Answers to Questions - Forensic Applications of Gas Chromatography - page 150

Answer: The molecular ion may not be the most stable fragment
formed and therefore may not always be the most abundant.
7. When extracting ions to identify the aromatic components from
the chromatogram from fire debris that may contain an ignitable
liquid, two ions, 91 and 105 (as well as 119) Da, are used. Why use
these ions?
Answer: Alkylbenzene to tropylium ion = 91 Da; 1,2,4-trimethyl-
benzene to methyltropylium ion = 105 Da.
8. Is there a difference between an accelerant and an ignitable liquid?
Answer: Yes. An accelerant is used to help increase the rate at which
fire will spread or can make the fire more intense. An ignitable
liquid is one that will ignite readily in the presence of a source
of ignition. Not all ignitable liquids are used as accelerants; they
can be present normally in the fire environment (e.g., paint thin-
ners and paint brushes stored inside a garden shed that catches
fire). The paint thinners were being stored in the shed rather than
being used to accelerate the fire.
9. In the example of the fire debris analysis by GC-MS (Section 8.4.7),
SPME was used as the sample introduction technique with a car-
boxen/PDMS fibre. Why was this fibre chosen?
Answer: The carboxen/PDMS SPME fibre is a bipolar fibre that is
typically used with gases and low molecular weight (typically
30-250 Da) compounds.
10. Why is py-GC the 'last resort' when carrying out forensic analysis of
paint?
Answer: py-GC is a destructive technique and these types of tech-
niques should only ever be used as a last resort since the sample
is used up in the analysis.
11. Why are molecular ions not investigated in py-GC analysis of paint?
Answer: As pyrolysis is used to break down components in the paint
sample thermally, we will not use molecular ions to identify
paint components; instead, we will consider the pyrolysed prod-
ucts from the paint components.
12. Why is urine sometimes added to adulterated perfume?
Answer: Urine has been added as a stabiliser in fraudulent perfume.
Stabilisers are usually added to stop or slow the degradation of
components in perfume. (These compounds are also used in
food products for the same reasons.)
13. Why are high-value goods adulterated?
Answer: By their very nature, high-value goods are open to adul-
teration because they are generally sought after and are products
that cost much money to produce or harvest. By adding lower