Dual column gas chromatographic system for use in mass spectral determination of nitrosamines

A packed gas chromatographic column and a support coated open tubular (SCOT) column are connected in series. Between the columns are two micro-volume switching valves, one enabling solvent to be vented. Short retention nitrosamines are passed through both columns, whereas longer retention nitrosamines by-pass the SCOT column by means of the other switching valve.