Mobility differences: Cocaine cutting agents identified
- Published: May 1, 2012
- Author: Steve Down
- Channels: Base Peak
Cutting it fine
The differential mobility spectrometer
located at the front of the mass spec
Image: courtesy Paul Vouros
Street cocaine is rarely pure. It is generally modified with other chemicals known as cutting agents which are added for one of two reasons. Adulterants emulate some of the biological effects of cocaine to increase its perceived quality, whereas diluents act as simple fillers to increase the bulk and subsequently the profits of the drug dealer.
Typical adulterants are caffeine, which gives a kick and can mislead the drug taker, and anaesthetics like procaine and lidocaine. The veterinary drug levamisole is another common adulterant, being found in 82% of seizures up to 2011, as reported by the US DEA. Levamisole looks like cocaine, is a potential stimulant, and is thought to remain undetected in street purity tests. However, it has some unpleasant side effects, drying the user’s skin and causing it to flake off.
From a forensic point of view, identifying cutting agents is a key process because it can link different seizures of cocaine to one original batch. It is often accomplished by GC/MS or LC/MS methods but these have been coming under scrutiny lately due to their timescale. Although the performances of chromatographic methods are adequate, it can take anything from 10-40 minutes to complete one analysis.
Many drug testing labs have substantial backlogs of samples to analyse and this will be exacerbated in the USA where a recent legal judgement could require analysts to appear in court, rather than simply submitting a written report.
One group of scientists has been looking at alternative strategies for analysing cocaine and its cutting agents. Adam Hall from Boston University School of Medicine, and colleagues Stephen Coy, Erkinjon Nazarov and Paul Vouros from Northeastern University, Boston and the University of Southern Florida, elected to try a differential mobility spectrometry-mass spectrometry (DMS-MS) procedure, which they described in the Journal of Forensic Sciences.
Differential ion mobility
DMS-MS has been used in their lab since 2006 and they have proven that it can replace chromatographic separation, greatly reducing analytical time. The researchers have demonstrated its ability to separate peptides, oligosaccharides and metabolomic biomarkers of radiation exposure, and they have now extended the scope to illicit cocaine.
In practical terms, "the interfacing of DMS to an ion trap and to a triple quadrupole mass spectrometer is very simple. In principle it should be easy to place in front of the ion transfer capillary or skimmer in any mass spectrometer, depending on the geometry of the flanges, etc.," Vouros told spectroscopyNOW.com.
They used a commercial DMS unit measuring just 10.0 x 3.0 x 0.5 mm which could be switched on for filtering the electrosprayed ions or switched off for “transparent mode” and no prefiltration. It was connected to an ion trap mass spectrometer but has also been used with a triple quadrupole mass spectrometer.
So, how does this prefiltration of ions work? In the parent technique, ion mobility MS, the ions are passed through a drift tube in an electric field with an inert buffer gas at atmospheric pressure flowing in the opposite direction. The speed of an ion depends upon its mass and geometric structure and the strength of the electric field.
However, in DMS, an asymmetric electric field is applied to the ions as they pass through the drift gas between two plate electrodes. This field switches regularly from a strong field of one polarity to a weaker field of opposite polarity, applied for shorter and loner times, respectively. The ions will tend to move towards one of the plates but the application of a secondary small voltage, known as the compensation voltage (CV), will allow specific ions to pass through the drift tube towards the detector. When the CV is scanned, a range of ions will be detected in turn.
In this study, the sample solutions were injected to the electrospray source for ionisation then directed through the DMS inlet. Ethyl acetate was added to the solutions as a modifier, to enhance separation of compounds that share common structural features, such as cocaine and its primary metabolite benzoylecgonine, and allow baseline separation.
A mixture containing cocaine and five common adulterants (levamisole, tetramisole, lidocaine, benzocaine and procaine) was baseline-separated except for the isomeric levamisole and tetramisole. This was achieved by holding the CV values for each component for 5 seconds only, resulting in separation in 25 seconds.
A second experiment with cocaine mixed with adulterants and diluents (xylazine, hydroxyzine, creatinine and creatine) demonstrated resolution of all components. Separation of cocaine followed by its structural conformation by tandem mass spectrometry occurred within 5 seconds.
For a third mixture containing cocaine and 12 cutting agents, a complex mass spectrum was recorded when the DMS unit was switched off, but the common contaminant levamisole was detected at its characteristic CV, again within 5 seconds.
The sensitivity of DMS-MS was illustrated by mixing five adulterants with cocaine in a ratio of 5000:1. With DMS off, the protonated molecule of cocaine was not visible in the mass spectrum, but it was clearly present with DMS on.
So, DMS-MS shows great potential for the rapid analysis of cutting agents in illicit cocaine. Some method development is required to select the appropriate voltages for the device but this can be accomplished within 10 minutes and is quicker than the equivalent process for GC or LC methods.
Analysis times for typical drug samples is about 30 seconds, compared with 10-15 and 30-45 minutes for GC and LC methods, respectively, giving huge time savings. The device also has a distinct price advantage, costing a fraction of chromatographic systems.
The main problem for forensic labs at the moment is that DMS-MS is not recognised by the Scientific Working Group for the Analysis of Seized Drugs (SWGDRUG) or the US courts. However, "as a variant of IMS, DMS may be accepted by SWGDRUG as a Category B analytical technique in the future with additional published applications and acceptance by the forensic science community as a valid technique," said Vouros.
The next stage for the research team is to establish the quantitative performance of the DMS system for the cutting agents in cocaine, and to extend the method to the rapid quantitation of drugs and their metabolites in biological media.
Journal of Forensic Sciences 2012, 57, 750-756: "Rapid separation and characterization of cocaine and cocaine cutting agents by differential mobility spectrometry-mass spectrometry"
Article by Steve Down
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.