Chemical Analysis of Zooplankton by Calibration-Free Laser-Induced Breakdown Spectroscopy
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TROSCOPY AND PHYSICS OF ATOMS AND MOLECULES
Chemical Analysis of Zooplankton by Calibration-Free Laser-Induced Breakdown Spectroscopy N. I. Sushkova, N. V. Lobusb, I. V. Seliverstovaa, and T. A. Labutina, * a
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Faculty of Chemistry, Moscow State University, Moscow, 119234 Russia Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, 117997 Russia * e-mail: [email protected] Received October 20, 2019; revised February 7, 2020; accepted May 29, 2020
Abstract—Ratios of light metals (Li, Na, K, Mg, and Ca) in zooplankton (Calanus spp.) were determined by laser-induced breakdown spectroscopy without reference samples under the assumption of local thermodynamic equilibrium. The temperature of plasma formed as a result of laser ablation of zooplankton was estimated from the CN rotation–vibration bands, and the electron density was estimated from the Stark broadening of Mg I 383.23 nm, Li I 610.37 nm, and Ca II 396.85 nm lines. These experimentally determined parameters were used for plasma modeling, taking into account the transport of radiation. This made possible a selection of analytical emission lines free from self-absorption. The results are compared with the results of elemental analysis by atomic emission and inductively coupled plasma mass spectrometry. The influence of the ionization equilibrium on the correctness of results is discussed. The described method is proposed to be used for direct semiquantitative determination of Li, Mg, and Ca ratios in zooplankton. Keywords: laser-induced breakdown spectroscopy, calibration-free analysis, plasma diagnostics, light metals, ionization, zooplankton DOI: 10.1134/S0030400X20090222
INTRODUCTION The analysis of biological samples is hampered by the high variability of the composition and heterogeneity of the matrix. In addition, a rapid, not highly time- and resource-consuming analysis is of interest. Usually, biological samples are analyzed by atomic spectroscopy for heavy metals, which is necessary for the needs of environmental monitoring [1]. However, there is a need to determine light elements as well. In particular, the ratio of Mg and Ca contents serves as an indicator of the growth conditions of marine organisms [2]. In addition, in recent years it has been established that the Arctic copepod species of the genus Calanus accumulate high amounts of lithium (along with arsenic and uranium) [3, 4]. It should be noted that the ratio of light metals Li/Na serves as an indicator of the physiological state of an organism in diapause [5]. In direct determination of light elements in planktonic organisms, laser-induced breakdown spectroscopy (LIBS) has marked advantages over other methods. This rapid method does not cause significant destruction of the sample, does not require complex sample preparation, and is characterized by a good sensitivity to light elements (in contrast to X-ray fluorescence spectroscopy typically allows only reliable determination of elements heavier than Na [6, 7]).
There are already a number of studies devoted to the
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