DART scores direct hit: Applying ambient mass spectrometry to forensic drug analysis

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  • Published: Oct 17, 2011
  • Author: Jon Evans
  • Channels: Detectors
thumbnail image: DART scores direct hit: Applying ambient mass spectrometry to forensic drug analysis

What have we got here

US forensic scientists have shown that combining thin layer chromatography (TLC) with a novel form of ambient mass spectrometry known as direct analysis in real time (DART) provides a particularly efficient and effective way to analyze unknown drug samples.

TLC is already commonly used by forensic laboratories to analyze drug samples, with separated analytes often initially identified by spraying them with fluorescent reagents that emit light when reacting with certain compounds. This technique is quick, cheap and simple, but also rather rough and ready, because analytes are often not clearly separated and a single reagent usually reacts with more than one analyte.

So proposed identifications made by TLC need to be confirmed by more accurate and robust methods, usually gas chromatography-mass spectrometry (GC-MS). But this means that the benefits of speed and ease offered by TLC are pretty much negated by having to analyse the unknown drug by GC-MS, which can take around an hour.

It would be much simpler if the more robust and accurate analytical technique needed to confirm the identifications could be applied directly to the TLC plate. Now, with DART, scientists at the Virginia Department of Forensic Science (Va DFS) in Richmond led by Robert Steiner think they have found a way to do this.

Fetch the DART

In DART, an electrical potential is applied to a stream of gas (usually helium or nitrogen), transforming it into a plasma, which is then fired at the surface of a sample. This converts atoms and molecules at the surface into ions, which are directed into a time-of-flight mass spectrometer (TOF-MS). Because DART is able to ionize samples out in the open air, it potentially offers an ideal way for compounds separated by TLC to be analyzed and identified by mass spectrometry, removing the need for a separate GC-MS step.

Although several researchers have already tried combining TLC with similar forms of ambient mass spectrometry (see TLC samples frolic in the water spray), no one had tried combining TLC with DART for forensic drug analysis. To test the potential for doing this, Steiner and his colleagues used TLC and DART to analyze three simple painkiller mixtures and compared it with GC-MS.

The three painkiller mixtures were oxycodone with acetaminophen (paracetamol), hydrocodone with acetaminophen, and codeine with acetaminophen, and each were formulated as tablets. For each mixture, the two painkiller compounds were clearly separated by TLC and were then each detected by DART-TOF at levels as low as 0.3-0.5mg/mL, making it just as sensitive and accurate as GC-MS. Furthermore, the DART-MS analysis took less than 10 minutes.

Cheaper, easier, simpler

Following this initial success, Steiner and his colleagues have since gone on to simplify the TLC-DART-TOF process. In the original version, the glass plate holding the silica gel stationary phase needed to be physically cut up to remove the segment containing the separated compounds and expose it to the DART instrument. Now, however, Steiner and his colleague have developed a way to scrape the entire silica gel layer off the glass plate, remove the separated compounds with a solvent and then expose them to DART.

'Much easier, safer (no working with cut glass plates) and very efficient as the entire spot is now available for analysis via the DART,' Steiner explains. As a consequence, he is now regularly using this improved TLC-DART-TOF process in his laboratory at Va DFS.

'It is relatively cheap and simple to perform, employing equipment used every day in our laboratory,' he told separationsNOW. 'We have found this technique very useful for not only purifying pharmaceutical standards but also to purify specific components of case samples to obtain "clean spectra" from the DART-TOF for structure elucidation of unknown drugs.'

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

DART scores direct hit: Applying ambient mass spectrometry to forensic drug analysis

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