Application Note AN 108-01-C
Analysis of Explosives by OPLC
Explosives are complex mixtures that produce heat and large volumes of gases in a short period of time when an appropriate ignition is provided. While these mixtures bstract have a legitimate use in a number of industries (e.g. mining, demolition of old buildings), they are also commonly used in terrorist activities. The analysis of residues after an explosion and the determination of the nature/origin of the explosive material(s) are a critical issue for law enforcement. OPLC provides the investigator with a very powerful tool to assist in the identification of the explosive. A significant benefit of OPLC is its ability to allow for the identification of both strongly retained and weakly retained compounds in a single run. Introduction
Experimental OPLC separations were performed with an OPLC 50 system (Bionisis, Le Plessis-Robinson, France) using a reverse phase (C18) sorbent bed (20 x 20 cm) (Bionisis) and toluene (0.5 mL/min). Detection was performed in the off-line mode; after the separation was completed, the plates were sprayed with KOH (3-5%in ethanol) or Griess’s reagent. The plates were monitored at 510 nm with a densitometer.
The separation of a number of explosive residues is shown in Figure 1 and 2. It is clear that the various explosives can be readily separated via OPLC. Samples 1-3 are compounds that were obtained from an armored vehicle that was destroyed by explosives (sample was removed from the debris by gauze). The chromatograms in Figure 2 present a major advantage of OPLC. In the upper chromatogram, the sorbent bed was sprayed with KOH and a number of compounds were identified. In the lower chromatogram the sorbent bed was sprayed with Griess’s reagent. This ability to use more than one means of detection can be an extremely powerful benefit to the analyst who is looking for confirmation of the presence of an explosive. We note that the standards and the samples were run at the same time; this provides a significant reduction in time and solvent, relative to the use of HPLC, where the various samples must be run individually. Visualisation via Griess’ Reagent 15 14 13 12
Figure 1: Separation of explosive residues by OPLC. Stationary Phase: HTSorb RP18, Mobile Phase: Toluene (0.5 mL/min). Spotting vol: 1µl. Separation Time : 8 min.
SPOTS: 1) NG (nitroglycerin) 2) NGL (nitroglycol), 3) NC (nitrocellulose) 4) TNT (trinitrotoluene) 5) 3,4DNT (3,4-dinitrotoluene) 6) 2,4DNT (2,4-dinitrotoluene) 7) 2,6DNT (2,6-dinitrotoluene) 8) HMX (hexogene) 9) TY(Tetryl) 10) Semtex (RDX[cyclonite]+PETN[pentrite]) 11) sample1 12) sample2 13) sample3 14) Active TNT 15) Active PETN Note: Standards having only one component (TNT,PETN,..) have been spotted as a direct sampling without any dilution or solvent
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When an explosive material is employed in a terrorist attack or other “non-authorized use”, a forensic chemist may be required to determine the type of explosive that was employed from residual materials that may be found at the scene. Optimum performance Laminar Chromatography™ (OPLC) provides the chemist with a rapid and reliable analytical procedure that allows for detection of the residue in a broad variety of matrices.
In addition, it should be noted that OPLC can be used for quantitation of analytes. In Figure 3 we present a part of the output from the densitometer when the sorbent plate was scanned. The spots refer to the compounds that are indicated by arrows in Figure 2.
Sample 1 from fig.2 (4 components)
Visualisation via KOH Sample 1 Semtex TNT NG+NGL 3,4+2,4+2,6 DNT Sample 1
HMX NC NGL NG 2,6 DNT 2,4 DNT
migration Visualisation via Griess’ Reagent
Figure 3: Densitometric scan of sorbent bed. Trace 1 is from KOH spray, Trace 2 is from Griess reagent spray. Both scans were recorded at 510 nm absorbance.
Sample 1 Semtex
TNT NG+NGL 3,4+2,4+2,6 DNT Sample 1
OPLC can provide a significant amount of information regarding the presence of an explosive as it can monitor strongly retained compounds as well as weakly retained compounds in a single run. It provides a number of benefits to other forms of chromatography since several samples can be separated in a single run (both standards and unknowns). In contrast only one sample can be run at a time in HPLC. In addition, OPLC provides much better resolution and efficiency than thin layer chromatography.
NGL NG 2,6 DNT 2,4 DNT 3,4 DNT
Figure 2: Separation of explosive residues by OPLC. Stationary Phase: HTSorbTM RP18, Mobile Phase: Toluene (0.5 mL/min). Spotting vol:1µl (except upper spot, upper figure > Sample 1 0.6 µL). Time analysis: 5 min. Note: components indicated by arrows are only seen with KOH reagent.
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