Frayret1, Barbora Svatosova3, Christophe Pécheyran1 1- CNRS/ University of Pau & Pays de l’Adour, Institut des Sciences Analytiques et de Physico-chimie pour l’Environnement et les Matériaux, UMR5254, 2 Avenue du Président Angot, 64000 Pau, France 2- CEA, DAM, DIF, F-91297 Arpajon, France 3- Masaryk University Brno, Kotlarska 2, 61137 Brno, Czech Republic ABSTRACT: The characterization of the aerosol (size, composition and concentration) generated by Laser Ablation is of great interest due to its impact on the analytical performances when coupled to Inductively Coupled Plasma Mass Spectrometry (ICPMS). The capabilities of High Resolution ICPMS as a direct tool to characterize nanoparticles produced by femtosecond Laser Ablation of pure copper are presented. An analytical protocol, similar to the ‘single particle ICPMS’ technique used to characterize the size distribution of nanoparticles in solution, was developed in order to observe the signals of individual particles produced by a single ablation shot. A Visual Basic for Applications (VBA) data processing was developed to count and sort the particles as a function of their size and thus determine the particle size distribution. To check the reliability of the method, the results were compared to a more conventional technique, namely Electrical Low Pressure Impaction (ELPI) for 4 000 shots. Detection limit for the particles produced by the laser ablation of a copper foil is of a few attograms corresponding to a nanoparticle of 14 nm The direct on-line determination of particle size by ICPMS gave similar results than ELPI for copper particles ejected during the ablation shot by shot at a fixed spot, from 1 to 100 shots. Particles larger than 159 nm represented less than 1% of the aerosol whose distribution was centred on 25-51 nm.

Because of the increasing use of nanoparticles in industry, the need for sensitive techniques to characterise their elemental composition, size, number or concentration has emerged. Over the last decade, interest has grown in using inductively coupled plasma mass spectrometry (ICP-MS) as an analytical tool for its ability to size and characterise nanoparticles. The high sensitivity, elemental and isotopic measurement capabilities of the ICP-MS are the main advantages of this technique compared to other particle characterization techniques such as light scattering techniques, spectroscopic analysis, electron and dynamic force microscopy and field flow fractionation 1, … Capability of ICP-MS to observe the signal produced by a single nanometre-sized particle, here referred to as “Single Particle ICP-MS” (SP-ICP-MS) analysis, is based on the observation that the analyte contained in a particle which arrives into the plasma remains spatially concentrated within the plasma even after the particle’s destruction2,3. Therefore, the introduction of a single particle into the plasma produces a gaseous ion cloud which is transferred from the ICP to the mass spectrometer4. The time scale for detection of the ion cloud in the detector is a few hundreds of microsecond5–7. The resulting signal peak measured, is related to the number of atoms that composed the analyte3. SP-ICP-MS has already been applied to different types of samples: airborne particles8,9, aqueous suspension3,7,10 and monodisperse droplets11,12. Characterization of laser-produced aerosol is of great interest for fundamental studies on the laser ablation ICP-MS (LA-ICPMS) technique. Particle size distribution and elemental

characterization of laser-produced aerosols are critical parameters for studying fractionation effects (nonstoichiometric sampling), regarded as the main limitation for a more widespread use of LA-ICP-MS. The use of nanosecond laser pulses produces an aerosol composed of particles with heterogeneous size distribution and elemental composition 13. During the transport to the ICP-MS, larger particles can be lost on impact with the tubing or on reaching the ICP. Moreover, they may not be completely atomised, resulting in nonstoichiometric sampling. Over the last decade, the reduction in the laser pulse duration by using the femtosecond laser has been the most promising technique for reducing fractionation effects14. Femtosecond laser-produced aerosol studies are essential in order to understand the mechanisms that control its formation15,16. Several instruments are available to characterise the particle size distribution of an aerosol. The use of an Electrical Low-Pressure Impactor (ELPI) as an external technique, allows direct measurement of the particle size distribution. Electrically charged particles impact on the different parts of the instrument as a function of their aerodynamic diameters and produce an electrical current proportional to the number of particles. When the impaction stages are fitted with collection disks (filters, aluminium foils etc…), the impacted particles can be collected for additional characterisation (Scanning Electron microscopy, trace analysis, etc…)17–19. An Optical Particle Counter (OPC) is an online technique installed at the exit of the laser ablation system which will characterise, by using light scattering measurements, the size distribution of the particles transported to the ICP16. When

