English to Portuguese Tech/Engineering Engineering (general) Liquefied Petroleum test

Switching (Backflush) Valve (optional)—The valve rests in the “off” state, allowing a continuous back flush flow through the pre-column.
Before or upon injection of the sample, the valve should be rotated to the “on” position so that the pre-column is placed at the head of the flow path from the sample valve.
At a time which must be empirically determined and which is dependant upon the length and type of pre-column used, the valve must be returned to the “off” position, causing the flow to back flush through the pre-column and flush to the detector ahead of components eluting from the analytical column.

This pre-column acts to keep the heavier components away from the analytical column and to back flush the heavier components as a composite peak to the detector for quantitation.