ИСТИНА

Since the knowledge of heavy metal concentrations allows the decision of several scientific tasks in oceanology, the study of slurries and sediments is of great importance in oceanological research. Here some of the abovementioned tasks should be listed: (i) description of geochemistry of the oceanic sedimentary process that allows one to create geochemical model of the modern ocean; (ii) development of a basis for comparative lithological study for modern and ancient oceanic sedimentary formations; (iii) creation of a general scheme for the ore formation in the ocean since new mineral deposits are one of the criteria by which one can judge about the features of a particular type of lithogenesis; (iv) monitoring the amount of trace elements because many of them are of interest to their industrial mining (for example, rare-earth elements). The main markers to search for ore deposits within pelagic sediments are often the ratios of main components, e.g. the ratio of iron to manganese in iron-manganese nodules (IMN) samples. This value is used to determine the age of the sample that is related to its price as time accumulates rare-earth metals in the nodules. Determination is complicated by at least two factors: strong matrix effects and the lack of suitable reference samples for reliable calibration. Therefore in present work the analysis of nodule samples has been performed by calibration-free LIBS technique (CF-LIBS). In order to determine detector's delay (2.5 µs) and gate (0.25 µs) corresponding to local thermodynamic equilibrium, the study of plasma evolution was carried out: plasma temperature was determined by the two lines method (Mn I lines at 447.28 nm and 447.93 nm), and electron density was calculated using Stark broadening parameters of Fe I lines at 517.15 nm. A pulsed laser with a repetition rate of 5 Hz and an energy of 45 mJ/pulse has been used in the experiment. The emission of plume has been projected onto a slit of high-aperture Czerny-Turner spectrometer («HR 320» (ISA, USA) resolution of 45 000 at 400 nm and 120 000 at 520 nm) through the optical fiber. The possibility of semi-quantitative determination of basic components ratio in iron-manganese nodules samples by calibration-free LIBS has been demonstrated: the ratio has varied from 0.2 to 4.2. Comparison between external standard method and CF-LIBS has been performed, and the advantages for CF-LIBS technique have been established.