Troubleshooting in Gas Chromatography - Forensic Applications of Gas Chromatography - page 59

6

Troubleshooting in
Gas Chromatography

6.1 Introduction

The key to success in troubleshooting for GC is largely down to personal and
practical experience over a period of time. Nevertheless, this chapter seeks to
identify some common problems and highlight possible remedies. In most
cases, apart from identifying an issue relating to the separation capability of
the instrument and its performance, the remedy can often only be done by a
trained technician or scientist or representative from the instrument manu-
facturer. To be able to carry out any form of troubleshooting requires that the
individual have a basic 'GC troubleshooter's tool kit' consisting of the follow-
ing (see also Further Reading section):

• Flow meter (capable of measuring lows in the range of 10 to 500 mL/
min): An example of a commercially available flow meter is shown
in Figure 6.1.
• A spare GC syringe (either for manual injection or autosampler,
depending on what is normally used): This syringe should not have
been used before (and should also be in full working order).
• A source of methane or butane: hese are obtainable either from the
natural gas supply in the laboratory or a cigarette lighter, respec-
tively. Their purpose is to provide a position in the chromatographic
run time when the unretained compound—that is, t o (see Section
3.2)—appears. Either gas can be introduced into the hot injection
port using a gas tight syringe (see Figure 7.5).
• New septa, ferrules and injection liners: A septum (see Figure 2.6 or
2.7) should be replaced after every 50-100 injections; the repeated
injection process, which uses the syringe to pierce the septum, even-
tually causes the septum to have a larger hole than initially made
by the syringe. Ultimately, therefore, the septum no longer self-seals
and air can be introduced into the instrument. Also, the repeated
injection process causes parts of the septum to break away and enter
the injection port where they must be removed. A graphite ferrule
is used to attach the capillary column to the output of the injection
port and input of the detector. An air-leak-proof seal is required. On
that basis, new ferrules should be used when the column is replaced.