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Agilent 6890 Gas Chromatograph Service Manual
250 Volatiles Interface
Theory of operation
The volatiles interface (VI) introduces a gas sample into the gas
chromatograph (GC) from an external device such as the headspace, purge
and trap, or air toxics samplers. The interface has a small volume and is highly
inert, thus ensuring high sensitivity and resolution for applications requiring
trace level detection.
Total flow to the interface is measured by a flow sensor and is divided into
two streams. One stream connects to the purge regulator; the other connects
to a frit block. At the frit block, the flow is further divided. The first stream
goes to the gas-phase sampler and from there is introduced into the interface.
The second stream, called the pressure sensing line, passes through the frit
block and is measured by a pressure sensor. This stream also provides a trickle
flow to the interface.
There are three modes of operation available—split, splitless, and direct. The
pneumatics vary for each operating mode and are discussed in more detail
later in this chapter. The volatiles interface is not supplied with manual
pressure control.
Split mode
When you introduce a sample in the split mode, a small amount of the sample
enters the column while the major portion exits from the split vent. The ratio
of split flow to column flow is controlled by the user. The split mode is
primarily used for high concentration samples when you can afford to lose
most of the sample out the split vent and for samples that cannot be diluted.
During Pre Run, during sampling, and after sampling, total flow to the
interface is measured by a flow sensor and controlled by a proportional valve.
Flow at the head of the column is back-pressure regulated. Pressure is sensed
upstream from the proportional valve.
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