Fentin acetate is a fungicide used in preventing and curing beet cercospora leaf spot, pod and stem blight of soybean, brown spot, purple blotch, false smut of rice, sheath and culm blight of rice, rice narrow brown leaf spot, alternaria leaf rot of onion, leaf spot of celery and alga, snail, of rice. Fentin acetate accumulates in soil and leaches into ground water. Easy and fast methods for determination and quantitation of fentin acetate in soil and other matrices are developed. Methods utilize mixed-mode chromatography and can be used with LC/MS, UV and ELSD. Ammonium formate and other buffers can be used instead of sulfuric and phosphoric acid. Sample preparation requires use of mixed-mode SPE or mixed-mode guard columns.