We report a simple sub parts per million (sub-ppm) detection method for heavy metals in water by laser-induced breakdown spectroscopy (LIBS). Filter papers were used as the substrates for both transforming aqueous solutions to solid samples and for pre-concentrating dissolved heavy metal ions. The amount of sample uptake was 1.2 g by soaking a filter paper. This provides limit-of-detection (LOD) values of 2.7 and 0.36 ppm for Pb and Cr, respectively. The LODs could be improved remarkably by pre-concentrating the heavy metal ions. When a 40 g sample solution was evaporated on a filter paper, the LODs of 75 and 18 parts per billion (ppb) were obtained for Pb and Cr, respectively. Moreover, by either increasing the evaporated amount of sample solution or applying an argon gas flow, further improvement of the LODs was found to be very promising. The LIBS spectra of tap water were recorded using the pre-concentration method and are discussed in comparison with the results from inductively coupled plasma atomic emission spectroscopy (ICP-AES). We could observe strong emission lines of Ca, Mg, K, Cu, and Sr in the tap water, of which concentrations were determined to be 6.3, 1.3, 1.1, 0.64, and 0.046 ppm, respectively, by ICP-AES. Our method shows promise as a fast, reliable, water-quality monitoring tool for heavy-metal concentrations as well as for hardness.

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