Assessment of a low-energy high repetition rate DPSS laser for quantitative analysis of marine ferromanganese nodules by LIBS

Abstract

Analysis of oceanographic objects aboard a ship requires a method capable of express direct analysis. Laser-induced breakdown spectroscopy (LIBS) can be an adequate solution for this task. However, conventional laboratory setups are too bulky and fragile for on-board application. For this reason, we present a study on the application of a prototype mobile setup for determination of Mn, Fe, Ca, Sr and Ti in marine ferromanganese nodules (FMNs) from the Kara Sea. We investigated the miniaturisation of the experimental setup and considered possible inhomogeneity of FMNs by using a portable air-cooled DPSS laser (λ = 527 nm, E = 0.25 mJ per pulse) with high repetition rate. Additional miniaturisation was studied by comparing two different detection systems – a portable non-gated CCD spectrometer (spectral range of 400–505 nm, resolution up to 5000, 2048 pixels) and an ICCD camera coupled with a high-aperture Czerny–Turner spectrograph (spectral range of 200–530 nm, resolution up to 150 000). Both detectors showed fairly similar R² of calibration – 0.971–0.999 for the CCD and 0.984–0.999 for the ICCD. We compared the LIBS results with those obtained by energy-dispersive X-ray fluorescence (EDXRF) and used ICP-AES as a reference method. It is shown that determination of Fe and Mn by EDXRF and LIBS with an iCCD camera provides similar results while miniaturisation by means of a compact CCD spectrometer leads to a decrease in accuracy by ∼65%. On the other hand, LIBS is able to provide up to 3 times better results than EDXRF for Ca, Sr and Ti. Further statistical treatment shows that LIBS results are generally closer to the reference rather than EDXRF in terms of median values. The difference between the CCD and iCCD here lies in the interquartile range which is wider for the majority of elements in case of the portable spectrometer. Thus, LIBS can be considered a valuable tool for rapid onboard analysis of complex marine FMN samples prior to more accurate yet time-consuming analysis via methods such as ICP-AES/MS.