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dealing with a laser-produced aerosol, the use of classical optical sizing techniques is limited because of their inability to detect particles smaller than 50-100 nm. A Condensation Particle Counter (CPC) increases the particle detection range allowing the detection of particles down to 2 nm20. Using a Scanning Mobility Particle Sizer (SMPS), electrically charged particles are selected according to their mobility in an electric field and are size-characterised by CPC21. Microscopy techniques such as Scanning Electron Microscopy (SEM)17,19,22, Transmission Electron Microscopy (TEM)18 or Atomic Force Microscopy (AFM)23 are also used to characterise the particle size composition. Moreover, the elemental composition of the particles can be determined by means of Energy Dispersive Xray spectroscopy (EDX), associated to a SEM or a TEM24,25. All of these techniques require an external device to study the laser-produced aerosol. In this publication, we present an original method of characterizing the particle size distribution of the aerosol produced by a single laser shot on a pure copper foil. This is achieved with an IR femtosecond laser directly coupled to a high resolution Sector-Field Mass Spectrometer (ICP-SFMS). A single isotope was recorded with the instrument’s lowest possible measurement time (100 μs), and intensities of the peak signals produced by the introduction of the particles in the plasma were measured to estimate the particle size. In the conventional SP-ICP-MS method, typically designed for aqueous suspensions, the concentration of particles can be decreased until the probability of simultaneously detecting two or more particles is negligible. In such a case, particles are well-separated and the determination of the particle sizes through measurement of the signal intensity is straightforward. This procedure is not achievable for a solid aerosol like the one produced by laser ablation for which particles are highly concentrated. In our study, we wanted to directly observe the arrival of the ions on the detector resulting from the laser-produced particles, with the appropriate time scale (hundreds of microseconds) and thus to estimate the size of these ablation fragments. The high density of the particles reaching the plasma could produce a mixed signal if several particles were detected almost at the same time. An ExcelTM VBA macro was developed to measure intensities of the particle signals and to determine the size distribution of the aerosol. In this paper, two models used for calculating the intensity of the particle signal are discussed, especially when several particles arrive within a very short time interval. To evaluate the results obtained by this SP-ICPMS technique, the size distribution of the laser-produced aerosol was also measured by ELPI. In addition, the sizes of the particles ejected during ablation at a fixed spot were studied shot by shot (from 1 to 100 shots) using both techniques. THEORY OF SP-ICPMS According to the theory developed by Degueldre et al.26,27, the introduction of a particle into the plasma of an ICP produces a cloud of ions which is then transmitted to the mass spectrometer detector. If the dwell time or unitary time of measurement per element is shorter than the time of arrival on the detector of the entire ion cloud, a peak composed of several points corresponding to fractions of the ion cloud is measured. The detection of this ion cloud is called an “event” in this study. The resulting area of the peak is directly proportional to the number of atoms of the element of interest introduced into the instrument (provided the particles are fully atomized in the

Page 2 of 10

plasma). If the shape and the density of the particle are known or assumed, the size of the particle (i.e. the equivalent diameter assuming a spherical shape) can be calculated. Various approaches can be considered to calibrate the relationship between the signal and the analyte mass for each particle. A calibration curve can be drawn from the analysis of several suspensions of standard monodisperse nanoparticles5,26. However, suspension of standard nanoparticles is not available for all chemical elements. Monodisperse droplets of a standard solution can also be used to perform the calibration, using a micro-droplet generator8,9. As an easier alternative, an accurate solution of the element of interest can be nebulised and introduced into the ICPMS28–30. This approach was used in our study. A key parameter is the “transmission yield” ftrans defined here as the ratio of the number of counts detected to the number of atoms of the analyte introduced into the nebuliser via the solution. It is calculated using the following equation: 𝑓𝑡𝑟𝑎𝑛𝑠 =

𝐼𝑆𝑆 ×𝑀𝑒 𝑓𝑛𝑒𝑏 ×𝐴𝑖 ×𝑄𝑛𝑒𝑏 ×[𝐶]𝑒×𝑁𝑎𝑣𝑜

(1)

Where Iss is the intensity measured for the standard solution of known concentration corrected for the background (in cps), M e is the exact molar mass of the element of interest (in g mol-1), fneb is the nebulisation yield, Ai is the abundance of the isotope of the element recorded, Qneb is the flow rate of the nebulisation (in L s-1), [C]e is the concentration of the element of interest (in g L-1) and Navo is the Avogadro’s number equal to 6.02×1023 mol-1. The “nebulisation yield” is considered to be the ratio of the number of atoms introduced into the plasma to the number of atoms in solution introduced into the nebuliser. The nebulisation yield was determined by the waste collection method (WC)29. Vessels containing the standard solution and the waste were weighed before and after taking the measurement. Nebulization yield fneb is calculated using the equation: 𝑓𝑛𝑒𝑏 = 1 −

𝑚𝑤 𝑚𝑠

(2)

Where mw is the mass rate of the standard solution redirected to the waste (in g s-1), and ms is the mass rate of standard solution sent to the nebulisation chamber (in g s-1). This approach implies the use of very robust ICP conditions in order to ensure that dry and wet particles originating from the laser ablation and the solution nebulisation are completely atomised into the plasma, thus ensuring a reliable calibration of the dry particles from the solution nebulisation. Centre gas flow rate, sampling depth, power and inner diameter of the torch injector are the key parameters determining the location where a particle starts to vaporise in the plasma7. The maximum signal is reached when the particle is completely vaporised and before extensive diffusion has occurred. If the vaporization process in the plasma starts too soon, the particle will be completely vaporised but an extensive diffusion process will produce an ion cloud that is too dispersed to be completely extracted by the sampler cone. Ions located off the radial centre of the sampling orifice will not be transferred to the ICPMS resulting in a lower signal. Additionally, Lee et al.31 showed that depending on the vaporization rate, large particles can be underestimated due to incomplete vaporization. However, particles of low density, molecular weight and boiling point are more likely to produce linear calibration curve (intensity versus diameter). In their article, gold (whose properties are 19.3 g cm-3 density, 196.97 g mol-1 molecular weight and 3129 K boiling point) showed a


