organic GSR Flashcards
1
Q
organic GSR
A
o-GSR
2
Q
GSR derived from
A
the combustion of propellant materials: • Cellulose Nitrate • Nitroglyercol • Nitroguanidine • Most importantly other propellant components such as additives
3
Q
Organic GSR
A
- Arguably of significantly less evidential value than p-GSR (inorganic)
- More common in the environment
- Requires a very different analytical approach
- SEM-EDX inappropriate
4
Q
propellants
A
- Cellulose nitrate alone does not contain the necessary oxygen to combust fully
- Negative oxygen balance
- Ideal combustion of pure cellulose hexanitrate would produce…
- C12H14(NO3)6O7 = 9CO2 + 3CO + 7H2O + 3N2
5
Q
in practice complete combustion does not occur
A
- Some unburned material leaves the barrel without further oxidation often due under-ignition
- Combustion completes upon combination with the atmosphere producing characteristic flash
6
Q
residues of partial combustion
A
• Nitrates and Nitrites along with unchanged additive component
7
Q
Incomplete combustion products
A
- Most commonly
- Unchanged additive components such as Ethyl Centralite
- Nitrates – R-NO3 – In many cases Nitrocellulose itself!
- Nitrites – R-NO2
- These may be of evidential value but this is limited by their presence in the environmental in numerous guises
- Various sources for both classes of compounds
8
Q
Presumptive tests
Dermal nitrate test – Nitrates
A
- Used for many years for the detection of explosives
- Hands or suspect areas covered in a layer of molten paraffin wax
- This involves a suspect dipping hands in a bucket of heated wax (H&S!)
- Allowed to cool and solidify
- Wax ‘gloves’ removed
9
Q
Dermal nitrate test – Nitrates
cast treated
A
- 3% diphenylamine in 95% sulphuric acid
- This is pipetted on drop by drop and observed
- A positive reaction gives a blue colouration in areas where nitrates are present
- Normally tiny blue specks
10
Q
Dermal nitrate test – Nitrates
reaction mechanism
A
- Quite a complex reaction in real terms
- Sulphuric acid reacts with nitrates forming nitric acid
- H2SO4 + 2NO3- = SO42- + 2HNO3
- Nitric acid formed reacts with diphenylamine to form a blue coloured compound diphenylbenzidine blue
11
Q
Dermal nitrate test – Nitrates
considered unreliable
A
- Numerous false negatives
- False positives
- Ferric ion
- Nitrite
- Dichromate
12
Q
Presumptive tests Griess Test (No longer used) – Nitrites
A
- Used for many years for the detection of explosives
- Hands or suspect areas swabbed using dilute Nitric acid
- Swab treated with Griess reagent – diazotisation
- Sulfanilamide (2%)
- Naphthylethylenediamine dihydrochloride (0.2%)
- Phosphoric acid (5%)
- Colour change to pink indicates presence of nitrites
13
Q
Griess Test (No longer used) – Nitrites
considered unreliable
A
- Numerous false negatives
- False positives
- Poor reputation (Birmingham six)
14
Q
Common test for nitrites
Modified Griess test – Nitrites
A
- Used as a presumptive test for location of potential GSR
- Developed to replace the suspected carcinogenic used in the Griess test
- Improvement in colour reaction compared to Griess
• Sulphanilic acid
• Alpha napthol
• Combined and absorbed onto medium
• Acetic acid (15%) steam activates the reaction - Colour change to orange indicates presence of nitrites
15
Q
Modified Griess test – Nitrites
Not to be used a confirmatory test
A
- Numerous false negatives
- False positives
