a two - dimensional gas chromatograph with a primary column and dual secondary columns is described . flow rates in the primary column are less than those in the secondary columns due to an accumulation valve . typically the ratio of second and third column flow capacities combined to primary column flow capacities between about 10 to 1 and 30 to 1 . volatile organic compounds are detected in environmental samples of air water , soil and in body fluids of animals and humans .