Page 3 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


linear range up to 150 nm. Therefore, we can assume that copper calibration curve (density of copper 8.96 g cm-3, molecular weight of 63.5 g mol-1 and boiling point of 2835 K) is likely to be linear in the range of the particles measured. In this study the selected element of interest was Cu measured by recording the isotope 65Cu. Under our ICPMS conditions (see below), a solution of 1 μg L-1 produced a signal intensity of 61 000 cps with a nebulisation flow rate of 1.2×10-6 L s-1 and a nebulisation efficiency of 31.5%. The transmission yield calculated with equation (1) was 0.0055%. The number of counts detected per particle Ipart can be related to the mass of the particle mpart reaching the plasma. Assuming that the particle is a dense sphere of known density ρ, the diameter dpart of an equivalent spherical particle can be calculated with the equation: 𝑑𝑝𝑎𝑟𝑡 = (6 ×

𝐼𝑝𝑎𝑟𝑡 ×𝑀𝑒 𝐴𝑖 ×𝑓𝑡𝑟𝑎𝑛𝑠 ×𝑁𝑎𝑣𝑜

⁄(𝜋 × 𝜌))

1 3

(3)

EXPERIMENTAL SECTION Instrumentation. Chemicals and samples. Aqueous copper standard solution (Cu Plasma Cal, 998 ± 3 μg mL-1 4% HNO3, SPC Science) was used to prepare a solution of 1 μg L-1 by dilution with demineralised water and 2% HNO3 (JT Baker Instra-analysed, 69.0-70%) previously purified by sub-boiling distillation in a Savillex DST-1000 sub-boiling still. Ablations were performed on a 2 mm copper foil (99.9% purity, Good Fellow). ICP-MS. The High Resolution ICP-SFMS (Element XR, Thermo Scientific, Bremen, Germany) used in this study, was fitted with the “Jet Interface” (high capacity dry interface pump and a specially designed set of cones, X-Skimmer and JetSampler). The general operating conditions are presented in Table 1. A Peltier cooled cyclonic chamber (PC3, ESI, Omaha, United States) was used for double sample introduction of a 2% HNO3 solution and the laser-produced particles transported from the ablation cell by a 600 mL min-1 helium stream. Only the isotope 65Cu was recorded. Optimisation was performed on a daily basis to meet the two conditions required for this application: i) the unitary time of measurement by isotope must be as small as possible (see following section “data acquisition”) ii) only the pulse counting detector must be used, meaning that the sensitivity had to be restrained so that count rates are below 106 cps. Particle aerosols should also be completely atomised so that the tuning of the instrument with a standard solution is meaningful. Adjustment of the analytical parameters is a three-step process. The instrument parameters were first adjusted to obtain the highest sensitivity for a Cu solution of 1 μg L-1. Secondly, a glass standard CRM NIST 612 (transect trajectory) was ablated (while 2% HNO3 was nebulized) in order to optimize the 238 U/232Th ratio as close as possible to 1 ± 0.05, which is regarded as a good indicator of complete atomisation and ionisation of most of the elements, including copper, into the plasma32,33. Thirdly, the 1 μg L-1 Cu solution was nebulised again without laser ablation and the optical lenses were defocused to decrease the sensitivity to ~500 000 cps in low resolution mode. Laser Ablation System. The femtosecond laser ablation station LAMDBA III (Nexeya SA/Amplitude Systèmes, Bordeaux, France) which is fitted with a diode-pumped Yb:KGW crystal

laser source (HP2, Amplitude system, Pessac, France) was used. Three wavelengths can be selected: 1030 nm (fundamental), 515 nm (2nd harmonic) and 257 nm (4 th harmonic). The 1030 nm wavelength was the only one used in this study. At this wavelength, the laser operates within a large range of repetition rates from 1 Hz to 100 kHz at low energy ranging from 2 mJ per pulse under 1 kHz to 85 µJ at 100 kHz. By combining a high repetition rate and the use of 2 fast galvanometric scanners, virtual beam shaping can be realized by moving the laser rapidly (up to 2 m s-1) across the surface. The so-called virtual beam shaping refers to the fact that the apparent resulting ablation beam is shaped accordingly to the design trajectory. Table 1. Optimised ICPMS and laser ablation operating parameters Laser ablation system parameters Wavelength

1030 nm

Pulse duration

< 400 fs

Spot Size, Fluence

20 μm, 4 J cm-2

He flow rate

600 mL min-1

Ablation scheme

- 1 Hz, static, single shot (ICPMS) - 1000 Hz, matrix of 63 x 63 = 3969 spots (3.7 x 3.7 mm), ~4s (ELPI)

