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Trace Explosives

Analysis

Using an Rtx

-1 GC Column

by Gordon McMillen and Ann Irwin, Forensic Science Northern Ireland**

Background

For more than 30 years the forensic science labora-

tory in Northern Ireland has undertaken explosives

analysis to cope with the continual use of explosives

by terrorists. In many terrorist cases explosives and

firearms are used together, so the laboratory devel-

oped methods to collect and examine both types of

evidence. The trace residue from the discharge of

firearms (cartridge discharge residue [CDR]) com-

prises two components: the primer residue (e.g.,

inorganic metallic particles) and organic residue

(e.g., nitroglycerine from the propellant).

Collection procedure

Two types of laboratory swabbing kits

are manufac-

tured for collecting CDR and explosive residue: one

for suspects and one for scene locations. They each

contain a ball of acrilan fiber moistened with iso-

propanol and heat-sealed in foil envelopes. Gloves

and a disposable boiler suit also are included to

prevent cross-contamination from the analyst during

the collection process. After use, each swab is

placed in a custom-made plastic swab holder

resembling a miniature syringe body, which is then

capped at both ends. Items in the laboratory, such

as clothing, are sampled with balls of acrilan fiber

or, more likely, vacuum sampled using a 25mm

diameter deldrin filter holder (Gelman cat.# 1109)

and a 0.5µm pore size Fluoropore

membrane filter

(Millipore cat.# FHLP02500).

Extraction procedure

The swabs and filter samples can be examined for

explosives only, or for particulate CDR residue and

explosives. An extract is produced for explosives

analysis by centrifuging the samples twice, once

after the addition of an internal standard (1,3-dini-

trobenzene) and once after the addition of an

aliquot of diethyl ether. The final 1mL extract is col-

lected in a 1.5mL gas chromatography (GC)

autosampler vial.

The inorganic extract for particulate CDR examina-

tion is produced by ultrasonicating the swabs or fil-

ter residue in petroleum ether 140-165 for 30 min-

utes. The suspension then is decanted through a

25µm wire mesh filter in a Swinnex

holder

(Millipore cat.# SX 0001300) to remove heavy

debris, then through a 12.5mm diameter Swinnex

holder containing a 1µm pore size Fluoropore

membrane filter (Millipore cat.# FALP01300). After

filtration, the 1µm filter is removed and placed on a

12.5mm diameter aluminium stub, carbon-coated

and examined by automated scanning electron

microscopy/energy dispersive x-ray (SEM/EDX)

analysis.

GC analysis for organic explosives

The ether extracts are analyzed by GC fitted with a

thermal energy analyzer (TEA

, Thermo Orion

Model 543). An Agilent 5890 Series II GC, which has

the pyrolyzer for the TEA

fitted through a hole cut

in the left side of the oven, was used. The pyrolyzer

and TEA

have been modified following some of the

suggestions by Douse.

A Restek Rtx

-1 capillary col-

umn (cat.# 10120, 15m 0.25mm ID 0.25µm df) is

butt-connected to a length of deactivated, 0.25mm

ID fused silica tubing (Restek cat.# 10012) using a

universal Press-Tight

connector (Restek cat.#

20400). This passes through the pyrolyzer directly

into the reaction chamber of the TEA

detector,

alongside a similar length of uncoated fused silica

tubing originating directly from the ozone outlet.

The second inlet on the reaction chamber is sealed

with a blanking plug.

7 8

1. nitrobenzene (NB)

2. orthonitrotuluene (ONT)

3. nitroglycerine (NG)

4. 1,3-dinitrobenzene (1,3-DNB) (internal standard)*

5. 2,6-dinitrotoluene (2,6-DNT)

6. 2,3-dinitrotoluene (2,3-DNT)

7. 2,4-dinitrotoluene (2,4-DNT)

8. 3,4-dinitrotoluene (3,4-DNT)

9. trinitrotoluene (TNT)*

10. penta erythritol tetranitrate (PETN)*

11. cyclo trimethylene trinitramine (RDX)*

Standards are injected at a concentration of 0.1ng/µL,

except for those marked *, which are 0.2ng/µL.

Oven temp:

100°C (hold 3 min.) to 165°C (hold 3 min.) at 35°C/min. to 195°C (hold 3 min.) at 30°C/min.;

Carrier gas:

helium, 2.5mL/min.;

Split ratio:

5.6:1;

Inj.:

198°C;

TEA

conditions:

Interface temp.:

250°C;

Pyrolyzer temp.:

800°C;

The signal output of the TEA

detector is fed to a chromatography data system, which comprises a 4-channel A/D con-

verter and Atlas software from Thermo Labsystems Ltd., a Thermo Electron Corporation Company

Figure 1

An Rtx-1

column is ideal for assessing performance and response for explosives standards.

Five microliters of sample and standard are injected

from the autosampler into an injector port fitted with

a cup-split liner containing a small amount of fused

silica wool (Restek cat.# 20790). The system is cali-

brated to assess performance and response with a

combined standard solution (Figure 1). Casework

samples are analysed without further cleanup and

those samples with peaks lying within a specified

retention time window are subject to confirmation.

HPLC Confirmation analysis

Positive results indicated on the GC/TEA

system must

be confirmed by a second analytical system before the

results can be reported in a statement to the police.

The method of choice is high performance liquid

chromatography (HPLC) with electrochemical detec-

tion (ECD) at a pendant mercury drop electrode

using an autosampler.

Prior to HPLC analysis, the

samples are cleaned using SPE extraction

and a col-

umn containing 40mg of a mixture of Chromosorb

104 and Amberlite

XAD

-4 in the ratio 3:1.

Conclusion

The GC/TEA

system has been the Forensic Science

Northern Ireland “workhorse” for many years, pro-

cessing many thousands of samples a year during our

busiest periods. We have been using Restek columns

and accessories for a number of years and have

found the products to be very reliable, producing a

consistent analysis. There also is a first-rate customer

backup service, provided by Restek Ireland.

References

Improved method for the trace analysis of explosives by silica

capillary column gas chromatography with thermal energy

analysis detection.

J.M.F. Douse, Journal of Chromatography, 410

(1987) 181-189.

Automated method for the analysis of organic explosive

residues by HPLC with a pendant mercury drop electrode detector.

WJ McKeown and SJ Speers, Science & Justice 1996: 36: 15-20.

Evaluation of improved methods for the recovery and detec-

tion of organic and inorganic cartridge redsidues.

Speers SJ,

Doolan K, McQuillan J and Wallace JS, Journal of Chromatography

1994; A, 674: 319-327.

References not available from Restek.

**Gordon McMillen and Ann Irwin, Forensic Science Northern Ireland, 151 Belfast Road, Carrickfergus, Co Antrim. Tel +44 (0) 28 9036 1835; Fax +44 (0) 28 9036 1900.

g.mcmillen@fsni.gov.uk

✔

Rtx

-1

(fused silica)

Crossbond

100% dimethyl polysiloxane

ID df (µm) temp. limits

15-Meter

0.25mm 0.25 -60 to 330/350°C 10120

For a complete Restek GC column offering,

refer to the annual chromatography

products guide, lit. cat.# 59960.

GC_MS00513