LIBS Analysis of Liquids and of Materials Inside Liquids

Abstract

Abbreviations and Symbols CILaser induced plasma formation on or inside liquids is characterized by large energy losses due to liquid evaporation. Ablation of a liquid surface is followed by hydro-dynamical instabilities and splashing. Plasma generation inside bulk liquids is affected by the light absorption and scattering, and it is accompanied by intense pressure waves and successive vapor cavitation. Efficient LIBS analyses in presence of liquids require different considerations; the examples are reported and discussed in following. IntroductionLaser induced plasma formation has the irradiance threshold dependent on the sample density, and generally it progressively reduces when passing from gases, to liquids and then to solids. Excluding the losses along the beam path, this means that less laser energy is required for ablation of dry or submerged solid materials than for ablation of a liquid or for generating breakdown inside liquids. Plasma formation in presence of liquids is not efficient because a great portion of the laser energy is expended for the liquid vaporization; this strongly reduces the energy available for the plasma excitation. It has been estimated that about 75 % of the input radiation is consumed for vaporization during laser induced breakdown (LIB) inside water [1]. Furthermore, both water and organic solutions contain hydrogen, which contributes to a rapid thermalization and cooling of the formed plasma. Once the plasma is created in liquids, the high density and nearly incompressible medium strongly confines the plume; the corresponding effects on the plasma evolution and LIBS signal are recently reviewed [2]. Laser ablation (LA) of liquids by short, intense pulses is a complex process where the liquid surface is no longer in equilibrium with the surrounding vapor; this results in a very high net mass flux from the sample surface to the surrounding [3 and references therein]. The high mass flux increases the pressure at the target surface and so raises the boiling temperature. Once the normal boiling is established, the presence of volumetric energy densities slightly higher than that equivalent to the saturation temperature causes the formation and growth of a vapor bubble. If the deposition rate of volumetric energy density by a laser pulse exceeds the energy consumed by vaporization, the liquid is driven to a metastable state until the spinoidal temperature is reached. At this temperature, the liquid undergoes so called ''spinoidal decomposition'' where the entire superheated liquid volume separates into saturated liquid and vapor, which are ejected into atmosphere. This process occurs normally during LIBS sampling of a liquid surface. In case of water ablation it has been estimated that less than 50 % of the deposited energy transforms into vapor while the remaining part is consumed for ejection of droplets [3]. The energy loss on droplet expulsion i.e. splashing further degrades the LIBS signal.LIBS measurements are often performed on water solutions or inside the same. Laser intera...

Signal Enhancement Evaluation of Laser-Induced Breakdown Spectroscopy of Extracted Animal Fats Using Principal Component Analysis Approach

“…18 Literature shows that the conversion of a sample from liquid to solid could reduce splashing and the threshold of plasma formation and result in stronger signal intensity as well as increase the analytical sensitivity. 19…”

Dispersive micro solid-phase extraction (DµSPE) with graphene oxide as adsorbent for sensitive elemental analysis of aqueous samples by laser induced breakdown spectroscopy (LIBS)

“…In an attempt to overcome this problem, different strategies for LIBS measurement of liquids have been developed by many authors. For instance, it has been proved that LODs are largely improved when LIBS measurements are performed on the surface of a liquid or on liquid jets, aerosols or isolated droplets, in comparison with those performed in the bulk liquid [3,4]. In addition, further improvements can be achieved by using the double-pulse LIBS modality (DP-LIBS) [5,6].…”

Laser-Induced Breakdown Spectroscopy Applied on Liquid Films: Effects of the Sample Thickness and the Laser Energy on the Signal Intensity and Stability

“…Rapid chemical analysis of liquids by LIBS [9] has many potential applications; among them, there are monitoring of waters [10][11][12][13], of quality in pharmaceutical or food industry [14][15][16], medical diagnostics [17,18], and control of industrial processes or mechanical equipment [19,20]. Laser-induced plasma on or inside liquids generates intense shockwaves, bubbling, and expulsion of droplets, which together with the laser energy losses due to liquid vaporization reduce the available energy for the plasma excitation.…”

“…Laser-induced plasma on or inside liquids generates intense shockwaves, bubbling, and expulsion of droplets, which together with the laser energy losses due to liquid vaporization reduce the available energy for the plasma excitation. LIBS measurements on liquids usually have low analytical sensitivity, which could be improved by proper experimental approach and sample preparation, as reviewed in [9,21] and updated in [22].…”

“…Here, the breakdown initiates on the substrate and the plasma atomizes and ionizes the liquid layer. In this way, the laser-induced splashes, the plasma formation threshold, and the matrix effect (very strong in case of a direct plasma formation on liquids [9]) are importantly reduced. The LIBS signal strongly gains in intensity and so in analytical sensitivity.…”