HR ICP-SFMS parameters Cooling gas flow rate

16.2 L min-1

Auxiliary gas flow rate

0.96 L min-1

Nebuliser gas flow rate

0.85 L min-1

RF power

1100 W

Resolution

2000

Recorded isotope

65Cu

Samples per peak

1000

Mass window

100 %

Sample time

0.1 ms

Settling time

1 ms

Electrical Low Pressure Impactor. To provide a comparison with the size distribution of the fs-LA aerosol obtained by SPICPMS, an ELPI (Dekati, Finland) was used. This device is composed of 13 impactor stages. Particles in the gas flux are electrically charged at the entrance of the instrument and impact on the different stages depending on their aerodynamic diameters. Electrical currents generated by the impact of particles are recorded, thus allowing online measurement of the particle size distribution. The device is designed to collect airborne particles between 7 nm and 10 µm (equivalent aerodynamic diameters). The equivalent aerodynamic diameter is defined as the diameter of a sphere having a density of 1 g cm-3 with the same velocity as the particles of interest. As explained in the “Theory of SP-ICPMS” section, the SP-ICPMS method is based on the calculation of a theoretical spherical dense particle with a density of pure copper (8.96 g cm-3). To compare with the SP-ICPMS method, the diameter range for the ELPI was recalculated by the software using Stokes diameter formula. The Stokes diameter is the diameter of a sphere with the same density and the same velocity as the particle of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

interest. The cut Stokes diameters of the instrument calculated for the density of pure copper were: 3 nm, 7 nm, 13 nm, 25 nm, 51 nm, 85 nm, 159 nm, 269 nm 486 nm, 750 nm, 1.29 µm and 3.27 µm. The ELPI was operated under a pressure gradient of 900 mbar resulting in an inlet gas flow of 9.89 L min-1. The helium flow rate coming from the laser ablation chamber (0.6 L min-1) was supplemented with filtered air using a Y connection. The selected integration time was 1 s. Ablation schemes. To determine the particle size distribution by means of the SP-ICPMS technique, 100 single shots were performed at the same ablation spot using the settings mentioned in Table 1 in order to follow the evolution of the particle size distribution according to the shot number. Since here neither the scanner nor the XY stage was moved, this ablation scheme is called “static”. More precisely, particle size distributions produced by the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 15th, 20th, 30th, 40th, 50th, 60th, 70th, 80th, 90th, and 100th shots were individually recorded by SP-ICP-MS. The use of the same ablation schemes was not possible for the ELPI measurement because the quantity of material ablated could not be detected by the instrument due to higher detection limits. However, comparison of results from the two techniques were possible due to the unique laser feature which allows large ablation in a very short period of time by combining high repetition rate with fast laser beam movement (using the scanner). A matrix composed of, 3969 discretized and identical laser shots was performed at 1 kHz, in about 4 seconds (spots are spaced by 60µm). We assumed that this strategy allowed amplifying the signal of a single shot (in terms of particles number and distribution) by a factor 3969 then making possible the ELPI detection. Due to the high precision of the scanner, successive ablations at the same location according to this matrix pattern resulted in a matrix of well-defined craters as shown in Figure 1. In this study, the matrix pattern was repeated 100 times at the same location in order to study the particle size distribution shot by shot as described for the SP-ICP-MS technique.

(a)

(b)

Figure 1: Typical example of well separated spots performed for ELPI detection, (a) 10 shots and (b) 100 shots.

Data acquisition. ICPMS method. The ‘Element XR’ instrument is not specifically designed for single particle analysis. The elementary measurement time during which counts are integrated to give an average count rate, called ”sample time”, usually ranges between 1 ms and hundreds of ms, i.e. measurement times are longer than the duration of the signal produced by the ion cloud which results from the ionization of a single particle in the plasma. In this study, to obtain smaller measurement times, the methodology developed by Shigeta et al.11 for the analysis of single cells was used. The sector field

Page 4 of 10

electric analyser (ESA) was used because this is the fastest scanning mode available on this instrument. Only one isotope (65Cu) was recorded to avoid the loss of counting time by switching from one mass to the other (i.e. settling time). To explain the implementation of this measurement method, a good understanding of the significance of the key acquisition parameters of the ‘Element XR’ software is necessary. Measurement of each isotope is defined within a mass range called “peak” automatically defined by the software according to the chosen mass resolution (terms in inverted commas are the terms defined by the ‘Element XR’ software). Each mass range is divided into a number of “samples” which represent the unitary acquisition time of the detector. Users choose the number of “samples per peak” and the “sample time”. The “mass window” is set up to define the percentage of the mass range which is ultimately measured. With the ‘Element XR’ the smallest possible “sample time” is 100 μs when using the pulse counting mode and 1 ms when using the detector’s analogic mode. Therefore, the analogic mode should be avoided. The highest possible number of “samples per peak” is 1 000. It should be noted that there is no delay between the measurements of each “sample”. However, at the end of the mass window measurement, a “settling time” of 1 ms is required before starting the next measurement. 500 mass window measurements were performed to obtain a total acquisition time of 50s. The 1 000 successive unitary 100 µs measurements for the 65Cu peak must be performed only on the part of the peak for which a constant intensity is obtained. This is achieved by selecting a medium mass resolution of 2 000 for which a flat top peak is obtained around the peak centre. The slits which give the mass resolution of 2 000 are installed instead of the slits for a high mass resolution of 10 000. As a consequence, the software defines a very narrow mass range, normally used for a resolution of 10 000, which is smaller than what would normally be used with a mass resolution of 2 000. Thanks to this artifice, the 1 000 successive unitary (“sample”) measurements on the whole mass range (“mass window” of 100%) are performed on the flat top part of the peak and provide a temporal acquisition of the signals produced by the measurement of the LA fragments. The raw data are recovered and processed through laboratory-developed software. Hence, with a “sample time” of 100 µs and 1 000 “samples per peak”, the signals due to measurement of LA fragments were recorded for 99% of the total time (with exception of the 1 ms corresponding to the settling time of the ESA). Models of data treatment for detection of a single particle. Raw signal on 65Cu obtained for a single shot on the copper foil is shown in Figure 2. The signal is composed of several peaks each of hundreds of microseconds, and is clearly extremely noisy, due to the random arrivals of ablation fragments of various sizes.


