Varian 450-GC, User Manual

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Maintenance
Page: 428 User Manual 450-GC Varian, Inc.
General GC Maintenance
The common general GC maintenance tasks are checking and changing gas
supply cylinders, leak checking and changing gas purifier filters. These should
be performed at the frequency suggested above. The following procedures are
critical to the successful long term operation of a Gas Chromatograph. It is very
important to leak check the system on a routine basis and to ensure the quality
of gas supplies, particularly the carrier gas and H
2
(Safety Reasons).
Check and Renew Gas Supplies
The pressure of the various GC gas supplies should be checked on a weekly
basis and the following guidelines used for frequency of renewing supplies:
Carrier gas: The carrier gas supply cylinder should be changed when its
pressure drops below 200 psi. This ensures that high purity carrier gas is
always supplied to the instrument. With typical usage on one Gas
Chromatograph an A-size cylinder of carrier gas should last for 3 to 6 months.
Note that this usage includes also using the carrier gas as the supply of make-
up to the detectors. When a new cylinder is installed the regulator and tubing
should be purged with carrier gas before connecting to the GC. This will avoid
introducing a large amount of air into the GC. Flush the GC for a few minutes
before heating the Column.
Detector fuel gases: The typical detector fuel gases are hydrogen and air.
These should be changed when the supply pressure drops below 200 psi.
Typically an A size cylinder of hydrogen used on one GC with one Flame
Ionization Detector lasts about 6 months. However, due to the high flow rate of
air used in a Flame Ionization Detector this supply will need renewing more
frequently. For this reason it is worthwhile to consider a clean supply of
compressed air for FID equipped instruments.
Leak Check
Leak checking is one of the most important maintenance tasks for any Gas
Chromatograph. The following information refers to general leak checking
procedures for the 450-GC. Specific information regarding individual 450-GC
components will be presented in the relevant part of this maintenance section.
Leaks in the GC system can lead to poor chromatographic performance or
damage components such as the analytical column. The presence of oxygen in
the GC carrier gas at elevated temperatures can lead to permanent column
phase degradation. While the use of an oxygen filter on the carrier supply to the
instrument can help, leaks downstream of the filter are generally more likely to
be the problem.