SPME turns up the heat

When I was entering Australia recently, waiting for my luggage to come crashing down the carousel, customs officers with sniffer dogs were patrolling amongst the waiting passengers. I was looking on with interest, anything to stay awake after 24 hours of travelling, when one of the dogs made a beeline for my hand baggage and gave a nod and a wink to its handler. No, I wasn't carrying narcotics but I had been carrying an apple which had already been disposed of.

Sniffer dogs are an invaluable companion to customs personnel and law enforcement officers. They have been trained to detect many different types of volatile organic compounds (VOCs), including those originating from human scent, human remains, drugs and explosives. Although they are highly successful in detecting buried corpses and matching the scent of an individual to that from a piece of evidence, there still remains one key legal problem.

Scent detection is not accepted in court as evidence, although it may be accepted in corroboration. One reason for this legal standpoint is the lack of national standards for scent-discriminating dogs.

A group of scientists in the USA decided that canine evidence could be supported if scent samples were collected and analysed in the field, under similar conditions in which the dog operates. That is, they are collected rapidly over a few seconds at a high sampling rate. This led them to develop a new solid-phase microextraction (SPME) device which can sample air at several hundred cm/s while providing improvements in sensitivity over existing SPME setups.

Brian Eckenrode from the FBI Academy in Quantico, VA, and colleagues from RVM Scientific, Inc., Santa Barbara, CA, the Uniformed Services University of the Health Sciences, Bethesda, MD and the Defense Threat Reduction Agency, Albuquerque, NM collaborated on the project. Their solution involved a high surface area carboxen-polydimethylsiloxane (PDMS) stationary phase coated on a nickel alloy wire.

The 10-cm wire was chosen to have a high resistance to oxidation under repeated cycles of ballistic heating and produced a polymeric surface area that was ten times greater than that of a typical straight SPME fibre. A second essential advantage over fibres was the ability to manipulate the wire. Specifically, it was wound around the outer surface of a glass tube at a constant pitch and the tube was positioned within a second glass tube.

The calculated ratio of the surface area to the gas volume was 1:3, compared with 1:40 for a straight 10-cm fibre having the same phase thickness. Air was drawn through the space between the glass tubes, contact also being aided by the restrictive flowpath, allowing non-equilibrium sampling.

The SPME device was tested on the analysis of BTEX (benzene, toluene, ethylbenzene and o-, m- and p-xylene) as part of a commercial 39-component mixture. The mixture was sampled using two air sampling pumps that achieved reproducibly low and high linear air velocities of 3-140 and 275-1100 cm/s, respectively, corresponding to volumetric flow rates of 0.01-0.5 and 1-4 L/min.

BTEX sampling for 10 s showed maximum efficiency at 40 cm/s, with additional uptake possible at velocities up to 639 cm/s. This high rate allowed controlled extractions in relatively short periods of time.

The trapped analytes were thermally desorbed from the SPME device by resistively heating the wire and transferred to a novel focusing preconcentrator to inject a sharply focused band into a gas chromatograph for GC/MS analysis.

Using quantitation with 4-bromofluorobenzene as an internal standard, detection limits of 0-2-6.9 ppt by volume were achieved for the BTEX components by selected ion monitoring, an improvement of several orders of magnitude over standard SPME with a commercial carboxen PDMS fibre.

Since the SPME device was intended for use in the field, power requirements are an issue. The sampler was designed to be "self-heating" with the ultimate aim of eliminating the conventional gas chromatograph inlet. In connection to a low thermal mass gas chromatograph, the average standby power was 52 W and consumption rose to 101 and 88 W, respectively, for heating the SPME and the preconcentrator. A portable battery could provide 1.5 hours of energy for seven analyses of 10 min each.

The prototype was tested "in the field" in the weapons preparation room at the FBI Academy, where guns are cleaned daily by a solvent containing trichloroethylene and substituted benzenes. The samplers were loaded with internal standard and the air was extracted. The BTEX concentrations from the new sampler agreed with those determined using the commercial fibre.

The heated SPME device allows rapid uptake from the air using a hand-held pump for analysis on a portable, low-power field GC/MS instrument at high sensitivity. The researchers concluded by declaring that it "shows promise for reaching the canine capability model of field sampling." Apart from the forensics scene, it would be suitable for other scenarios such as public safety, border security and military defence.

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