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Efforts to manage gas emissions from animal feeding operations are expected to create a need for economical and effective methods to assess mitigation practices. The purpose of this project was to compare gas concentrations measured using semi-continuous sampling with dedicated gas analyzers versus concentrations obtained using a system that fills 50 L bags over 24-h periods. Sampling was done on two flat-bed biofilters with new media, 5 cm or 10 cm, treating air from swine nurseries. Concentration ratios and percent reductions were calculated and compared using concentrations from both sampling methods. The results indicate that 24-h bag concentrations and percent reductions tracked the semi-continuous concentrations and percent reductions well. Ratio means for NH3, CO2 and N2O were close to one indicating similar concentrations using the two methods. The H2S ratio mean was over 1.65 and CH4 ratio means were less than one. Mean percent reduction differences were mixed. CH4 and N2O mean percent reduction differences were near zero but H2S mean percent reduction differences were negative and NH3 mean percent reduction differences were inconsistent. The flat-bed biofilter with 10 cm media removed 61 to 73% of the NH3 emitted while the flat-bed biofilter with 5 cm media removed between 6 and 15%. Both biofilters removed between 42 and 65% of the H2S. The 10 cm media did not generate N2O.