Answers to Questions - Forensic Applications of Gas Chromatography - page 138

5. What is an internal standard?
Answer: An internal standard is a known compound that is not
present (and is never likely to appear) in the sample. Ideally, it has
a similar chemical structure to the compounds of interest. It is
added, at the same concentration, to both standards and samples
prior to analysis. By measuring its signal response (peak area)
compared to that of the unknown compounds, it is possible to
eliminate signal variation due to imprecise injection technique.
In GC it is common practice when plotting a calibration graph
to plot the peak area (on the y-axis) as the ratio of the peak area
of the compound under investigation divided by the peak area
of the internal standard versus concentration of the compound
under investigation.
6. What is an unreacted silanol group?
Answer: An unreacted silanol group is essentially the -OH (hydroxyl
group) that is present on the surface of silica, which has the
potential to ionize, generating the -O - species, which can itself
interact with polar compounds.
7. What happens to the vaporized gaseous material that does not go on
to the GC column?
Answer: Depending upon the split ratio valve setting, a significant
portion of the vaporized sample goes to waste and not onto the
column. Typically, one part of the vaporized sample goes onto
the column and 50 or 100 parts go to waste. Fortunately, the
waste does not vent directly into the laboratory, as this would be
very dangerous for the user (the inhalation pathway is a signifi-
cant exposure pathway to humans); it passes through a trap that
removes the often toxic organic compounds from the sample or
standard.
8. How much of the GC column stationary phase do you think will be
damaged by the insertion of the syringe needle?
Answer: An approximate 5 cm length of the GC stationary phase
will be damaged by the insertion of the syringe needle. This is not
that important when you consider that the column will typically
be 30 m long. What is more important is what happens to the
removed stationary phase. It will eventually work its way down
the entire length of the column and contaminate the detector,
leading to response issues (see Chapter 6).
9. What might a typical PTV temperature programme look like?
Answer: A typical temperature programme for a PTV injector might
be the following: initial temperature 50°C for 30 s, then 200°C/
min to 250°C, followed by introduction to the column.