Review
From single pulse to double pulse ns-Laser Induced Breakdown Spectroscopy under water: Elemental analysis of aqueous solutions and submerged solid samples

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Abstract

In this paper the developments of Laser Induced Breakdown Spectroscopy (LIBS) underwater have been reviewed to clear up the basic aspects of this technique as well as the main peculiarities of the analytical approach. The strong limits of Single-Pulse (SP) LIBS are discussed on the basis of plasma emission spectroscopy observations, while the fundamental improvements obtained by means of the Double-Pulse (DP) technique are reported from both the experimental and theoretical point of view in order to give a complete description of DP-LIBS in bulk water and on submerged solid targets.

Finally a detailed description of laser–water interaction and laser-induced bubble evolution is reported to point out the effect of the internal conditions (radius, pressure and temperature) of the first pulse induced bubble on the second pulse producing plasma. The optimization of the DP-LIBS emission signal and the determination of the lower detection limit, in a set of experiments reported in the current scientific literature, clearly demonstrate the feasibility and the advantages of this technique for underwater applications.

Introduction

Laser Induced Breakdown Spectroscopy (LIBS) is defined as the optical emission spectroscopy of the plasma produced by laser–matter interaction, and has been largely investigated in the last two decades in many applications of Laser Induced Plasma (LIP) [1], [2]. Several works on LIBS as an analytical tool for in-situ determination of the elemental composition of solid samples have highlighted a great number of advantages including fast response and high sensitivity (generally in the ppm range), the wide range of materials that can be investigated avoiding the use of both a pumping system and a chamber for controlled background environment, no preliminary sampling or surface treatment, flexibility of the experimental set-up configuration [3], [4], [5], [6], [7]. LIBS has been also employed in remote sensing with a good spatial resolution for distances up to hundreds of meters by setting up suitable optical systems for laser focusing and emission detection [8], [9]. Furthermore, LIBS has been used for the characterization of LIP processes occurring at low pressures during the production of thin solid films and nano-particles of different materials [10], [11], the control of industrial processes and medical applications [12], [13], [14], [15]. These investigations have led to a good understanding of the fundamental processes occurring in various experimental conditions particularly the distribution of excited-state species in the plasma depending both on the expansion fluidynamics of ablated particles and the balancing of elementary processes [16]. These aspects have been described extensively both theoretically and experimentally, and related models have been applied successfully to the interpretation of data obtained in a wide range of conditions [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. In particular, LIBS analytical peculiarities and the extended knowledge of LIP characteristics suggest that this technique can be efficiently applied in automatic and compact systems to be employed in hostile environments [28], [29], [30].

Recently, double-pulse LIBS (DP-LIBS) is attracting the interest of many research groups [31], [32], [33], [34], [35], [36], [37], because of its better sensitivity and more stable emission signal that can be one to two orders of magnitude greater than that of conventional LIBS, depending on the physical properties of the target and on the energy associated to the upper excited level of the analyte transition. Although the theoretical aspects related to the emission signal improvement are not yet completely understood, the following effects are believed to occur:

  • (a)

    the amount of matter ablated by DP-LIBS is considerably greater than that obtained by conventional single pulse LIBS (SP-LIBS). This effect occurs because, as a consequence of the first laser pulse, the gas behind the induced shockwave is hot enough to warm up the target surface, thus decreasing its reflectivity and increasing penetration depth, which in turn produces, in the case of conductors and semi-conductors, a more efficient ablation operated by the second laser pulse.

  • (b)

    the plasma induced by the second laser pulse expands inside a very hot (temperatures of several thousands Kelvin) and rarefied environment obtained thanks to the plasma expansion of the first pulse. This effect determines a longer decay time because the energy transfer from plasma to the surrounding environment occurs slower than in conventional LIBS, thus causing a higher average temperature and a more stable emission signal.

In case of under water measurements, the improvement of the emission signal obtained by DP-LIBS represents a unique feature with respect to SP-LIBS, inducing plasma expansion in a hot water vapor bubble instead of in the liquid water environment [38], [39]. Therefore, DP-LIBS enables LIBS applications in submerged environment where the possibility of obtaining fast elemental analysis in-situ, thus without clawing back to the laboratory on land, is extremely attracting, especially in oceanographic investigations [40], archeological diagnosis of submerged treasures [41], on-line control of cooling systems in plants and industrial processing, and various biological and medical applications [42].

The aim of this review is to provide a general description of the basic aspects of underwater LIBS and of the peculiarities of DP-LIBS as an invaluable analytical tool for the elemental analysis of bulk water and submerged solid samples.

Section snippets

Experimental

In general, the experimental set-up configuration of LIBS is extremely flexible according to the specific requirements of the concerned application. Basically, it consists of a pulsed laser source able to deliver enough irradiance (> 0.9 GW cm 2) [2] to produce the breakdown of sample surface and a spectrograph for the detection of the emission signal. Several laser sources with different characteristics are employed, including fundamental and doubled Nd:Yag, excimer, ruby, gas laser, etc. [1],

Laser–water interaction

The interaction between a laser beam and water has been widely studied in several applications. The massive use of laser sources in medical issues pushed research on related phenomena such as formation of LIP in liquids and laser-induced cavitation bubbles [2], [42], [57]. Laser–water interaction at high irradiance (≈ 1012 W cm 2) includes a wide range of phenomena such as non-linear effects, molecule orientation and liquid contraction [2], electron hydration [58], [59], laser filamentation and

Plasma emission spectra

Conventional SP-LIBS has been applied for bulk aqueous solution analysis [68], [69], [70], [71], [72], [73]. The laser-induced breakdown in bulk water is characterized by visible plasma emission, bubble formation and shock waves. The breakdown threshold is reached when the energy of the laser pulse can produce plasma hot and dense enough to yield a visible emission [42]. The breakdown threshold in water is substantially higher than that in the solid phase since a great amount of the laser pulse

Inter-pulse delay effects on the DP-LIBS emission spectrum

The DP-LIBS in bulk water is obtained by focusing two suitably delayed laser pulses in the same volume, directly into the water solution or onto a submerged target. For the following discussion we refer to the collinear geometry configuration mainly applied to a solid target [38], [39], [41], [46].

Considering that the duration of LIP in the bubble is of few microseconds, while the life-time of the bubble is in the order of a few hundreds microseconds, the plasma obtained by the second pulse can

Conclusion

In this review, the fundamental aspects of under water LIP have been discussed, and the plasma produced by a single laser pulse under water has been shown to extinguish quickly thus producing a cavitation bubble. The laser-induced bubble has been shown to be an environment that promotes the production of successive LIP by application of a second laser pulse. The temporal evolution of the bubble has been discussed and it has been shown that, during the expansion, both pressure and temperature in

Acknowledgments

The authors would like to thank Dr. A. Casavola, Dr. F. Taccogna, Dr. G. Colonna and Dr. G. Senesi from CNR-IMIP sec. Bari for taking part in the scientific discussion. This research has been partially supported by M.I.U.R under the contracts MIUR PON, TECSIS “Tecnologie diagnostiche e sistemi intelligenti per lo sviluppo dei parchi archeologici del sud d'Italia”, Prot. MIUR 12905 and MIUR FIRB Prot. RBAU01H8FW “Dinamica microscopica della reattività chimica”.

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