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Feasibility of nanoparticles enhanced laser ablation inductively coupled plasma mass spectrometry Markéta Holá, Zita Salajková, Aleš Hrdli#ka, Pavel Po#ízka, Karel Novotny, Ladislav #elko, Petr Šperka, David Prochazka, Jan Novotný, Pavlína Modlitbová, Viktor Kanicky, and Jozef Kaiser Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01197 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018
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Analytical Chemistry
Feasibility of nanoparticles enhanced laser ablation inductively coupled plasma mass spectrometry Markéta Holᆇ, Zita Salajková§, Aleš Hrdlička*†‡, Pavel Pořízka§, Karel Novotný†‡, Ladislav Čelko§, Petr Šperka$, David Prochazka§, Jan Novotný§, Pavlína Modlitbová§, Viktor Kanický†‡, Jozef Kaiser§. † ‡
Department of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic. Central European Institute of Technology (CEITEC), Masaryk University, Kamenice 753/5, 625 00 Brno, Czech Republic
§
Central European Institute of Technology, Brno University of Technology, Purkyňova 656/123, CZ-612 00 Brno, Czech Republic $
Institute of Machine and Industrial Design, Faculty of Mechanical Engineering, Brno University of Technology, Technická 2896/2, CZ-61669 Brno, Czech Republic * E-mail:
[email protected].
Fax: +420 54949 2494
ABSTRACT: Nanoparticles (NPs) applied to surface of some solids can increase signals in inductively coupled plasma mass spectrometry (ICP-MS). Drops containing 20 and/or 40 nm nanoparticles of Ag and/or Au were deposited on metallic and ceramic/glass samples and, after being dried, both the samples treated with NPs and plain targets were ablated by one pulse per spot. The LA-ICP-MS signals were enhanced for metallic samples modified with NPs in comparison to signals produced at plain, untreated surface. Maps of LA-ICP-MS signals recorded for several laser fluences show that the NPs-induced signal enhancement exceeds even two orders of magnitude for metallic samples. No enhancement was achieved for non-conductive samples. This enhancement is limited to peripheral annular region of dried droplet area where NPs are concentrated due to “coffee stain” effect. Ablation crater profilometric inspection revealed a more uniform material rearrangement over NPs-treated surface compared with ablated plain target. However, besides smoother crater bottom no other evidence of NPs enhancing effect was noticed, although increased ablation rate was anticipated. Limits of detection dropped by one order of magnitude for the minor elements in the presence of NPs. Observed phenomena depend only on the NPs surface concentration but not on the material or size of NPs. Electron microprobe study of collected ablation aerosol has shown that aerosol particles consisting of target material are aggregated around NPs. The hypothesis is, that such aggregates exhibit better transport/vaporisation efficiency thus enhancing signals for metallic samples. A detailed study of suggested mechanism will be continued in ongoing work.
