Tipping point: Wooden toothpicks for ambient ionisation of illegal drugs

Recovery operations

The ambient mass spectrometry techniques developed in recent years have simplified the steps needed for sample preparation to such an extent that some samples can be analysed directly, without any pre-treatment at all. They are merely placed in position near the ionisation source and analysed. This applies to many types of solids like foodstuffs, pharmaceuticals, plants and human tissue.

Some samples are smaller or difficult to collect, which introduces a preliminary step while the sample is harvested and positioned on an appropriate holder. However, sample collection and manipulation can be united into a single step, using a novel technique devised recently by Asian scientists. In 2011, Zhong-Ping Yao and coresearchers from the Hong Kong Polytechnic University reported the use of a wooden toothpick as sampler.

In a technique they called wooden-tip electrospray ionisation mass spectrometry, the toothpicks can be used to pick up the sample which might be in an awkward place, such as dust in a corner. Alternatively, liquids can be pipetted onto the tip. Good mass spectra were obtained when a high voltage was applied to the tip. The gentle ionisation process was shown to be suitable for a range of compounds, including peptides, proteins, organic and inorganic compounds, slurries and powders.

Now, Yao has used the method to measure the levels of the veterinary drug ketamine and its principal metabolite norketamine in human urine and oral fluid. Ketamine is a widely abused substance and drug testing labs need rapid and accurate methods for detecting and measuring it in biological samples. The ambient methods offer one quick way to achieve these goals.

Sharpened toothpicks

Commercial toothpicks made from birch wood with pointed ends were bought from a supermarket to act as tips. In order to produce good signals, they were sharpened further at one end. Although the modified shape was not perfectly rounded, this did not appear to have any ill effects on the ion currents.

A tip was fitted into the capillary holder of the ion source of a triple quadrupole mass spectrometer. Raw urine and oral fluid were spiked with the two compounds as well as deuterated norketamine to act as an internal standard. A small amount of the solution was added to the toothpick and a voltage of 3.5 kV was applied to the opposite end of the tip, resulting in ionisation of the drugs.

The mass spectrometer was operated in selected reaction monitoring mode, following one specific fragmentation reaction for each of the three compounds. Distinct mass spectral peaks were observed within 20-40 seconds of the voltage being applied and their intensities varied with the concentrations of drugs. The peak heights were readily determined and measurements were complete within one minute.

Illegal drug testing

The calibration curves for ketamine and norketamine were linear over the range 50-5000 ng/mL, which is comparable to those reported for GC/MS and LC/MS studies and fits nicely with the expected concentrations in urine and oral fluid of drug users.

The detection and quantitation limits for both compounds were 20 and 50 ng/mL, respectively. The other analytical data, including accuracy, precision and reproducibility, were also acceptable, confirming that the method is suitable for real applications.

The identities of the ketamine and its metabolite could be confirmed, if necessary, by carrying out product-ion scans. The fragmentation patterns deduced from the spectra were consistent with both compounds and could be measured at low concentrations, typically 50 ng/mL. So, when the wooden tip experiments are routinely carried on a mass spectrometer capable of taking product-ion scans, the presence and levels of ketamine could be determined with ease.

The one-step sample preparation procedure, simply dipping the toothpick into the fluids, transfers sufficient sample for analysis. The shape of the tip prevents significant spreading of the sample, unlike paper spray mass spectrometry, thereby extending the acquisition time.

Sample preparation and analysis are both simple and fast and the wooden toothpicks are very cheap to buy, being about US$1 for 500 in Hong Kong. Remarkably, the toothpicks need no preparation apart from sharpening. Any endogenous compounds present did not interfere with the analysis for ketamine. Once the new procedure has been optimised, it could become an important addition to forensic and drug testing labs.