Crystals reveal the answer: Detecting drugs with a quartz crystal microbalance
Balancing in the heat
A novel type of high temperature quartz crystal disc could make the perfect detector for small, portable gas chromatography (GC) systems, say US researchers.
Such quartz crystal discs form the centrepiece of quartz crystal microbalances (QCMs), which are widely used as gas sensors and to study surface binding. When a thin quartz crystal disc is hooked up to an alternating electric current it starts to vibrate at a specific frequency. Any analytes that subsequently bind with the surface of the disc alter this frequency, allowing the analytes to be detected, as also happens with microcantilevers.
Although QCMs can happily work with analytes in both gases and liquids, they aren’t commonly used as detectors for liquid or gas chromatography. Part of the reason for this is that they tend to become much less sensitive at high temperatures (above 100°C), because their vibration frequency starts to vary randomly at such temperatures, masking any frequency change caused by the analytes. When combined with GC, this has restricted QCMs to detecting analytes that are volatile at room temperatures.
Polymer with nanotubes
Recently, however, more robust quartz crystal discs have been developed by companies such as Colnatec that can maintain a stable frequency up to 500°C, raising the possibility of using a QCM to detect non-volatile analytes separated by GC at high temperatures. In which case, a QCM would be the ideal detector for a small, portable GC system being developed by scientists from the University of San Diego and Seacoast Science, a Californian chemical sensor company.
‘QCM is known to be highly sensitive, which was a requirement for this study,’ explains Marcel Benz from Seacoast Science. ‘Secondly, the applied QCM model allows operation at high temperatures, which was another must for the project. The detector also needs to be small-sized, lightweight and operable in ambient air as well as inert gas. Finally, no specialty gases (i.e. H2) or vacuum system would be feasible for this mobile instrument.’
To get analytes to bind to the surface of a QCM, the disc is usually covered with a substance that promotes binding, preferably a substance with a large surface area. In this instance, Benz and his colleagues were specifically developing their portable GC-MS system to detect illicit drugs collected by wiping surfaces with swabs. So they covered high-temperature quartz crystal discs in a variety of coatings, including polymer thin films, metal organic framework nanoparticles and carbon nanotubes, and then tested their ability to bind a structural analog of cocaine known as tropine.
What they discovered was that a mixture of a porous polymer film and carbon nanotubes produced the best response at 180°C, increasing the detection sensitivity by three orders of magnitude compared to uncoated crystals. Next, they showed that a combination of GC and QCM with this type of coated high-temperature disc could separate and detect the components of a specially prepared mixture of several illicit drugs, including methamphetamine, MDMA and cocaine. Despite the GC needing to be conducted at temperatures that increased from 120°C to 220°C to separate the drug compounds, the QCM was still able to detect each of them at sub-microgram quantities, with the whole analysis taking less than 10 minutes.
Usually, this kind of on-site drug detection is conducted by ion mobility spectrometry, but the portable GC-QCM system potentially offers a cheaper alternative. Nevertheless, this work was solely done to test the technology and Seacoast Science has no current plans to commercialize the GC-QCM system.
Crystals reveal the answer: Detecting drugs with a quartz crystal microbalance Published: Mar 16, 2015 Author: Jon Evans Channels: Detectors