Page 5 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. 65Cu signal corresponding to the arrival of particles produced by a single shot on a copper foil

Two models of data treatment using VBA macro were developed to calculate the intensity of each “event”, count the number of particles and estimate the particle size distribution of the aerosol (according to equation (3)). On both models, an event is defined as being the sum of consecutive intensities until the background level is reached. For comparison with the size distribution measured by ELPI, the same diameter ranges were chosen to produce the distribution charts. Model 1 Model 1 is the simplest model with one event corresponding to one particle. In this study no background signal was detected as a continuous baseline, so the defined threshold value Th was 0. The intensity of an event Ie is then defined by the following formula: 𝐼𝑒 = ∑𝑖𝑛=1 𝐼𝑐𝑖 𝑤𝑖𝑡ℎ 𝐼𝑐𝑖 > 𝑇ℎ = 0 𝑎𝑛𝑑 𝑖 > 2 (4) Ici being consecutive intensities. Model 2 As the aerosol produced by a single shot of LA is relatively dense, the intervals between the arrivals of the particles on the detector are not always large enough to allow the signal of one particle to reach the background before the arrival of the next one. Therefore, in addition to the signal of single particles, signals corresponding to clusters of particles are observed (see Figure 3). Model 1 does not allow the discrimination of these two different kinds of signal. Therefore, part of the events detected with model 1 is the sum of the intensities of several particles, resulting in an overestimation of the intensity (and of the particle size) and underestimation of the number of particles. In Model 2, an event is not considered as a single particle. The determination of the number of particles included in an event is estimated by counting the number of apexes n c,. Apexes are detected according to the following equation: 𝐼𝑓 𝐼𝑐𝑖−1 < 𝐼𝑐𝑖 𝑎𝑛𝑑 𝐼𝑐𝑖 > 𝐼𝑐𝑖+1 𝑡ℎ𝑒𝑛 𝐼𝑐𝑖 = 𝐼𝑣𝑗

(5)

Ivj, is the intensity of an apex. To estimate the intensity Ij associated with each particle of the cluster, the relative intensity of each apex is calculated as: 𝐼

𝐼𝑗 = ∑𝑛𝑐𝑣𝑗

𝑖=1 𝐼𝑣𝑗

× 𝐼𝑒

(6)

Figure 3. Example of signal measured for 65Cu, for (a) a single particle well separated from the other particles, and (b) “cluster” of particles which were analysed within a very short time interval.

RESULTS AND DISCUSSION SECTION Model evaluation. Limit of detection. In this study the limit of detection is defined as the smallest signal detectable that can be considered as an event. An event is defined when the intensities of at least two consecutive points are greater than the chosen threshold (here 0 count). Thus the limit of detection is 2 counts (two times 1 count), which for Cu (with a transmission factor of 0.0055 %), corresponds to a mass of ~12 ag or an equivalent Stokes diameter of 13.9 nm. Influence of the model on the particle size distribution measured. To study the responses of the models for an aerosol produced by fs-IR-LA, the size distributions of the LA aerosol produced after a single shot on Cu foil were determined using both models.

Figure 4. Percentage of particles detected by diameter range in the ablation aerosol produced by a single shot on a copper foil using: (a) Model 1 (b) Model 2. Error bars correspond to standard deviation calculated from 3 replicates.

As it can be seen in Figure 4, particles larger than 159 nm are very scarce, representing only 1% of the total number of detected particles using both models. Using model 2, the percentage of small particles increases and the percentage of large particles decreases compared to model 1. Larger particles detected using model 1 must have been clusters of particles that arrived on the detector at almost the same time. Those aggregated particles are counted individually using model 2. Due to the counting of particles from clusters, the total number of particles detected with model 1 (3800 particles) is lower than for model 2 (7580 particles). Concerning model 2, the particle size distribution is centred on the 13-25 nm range. Additionally,



Table 2. Clusters simulation: sum of counts and particle diameter calculated with model 2 for each particle composing 3 theoretical clusters A, B and C. Apexes of single particle which compose the cluster are moved closer by 100 µs increments. Error on the expected particle diameter greater than 10% are highlighted in italics

Figure 5. Theoretical signal intensities for cluster A. Apexes of the 3 particles composing the clusters are moved closer by 100 µs increments.

600 µs

I (counts) d (nm)

P1 7 21

Particle P2 16 28

P3 11 24

500 µs

I (counts) d (nm) Error on d

7 21 1.3%

15 27 -3.1%

12 25 3.3%

400 µs


7 21 1.3%

15 27 -3.1%

12 25 3.3%

300 µs


-

19 29 5.0%

15 27 12%

200 µs


-

194 29 6.7%

15 27 10%

100 µs


-

34 37 34%

-

600 µs

I (counts) d (nm)

24 32

485 86

35 36

500 µs


29 34 6.2%

479 86 -0.4%

36 36 0.9%

400 µs


29 34 6.2%

479 86 -0.4%

36 36 0.9%

300 µs


-

544 89 3.9%

-

200 µs


-

544 89 3.9%

-

100 µs


-

544 89 3.9%

-

600 µs

I (counts) d (nm)