Laser ablation (LA) became an effective and widespread sampling technique for analysis of mainly solid samples. Numerous modifications have been developed during last four decades. Laser beam interaction with a target causes releasing of material in a form of vapour and liquid or solid particles, finally creating dry aerosol, which is introduced into inductively coupled plasma source for mass or optical emission spectrometer (ICP-MS or ICP-OES)1. Another laser/sample interaction effect is microplasma formation which in situ atomizes and ionizes the sample. This principle is the basis for the Laser-Induced Breakdown Spectroscopy (LIBS)2. Apart from the discussion about pros and cons of both methods, which can be found reviewed elsewhere3-5 it is necessary to emphasize a permanent effort to improve sensitivity and limits of detection (LOD). Each analytical method can be subjected to a certain optimization for achieving better sensitivity and LOD, nevertheless, the best
yielded values are limited by a particular instrumentation and physical limits. Namely, a level of the collected useful radiation presented as a signal-to-background ratio is lowered by presence of strong radiation continuum such as Bremsstrahlung for LIBS, ICP-OES and other OES techniques. On the other hand, the MS detection is disturbed practically only by the detector noise and non-analyte particles (flying in the inner space) or energetic particles (coming from the outer space of the mass spectrometer). The latter must not be confused with interference of different ions with similar or the same m/z. Signal enhancement at the presence of nanoparticles (NPs) has been observed and used for many years at Surface-Enhanced Raman Spectroscopy (SERS). The main effect is supposed to be an intensification of the local electric field induced by incident photons on edges, tips, NPs and other nanostructures
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on the surface which could even be amplified by a surface plasmon resonance6-9. In any case a strong electric field is formed in the gap between the NP and the substrate plain10 enhancing the emission of seed electrons. It was also found that NPs are fragmented into smaller ones under the laser irradiation on a 0.1 ns time scale for gold and on a fs time scale for TiO2 NPs. This change of their dimension influences the resulting signal amplification11. Moreover, another effect of the ion signal enhancement has been observed and used in Laser Desorption Ionization (LDI) techniques. Thermal mechanisms are more dominant for low irradiances at LDI than for high irradiances (109 W cm-2) at LA. Very important is the heat transfer influenced by the thermal conductivity of NPs12. Effort to obtain signal enhancement led to the use of NPs also in the field of LIBS. The first study presented a signal increase by the presence of NPs on the sample surface at LIBS of nondesiccated plant leaves13. Without NPs, the yielded LODs were inferior compared to ICP. Later, the following two aspects were theoretically suggested and experimentally confirmed with ns Nd:YAG lasers - the ablation threshold is significantly lowered on metallic materials14; the magnitude of signal amplification induced by the presence of NPs depends on the diameter of NPs, distances between NPs, the incident laser wavelength and the irradiance15-17. However, the two-fold amplification for the lines Cu I 510.6, 515.32, 521.86 and 578.29 nm was also observed on a copper target using a 50 fs laser18. An intensification of molecular bands of AlO B2Σ+→Χ+Σ+ band system was also observed while using AgNPs19. The amplification of the band emission at longer gate delays - around 50 µs - was due to the higher primary concentrations of Al atoms enhanced by NPs surface effects mentioned above. The LIBS sensitivity and LODs at analysis of solutions were substantially enhanced by two effects: the preconcentration effect of drying a solution drop on a substrate and the NPs effect, when NPs were deposited onto the substrate prior to the sample drop20. A recent study on the effect of Ag-NPs with combination of lowered pressure in air showed best NPs-induced signal enhancement at the pressure of 400 mbar for the Pb I 405.78 nm line21. Among the other parameters such as the gate delay and the pulse energy, it was only the ambient pressure which dominantly influenced the signal enhancement by NPs. An optimization of the LA process, namely the part of a lasersample interaction, lies in the elimination or substantial suppression of matrix effects, the production of regular craters (best smooth cylinder without any melting and cracks) and the best level of analytical signal. To fulfil these requirements, short laser pulses, short lasing wavelengths and a sufficiently high irradiation are recommended. That is because of a shorter penetrating depth and a shorter heat propagation path in the sample body. Non-thermal ablation is thus required to reduce selective evaporation5, 22, 23. These items are mostly valid for both LIBS and LA-ICP-OES/MS techniques. While the irradiation could be relatively easily controlled and optimized, the laser wavelength depends on the quality of a particular laboratory’s equipment. , Furthermore, the LA process should ideally produce an aerosol. Its particles’ elemental
