Ethyl viologen dibromide (1,1-diethyl- 4,4-bipyridinium dibromide) has been used as an internal standard in analysis of paraquat and diquat in the serum using high-performance liquid chromatography. It has also been used as as an internal standard in the quantitative analysis of paraquat in biological samples by liquid chromatography- electrospray ionization-mass spectrometry. Ethyl viologen dibromide (1,1-diethyl- 4,4-bipyridinium dibromide) has been used as an internal standard in analysis of paraquat and diquat in the serum using high-performance liquid chromatography. It has also been used as as an internal standard in the quantitative analysis of paraquat in biological samples by liquid chromatography- electrospray ionization-mass spectrometry.