GC-MS (Gas Chromatography-Mass Spectrometry) is a powerful technique for analyzing nicotine in various samples like tobacco, e-liquids, and biological fluids (urine, blood) by separating compounds, identifying nicotine using its mass spectrum, and quantifying it accurately, often requiring sample extraction (liquid-liquid, MEPS) and using techniques like SIM (Selected Ion Monitoring) for high sensitivity and specificity, providing reliable results for quality control, product safety, and exposure studies.