The use of biogas in the energy sector requires efficient removal of harmful trace levels of impurities. Reliable and robust online analytical equipment to measure these impurities is necessary and must be validated using offline techniques. In this work, total silicon and siloxanes were measured at an industrial solid oxide fuel cell fed by biogas. SOFC anodes are sensitive to silicon-based contaminants even at parts-per-billion levels. Online measurements of total silicon were carried out using a nondispersive infrared sensor (NDIR). Different sampling methods, including chemical adsorption, liquid quenching, and solid-phase adsorption were used at six biogas sampling points. Organic silicon compounds in the samples were determined by offline methods including GC-FID and GC-ICP-MS. The online analyzer's performance was satisfactory in both untreated biogas, where it agreed within the error margin of the most reliable offline technique used, and in clean biogas, where <0.1 mg_Si/m³ siloxanes were detected. Several operational issues with different offline sampling and analytical methods are discussed, as is the performance of the sorbent-based biogas cleaning system used.