Detection of leachable components from conventional and bulk fill resin composites (high and low viscosity) using liquid chromatography – tandem mass spectrometry (LC-MS/MS) method Aim: Light cure dental composites are the materials of choice in restorative dentistry due to the their esthetic properties, mechanical strength and applicability in minimally invasive procedures. The time-consuming incremental technique has recently been replaced by a bulk technique thanks to the discovery of bulk – filling composites. Ten percent of the elution of resin composites is caused by free monomers. The aim of this study was to investigate the amount of leachable components (monomers) in high and low viscosity dental bulk-fill resin composites and conventional resin composite materials after polymerization, because it is indicated that they can be eluted in oral environment and may cause cytotoxic reactions on pulp and living tissues. Materials and methods: In this study, six commercially bulk fill dental composites were investigated and compared with six conventional composites. Low and high viscosity composite samples were prepared and light cured according to the standard protocol (20 sec of irradiation with 1200 mW/cm3 in a wavelength range of 380-550 nm) using a LED-curing device, Bluephase G2 and immersed in 75% ethanol solution at 37 °C. After a dark storage period of 24 h, 7 days or 28 days, the eluates were collected and analysed using liquid chromatography coupled with triple quadrupole tandem mass spectrometry (LC-MS/MS). Results: Detectable amounts of 2-Hydroxyethylmetacrylate (HEMA) were found in both Gradia materials and the amount observed across different time points was statistically different (p<0,05), with the amount in solution increasing for Gradia and decreasing for Gradia Direct flo. Bisphenol A diglycidildimethacrylate (BiS-GMA) was found in Filtek and and Tetric materials. Triethylene glycoldimethacrylate (TEGDMA) was detected in all materials. On the other hand, there were no statistically significant differences in the amounts of TEGDMA detected across different time points in either of the tested materials. Conclusion: Monomers HEMA, TEGDMA, 4 –dimethylaminobenzoic acid ethyl ester (DMA BEE) and BiS GMA in bulk fill and conventional composites (high and low viscosity) can be eluted into the oral environment after polymerization. The results indicate that the effect may be ambiguous, as apparently materials from different manufacturers release some monomers more than others.