655 95

27 33

485 86

500 µs


727 98 3.5%

25 32 -2.6%

415 82 -5.0%

400 µs


727 98 3.5%

25 32 -2.6%

415 82 -5.0%

300 µs


743 99 4.3%

-

424 82 -4.4%

200 µs


742 99 4.2%

-

426 82 -4.3%

100 µs


710 98 2.7%

-

457 84 -2.0%

CLUSTER A

Separation time

CLUSTER B

by comparing the number of particles obtained with method 1 with the number of clusters detected with method 2, we can consider that 64% (for the first shot whose density of particles is higher) to 89% of the detected events are single particles. Those single particles represent 31 to 60% of the particles detected, meaning that many clusters are composed of multiple particles. Accuracy of the distribution obtained with the models. To evaluate the capabilities of both models, the signals of 3 theoretical individual particles are moved closer together (based on their apexes) in 100 µs increments to produce clusters. Three different types of theoretical clusters are tested: cluster A which is composed of three particles of similar size (see Figure 5), cluster B which is composed of one big particle surrounded by two smaller particles and cluster C which is composed of one small particle surrounded by two large particles. The sum of intensities and diameter of each particle are calculated using both models and compared to the expected sum intensities and diameter of the individual particles. Table 2 shows the results obtained for model 2. When using model 1, as soon as the particles are not separated anymore (from 500 µs to 100 µs separation), the cluster is considered as a single particle. Since clusters of particles are often produced during laser ablation process, model 1 is not adapted and will not be considered below unless otherwise stated. Considering model 2, when particles arrive on the detector too close together in time, the signal tends to produce only one peak, resulting in an overestimation of the intensity for a single particle (for clusters A and B, when the apexes are separated by 100 µs and 300 µs respectively).

CLUSTER C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 10


Page 7 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For cluster C, the smallest particle is not detected when the apexes are separated by less than 300 µs. However, as the equivalent diameter is proportional to the cube root of the intensity, the relative error in the diameter is far lower than for the sum of intensities. For instance, for cluster A with a separation of 100 µs between the apexes, the error in the sum of intensities is 47 % (34 counts instead of 16) but the relative error in the diameter is of 34 %. Cluster cannot be distinguished using Model 2 when several particles form a single peak. However, when several apexes are distinguished, the error in the diameter size can be regarded as acceptable for the particle size distribution (a maximum relative error of 6.2 % for the 3 theoretical clusters). The smaller particles are more likely to be undetected when their signals are not sufficiently separated than for larger particle signals (for clusters B and C). It can therefore be assumed that the particle distribution of the LA aerosol obtained with SP- ICPMS might underestimate the contribution of the small particles to the particle size distribution and slightly overestimate the size of the largest particles. Low Pressure impactor versus SP-ICPMS. The size distribution measured by SP-ICPMS for the ablation of a single shot on a copper foil was compared to the size distribution measured by ELPI. As discussed in the laser ablation scheme section, a single shot produces a number of particles below the detection limit for the ELPI. Therefore, we physically amplified the signal of a single shot by a factor 3969 by repeating 3969 spatially separated shots in a short period of time (4 s) to provide enough particles for ELPI measurement. Additionally, to compare the results, percentages of particles are displayed only for the particle size range detectable with both techniques. For the SP-ICPMS, only the distribution obtained with model 2, which is the closest to the ELPI distribution, is shown. As it can be seen in Figure 6, the general size distributions measured by the two methods are in good agreement. Both of them are centred on the 25-51 nm range with 34.7% and 36.4% of particles in that range for SP-ICPMS and ELPI respectively. It seems however, that the proportion of larger particles is overestimated for the 85-159 nm range and slightly underestimated for the 50-85 nm range by SP-ICPMS compared to ELPI. Three hypotheses can be suggested. First, during ELPI measurement, the particles travel a longer distance from the laser to reach the analyzer compared to the ICPMS. Therefore, part of the large particles can be lost by gravity or impaction in the transportation tube. Second, if we take into account a larger size range from 50-159 nm, the percentage for ELPI (33.7%) and SP-ICPMS is similar (34.1%). Therefore, the difference can be due to the wrong sorting of particles closed to the diameter threshold (50 nm). Finally, as stated in paragraph “Accuracy of the distribution obtained with the models”, SP-ICPMS might underestimate the contribution of small particles included in clusters. In any case, the global distribution is very similar. It is worth mentioning that precision of SP-ICPMS (from 5 to 9% RSD) is better than ELPI (from 2 to 19% RSD). However, using this specific ablation scheme, ELPI allows accessing lower particle size diameter, up to 7 nm. They are not shown here because this range was not detectable by the ICPMS and we chose to compare percentage of particles from the same particles diameter range. The reason why these 7 nm range particles were not detected by the ICPMS is due to the fact the ionic lenses have been defocused to lower the sensitivity of the instrument in order to keep the signal recorded by the pulse counting mode only. Moreover, to protect the detector from too

