Forensic Applications of Gas Chromatography - Forensic Applications of Gas Chromatography - page 104

8.3.6 Interpretation of Analytical Results
The interpretation of analytical results depends upon the purpose of analysis.
Interpretation of the analytical data requires access to data on the concentra-
tions of drugs or metabolites in biological samples and knowledge of their
effects, including other factors that may influence these effects.
One of the important points to consider is the condition of the biological
sample (i.e., the level of decomposition). Other factors, such as tolerance to
the drug, should also be considered.

8.3.6.1 A Toxicology Example
In Section 8.3.5.1, the LLE method for extracting both acidic and basic
compounds from a blood sample was provided. In this section, the result-
ing chromatograms and the associated mass spectra will be examined, to
establish what if anything is present in our sample. The method shown in
Table 8.3 was used in the analysis.
Figure 8.15 shows the total ion chromatogram (TIC) for the acidic
extract from the unknown blood sample. As can be seen, this is a particularly
complex chromatogram. ( Note: The method of pH adjustment and liquid-
liquid extraction applied will extract many of the fatty acid components of
the blood sample, along with any other acidic compounds present.) It is not
necessary to quantify these endogenous compounds; however, we must take
account of them.
Typically, we will analyse the mass spectra for each of the peaks in the
sample. With time, an analyst will become familiar with the mass spectra
for commonly encountered drugs. When examining the mass spectra for the
TIC in Figure 8.15, it was noted that the mass spectrum for diazepam may
have been present. In order to confirm the presence of diazepam, the princi-
pal ion for diazepam (285 Da) was extracted from the TIC and the result is
shown in Figure 8.16.
On examining the mass spectrum for the peak at 14.9 min (Figure 8.17),
it can be seen that the peak is consistent with a fatty acid. Again, analysts
will become familiar with commonly encountered endogenous compounds

Table 8.3 GC-MS Instrument Parameters for the Analysis of an 'Unknown'
Blood Sample a

GC Method Parameters Mass Spectrometry Parameters

Injection volume: 1 μL
Temperature program: 60°C held for 2 min,
then increase to 300°C at 15°C/min
Injection port temperature: 250°C
Carrier gas: Heat a flow rate of 1.5 mL/min
MS transfer line temperature: 300°C

Ion source temperature: 250°C
Mode: positive ion
Full scan range: 40-450 Da

MS detector switched on at a run time of
4.5 min

a Provided courtesy of Dr Alan Langford, Northumbria University.