Electronic Pneumatic Control0 pages

Electronic Pneumatic Control (EPC) of Carrier Gas
Using UNITYe configured with an Agilent 6890GC and Split/Splitless Injector
Introduction
Electronic Pneumatic Control (EPC) was a revolution in
carrier gas control for capillary gas chromatography, when it
was pioneered, in the early 1980s, by Agilent Technologies*
(formerly part of HP). Key advantages of EPC for general GC
methods include programmability of flows, maintenance of
constant flow/pressure throughout a run, flow/pressure
programming (for improving chromatographic speed and the
resolution of late-eluting compounds) and independent
regulation of column and split flow. With EPC, flow rates may
be stored and re-called with each individual GC method. GC
programs thus become more resettable and reproducible;
independent of operator, laboratory environment, column,
etc. Ultimately, the development of EPC has been refined by
Agilent to such an extent that it now facilitates 'Retention
Time Locking' - see below. In effect, chromatographic
separations become almost as stable as spectral data,
allowing the comparison of results from GC systems all over
the world and library searching of retention time databases.
Though desirable for enhanced analytical performance, it
has previously proved difficult and expensive to incorporate
EPC into method-compliant thermal desorbers. This is
because of the many valve- and carrier gas flow path
changes that occur during the analytical sequence (eg. stopflow
leak testing, dry-purging, pre-purging, tube desorption,
backflush trap desorption, split on in standby, etc.). However,
after a collaborative development programme between
Agilent and Markes engineers, back-pressure regulated EPC
has now been successfully and cost-effectively incorporated
into the UNITY thermal desorption platform. This
development should revolutionise thermal desorption-GC
analysis, particularly for complex sets of target analytes. Its
impact on thermal desorption applications is likely to be as
significant as that seen for conventional GC methods when
EPC was first introduced.
Implementation of EPC with UNITY - UNITYe
Back-pressure regulated EPC control is implemented on
UNITY using the standard electronic pneumatics control
module supplied with every 6890 GC split/splitless (S/S)
injector. Some modifications are required on a standard
UNITY, to make it EPC-compatible - UNITYe. The most
important of which involves incorporation of a 'T' piece in the
carrier bypass line, close to the connection point with the
fused silica column/transfer line. This allows accurate
monitoring and closed-loop control of column head pressure,
whatever the stage of thermal desorption operation and
whatever the required flow. Use of a low-volume 3-way valve
on each of the two gas connection lines would allow one
EPC module to be applied, interchangeably between TD and
conventional liquid injection, if required.
Once configured, a UNITYe-based TD system coupled to a
6890 GC or GC-MS system offers all the performance
enhancements, associated with constant/programmed
pressure control on standard liquid injection systems.
Registered in England No. 3414783 VAT Registration 700 8403 76
Mi BEPC
Figure 1. Schematic of UNITYe showing primary desorption step
Agilent EPC Module
Markes TDU
closed loop back pressure feedback
Split line not used
Split/re-collection tube To column
Split vent
Trap vent
Flow
sensor
Proportional
valve PV1
Flow
limiting frit
Cold trap
Sample tube
Pressure
sensor
Septum purge
regulator
* Based in Palo Alto California, Agilent Technologies is the world's leading supplier of GC and GC-MS technology and a close partner of
Markes international Ltd. Markes thermal desorption instruments UNITY, UltrA-UNITY and the Air Server are resold by Agilent Technologies,
for environmental health and safety applications, throughout Western Europe.