composition should fully reflect the elemental composition of the original sample, i.e. stoichiometric representation of the sample5. This requirement applies mainly to LA-ICPOES/MS. To fulfil this requirement the particle size distribution of the aerosol must allow the complete evaporation of the particles in ICP. Also, the ablation cell flushing and transport tubing should not change the particle transport efficiency by their size and composition22. The particles’ sizes and their size distribution are strongly influenced by the laser parameters, the sample properties, and ablation atmosphere (gas, pressure)5. There are also fluctuations in the arrival time and the size of individual ablated particles transported to the ICP24. Surface phenomena at LA-ICP-MS have been studied in terms of aerosol redeposition around the ablation crater25. The redeposition is efficiently reduced by the use of a helium atmosphere instead of an argon one. Also, the sensitivity of the LA-ICP-MS analysis increases. The difference between the ablation yielded from the first pulse on the fresh surface and the one yielded from other pulses may not be as critical as in the case of LIBS. The reason is that the aerosol formation, its flush out of the interaction cell and dispersion in the tubing between this cell and ICP lead mostly to a mix of material from several pulses. Besides, the sample surface is often pretreated to obtain a smooth area. However, monitoring the signal peaks from individual pulses (i.e. mapping the sample surface) at a low repetition rate is possible26. Although NPs in solution are nowadays routinely analyzed with ICP-MS (so NPs are analyte), their effects on the ablation of the sample surface have not been investigated yet. Therefore, our work aims to demonstrate the effect of silver and gold NPs on the measured LA-ICP-MS signal of elements which are constituents of smoothed reference aluminium alloy, brass, glass and glaze samples. EXPERIMENTAL SECTION The used equipment consisted of an ICP-MS Agilent 7500ce (Agilent Technologies, USA) with a quadrupole analyzer and an octopole reaction cell. This instrument was operated at the forwarded power of 1500 W, Ar gas flow rates 15 (outer plasma gas), 0.7 (auxiliary), He carrier 1.0 and Ar make-up gas 0.6 l min-1. The ablated material was transported through a polyurethane tube (i.d. 4 mm, length 1 m) to ICP-MS. For minor elements (isotopes: 208Pb, 63Cu, 26Mg, 57Fe) the dwell time per isotope was adjusted to 0.1 s, for major elements (isotopes: 27Al, 107Ag) to 0.01 s. The total length of one cycle was then 0.246 s. Laser ablation was performed with a New Wave UP 213 system (New Wave Research, Fremont, CA) equipped with a frequency-quintupled pulsed Nd:YAG laser emitting the wavelength of 213 nm at the pulse duration of 4.2 ns FWHM with a flat-top beam profile, and a heliumflushed xyz movable Supercell. We chose an ablation spot of 100 µm in diameter to ensure a sufficient isotopic signal and a representative area containing NPs on the sample surface. The line scan was performed, with ablation speed of 0.5 mm s-1 at the repetition rate of 5 Hz, i.e. a chain of adjacent 100 µm ablation spots was produced and the NPs area was crossed by about 25-32 ablation spots close to the equator. The applied
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Analytical Chemistry pulse energy was varied corresponding with the need of the experiment so that the fluences were: 4.5, 3.7, 3, 2, 1, 0.4, 0.3 and 0.2 J cm-2.
experiment Aluminium alloy AW 2030 was used. The influence of nanoparticles type (composition and size) and the laser fluence is discussed below.
The shape, structure and composition of laser-generated particles were studied after their collection on a polycarbonate membrane filter (25 mm in diameter and 0.2 µm pores, Cyclopore,Whatman) using Scanning Electron Microscopy (SEM, Mira 3, Tescan Orsay Holding, Tescan Brno, Czech Republic; 10 kV, backscattered electrons mode). The shapes and profiles of ablation craters were studied by an optical 3D microscope (Contour GT-X8, Bruker, USA). Aluminium alloy AW 2030 was used as a model sample. The composition was (wt. %): Al (92), Cu (3.9), Pb (1.2), Mg (0.8), Mn (0.6), and Fe (0.1). The sample surface was carefully polished (FEPA 800, 1000, 4000, diamond pastes 3 and 1 µm, cleaned with IPA) prior to the application of drops containing NPs. A drop corresponded to 5 µl of a water suspension containing spherical NPs and was freely dried in the open air onto the sample surface. Typically the created drop was shaped in an almost circular area with a diameter of about 2.73 mm. Other materials has been tested to conclude about the effect of physical properties on the signal enhancement: alloy AW 6082 Al (97), Cu (
