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the range being from about m/e 206 to about rnle 520. The parent peak with the highest intensity occurred a t rnle 274,

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A simple cutting, holding and back-flushing arrangement, involving only onoff gas valves, is described, which can be fitted to commercial dual flameionisation gas chromatographs and can be operated from a single pressurecontrolled carrier-gas supply. It does not interfere with the normal independent operation of either column. Any peak or portion of a peak emerging from the first column can be transferred either directly or via an interceptive trap to the second column for further examination (cutting operation). Simultaneously, by equilibrating the pressure across the first column, material remaining on this column can be held (holding operation) for subsequent elution or examination on the second column or, by releasing the pressure at the inlet to the first column, can be eluted in a reverse direction (back-flushing operation). The arrangement has proved particularly useful for determining trace impurities that are not completely separated from the tail of a major component.