intense ion currents, the detector cannot operate in the pulse counting mode when two consecutive “numbers of counts” higher than 625 counts are recorded in a row within a time interval of 125 µs. In such a case, the detector is normally operated in the analogic mode or the ion beam is redirected to a Faraday cup. In our case, only signals measured in the pulse counting mode were recorded. As a consequence, part of the signal obtained for the largest clusters (or for large single particles) can be missing. For the different aerosols studied, these missing particles represent less than 1% of the total number of particles. Finally, the size distributions measured in this study are consistent with the results reported in the literature for femtosecond laser ablation of metallic material. Koch et al. 34 has characterised the size distribution of aerosols produced by femtosecond ablation (170 fs, 755nm) of brass, for different fluences by ELPI. At low fluence (2.5 J cm-2) the aerosol distribution is an ultra-fine monomodal distribution centred on an equivalent diameter (Stokes diameter) of 10 nm. At higher fluence (15 J cm-2) the aerosol is polydispersed and a bimodal distribution centred on 20 nm and 1 μm equivalent diameters is observed. The fluence used in this study was 4 J cm -2 which is close to the lowest fluence employed by Koch et al.34 that allows obtaining a monomodal distribution but slightly higher which could explain the higher central range of 25-51 nm. Additionally, Diwakar et al.20 has shown, by studying the particle size distribution produced by femtosecond laser ablation of brass, that an increase of the laser pulse energy shifts the size distribution towards larger particle size.

Figure 6. Comparison of the size distributions of the first ablation shot on a copper foil by (a) ELPI and (b) SP-ICPMS.

Shot-by-shot evolution of the size distribution. The SPICPMS allows for the measurement of the particle size distribution in the aerosol produced by a single shot. To study the evolution of this distribution produced by several laser ablation shots carried out at the same spot, each shot was recorded separately by SP-ICPMS and ELPI (Figure 7). To compare the two detection methods, the number of particles is normalized to the number of particles within the range 25-51 nm of shot 1. Moreover, only the particle sizes detected with both methods are used for comparison. For both methods, particle diameters are centred in the range 25-51 nm. However, a significant variation in the total number of particles can be observed. The number of particles produced during the second shot is lower than the initial one. Then it is stable for the next two shots and increase slowly up to the last 100th shot. A possible explanation lies in a modification of the structure of the copper foil during the first pulses. Wang et al.35 studied the ablation of a copper foil by a single shot with a



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

femtosecond laser ablation system (800 nm, 120 fs) for different fluences. In their study, they suggested that multiple-pulse irradiation of a spot could induce a change in laser beam reflectivity due to changes in the surface structure properties and the inclined walls of the crater thus formed. This change in laser beam reflectivity might lead to a variation in the quantity of material removed for each shot. In addition, the Gaussian profile of the laser beam likely affects the amount of material removed, as the crater tends to enlarge as a function to the depth. Considering the ELPI detection, this increase of the number of particles is more pronounced than for SP-ICPMS. It might be due to the fact that the ablation performed for the ELPI measurement is larger than for SP-ICPMS. The focus has been made at the center of the matrix and the ablation spots located farther from the center can be slightly defocused if the sample is not perfectly in horizontal position and induce a change in particle size distribution. It can also be highlight that for SPICPMS, after the 20th, shot the distribution is slightly shifted toward 13-25 nm range while the distribution recorded by ELPI remains centered on 25-51 nm range. Shift of particle size distribution toward thin particles during spot ablation have been reported by Kuhn et al.36, which is consistent with the results obtained by SP-ICPMS. The aggregation of particles, that is more likely to occur during ELPI measurement first because particle density is much higher (by a factor less than 3969) and second because they travel a long distance before reaching the ELPI, can be one explanation.

Page 8 of 10

SP-ICPMS. Nanoparticles detected by this method are counted, their sizes are calculated and they are attributed to a given size range using a laboratory-developed VBA Excel programme. The SP-ICPMS method allows for direct measurement of the LA aerosol size distribution and is extremely sensitive with a detection limit of a few attograms corresponding to a nanoparticle of 14 nm. Lower LOD could be obtained by selecting the 63Cu isotope and by improving the sensitivity. However, this would limit the detection of large particles since only the pulse counting mode can be used. Two models for particle identification and size-measurement have been tested. The second model, which allows counting particles from clusters or particles that reach the detector within a very short time interval has been preferred due to the dense aerosol obtained by laser ablation. The particle size distributions of laser-produced aerosols on a copper foil, measured by both SPICPMS for a single shot and ELPI for 4 000 shots, were consistent. In addition, the variation of the particle size distributions during repetitive ablation shots forming an ablation crater was similar using both techniques although some difference were noticed concerning a shift toward thin particles that was evidenced only with the SP-ICPMS.

Figure 7. Evolution of the particle size distributions of aerosols produced during the ablation of a crater in a copper foil (from 1 to 100 shots) measured by (a) SP-ICPMS and (b) ELPI. Number of particles is normalised to the highest number of particles measured in the 25nm class range of the first shot.

