Primesep 100 separates guanidine from its sodium counterion on a short 50 mm column by cation exchange. Guanidine is found in urine as a product of protein metabolism and is also used in the manufacture of plastics and explosives. Guanidine is detected by UV detection at 200 nm, but UV cannot detect the sodium ion. If an evaporative light scattering detector (ELSD) is used, both components can be detected. The separation method uses a mobile phase mixture of water, acetonitrile (MeCN, ACN) and trifluoroacetic acid (TFA).