The objective of this research was to develop a quantitative assay for analysis of electrolytes (Na, K, Ca, Mg) in blood serum by use of simple procedure and single pulse Laser-Induced Breakdown Spectroscopy (LIBS). Serum samples were homogenized and encapsulated in a polyacrylamide gel drop during gel formation and fixed on a silicon (polysiloxan) glue coated glass planch, with two internal standards for signal normalization, better reproducibility and accuracy: Rb was chosen for alkali metals and Ba for alkali earth metals signal correction. Water was gently evaporated on a hot plate at 80oC together with gel formation and fixation, and the resulting gel film, containing the sample, analysed by repeatedly shooting the laser on its surface and accumulating spectra. No additional sample preparation was necessary with 5 to 20 μl of Serum consumption. Limits of detection of 6, 0.6, 1.9 and 1.7 μg/mL for Na, K, Ca and Mg respectively were obtained with the proposed setup. The resulting linearity for the four elements ranged from 3 times below to 2 times above the expected blood serum concentration levels variation, at less than 10% RSD. Finally, the methodology was assessed in the determination of the metals in real samples of human blood serum and the performance compared to a standard, automated clinical laboratory assay where Ca and Mg are determined by complexometry/colorimetry, and Na and K – electrochemically by use of ion selective electrodes, in a biochemical analyzer assembly.