CONCLUSION A new methodology to determine the particle size distribution of an aerosol produced by laser ablation has been developed by


Page 9 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1 M. Hassellöv, J. W. Readman, J. F. Ranville and K. Tiede, Ecotoxicol. Lond. Engl., 2008, 17, 344–361. 2 M. S. Jiménez, M. T. Gómez, E. Bolea, F. Laborda and J. Castillo, Int. J. Mass Spectrom., 2011, 307, 99–104. 3 F. Laborda, J. Jiménez-Lamana, E. Bolea and J. R. Castillo, J. Anal. At. Spectrom., 2011, 26, 1362–1371. 4 F. Laborda, J. Jiménez-Lamana, E. Bolea and J. R. Castillo, J. Anal. At. Spectrom., 2013, 28, 1220–1232. 5 S. Gschwind, L. Flamigni, J. Koch, O. Borovinskaya, S. Groh, K. Niemax and D. Günther, J. Anal. At. Spectrom., 2011, 26, 1166–1174. 6 J. Liu, K. E. Murphy, R. I. MacCuspie and M. R. Winchester, Anal. Chem., 2014, 86, 3405–3414. 7 J. W. Olesik and P. J. Gray, J. Anal. At. Spectrom., 2012, 27, 1143–1155. 8 H. Kawaguchi, N. Fukasawa and A. Mizuike, Spectrochim. Acta Part B At. Spectrosc., 1986, 41, 1277–1286. 9 T. Nomizu, H. Hayashi, N. Hoshino, T. Tanaka, H. Kawaguchi, K. Kitagawa and S. Kaneco, J. Anal. At. Spectrom., 2002, 17, 592–595. 10 G. Cornelis and M. Hassellöv, J. Anal. At. Spectrom., 2013, 29, 134–144. 11 K. Shigeta, H. Traub, U. Panne, A. Okino, L. Rottmann and N. Jakubowski, J. Anal. At. Spectrom., 2013, 28, 646–656. 12 K. Shigeta, G. Koellensperger, E. Rampler, H. Traub, L. Rottmann, U. Panne, A. Okino and N. Jakubowski, J. Anal. At. Spectrom., 2013, 28, 637–645. 13 M. Guillong and D. Günther, JAAS, 2002, 17, 831–837. 14 B. Fernández, F. Claverie, C. Pécheyran, O. F. X. Donard and F. Claverie, TrAC Trends Anal. Chem., 2007, 26, 951– 966. 15 J. Koch, S. Schlamp, T. Rösgen, D. Fliegel and D. Günther, Spectrochim. Acta Part B At. Spectrosc., 2007, 62, 20– 29. 16 J. Koch, M. Wälle, J. Pisonero and D. Günther, J. Anal. At. Spectrom., 2006, 21, 932–940. 17 B. Fernandez, F. Claverie, C. Pécheyran, J. Alexis and O. F. X. Donard, Anal. Chem., 2008, 80, 6981–6994. 18 F.-X. d’Abzac, A.-M. Seydoux-Guillaume, J. Chmeleff, L. Datas and F. Poitrasson, Spectrochim. Acta Part B At. Spectrosc., 2011, 66, 671–680. 19 F. Claverie, C. Pécheyran, S. Mounicou, G. Ballihaut, B. Fernandez, J. Alexis, R. Lobinski and O. F. X. Donard, Spectrochim. Acta Part B At. Spectrosc., 2009, 64, 649–658. 20 P. K. Diwakar, S. S. Harilal, N. L. LaHaye, A. Hassanein and P. Kulkarni, J. Anal. At. Spectrom., 2013, 28, 1420–1429. 21 J. J. Gonzalez, A. Fernandez, D. Oropeza, X. Mao and R. E. Russo, Spectrochim. Acta - Part B At. Spectrosc., 2008, 63, 277–286. 22 C. Liu, X. L. Mao, S. S. Mao, X. Zeng, R. Greif and R. E. Russo, Anal. Chem., 2004, 76, 379–383. 23 H. Wu, N. Zhang and X. Zhu, Appl. Surf. Sci., 2014, 317, 167–171. 24 R. Glaus, R. Kaegi, F. Krumeich and D. Günther, Spectrochim. Acta Part B At. Spectrosc., 2010, 65, 812–822. 25 H. R. Kuhn and D. Günther, Anal. Chem., 2003, 75, 747–753. 26 C. Degueldre, P.-Y. Favarger and S. Wold, Anal. Chim. Acta, 2006, 555, 263–268. 27 C. Degueldre, P.-Y. Favarger, R. Rossé and S. Wold, Talanta, 2006, 68, 623–628. 28 J. Tuoriniemi, G. Cornelis and M. Hassellöv, Anal. Chem., 2012, 84, 3965–3972. 29 J. Tuoriniemi, G. Cornelis and M. Hassellöv, J. Anal. At. Spectrom., 2014, 29, 743–752. 30 Y. Suzuki, H. Sato, S. Hikida, K. Nishiguchi and N. Furuta, J. Anal. At. Spectrom., 2010, 25, 947–949. 31 W.-W. Lee and W.-T. Chan, J. Anal. At. Spectrom., 2015, 30, 1245–1254. 32 B. Hattendorf, C. Latkoczy and D. Günther, Anal. Chem., 2003, A pages, 342A–347A. 33 D. Günther and B. Hattendorf, TrAC Trends Anal. Chem., 2005, 24, 255–265.

34 J. Koch, A. von Bohlen, R. Hergenröder and K. Niemax, J. Anal. At. Spectrom., 2004, 19, 267–272. 35 S. Y. Wang, Y. Ren, C. W. Cheng, J. K. Chen and D. Y. Tzou, Appl. Surf. Sci., 2013, 265, 302–308. 36 H.-R. Kuhn, M. Guillong and D. Günther, Anal. Bioanal. Chem., 2004, 378, 1069–1074.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

. TOC


Page 10 of 10

Direct Online Determination of Laser-Induced Particle Size Distribution

Recommend Documents